International Journal of Photoenergy

Volume 2013 (2013), Article ID 456583, 7 pages

http://dx.doi.org/10.1155/2013/456583

Research Article

Cosensitization Properties of Glutathione-Protected Au₂₅ Cluster on Ruthenium Dye-Sensitized TiO₂ Photoelectrode

Kazuya Nakata,1,2 Sho Sugawara,1,3 Wataru Kurashige,⁴ Yuichi Negishi,2,4 Morio Nagata,⁵ Satoshi Uchida,⁵ Chiaki Terashima,² Takeshi Kondo,2,3 Makoto Yuasa,2,3 and Akira Fujishima²

¹Department of Applied Biological Science, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan

²Research 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

⁵Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan

Received 1 July 2013; Revised 6 October 2013; Accepted 8 October 2013

Academic Editor: Xie Quan

Copyright © 2013 Kazuya Nakata et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Cosensitization by glutathione-protected Au₂₅ clusters on Ru complex, N719-sensitized TiO₂ photoelectrodes is demonstrated. Glutathione-protected Au₂₅ clusters showed no significant changes in properties after adsorption onto TiO₂ particles, as confirmed by optical absorption spectroscopy, transmission electron microscopy, and laser desorption/ionization mass spectrometry. Adsorption property of the glutathione-protected Au₂₅ clusters depends on the pH, which affects the incident photon-to-current conversion efficiency (IPCE) of the TiO₂ photoelectrode containing Au₂₅ clusters. When pH < 5, the IPCE increases with pH. Conversely, the IPCE decreases with pH when pH > 7. The IPCE of a TiO₂ photoelectrode sensitized by both glutathione-protected Au₂₅ clusters and N719 was increased compared with photoelectrodes containing either glutathione-protected Au₂₅ clusters or N719, which suggests that glutathione-protected Au₂₅ clusters act as a coadsorbent for N719 on TiO₂ photoelectrodes. This is also supported by the results that the IPCE of N719-sensitized TiO₂ photoelectrodes increased upon addition of glutathione. Furthermore, cosensitization by glutathione-protected Au₂₅ clusters on N719-sensitized TiO₂ photoelectrodes allows that wavelength of photoelectric conversion was extended to the near infrared (NIR) region. These results suggest that glutathione-protected Au₂₅ clusters act not only as a coadsorbent to increase IPCE but also as an NIR-active sensitizer.

1. Introduction

Dye sensitization of wide bandgap semiconductors such as TiO₂, ZnO, and SnO₂ 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 [3–6], perylene bisamides [7–9], xanthenes [10, 11], hemicyanines [12–14], and porphyrins [15–18], 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 (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 [21–23], 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., ), 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 TiO₂ 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 TiO₂ photoelectrodes with N719 and glutathione-protected Au₂₅ clusters which act as both adsorbent and sensitizer. Au₂₅ 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, thiolate-protected Au₂₅ clusters have been studied extensively because of their thermodynamic stability, allowing clarification of their molecular and electronic structures [25–29]. Sakai and Tatsuma reported that glutathione-protected Au clusters adsorbed on TiO₂ electrodes exhibited anodic photocurrent in response to VIS and NIR light ( 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 Au₂₅ clusters on cosensitization of TiO₂ photoelectrodes with N719 dye.

2. Experimental Sections

2.1. Preparation of Au₂₅ Clusters

Glutathione-protected Au₂₅ clusters were synthesized according to a procedure reported in the literature with some modifications [31, 32]. Firstly, Au₁₁ clusters were prepared as a precursor of Au₂₅ clusters. A mixture of HAuCl₄·4H₂O (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 NaBH₄ (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 NaBH₄. The precipitate was dissolved in chloroform and evaporated to completely remove water.

To obtain Au₂₅ clusters, the Au₁₁ 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 Au₂₅ 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.

2.2. Preparation of Dye-Sensitized TiO₂ Photoelectrodes

TiO₂ photoelectrodes were prepared by screen printing a TiO₂ paste (Ti-Nanoxide T/SP, Solaronix SA) on fluorine-doped tin oxide/glass (FTO) substrates (Solaronix SA). The TiO₂ 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 TiO₂ layer.

For the experiment examining the pH dependence on the IPCE of the glutathione-protected Au₂₅ cluster-sensitized TiO₂ photoelectrodes, the above TiO₂ photoelectrode with a thickness of 3.4 μm (confirmed by SEM as shown in Figure  S2) was soaked in an aqueous solution (5 mL) of glutathione-protected Au₂₅ 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 μm thick Himilan film was used to assemble the TiO₂ 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 Au₂₅ clusters and N719, the preparation procedures were the same as above, except that 1.7 μm thick TiO₂ photoelectrodes were soaked in solutions containing Au₂₅ clusters (in 5 mL of H₂O, 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 μL). Acetic acid (50 μL) 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 TiO₂ photoelectrodes, the preparation procedure was the same as above, except that a 1.7 μm thick TiO₂ 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 μL) was added to adjust the pH to 3 before the addition of glutathione.

3. Results and Discussion

To check stability of the glutathione-capped Au₂₅ clusters on a TiO₂ photoelectrode, optical absorption spectra were measured. An optical absorption spectrum of water solution containing the glutathione-capped Au₂₅ clusters is characteristic of thiol-capped Au₂₅ clusters and indicates a quantum confinement effect of the electrons in the Au₂₅ 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 Au₁₃ core of Au₂₅ [25, 26, 31, 33]. Strong absorption in the VIS range characteristic of surface plasmon resonance originating from gold nanoparticles was not observed. After a TiO₂ electrode was soaked in an aqueous Au₂₅ cluster solution, a peak at 667 nm was also observed in an optical absorption spectrum of the TiO₂ electrode containing the Au₂₅ cluster, which indicates that electronic properties of Au₂₅ clusters were maintained on the TiO₂ electrode. Structural stability of the Au₂₅ clusters was evidenced by the transmission electron microscopy (TEM) image, as shown in Figure 1(b). The Au₂₅ clusters were almost uniform and dispersed over the TiO₂ particles. The average diameter of the Au₂₅ clusters was 1.7 nm, which is slightly larger than those previously reported (1–1.2 nm) [34, 35]. This is probably caused by aggregation of Au₂₅ clusters induced by the electron beam during TEM measurement. Note that the morphology of the Au₂₅ clusters was stable after the IPCE measurements as shown in Figure  S3. To further confirm the formation of Au₂₅ clusters, the TiO₂ electrode decorated with Au₂₅ clusters was studied by laser desorption/ionization mass spectrometry (LDI-MS) in the negative ion mode. The most abundant peak was centered at 5308 m/z and formed a pattern resulting from a series of fragments and recombined ion peaks that were consistent with Au₂₅(glutathione)₁₈ (Figure 1(c)) [34]. Other Au clusters and aggregates of Au₂₅ clusters on the TiO₂ particles were not detected. These results suggest that the structure of the Au₂₅ cluster does not change dramatically during adsorption onto the TiO₂ particles.

(a)

(b)

(c)

Figure 1: (a) Absorption spectrum of Au₂₅ clusters in water, (b) TEM image of Au₂₅ clusters on TiO₂ particles (scale bar is 20 nm), and (c) LDI-MS spectrum of Au₂₅ clusters on TiO₂ particles (negative ion mode).

The adsorption properties of the Au₂₅ cluster onto the TiO₂ electrode depend on the solution pH. To examine the adsorption properties, the IPCE of the Au₂₅ cluster-sensitized TiO₂ 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 TiO₂ and to allow comparison between electrodes, we prepared thin TiO₂ layers with a thickness of 3.4 μm on FTO substrates, which are thinner than a typical DSSC (~20 μm). The IPCE of the TiO₂ photoelectrode containing Au₂₅ in the pH range 1–7 exhibited a peak around 670–690 nm, which is related to the absorption spectrum of Au₂₅ on TiO₂ photoelectrodes prepared in various pH (Figure  S4) and indicates that photoinduced electron injection into the conduction band of TiO₂ occurs via the excited state of Au₂₅. Note that no peaks in absorption spectra in the range >450 nm for a TiO₂ electrode support the indication of photoinduced electron injection of Au₂₅ (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 Au₂₅ to the TiO₂ conduction band. The IPCE at  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+2.³⁷ In 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 TiO₂ (anatase) surface is 6.89 [37]. Thus, the Au₂₅ clusters adsorb onto the TiO₂ electrode when the pH of the Au₂₅ solution is in the range of 2–6 because the negatively charged −COO⁻ groups of glutathione and the positively charged TiO₂ surface interact electrostatically. In contrast, at pH = 1, almost all carboxyl groups are protonated, so they are not electrostatically attracted to the positively charged TiO₂ surface; therefore, the IPCE is reduced. Furthermore, when the pH > 7, the TiO₂ surface is negatively charged, which strongly suppresses the interaction between negatively charged TiO₂ and negatively charged −COO⁻ groups.

Figure 2: IPCE spectra of Au₂₅ cluster solutions at different pHs.

To examine the effects of the addition of glutathione-protected Au₂₅ clusters on N719-sensitized TiO₂ electrode, the IPCE of glutathione-protected Au₂₅ clusters and N719 coadsorbed onto a TiO₂ electrode were measured (Figure 3). As reference, the IPCE of glutathione-protected Au₂₅ clusters or N719 adsorbed onto TiO₂ electrodes were also measured, respectively. The IPCE of the TiO₂ photoelectrode containing Au₂₅ clusters or N719 exhibited peaks around 675 nm and 510 nm, respectively. The IPCE of the TiO₂ photoelectrode sensitized with both glutathione-protected Au₂₅ clusters and N719 was significantly greater than the ICPE of the TiO₂ photoelectrode sensitized with only N719 and exhibited the characteristic peak originating from N719 at 510 nm. A peak at 675 nm originated from the glutathione-protected Au₂₅ clusters may be hidden in large values of IPCE. It should be noted that the values of the IPCE are higher than the values calculated for addition of Au₂₅ clusters and N719, which indicates that the glutathione-protected Au₂₅ clusters may suppress competitive adsorption and aggregation of N719 and behave as a coadsorbent. The photocurrent-voltage characteristic for the TiO₂ photoelectrode sensitized with both glutathione-protected Au₂₅ clusters and N719 under AM 1.5 solar light illumination revealed that the photoenergy conversion efficiency was 0.034%, which is greater than those of 0.005 and 0.007% for the TiO₂ electrodes sensitized with Au₂₅ or N719, respectively (Figure  S6). To examine the effects of glutathione ligand on Au₂₅ clusters, the IPCE of N719-sensitized TiO₂ photoelectrodes prepared from N719 solutions at pH = 3 with or without glutathione were measured, as shown in Figure 4. The IPCE of the N719-sensitized TiO₂ photoelectrode increased with the addition of glutathione (10 mM). However, further addition of glutathione (100 mM) reduced the IPCE of the N719-sensitized TiO₂ photoelectrode because glutathione began to occupy the TiO₂ surface instead of N719. The amount of N719 molecules on TiO₂ photoelectrodes is 4.1 × 10⁻⁸, 1.2 × 10⁻⁸ and 9.4 × 10−9 mol/cm² for N719, N719 with 10 mM and 100 mM of glutathione, respectively. 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 [21–23]. In this case, glutathione molecules anchored on the TiO₂ 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 TiO₂ photoelectrode sensitized with both glutathione-protected Au₂₅ clusters and N719 is not due to the increased amount of N719. Typically, the amount of N719 molecules on a TiO₂ photoelectrode is calculated from absorption spectra of a solution containing N719 after N719 molecules are desorbed onto the TiO₂ photoelectrode in NaOH aqueous solution [38]. In this case, it was difficult to calculate the amount of N719 molecules adsorbed on the TiO₂ photoelectrodes, because Au₂₅ 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 TiO₂ photoelectrode exposed to N719 alone may be higher than that on the Au₂₅ cluster and N719 coadsorbed TiO₂ photoelectrode. This is because adsorption of N719 onto the TiO₂ surface will be suppressed by adsorption of glutathione-protected Au₂₅ clusters in the TiO₂ photoelectrode containing coadsorbed dyes.

Figure 3: IPCE spectra of TiO₂ photoelectrodes sensitized by glutathione-protected Au₂₅ clusters, N719 and coadsorbed glutathione-protected Au₂₅ clusters and N719, and the calculated Au₂₅ clusters and N719 spectrum.

Figure 4: IPCE of (a) N719-sensitized TiO₂ photoelectrode and after addition of (b) 1 mM and (c) 0.1 mM of glutathione.

Furthermore, the value of IPCE in the NIR region increased in the TiO₂ photoelectrode sensitized with both glutathione-protected Au₂₅ 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 Au₂₅ clusters act as a co-adsorbent to increase the IPCE as well as an NIR-active sensitizer.

4. Conclusions

In conclusion, cosensitization by glutathione-protected Au₂₅ clusters on N719-sensitized TiO₂ photoelectrodes was achieved. Glutathione-protected Au₂₅ clusters were stable after adsorption onto TiO₂ photoelectrodes, as confirmed by absorption spectra, TEM, and LDI-MS measurements. The IPCE of the TiO₂ photoelectrode with adsorbed glutathione-protected Au₂₅ clusters depended on the pH of the preparation solution. Addition of glutathione-protected Au₂₅ clusters increases the IPCE of the N719 adsorbed TiO₂ electrode, and the wavelength of photoelectric conversion was extended to 900 nm in the NIR range. This result suggests that glutathione-protected Au₂₅ 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.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This research was partially supported by the Japan Society for the Promotion of Science (JSPS) through its “Funding Program for World-Leading Innovative R&D in Science and Technology (FIRST Program)” and the Yazaki Memorial Foundation for Science and Technology.

References

B. O’Regan and M. Grfitzeli, “A low-cost, high-efficiency solar cell based on dye-sensitized,” Nature, vol. 353, pp. 737–740, 1991.

A. Fujishima and K. Honda, “Electrochemical photolysis of water at a semiconductor electrode,” Nature, vol. 238, no. 5358, pp. 37–38, 1972.

M. K. Nazeeruddin, R. Humphry-Baker, M. Grätzel et al., “Efficient near-IR sensitization of nanocrystalline TiO₂ films by zinc and aluminum phthalocyanines,” Journal of Porphyrins and Phthalocyanines, vol. 3, no. 3, pp. 230–237, 1999.

M. K. Nazeeruddin, R. Humphry-Baker, M. Grätzel, and B. A. Murrer, “Efficient near IR sensitization of nanocrystalline TiO₂ films by ruthenium phthalocyanines,” Chemical Communications, no. 6, pp. 719–720, 1998.

J. He, A. Hagfeldt, S.-E. Lindquist et al., “Phthalocyanine-sensitized nanostructured TiO₂ electrodes prepared by a novel anchoring method,” Langmuir, vol. 17, no. 9, pp. 2743–2747, 2001.

J. He, G. Benkö, F. Korodi et al., “Modified phthalocyanines for efficient near-IR sensitization of nanostructured TiO₂ electrode,” Journal of the American Chemical Society, vol. 124, no. 17, pp. 4922–4932, 2002.

S. Ferrere, A. Zaban, and B. A. Gregg, “Dye sensitization of nanocrystalline tin oxide by perylene derivatives,” Journal of Physical Chemistry B, vol. 101, no. 23, pp. 4490–4493, 1997.

H. Tian, P.-H. Liu, W. Zhu, E. Gao, D.-J. Wu, and S. Cai, “Synthesis of novel multi-chromophoric soluble perylene derivatives and their photosensitizing properties with wide spectral response for SnO₂ nanoporous electrode,” Journal of Materials Chemistry, vol. 10, no. 12, pp. 2708–2715, 2000.

H. Tian, P.-H. Liu, F.-S. Meng, E. Gao, and S. Cai, “Wide spectral photosensitization for SnO₂ nanoporous electrode with soluble perylene derivatives and cyanine dyes,” Synthetic Metals, vol. 121, no. 1–3, pp. 1557–1558, 2001.

M. Matsumura, K. Mitsuda, N. Yoshizawa, and H. Tsubomura, “Photocurrents in the ZnO and TiO₂ photoelectrochemical cells sensitized by xanthene dyes and tetraphenylporphines. Effect of substitution on the electron injection processes,” Bulletin of the Chemical Society of Japan, vol. 54, no. 5, pp. 692–695, 1981.

K. Sayama, M. Sugino, H. Sugihara, Y. Abe, and H. Arakawa, “Photosensitization of porous TiO₂ semiconductor electrode with xanthene dyes,” Chemistry Letters, vol. 27, no. 8, pp. 753–754, 1998.

Z.-S. Wang, F.-Y. Li, C.-H. Huang et al., “Photoelectric conversion properties of nanocrystalline TiO₂ electrodes sensitized with hemicyanine derivatives,” Journal of Physical Chemistry B, vol. 104, no. 41, pp. 9676–9682, 2000.

Z. Wang, Y. Huang, C. Huang, J. Zheng, H. Cheng, and S. Tian, “Photosensitization of ITO and nanocrystalline TiO₂ electrode with a hemicyanine derivative,” Synthetic Metals, vol. 114, no. 2, pp. 201–207, 2000.

Z.-S. Wang, F.-Y. Li, and C.-H. Huang, “Photocurrent enhancement of hemicyanine dyes containing RSO₃⁻ group through treating TiO₂ films with hydrochloric acid,” Journal of Physical Chemistry B, vol. 105, no. 38, pp. 9210–9217, 2001.

R. B. M. Koehorst, G. K. Boschloo, T. J. Savenije, A. Goossens, and T. J. Schaafsma, “Spectral sensitization of TiO₂ substrates by monolayers of porphyrin heterodimers,” Journal of Physical Chemistry B, vol. 104, no. 10, pp. 2371–2377, 2000.

F. Fungo, L. Otero, E. N. Durantini, J. J. Silber, and L. E. Sereno, “Photosensitization of thin SnO₂ nanocrystalline semiconductor film electrodes with metallodiporphyrin,” Journal of Physical Chemistry B, vol. 104, no. 32, pp. 7644–7651, 2000.

F. Fungo, L. A. Otero, L. Sereno, J. J. Silber, and E. N. Durantini, “Synthesis of porphyrin dyads with potential use in solar energy conversion,” Journal of Materials Chemistry, vol. 10, no. 3, pp. 645–650, 2000.

S. U. Son, K. H. Park, S. J. Lee, H. Seo, and Y. K. Chung, “Synthesis of planar chiral (1,3-disubstituted arene)Mn(CO)₃⁺ cations via addition of nucleophiles to (oxocyclohexadienyl)Mn(CO)₃ in the presence of chiral ligands. Electronic supplementary information (ESI) available: experimental section,” Chemical Communications, no. 11, pp. 1230–1231, 2002.

M. K. Nazeeruddin, F. de Angelis, S. Fantacci et al., “Combined experimental and DFT-TDDFT computational study of photoelectrochemical cell ruthenium sensitizers,” Journal of the American Chemical Society, vol. 127, no. 48, pp. 16835–16847, 2005.

M. Amirnasr, M. K. Nazeeruddin, and M. Grätzel, “Thermal stability of cis-dithiocyanato(2,2′-bipyridyl4,4′dicarboxylate) ruthenium(II) photosensitizer in the free form and on nanocrystalline TiO₂ films,” Thermochimica Acta, vol. 348, no. 1-2, pp. 105–114, 2000.

A. Kay and M. Grätzel, “Artificial photosynthesis. 1. Photosensitization of titania solar cells with chlorophyll derivatives and related natural porphyrins,” Journal of Physical Chemistry, vol. 97, no. 23, pp. 6272–6277, 1993.

M. K. Nazeeruddin, P. Péchy, and M. Grätzel, “Efficient panchromatic sensitization of nanocrystalline TiO₂ films by a black dye based on a trithiocyanato-ruthenium complex,” Chemical Communications, no. 18, pp. 1705–1706, 1997.

F. Odobel, E. Blart, M. Lagrée et al., “Porphyrin dyes for TiO₂ sensitization,” Journal of Materials Chemistry, vol. 13, no. 3, pp. 502–510, 2003.

R. Y. Ogura, S. Nakane, M. Morooka, M. Orihashi, Y. Suzuki, and K. Noda, “High-performance dye-sensitized solar cell with a multiple dye system,” Applied Physics Letters, vol. 94, no. 7, Article ID 073308, 2009.

M. Zhu, C. M. Aikens, F. J. Hollander, G. C. Schatz, and R. Jin, “Correlating the crystal structure of a thiol-protected Au₂₅ cluster and optical properties,” Journal of the American Chemical Society, vol. 130, no. 18, pp. 5883–5885, 2008.

C. M. Aikens, “Origin of discrete optical absorption spectra of M₂₅(SH)₁₈⁻ nanoparticles (M = Au, Ag),” Journal of Physical Chemistry C, vol. 112, no. 50, pp. 19797–19800, 2008.

M. W. Heaven, A. Dass, P. S. White, K. M. Holt, and R. W. Murray, “Crystal structure of the gold nanoparticle [N(C₈H₁₇)₄][Au₂₅(SCH₂CH₂Ph)₁₈],” Journal of the American Chemical Society, vol. 130, no. 12, pp. 3754–3755, 2008.

J. Akola, M. Walter, R. L. Whetten, H. Häkkinen, and H. Grönbeck, “On the structure of thiolate-protected Au₂₅,” Journal of the American Chemical Society, vol. 130, no. 12, pp. 3756–3757, 2008.

Z. Wu, E. Lanni, W. Chen, M. E. Bier, D. Ly, and R. Jin, “High yield, large scale synthesis of thiolate-protected Ag₇ clusters,” Journal of the American Chemical Society, vol. 131, no. 46, pp. 16672–16674, 2009.

N. Sakai and T. Tatsuma, “Photovoltaic properties of glutathione-protected gold clusters adsorbed on TiO₂ electrodes,” Advanced Materials, vol. 22, no. 29, pp. 3185–3188, 2010.

H. Qian, M. Zhu, E. Lanni, Y. Zhu, M. E. Bier, and R. Jin, “Conversion of polydisperse Au nanoparticles into monodisperse Au₂₅ nanorods and nanospheres,” Journal of Physical Chemistry C, vol. 113, no. 41, pp. 17599–17603, 2009.

Y. Shichibu, Y. Negishi, T. Tsukuda, and T. Teranishi, “Large-scale synthesis of thiolated Au₂₅ clusters via ligand exchange reactions of phosphine-stabilized Au₁₁ clusters,” Journal of the American Chemical Society, vol. 127, no. 39, pp. 13464–13465, 2005.

M. Zhu, W. T. Eckenhoff, T. Pintauer, and R. Jin, “Conversion of anionic [Au₂₅(SCH₂CH₂Ph)₁₈]- cluster to charge neutral cluster via air oxidation,” Journal of Physical Chemistry C, vol. 112, no. 37, pp. 14221–14224, 2008.

Z. Wu, C. Gayathri, R. R. Gil, and R. Jin, “Probing the structure and charge state of glutathione-capped Au₂₅(SG)₁₈ clusters by NMR and mass spectrometry,” Journal of the American Chemical Society, vol. 131, no. 18, pp. 6535–6542, 2009.

Z. Wu, J. Suhan, and R. Jin, “One-pot synthesis of atomically monodisperse, thiol-functionalized Au₂₅ nanoclusters,” Journal of Materials Chemistry, vol. 19, no. 5, pp. 622–626, 2009.

D. L. Rabenstein, “Nuclear magnetic resonance studies of the acid-base chemistry of amino acids and peptides. I. Microscopic ionization constants of glutathione and methylmercury-complexed glutathione,” Journal of the American Chemical Society, vol. 95, no. 9, pp. 2797–2803, 1973.

B. Ohtani, Y. Okugawa, S.-I. Nishimoto, and T. Kagiya, “Photocatalytic activity of titania powders suspended in aqueous silver nitrate solution: correlation with pH-dependent surface structures,” Journal of Physical Chemistry, vol. 91, no. 13, pp. 3550–3555, 1987.

G. Xue, Y. Guo, T. Yu et al., “Degradation mechanisms investigation for long-term thermal stability of dye-sensitized solar cells,” International Journal of Electrochemical Science, vol. 7, no. 2, pp. 1496–1511, 2012.