Nanocrystalline electrodes

Wang Y, Cheng H, Hao Y, Ma J, Li W, Cal S (1999) Photoelectrochemical properties of metal-ion-doped Ti02 nanocrystalline electrode. Thin Solid Films 349 120-125... 

Charge transport in nanocrystalline electrodes is clearly strongly influenced by the inter-penetration of the solid and liquid phases. If electron hole pairs are generated by band to band excitation, it is usually observed that one type of carrier is transferred to the solution, while the other is transported to the substrate contact. In the case of the dye sensitized nanocrystalline systems, an electron is injected into the conduction band from the photoexcited dye and is then transported to the substrate. The dye is regenerated by reaction of its oxidised state with a supersen-sitiser such as 1 as shown in Fig. 8.25. 

The photocurrent responses of nanocrystalline electrodes to stepped or pulsed illumination exhibit features on rather slow timescales. This is illustrated, for example, by Fig. 8.26, which is a set of photocurrent transients reported by Solbrand et al. [78] for band-band excitation at 308 nm of nanocrystalline Ti02 films of differing thicknesses permeated by 0.7 mol dm-3 LiC104 in ethanol. The 30 ns excimer laser pulse was incident from the solution side, and since the penetration depth of the light was much smaller than the film thickness, electron-hole pairs were effectively... 

If the penetration depth of the light is much smaller than the film thickness, illumination of the nanocrystalline electrode from the electrolyte side is expected to give a characteristic spiral in the high frequency IMPS response (Fig. 8.29). This spiral is typical for a constant time lag (i.e., frequency dependent phase shift), and it arises simply from the transit time required for carriers to move from the front face to the rear contact. By contrast, if the electrode is illuminated through the substrate, electrons are generated close to the contact and the transit time is much smaller (Fig. 8.29). This is reflected in the high value of o)min. 

The IMPS response of nanocrystalline electrodes can be strongly influenced by the RC time constant Tchi- The capacitance of nanocrystalline Ti02 electrodes is strongly dependent on potential, and under accumulation conditions capacitance values in the mF cm-2 range are common. RC attenuation alters the shape of the IMPS response, and in the limit Tceit > Td, the IMPS plot becomes semicircular and is dominated by Further details are given in ref. [72]. 

Nanocrystalline systems display a number of unusual features that are not fully understood at present. In particular, further work is needed to clarify the relationship between carrier transport, trapping, inter-particle tunnelling and electron-electrolyte interactions in three dimensional nan-oporous systems. The photocurrent response of nanocrystalline electrodes is nonlinear, and the measured properties such as electron lifetime and diffusion coefficient are intensity dependent quantities. Intensity dependent trap occupation may provide an explanation for this behaviour, and methods for distinguishing between trapped and mobile electrons, for example optically, are needed. Most models of electron transport make a priori assumptions that diffusion dominates because the internal electric fields are small. However, field assisted electron transport may also contribute to the measured photocurrent response, and this question needs to be addressed in future work. 

The electron concentration can be increased by forward biasing the nanocrystalline electrode electrolyte interface potentiostatically. The interface is thus driven into the accumulation regime for the majority carriers and if a transparent rear contact (e.g., F-doped, Sn02 or Sn-doped indium oxide) is used, the resultant blue... 

Figure 13. Energy scheme for f,u-[Ru (dcbpy)2(NCS)2] dye sensitizer adsorbed on various oxide semiconductors. Molecular levels are based on the values of the oxidation potential of the complex ( °(S /S) = +0.86 V/SCE and excitation energy AFq.o = 1-85 eV [21]. The flatband potentials ( fb(SC) of the different solid oxides were estimated by monitoring the optical absorption at 750 nm of transparent nanocrystalline electrodes in propylene carbonate as a function of applied potential [61].

Cherepy N. J., Smestad G. P., Gratzel M. and Zhang J. Z. (1997), Ultrafast electron injection implications for a photoelectrochemical cell utilizing an anthocyanine dye-sensitized Ti02 nanocrystalline electrode , J. Phys. Chem. B. 101, 9342-9351. 

Transient Photocurrent Response of Porous and Nanocrystalline Electrodes. 143... 

The simplest photoelectrochemical cells consist of a semiconductor working electrode and a metal counter electrode, both of which are in contact with a redox electrolyte. In the dark, the potential difference between the two electrodes is zero. The open circuit potential difference between the two electrodes that arises from illumination of the semiconductor electrode is referred to as the photovoltage. When the semiconductor and counter electrode are short circuited, a light induced photocurrent can be measured in the external circuit. These phenomena originate from the effective separation of photogenerated electron-hole pairs in the semiconductor. In conventional photoelectrochemical studies, the interface between the flat surface of a bulk single crystalline semiconductor and the electrolyte is two dimensional, and the electrode is illuminated from the electrolyte side. However, in the last decade, research into the properties of nanoporous semiconductor electrodes interpenetrated by an electrolyte solution has expanded substantially. If a nanocrystalline electrode is prepared as a film on a transparent conducting substrate, it can be illuminated from either side. The obvious differences between a flat (two dimensional) semiconductor/ electrolyte junction and the (three dimensional) interface in a nanoporous electrode justify a separate treatment of the two cases. 

Fig. 31. SEM showing a porous nanocrystalline electrode consisting of an interconnected assembly of 30 nm colloidal Ti02 particles. Electrodes of this type are used, for example, in dye sensitised solar cells.

Fig. 32. Comparison of photoexcitation mechanisms in nanocrystalline electrodes. Interband excitation produces electron hole pairs in the solid. Sensitisation involves in electron injection from the photoexcited state of a dye adsorbed on the surface of the solid. The dye is regenerated by the supersensitiser R.

Photogenerated electrons in a nanocrystalline electrode may react with redox species before they are collected at the back contact. Sodergren et al. [189] have considered generation, diffusion and back reaction of electrons with Ij ions in the... 

One of the key questions that remains unanswered is the role of temporary localisation of electrons in traps in determining the rate of electron transport in porous and nanocrystalline electrodes. Dlocik et al. [90] have taken electron trapping and detrapping into account by defining effective values for the electron diffusion coefficient and lifetime as... 

Use nanocrystalline electrode material for faster response time... 

Sensors with nanocrystalline electrodes were fabricated and tested... 

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