Photocatalytic cells, semiconductor

Doping is important for semiconductors in order to tune their optical and electrical properties for the potential applications in biotechnology and solar cells [65]. Ag-doped hexagonal CdS nanoparticles were successfully obtained by an ultrasound-assisted microwave synthesis method. Here, the doping of Ag in to CdS nanoparticles induced the evolution of crystal structure from cubic to hexagonal. Further support from photocatalytic experiment also clearly indicates the doping of Ag clusters into the CdS matrix. 

Of the photocatalytic systems and structures composed of a single active material, eventually coupled with redox catalysts and/or metals, only a wide band gap oxide semiconductor, like Pt/Ti02, requiring UV irradiation, showed some photoactivity for water photosplitting. Water splitting with visible light requires the irradiation of multiple band gap photoelectrochemical cells (PEC) or Z-scheme systems (like the photosynthesis system of plants etc.). 

Understand how energy-rich electron-hole pairs can be utilised to produce electricity in photovoltaic solar cells, to drive chemical reactions where water is split into hydrogen and oxygen, to take part in photocatalytic reactions and to change the surface of the semiconductor, resulting in the phenomenon of superhydrophilicity. 

There are several non-oxide semiconductor electrodes that have been studied for use in photoelectrochemical cells for water splitting. Materials used as photocathodes include p-Si, p-lnP, p-GaAs, and p-CdTe. Materials used as photoanodes include n-Si, n-InP, n-GaAs, n-CdTe and n-CdSe. Similar to CdS, unfortunately most of these non-oxide semiconductors undergo photocatalytic corrosion under the conditions of oxygen evolution. 

In a single-crystal semiconductor (n-type) based photoelectrochemical cell, the problem of achieving charge separation is easily overcome by applying an anodic bias as was first demonstrated by Honda and Fujishima [263]. Using a single crystal Ti02, they were able to carry out the photoelectrolysis of water under the influence of an anodic bias. This concept to manipulate the photocatalytic reaction by electrochemical method can be extended to nanostructured semiconductor thin films [39,116]. The principle of electrochemically assisted photocatalysis is illustrated in Fig. 10. 

Semiconductors have proven to be very important factors in the photovoltaic conversion of solar radiation into electric energy. They can also be used as photoelectrodes in photoelectrochemical cells producing power or fuels, and in photocatalytic reactions of high specificity, though these applications are still at the experimental stage. 

Titanium oxynitride nanoparticles were prepared by Gole [2] and used in solar cells and as a semiconductor-based photocatalytic component in fuel cells. 

The number of carriers collected (in an external circuit, for example) versus those optically generated defines the quantum yield (C>), a parameter of considerable interest to photochemists. The difficulty here is to quantify the amount of light actually absorbed by the semiconductor since the cell walls, the electrolyte and other components of the assembly are all capable of either absorbing or scattering some of the incident light. Unfortunately, this problem has not been comprehensively tackled, unlike in the situation with photocatalytic reactors involving semiconductor particulate suspensions where such analyses are available [204-207]. Pending these, an effective quantum yield can still be defined. 

Chapters 12 and 13 cover two of the most important novel catalytic applications of carbon materials, electrocatalysis and photocatalysis. In the first case, carbons are used mostly as supports for metal catalysts in fuel cells, while the synergistic effects of carbon-based composite semiconductor materials, such as C-TiOi, make them particularly effective in photocatalytic degradation reactions. 

FIGURE 1. The Photocatalytic Semiconductor Particle "Corrosion" Cell Illustrated by TiOs. Note that the Redox Couples may need to be Adsorbed if Reaction is to Compete with Recombination. 

---