**1.2 Metal nanoparticle modification of semiconductor electrode**

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.

Solar to Chemical Conversion

Using Metal Nanoparticle Modified Low-Cost Silicon Photoelectrode 235

 SiO2 + 6HF → SiF62- + 2H+ + 2H2O (5) Non-polished multicrystalline n-type Si wafers (cast, ca. 2 cm, 0.3 mm thick) were washed with acetone, and etched with 1 mol dm-3 sodium hydroxide or potassium hydroxide solution at 353 K for saw damage layer removal. Before deposition of Pt particles, the wafers were treated by one of two pretreatment methods (hereafter, we call these pretreatments method A and method B). Method A: the wafers were washed with acetone, immersed in a CP-4A (a mixture of hydrofluoric acid, nitric acid, acetic acid, and water) solution for three min, and then immersed in a 7.3 mol dm-3 hydrofluoric acid solution for two min. Method B: the wafers were immersed in 14 mol dm-3 nitric acid at 353 K for 30 min after method A treatment. The multicrystalline Si wafer was immersed in a 1.0x10-3 mol dm-3 hexachloroplatinic (IV) acid

Fig. 4. Typical scanning electron microscopic (SEM) images of multicrystalline n-Si wafers pretreated by method A (image a) or B (b and c) and immersed in the Pt displacement

Figure 4 shows typical scanning electron microscopic (SEM) images of Pt deposited multicrystalline Si wafers. Spherical Pt-nanoparticles were sparsely scattered on the multicrystalline Si surfaces. Thin Si oxide layer formed by immersing the multicrystalline Si wafers in nitric acid solution (method B) decreased the particle density from 4x109 to 0.9x109 cm-2 (Figs. 4a and b). Shortening the immersion time from 120 to 30 s decreased the average particle size from 87 to 67 nm (Figs. 4b and c). The size and particle density of electrolessly deposited Pt nanoparticles on multicrystalline Si can be controlled by changing the deposition conditions. This is consistent with our previously reported results on single-

**2.2 Porous silicon formation by metal-particle-assisted hydrofluoric acid etching**  The antireflection of the semiconductor surface is an effective method for improving the energy conversion efficiency of solar cells (Sze, 1981, Nelson, 2003). The surface texturization by anisotropic etching is a common antireflection method for single crystalline Si solar cells. However, multicrystalline Si cannot be uniformly texturized by the anisotropic etching caused by its variety of orientations of crystallites. The metal-particle-assisted hydrofluoric acid etching can form both macroporous and microporous layers on

solution containing 0.15 mol dm-3 hydrofluoric acid at 313 K for 30-120 s.

crystalline silicon aside from high particle density (Yae et al., 2007c).

deposition solution for 120 (a and b) or 30 s (c).

Si + 2H2O + 4h+ → SiO2 + 4H+ (5)

In this study, Pt-nanoparticle modified multicrystalline Si wafers and microcrystalline Si (c-Si:H) thin films (Yae et al., 2007a, b) are used as photoelectrodes in the photodecomposition of hydrogen iodide for low-cost and efficient production of solar hydrogen. We also attempt solar water splitting using a multi-photon system equipped with the microcrystalline Si thin film and metal-oxide photoelectrodes in series (Yae et al., 2007b).
