Photocathode surface

Standard equipment makes use of photodiodes working with the internal photo effect. The charge current is linear proportional to the number of photons falling onto the sensitive area. These photodiodes allow an arrangement in arrays. This type of detector is used in modem simultaneous detection systems (see Sec. 3.2.) and in chromatography [1,18]. Photomultipliers use the external photo effect. Incident radiation catapults out of the sensitive surface (photocathode) electrons which are multiplied (scintillation) and accelerated between dynodes on their way to the anode. Their detectivity is better and prices are higher [23]. 

The absolute and spectral sensitivities can often vary by up to 100% within a few millimeters on the surface of the photocathode [49]. Figure 19 illustrates this effect for a sideways and vertical adjustment of a photomultiplier, in addition slight maladjustment of the light entrance can lead to zero hne runaway as a result of thermal effects. 

The preparation of immobilized CdTe nanoparticles in the 30-60 nm size range on a Te-modified polycrystalline Au surface was reported recently by a method comprising combination of photocathodic stripping and precipitation [100], Visible light irradiation of the Te-modified Au surface generated Te species in situ, followed by interfacial reaction with added Cd " ions in a Na2S04 electrolyte. The resultant CdTe compound deposited as nanosized particles uniformly dispersed on the Au substrate surface. 

It has been already mentioned in preceding section that in process of ordering of disordered adsorbents the energy get released which is sufficient to brake the bonds in the surface compounds. Therefore, the emission of initially adsorbed active particles due to disorder relaxation should be studied in disorder surfaces. It is very convenient to use for such studies the amorphous antimony with adsorbed hydrogen atoms. The properties of thin antimony films have been studied in substantial detail due to their use in manufacturing of photocathodes [12]. 

Bradley et al.109 have combined a p-Si photocathode and homogeneous catalysts (tetraazamacrocyclic metal complexes, which had been shown to be effective catalysts for C02 reduction at an Hg electrode110) to reduce the applied cathode potential. The catalysts showed111 reversible cyclic voltammetric responses in acetonitrile at illuminated p-Si electrodes at potentials significantly more positive (ca. 0.4 V) than those required at a Pt electrode, where the p-Si used had surface states in high density and Fermi level pinning112 occurred. Electrolysis of a C02-saturated solution (acetonitrile-H20-LiC104 1 1 0.1 M) in the presence of 180 mM... 

Scheme V. Representation of the catalytic p-type Si photocathode for Ht evolution prepared by derivatizing the surface first with Reagent III followed by deposition of approximately an equimolar amount of Pd(0) by electrochemical deposition. The Auger/depth profile analysis for Pd, Si, C, and O is typical of such interfaces (49) for coverages of approximately 10 8 mol PQ2 /cm2.

The synthesis of catalytic photocathodes for H2 evolution provides evidence that deliberate surface modification can significantly improve the overall efficiency. However, the synthesis of rugged, very active catalytic surfaces remains a challenge. The results so far establish that it is possible, by rational means, to synthesize a desired photosensitive interface and to prove the gross structure. Continued improvements in photoelectrochemical H2 evolution efficiently can be expected, while new surface catalysts are needed for N2 and CO2 reduction processes. 

Fluorimeters use a photomultiplier (or PM) tube that is positioned perpendicular to the incident light beam to detect fluorescence emission by a sample. The photomultiplier has a light-sensitive surface (the photocathode) which is maintained at a negative potential of about 950-1500 volts. So-called photoelectrons are released from photocathode upon light absorption, and they are... 

Like other non-oxidic semiconductors in aqueous solutions, surface oxidized and photocorrosive InP is a poor photoelectrode for water decomposition [19,27,32,33], To enhance properties several efforts have focused on coupling of the semiconductor with discontinuous noble metal layers of island-like topology. For example, rhodium, ruthenium and platinum thin films, less than 10 nm in thickness, have been electrodeposited onto p-type InP followed by a brief etching treatment to achieve an island-like topology on the surface [27,28]. In combination with a Pt counter electrode, under AM 1.5 illumination of 87 mW/cm the metal (Pt, Rh, Ru) functionalized p-InP photocathodes [27] see a reduction in the threshold voltage for water electrolysis from 1.23 V to 0.64 V, and in aqueous HCl solution a photocurrent density of 24 mA/cm with a photoconversion efficiency of 12% [27]. 

Dominey RN, Lewis NS, Bruce JA, Bookbinder DC, Wrighton MS (1982) Improvement of photoelectrochemical hydrogen generation by surface modification of p-type silicon semiconductor photocathodes. J Am Chem Soc 104 467-482... 

A photoelectric cell consists of a photosensitive surface (the metal or alloy) deposited inside an evacuated glass bulb fitted with a second metal electrode kept at a positive potential (Figure 2.3). Electrons emitted by the photosensitive surface called the photocathode will then be captured by the positive anode and a current will flow in an external circuit which connects the two electrodes. 

PHOTOEMISSION AND PHOTOMULTIPLIERS. Photoemission is the ejection of electrons from a substance as a result of radiation filling on it Photomultipliers make use of the phenomena of photoemission and secondary-electron emission in order to detect very low light levels The electrons released from the photocathode by incident light are accelerated and focused onto a secondary-emission surface (called a dynode). Several electrons are emitted from the dynode for each incident primary electron. These secondary electrons are then directed onto a second dynode where more electrons are released. The whole process is repealed a number of times depending upon the number of dynodes used, In this manner, it is possible to amplify the initial photocurrent by a factor of 10s or more in practical photomultipliers. Thus, the photomultiplier is a very sensitive detector of light. 

At the n-type interface, the electric field generated causes photogenerated conduction band electrons to move into the bulk of the semiconductor, to the back metal contact, and into the external circuit. The valence band holes access the semiconductor interface due to the influence of the interfacial electric field (Fig. 28.2). Thus, redox species can be oxidized by the excited n-type semiconductor. These materials act as photoanodes. On the other hand, the electric field in a p-type material is reversed in potential gradient therefore, excited electrons move to the semiconductor surface, while holes move through the semiconductor to the external circuit (Fig. 28.2). These materials are photocathodes. The presence of an electric field at the semiconductor-electrolyte interface is usually depicted by a bending of the band edges as shown in Figure 28.2. Elec-... 

As recombination current in a photocathode was treated as an electron current to the photoanode surface, the photocathode recombination current may be viewed as a hole current. Correspondingly, OH- ions at the photocathode/electrolyte interface lower the energy barrier for hole transport to the photocathode surface. 

FIGURE 5.5 Schematic illustration of a photomultiplier tube. A single photon ejects an electron from the photocathode. The electron is accelerated by voltage differences, and knocks multiple electrons off each successive surface. The burst of electrons is collected at the end. 

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