**1.1 Solar hydrogen production with photoelectrochemical solar cells**

Solar hydrogen production, that is, water splitting by photoelectrochemical solar cells equipped with a titanium dioxide (TiO2) photoelectrode has been attracting much attention since Fujishima and Honda's report in 1972 (Fujishima & Honda, 1972). Photoelectrochemical solar cells have important and unique features (Licht, 2002, Nakato, 2000). They are fabricated simply by immersing a semiconductor electrode and a counterelectrode into an electrolyte solution (Fig. 1). They can convert solar energy not only to electricity (Fig. 1a) but also directly to storable chemical energy (Figs. 1b and 2) such as water splitting into hydrogen and oxygen (Arakawa et al., 2007, Fujishima & Honda, 1972, Grätzel, 1999, Khaselev & Turner, 1998, Lin et al., 1998, Miller et al., 2005, Park & Bard, 2005, Sakai et al., 1988), the decomposition of hydrogen iodide into hydrogen and iodine (Nakato et al., 1998, Nakato, 2000, Takabayashi et al., 2004, 2006), and the reduction of carbon dioxide to hydrocarbons or carbon monoxide (Hinogami et al., 1997, 1998). For the photovoltaic photoelectrochemical solar cell (Fig. 1a), two electrodes are immersed in a redox electrolyte solution. Opposite reactions such as the oxidation of the reductant to an oxidant on an n-type semiconductor electrode and the reduction of the oxidant to the reductant on the counterelectrode occur through solar illumination. Thus, the composition of redox electrolyte solution does not change, and only electricity is obtained as usual solid-state solar cells. For the photo to chemical conversion type of photoelectrochemical solar cells (Fig. 1b), different reactions occur on the electrodes, for example, the oxidation of iodide ions to iodine (triiodide ions) and

Solar to Chemical Conversion

hydrogen iodide with no external bias.

Using Metal Nanoparticle Modified Low-Cost Silicon Photoelectrode 233

Fig. 2. Photoelectrochemical solar cell produces hydrogen gas via photodecomposition of

Unfortunately, bare semiconductors can easily corrode or be passivated in aqueous solutions, and do not have enough catalytic activity for electrochemical reactions. Modifying the semiconductor surface with metal nanoparticles eliminates these problems without lowering the high energy barrier feature (Allongue et al., 1992, Hinogami et al., 1997, 1998, Jia et al., 1996, Nakato, 2000, Nakato et al., 1988, 1998, Nakato & Tsubomura 1992, Takabayashi et al., 2004, 2006, Yae et al., 1994a). The operation principle of this type of solar cells is explained as follows (Nakato et al., 1988, Nakato & Tsubomura 1992). Figure 3 shows a schematic illustration of cross section of a platinum (Pt)-nanoparticle modified n-type silicon (n-Si) photoelectrode. Photogenerated holes in n-Si transfer to the redox solution through the Pt particles, thus leading to a steady photocurrent. With no Pt particle, the photocurrent decays rapidly. The surface band energies of n-Si are modulated by the deposition of Pt particles. However, the effective barrier height is nearly the same as that for bare n-Si in case where the size of the Pt particles (or more correctly, the size of the areas of direct Pt-Si contacts) is small enough, much smaller than the width of the space charge layer. Thus, a very high barrier height, nearly equal to the energy band-gap, can be obtained if one chooses a electrochemical reaction with an enough high potential. Also, a major part of the n-Si surface is covered with a thin Si-oxide layer and passivated, and hence the electron-hole recombination rate at the n-Si surface is maintained quite low. For these reasons, very high photovoltage can be generated.

Fig. 3. Schematic illustration of cross section of a Pt-nanoparticle-modified n-Si photoelectrode.

**1.2 Metal nanoparticle modification of semiconductor electrode** 

hydrogen evolution. This decomposition of hydrogen iodide is an 'up-hill' reaction. Thus, solar energy is directly converted to chemical energy using this type of photoelectrochemical solar cells. The junction of an electrolyte solution and a semiconductor can generate a high-energy barrier, thus reaching a high photovoltage level even with a low-cost, low-quality semiconductor. However, water splitting using titanium dioxide encounters serious difficulty in achieving hydrogen evolution. There are three solutions to this difficulty: using another semiconductor with an energy band gap that is wider than titanium dioxide; using a multi-photon system equipped with multiphotoelectrodes in series or a tandem-type photoelectrode; and using a hydrogenproducing semiconductor, such as silicon (Si), and an oxidation reaction other than oxygen evolution, such as oxidation of iodide ions into iodine.

Fig. 1. Schematic illustrations of PEC solar cells. a) photovoltaic type; b) solar-to-chemical conversion type.

The Gibbs energy change for decomposition of hydrogen iodide (HI) into hydrogen (H2) and iodine (I2) (triiodide ion (I3–)) in an aqueous solution (see equations 1, 2 and 3) is smaller than that for water splitting. Thus, silicon photoelectrodes, which have a narrower energy band gap than titanium dioxide, can decompose hydrogen iodide with no external bias (Figs. 1b, 2 and equations (1)-(3)). The efficiency of a solar-to-chemical conversion via the photoelectrochemical decomposition of hydrogen iodide using single-crystalline silicon electrodes reached 7.4% (Nakato, 2000, Nakato et al., 1998, Takabayashi et al., 2004, 2006). Fuel cells using hydrogen gas and iodine solution can generate electricity via the reverse reaction of hydrogen iodide decomposition (equation (3)). Therefore, this system can form a solar energy cycle in a similar manner as the cycle consisting of water splitting and hydrogen-oxygen fuel cells.

$$\text{Anode}\tag{1}\tag{1}\tag{2}\text{I}\mathfrak{r}+\mathfrak{2}\mathfrak{h}^\*\to\mathfrak{I}\_3\tag{1}$$

$$\text{Cathode}\tag{2}\\
\text{H}^\* + 2\text{e}^\cdot \rightarrow \text{H}\_2\tag{2}$$

$$\text{Total reaction} \tag{3}$$

$$\text{2HI} \rightarrow \text{H}\_2 + \text{I}\_2 \text{(Ig)} \tag{3}$$

hydrogen evolution. This decomposition of hydrogen iodide is an 'up-hill' reaction. Thus, solar energy is directly converted to chemical energy using this type of photoelectrochemical solar cells. The junction of an electrolyte solution and a semiconductor can generate a high-energy barrier, thus reaching a high photovoltage level even with a low-cost, low-quality semiconductor. However, water splitting using titanium dioxide encounters serious difficulty in achieving hydrogen evolution. There are three solutions to this difficulty: using another semiconductor with an energy band gap that is wider than titanium dioxide; using a multi-photon system equipped with multiphotoelectrodes in series or a tandem-type photoelectrode; and using a hydrogenproducing semiconductor, such as silicon (Si), and an oxidation reaction other than

Fig. 1. Schematic illustrations of PEC solar cells. a) photovoltaic type; b) solar-to-chemical

Anode 3I-

Total reaction 2HI → H2 + I2 (I3

+ 2h+ → I3

Cathode 2H+ + 2e- → H2 (2)


(1)


The Gibbs energy change for decomposition of hydrogen iodide (HI) into hydrogen (H2) and iodine (I2) (triiodide ion (I3–)) in an aqueous solution (see equations 1, 2 and 3) is smaller than that for water splitting. Thus, silicon photoelectrodes, which have a narrower energy band gap than titanium dioxide, can decompose hydrogen iodide with no external bias (Figs. 1b, 2 and equations (1)-(3)). The efficiency of a solar-to-chemical conversion via the photoelectrochemical decomposition of hydrogen iodide using single-crystalline silicon electrodes reached 7.4% (Nakato, 2000, Nakato et al., 1998, Takabayashi et al., 2004, 2006). Fuel cells using hydrogen gas and iodine solution can generate electricity via the reverse reaction of hydrogen iodide decomposition (equation (3)). Therefore, this system can form a solar energy cycle in a similar manner as the cycle consisting of water splitting and

oxygen evolution, such as oxidation of iodide ions into iodine.

conversion type.

hydrogen-oxygen fuel cells.

Fig. 2. Photoelectrochemical solar cell produces hydrogen gas via photodecomposition of hydrogen iodide with no external bias.
