Cosensitization Properties of Glutathione-Protected Au 25 Cluster on Ruthenium Dye-Sensitized TiO 2 Photoelectrode

1 Department of Applied Biological Science, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan 2 Research Institute for Science and Technology, Photocatalysis International Research Center, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan 3Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan 4Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1–3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan 5 Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan


Introduction
Dye sensitization of wide bandgap semiconductors such as TiO 2 , ZnO, and SnO 2 is an attractive research field with considerable significance for solar energy utilization, including solar cells [1] and water splitting [2].As wide bandgap semiconductors absorb only ultraviolet (UV) light, by adding dyes which absorb visible (VIS) light, a larger proportion of solar light can be harnessed.To date, a number of organic dyes, such as phthalocyanines [3][4][5][6], perylene bisamides [7][8][9], xanthenes [10,11], hemicyanines [12][13][14], and porphyrins [15][16][17][18], have been used as dye sensitizers.One typical sensitizing dye is the ruthenium complex di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2  -bipyridyl-4,4  -dicarboxylato)ruthenium(II), known as N719, which assists in realizing high photoelectric conversion efficiency [19,20].There have been many attempts to further increase incident photon-to-current conversion efficiency International Journal of Photoenergy (IPCE) in dye-sensitized semiconductor photoelectrodes.One methodology is to use infrared (IR-) active dyes.Generally, dyes absorb UV and VIS light and are able to convert those photons to current.Solar light covers a wide range of wavelengths, including UV (3% of solar light), VIS (42%), and IR regions.Therefore, to improve the overall photoelectric conversion efficiency in semiconductor photoelectrodes, the whole range of solar radiation, including not only UV and VIS but also near infrared (NIR) light, needs to be accessed.Although the photovoltaic performance of semiconductor-based photoelectrodes sensitized by NIR-active dyes has been greatly improved recently, the spectral response of these cells in the NIR region remains insufficient.
The second methodology used to maximize IPCE is coadsorption reagents, which are typically molecules containing carboxylic end-groups such as chenodeoxycholic acid and taurodeoxycholate [21][22][23], which adsorb together with the dye on the semiconductor surface.The interfacial area of the semiconductor surface favors charge recombination, which should lead to the recapture of injected electrons by the oxidized species of the redox couple present in the electrolyte (e.g., I 3 − ), impairing the total light-to-electrical energy conversion efficiency of the device.Coadsorption suppresses this adverse back electron transfer from the semiconductor conduction band to the electrolyte, increasing the IPCE and total energy conversion efficiency.Furthermore, coadsorbents avoid the problems of competitive adsorption and aggregation of dyes that may induce unfavorable charge or energy transfer and quenching of photoexcited states.
The third methodology is cosensitization using multiple dyes.Ogura et al. reported that a DSSC containing two dyes, black dye and D131, showed a high energy conversion of 11.0% [24].Such high energy conversion was a result of independent contributions of electron transfer from each dye to the TiO 2 electrode.Although many combinations of organic sensitizer dyes have been studied in the context of cosensitization, success has been limited because electron transfer occurs between the dyes, leading to lower total photon energy conversion efficiencies.
In this work, we examine cosensitization on TiO 2 photoelectrodes with N719 and glutathione-protected Au 25 clusters which act as both adsorbent and sensitizer.Au 25 clusters with a diameter of less than 2 nm exhibit optical absorptions in the UV, VIS, and NIR regions because of their multiple narrow discrete electronic levels [25,26].In particular, thiolateprotected Au 25 clusters have been studied extensively because of their thermodynamic stability, allowing clarification of their molecular and electronic structures [25][26][27][28][29]. Sakai and Tatsuma reported that glutathione-protected Au clusters adsorbed on TiO 2 electrodes exhibited anodic photocurrent in response to VIS and NIR light (400 <  < 900 nm) [30].This made the electrodes applicable to the conversion of light to electricity.Each photocurrent action spectrum was consistent with the corresponding optical absorption spectrum because photoelectric conversion is based on the electronic transition between the HOMO and LUMO triggered by absorbed light.
Here, we examine the effects of glutathione-protected Au 25 clusters on cosensitization of TiO 2 photoelectrodes with N719 dye.

Experimental Sections
2.1.Preparation of Au 25 Clusters.Glutathione-protected Au 25 clusters were synthesized according to a procedure reported in the literature with some modifications [31,32].Firstly, Au 11 clusters were prepared as a precursor of Au 25 clusters.A mixture of HAuCl 4 ⋅4H 2 O (118 mg, 0.3 mmol), tetraoctylammonium bromide (190 mg, 0.348 mmol), water (5 mL), and toluene (10 mL) was stirred for 15 min.The organic phase was separated and centrifuged to completely remove the water phase.Triphenylphosphine (235 mg, 0.9 mmol) was added to the organic phase, which was then mixed with NaBH 4 (34 mg, 0.9 mmol) in ethanol (5 mL) and stirred for 2 h.After evaporation of the solvent, the precipitate was washed with water and hexane to remove excess triphenylphosphine and NaBH 4 .The precipitate was dissolved in chloroform and evaporated to completely remove water.
To obtain Au 25 clusters, the Au 11 clusters (4.7 mg) were dissolved in chloroform (7 mL) and then mixed with glutathione (reduced form, 136 mg, 0.4 mmol) in water (7 mL).The mixture was heated under reflux for 5 h.After cooling, the suspension was evaporated to obtain a powder of Au 25 clusters.The presence of glutathione was confirmed by FT-IR spectra as shown in Figure S1 available online at http://dx.doi.org/10.1155/2013/456583.

Preparation of Dye-Sensitized TiO 2
Photoelectrodes.TiO 2 photoelectrodes were prepared by screen printing a TiO 2 paste (Ti-Nanoxide T/SP, Solaronix SA) on fluorine-doped tin oxide/glass (FTO) substrates (Solaronix SA).The TiO 2 photoelectrodes were annealed at 200 ∘ C for 10 min and then at 500 ∘ C for 30 min, resulting in anatase films.Multiple heating steps were performed to avoid cracking of the TiO 2 layer.
For the experiment examining the pH dependence on the IPCE of the glutathione-protected Au 25 cluster-sensitized TiO 2 photoelectrodes, the above TiO 2 photoelectrode with a thickness of 3.4 m (confirmed by SEM as shown in Figure S2) was soaked in an aqueous solution (5 mL) of glutathione-protected Au 25 clusters (1.5 mg) for 24 h, washed with pure water, and then dried under an air flow.The pH of the aqueous solution was adjusted using acetic acid or aqueous NaOH prior to soaking.A 50 m thick Himilan film was used to assemble the TiO 2 electrode with an Au-sputtered FTO electrode (the thickness of the Au layer was 100 nm).The space between the electrodes was filled with a mixed electrolyte containing hydroquinone (55 mg, 0.5 mmol) and tetrabutylammonium perchlorate (34 mg, 0.1 mmol) in acetonitrile (20 mL).
For the experiment investigating cosensitization with glutathione-protected Au 25 clusters and N719, the preparation procedures were the same as above, except that 1.7 m thick TiO 2 photoelectrodes were soaked in solutions containing Au 25 clusters (in 5 mL of H 2 O, 3.0 × 10 −4 M) and/or N719 (3.0 mg) with a mixture of tert-butanol and acetonitrile (volume ratio of 1 : 1, 100 L).Acetic acid (50 L) was added to adjust the pH to 3, and then the mixture was left for 12 h.For the experiment determining the effect of glutathione as a coadsorbent for N719-sensitized TiO 2 photoelectrodes, the preparation procedure was the same as above, except that a 1.7 m thick TiO 2 photoelectrode was soaked in a mixture of tert-butanol and acetonitrile (5 mL, volume ratio of 1 : 1) containing N719 (3.0 mg, Dyesol) and glutathione (10 mM or 100 mM) for 12 h.Acetic acid (50 L) was added to adjust the pH to 3 before the addition of glutathione.

Results and Discussion
To check stability of the glutathione-capped Au 25 clusters on a TiO 2 photoelectrode, optical absorption spectra were measured.An optical absorption spectrum of water solution containing the glutathione-capped Au 25 clusters is characteristic of thiol-capped Au 25 clusters and indicates a quantum confinement effect of the electrons in the Au 25 clusters, as shown in Figure 1(a).A peak is observed at 667 nm (indicated by a filled circle), which is assigned to the HOMO-LUMO transition in the Au 13 core of Au 25 [25,26,31,33].Strong absorption in the VIS range characteristic of surface plasmon resonance originating from gold nanoparticles was not observed.After a TiO 2 electrode was soaked in an aqueous Au 25 cluster solution, a peak at 667 nm was also observed in an optical absorption spectrum of the TiO 2 electrode containing the Au 25 cluster, which indicates that electronic properties of Au 25 clusters were maintained on the TiO 2 electrode.Structural stability of the Au 25 clusters was evidenced by the transmission electron microscopy (TEM) image, as shown in Figure 1(b).The Au 25 clusters were almost uniform and dispersed over the TiO 2 particles.The average diameter of the Au 25 clusters was 1.7 nm, which is slightly larger than those previously reported (1-1.2 nm) [34,35].The adsorption properties of the Au 25 cluster onto the TiO 2 electrode depend on the solution pH.To examine the adsorption properties, the IPCE of the Au 25 cluster-sensitized TiO 2 photoelectrode prepared using different pH solutions is plotted as a function of excitation wavelength, as shown in Figure 2. To avoid fluctuation of IPCE originating from variation of the thickness of TiO 2 and to allow comparison between electrodes, we prepared thin TiO 2 layers with a thickness of 3.4 m on FTO substrates, which are thinner than a typical DSSC (∼20 m).The IPCE of the TiO 2 photoelectrode containing Au 25 in the pH range 1-7 exhibited a peak around 670-690 nm, which is related to the absorption spectrum of Au 25 on TiO 2 photoelectrodes prepared in various pH (Figure S4) and indicates that photoinduced electron injection into the conduction band of TiO 2 occurs via the excited state of Au 25 .Note that no peaks in absorption spectra in the range >450 nm for a TiO 2 electrode support the indication of photoinduced electron injection of Au 25 (Figure S5).The optical absorption peak around 670-690 nm is ascribed to the HOMO-LUMO transition.Thus, the corresponding IPCE is attributed to electron transfer from LUMO of Au 25 to the TiO 2 conduction band.The IPCE at  < 600 nm was also observed, which should be attributed to electron transition from a deeper level, such as HOMO-2 and HOMO-5, to LUMO and that from HOMO to LUMO+1 and LUMO+2. 37In the pH range of 1-5, the IPCE increased with increasing pH.In contrast, the IPCE decreased when the pH > 7. The pKa values of the carboxyl groups of glutathione are 2.05 and 3.40 [36] and the isoelectric point of the TiO 2 (anatase) surface is 6.89 [37].Thus, the Au 25 clusters adsorb onto the TiO 2 electrode when the pH of the Au 25 solution is in the range of 2-6 because the negatively charged −COO − groups of glutathione and the positively charged TiO 2 surface interact electrostatically.In contrast, at pH = 1, almost all carboxyl groups are protonated, so they are not electrostatically attracted to the positively charged TiO 2 surface; therefore, the IPCE is reduced.Furthermore, when the pH > 7, the TiO 2 surface is negatively charged, which strongly suppresses the interaction between negatively charged TiO 2 and negatively charged −COO − groups.
To examine the effects of the addition of glutathioneprotected Au 25 clusters on N719-sensitized TiO 2 electrode, the IPCE of glutathione-protected Au 25 clusters and N719 coadsorbed onto a TiO 2 electrode were measured (Figure 3).As reference, the IPCE of glutathione-protected Au 25 clusters or N719 adsorbed onto TiO 2 electrodes were also measured, respectively.The IPCE of the TiO 2 photoelectrode containing Au 25 clusters or N719 exhibited peaks around 675 nm and 510 nm, respectively.The IPCE of the TiO 2 photoelectrode sensitized with both glutathione-protected Au 25 clusters and N719 was significantly greater than the ICPE of the TiO 2 photoelectrode sensitized with only N719 and exhibited the characteristic peak originating from N719 at 510 nm.A peak Thus, the increase in IPCE is not due to increase in the amount of N719 molecules, which suggests that glutathione acts as a coadsorbent.Generally, coadsorbent molecules possess carboxylic end-groups because such groups can anchor to the semiconductor surface [21][22][23].In this case, glutathione molecules anchored on the TiO 2 surface may suppress back electron transfer from the semiconductor conduction band to the electrolyte and/or avoid competitive adsorption and aggregation of N719.Note that the increase in IPCE in the TiO 2 photoelectrode sensitized with both glutathioneprotected Au 25 clusters and N719 is not due to the increased amount of N719.Typically, the amount of N719 molecules on a TiO 2 photoelectrode is calculated from absorption spectra of a solution containing N719 after N719 molecules are desorbed onto the TiO 2 photoelectrode in NaOH aqueous solution [38].In this case, it was difficult to calculate the amount of N719 molecules adsorbed on the TiO 2 photoelectrodes, because Au 25 clusters also desorbed and were detected along with N719 in the absorption spectrum, preventing the exact amount of N719 from being calculated.However, the amount of N719 adsorbed on the TiO 2 photoelectrode exposed to N719 alone may be higher than that on the Au 25 cluster and N719 coadsorbed TiO 2 photoelectrode.This is because adsorption of N719 onto the TiO 2 surface will be suppressed by adsorption of glutathione-protected Au 25 clusters in the TiO 2 photoelectrode containing coadsorbed dyes.Furthermore, the value of IPCE in the NIR region increased in the TiO 2 photoelectrode sensitized with both glutathione-protected Au 25 clusters and N719, and an IPCE signal was detected up to 900 nm, which is an improvement compared with that sensitized with N719 alone (∼760 nm).It is suggested that the glutathione-protected Au 25 clusters act as a co-adsorbent to increase the IPCE as well as an NIRactive sensitizer.

Conclusions
In conclusion, cosensitization by glutathione-protected Au 25 clusters on N719-sensitized TiO 2 photoelectrodes was achieved.Glutathione-protected Au 25 clusters were stable after adsorption onto TiO 2 photoelectrodes, as confirmed by absorption spectra, TEM, and LDI-MS measurements.The IPCE of the TiO 2 photoelectrode with adsorbed glutathione-protected Au 25 clusters depended on the pH of the preparation solution.Addition of glutathione-protected Au 25 clusters increases the IPCE of the N719 adsorbed TiO 2 electrode, and the wavelength of photoelectric conversion was extended to 900 nm in the NIR range.This result suggests that glutathione-protected Au 25 clusters should behave as both a coadsorbent to increase IPCE and an NIR-active sensitizer, which opens new methodologies for the design of coadsorbents with sensitization properties.

Figure 1 :
Figure 1: (a) Absorption spectrum of Au 25 clusters in water, (b) TEM image of Au 25 clusters on TiO 2 particles (scale bar is 20 nm), and (c) LDI-MS spectrum of Au 25 clusters on TiO 2 particles (negative ion mode).

Figure 2 :
Figure 2: IPCE spectra of Au 25 cluster solutions at different pHs.