Photochemical diode

The platination of semiconductor powders is a method for producing semiconductor-metal type photochemical diodes with an energy level scheme as shown in Fig. 11.14a. This was demonstrated for the first time with platinized Ti02 powders which showed excellent photocatalytic activity for the photodecarboxylation of acetate (the photo-Kolbe reaction), a process which has already been discussed in detail in Section 9.2.3 [80]. Various attempts have also been made to photocleave water by using semiconductor powders on which a catalyst such as Pt or RUO2 has been deposited. The relevant experiments usually failed, either because the semiconductor was 

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Tacconi NR, Chenthamarakshan CR, Rajeshwar K, Tacconi El (2005) Selenium-modified titanium dioxide photochemical diode/electrolyte junctions Photocatalytic and electrochemical preparation, characterization, and model simulations. 1 Phys Chem B 109 11953-11960... 

Nozik, A.J., Photochemical diodes, Appl. Phys. Lett., 30,567,1977. 

In this type of cell both electrodes are immersed in the same constant pH solution. An illustrative cell is [27,28] n-SrTiOs photoanode 9.5-10 M NaOH electrolyte Pt cathode. The underlying principle of this cell is production of an internal electric field at the semiconductor-electrolyte interface sufficient to efficiently separate the photogenerated electron-hole pairs. Subsequently holes and electrons are readily available for water oxidation and reduction, respectively, at the anode and cathode. The anode and cathode are commonly physically separated [31-34], but can be combined into a monolithic structure called a photochemical diode [35]. 

Fig. 3.18 Energy diagram for p-n photochemical diode for water photoelectrolysis.

Nozik AJ (1977) Photochemical diodes. Appl Phys Lett 30 567-569... 

These systems are based on immersion of two photoactive electrodes in an electrolyte solution with connection via an external circuit. An overall solar-spectrum hydrogen conversion efficiency of 0.25% was found at zero bias for the n-Ti02/p-GaP cell. Nozik further designed a new type of cell, so-called photochemical diodes that do not require external wires and functions without electrical bias [26]. This device [26], consisting of a small sandwich-like structure, Fig.7.2, such as Pt/n-GaP, and n-Ti02/p-GaP connected through ohmic contacts, when suspended in an appropriate electrolyte causes decomposition of water upon exposure to light. 

Fig. 7.2 Sandwich (or stacked) configurations placed in an electrolyte solution (a) p-n type, (b) semiconductor-metal type photochemical diodes. Both p-type and n-type semiconductors are provided with ohmic contacts. In p-n type light is incident from both directions and ohmic contacts are connected through metal contacts.

Honda K and Frank AJ (1984) Polymer-catalyst modified cadmium sulfide photochemical diodes in the photolysis of water. J Phys Chem 88 5587-5582... 

Yoneyama H, Sakamoto H, Tamura H (1975) A photo-electrochemical cell with production of hydrogen and oxygen by a cell reaction. Electrochim Acta 20 341-345 A. J. Nozik (1977) Photochemical diodes. Appl Phys Lett 30 567-569... 

Photochemical Diodes. A CdS single crystal platelet used in conjunction with Pt to drive the HER from sulfide solutions. 489... 

Closely related is the so-called photochemical diode,489 consisting of either a metal/ semiconductor Schottky barrier or a p n junction, which generates the voltage needed on illumination, to split water. Photochemical diodes are discussed along with other twin photosystem configurations in the next Section. 

Photochemical Diodes and Twin-Photosystem Configurations for Water Splitting... 

As mentioned earlier, photochemical diodes489 can be either of the Schottky type, involving a metal and a semiconductor, or a p n junction type, involving two semi conductors (which can be the same, i.e., a homojunction or different, a heterojunc tion). Only the latter type is considered in this Section involving two irradiated semi conductor/ electrolyte interfaces. Thus n Ti02 and p-GaP crystal wafers were bonded together (through the rear Ohmic contacts) with conductive Ag epoxy cement.489 The resultant heterotype p n photochemical diode was suspended in an acidic aqueous... 

This type of device has been contrasted489 with a series connection of a photovoltaic p-n junction solar cell and a water electrolyzer. Unlike the latter which is a majority carrier system (i.e., the n-side of the junction is the cathode and the p-side becomes the anode), in a photochemical diode, minority carriers (holes for the n-type and electrons for the p-type) are injected into the electrolyte. This distinction translates to certain advantages in terms of the overall energetics of the solar energy conversion system (see Ref. 489). 

Fig. 11.14 Energy level diagrams for a) semiconductor-metal type photochemical diode, b) p/n type photochemical diode. (After ref. [69])...

The photovoltaic electrolysis cell has twice as many semiconductor layers as the photochemical diode. 

In the photovoltaic electrolysis cell the photoactive junctions are p-n junctions between two semiconductors, while in the photochemical diode the photoactive junctions are between semiconductors and aqueous solutions. 

In the photovoltaic electrolysis cell, the n-type region of the device, covered with a metal layer, becomes a cathode while the p-type region covered with a metal layer becomes an anode (i.e. it behaves like a majority carrier device) in the photochemical diode, the opposite is true, i.e. it is a minority carrier device with the n-type region acting as anode and the p-type region acting as a cathode. 

Several approaches to photoelectrolysis are possible, involving a photoactive semiconductor electrode electrically connected to a metal electrode, or photoactive anode (n-tyqre) and cathode (p-tyqre) acting either as separate electrodes, or coupled to form a monolithic structure called a photochemical diode. 

With PECs made of n-type and p-type semiconductors in contact with an electrolyte, the requirement of an external bias can be eliminated [11]. A first example of these p-n photoelectrolysis cells was reported by Nozik in 1976 [12] with an n-Ti02/p-GaP heterotype device whose efficiency at zero bias was 0.25%. The energy diagram of such a cell, also called photochemical diode, is... 

The liquid-junction semiconductor electrodes stabilized by polymer-coating can be used for photochemical conversion systems. The stabilized n-CdS coated with PP which incorporates RUO2 as catalyst was used for visible-light-induced water cleavage Photochemical diodes were fabricated by coating CdS with PP and polystyrene films, the latter containing metal dispersions such as Pt, Rh and RUO2 as a catalyst (Fig. 35). 

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