Question

7. Artificial photosynthesis is an emerging technology to produce chemical fuels from abundant renewable resources by using sunlight, based on principles learned from Nature For example, huge efforts have been made to develop photoelectrochemical system to produce hydrogen gas by electrolysis of water, also known as water-splitting. In principle, it is very clean and sustainable process because it uses water and sunlight as a raw byproduct. Answer the following questions. (total 25 pt) (e) In artificial photosynthesis, energy required to drive water electrolysis is provided from light energy in the presence of photocatalyst. What is the longest wavelength of light for successful water-splitting. Is it possible to do water-splitting with infrared light? If necessary, use the following values: Planck constant (h)-6.626x10 Js; speed of light (c)-2.99x10 ms; elementary charge (e)-1.6x101 C. (5 pt) (f) In reality, however, we have to use light with shorter wavelength than we calculated in (e). Explain why we have to use shorter-wavelength light. (5 pt) 34

Answer 1

Semiconductor Conduction band E(O2/H20)-1.23 v Valance band

Figure 1: Schematic representation of photocatalysis

generally artificial photosynthesis or any photoinduced process will be carried by using the semiconductor materials as catalyst.

the selection of semiconductor or modified semiconductor and the EMF value of H₂O oxidation and reduction will decide the what wavelength is suitable for the water splitting.

theoretically 1.23 V is the minimum energy required for the water splitting.

1.23 V equal to nearly 1000 nm (IR radiation)  

however, it is also important for the semiconductor selection,

rules for the semiconductor (catalyst) selection

1. Conduction band energy must be higher than E^o(H⁺/H₂)

2. valance band energy must be lesser than E^o(O₂/H₂O)

in realty till now, there in no semiconductor having band gap 1.23 eV and having the conduction band, valance band position as per the selection rule. so the catalyst band gap play important role in activation of water splitting reaction and the light absorbed will define the wavelength required. generally higher wavelength is required than the theoretical wavelength.