Interfacial Electron Transfer Dynamics of Photosensitized Zinc Oxide Nanoclusters

Abstract

We have prepared and characterized photosensitized zinc oxide (ZnO) nanoclusters, dispersed in methanol, using carboxylated coumarin dyes for surface adsorption. Femtosecond time-resolved emission spectroscopy allows us to measure the photo-induced charge carrier injection rate constant from the adsorbed photosensitizer to the n-type semiconductor nanocluster. These results are compared with other photosensitized semiconductors. IntroductionPhotosensitized nanoclusters of wide bandgap n-type semiconductors are promising systems for efficient solar energy conversion. A photosensitizing dye adsorbed onto the surface of the semiconductor can inject an electron into the semiconductor, which could possibly be used directly in a solar photovoltaic device or stored in a battery. A system design goal is to select a dyehemiconductor pair for which the ground electronic state of the dye w i l l lie in the bandgap between valence and conduction bands of the semiconductor, while the excited state of this dye will have an energy slightly higher than the sum of the conduction band edge and interfacial electron transfer reorganization energies. Several dyes have been used as photosensitizers, including organic laser dyes, porphyrin derivatives, and inorganic complexes of ruthenium, osmium, and iron. A number of semiconductor materials have been studied for this application, including Ti02, Sn02, Sn&, and ZnO. For an overview of these studies of dye-sensitized electron-transfer at the electrode intedace, we refer to the chapter by by Gerischer and Tributsch?** Photosensitizers included riboflavin, eosin, rose bengal, rhodamine B, and cyanine dyes. Higher efficiency was achieved in similar systems by Tsubomma, et aL6p6 using sintered porous disks of 50. This preparation was used in order to achieve a sufficiently high surface coverage of Zinc oxide electrodes photosensitized by a number of dyes were fist reported adsorbed dye on the ZnO electrode. Similar results using Rhodamine B and RuIIpolypyridyl dye sensitized sintered-porous ZnO were also reported by Alonso, et d 7 T i 0 2 has also been extensively used as a photosensitizer, with the first report for single-crystal rutile photosensitized by Ru(bpy)$+ and its derivatives given by Clark and Sutin.8 A number of researchers have prepared and characterized nanocrystalline ZnO, including Spanhel and Anderson: Bahnemann,lO and Cavaleri, et al. l1 The results mentioned here provide the foundation for the present study.A number of different types of photosensitizers for metal-oxide semiconductors have been studied, including both organic and inorganic complex chromophores. For strong binding between the photosensitizer and the metal-oxide semiconductor surface, a carboxylate, phosphonate, or sulfate group on the photosensitizer molecule is neededl2-I4. In the experiments presented here, we use 7-aminOCO~~narin chromophores that have a carboxylate substituent at the 3-position. The carboxylate, perhaps in concert with the 2-position carbonyl oxygen, binds to one or more metal-cat...

Quantum Dynamics of Photoinduced Electron-Transfer Reactions in Dye−Semiconductor Systems: First-Principles Description and Application to Coumarin 343−TiO2

“…This system has been investigated experimentally by a number of groups. 3,5,6,13,81 Recently, we have studied the electronic injection dynamics as well as the associated nuclear wave packet dynamics in this system employing a semiempirical Newns model. 41 Here, we apply the first-principles model outlined above.…”

“…3,5,6,13,81 This discrepancy can have a variety of reasons. As discussed above, the electron-injection dynamics are not a single-exponential decay but also exhibit a slower injection component as well as oscillatory structures, which may result effectively in an overall longer injection time.…”

Time-Dependent Density Functional Theory Investigation of the Ground and Excited States of Coumarins 102, 152, 153, and 343

“…7,[32][33][34][35][36][37] Another application of the 7-aminocoumarins, especially the 3-carboxylic acid derivatives, has been as photosensitizing dyes for wide band gap semiconductors such as TiO 2 and ZnO. 24,[38][39][40][41][42] Excited-state proton-transfer reactions have also been widely studied using 7-aminocoumarins. [43][44][45][46][47][48] We have recently published a theoretical study of coumarins 151 and 120.…”

A coumarin-derivative dye sensitized nanocrystalline TiO2 solar cell having a high solar-energy conversion efficiency up to 5.6%

“…coumarin 343 (C343) (Scheme 1), are recognized as good organic dye photosensitizers, injecting electrons effectively into the conduction band of semiconductors. 10,11 For example, Murakoshi et al observed an ultrafast electron injection ( < 20 fs) from C343 into the conduction band of TiO 2 . 11 The h values of nanocrystalline semiconductor solar cells using conventional coumarin dyes such as C343 as the photosensitizer is, however, much lower than that of solar cells sensitized by Ru complex photosensitizers because of the narrow absorption area in the visible region.…”