**3.1 Preparation of photoelectrodes and photovoltaic photoelectrochemical solar cells**

Hydrogenated microcrystalline silicon thin films were deposited onto polished glassy carbon (Tokai Carbon) substrates by the hot-wire catalytic chemical vapor deposition (cat-CVD) method (Matsumura et al. 2003). A 40-nm-thick n-type hydrogenated microcrystalline cubic silicon carbide (n-c-3C-SiC:H) layer was deposited on the substrates using hydrogendiluted monomethylsilane and phosphine gas at temperatures of 1700°C for the rhenium filament. An intrinsic hydrogenated microcrystalline silicon (i-c-Si:H) layer, with thickness of 2-3 m, was deposited on the n-type layer using monosilane gas at 1700°C for the tantalum filament. The microcrystalline silicon thin film electrodes were prepared by connecting a copper wire to the backside of the substrate with silver paste and covering it with insulating epoxy resin.

Solar to Chemical Conversion

**decomposition** 

(Fig. 1b and 2).

Using Metal Nanoparticle Modified Low-Cost Silicon Photoelectrode 247

For the photovoltaic photoelectrochemical solar cell (Fig. 1a), the Pt-nanoparticle-modified microcrystalline silicon thin film electrode and Pt-plate counterelectrode were immersed in a hydroiodic acid and iodine redox electrolyte solution as for the multicrystalline Si photoelectrodes (section 2.3). Figure 15 shows the photocurrent density versus potential (*j-U*) curves for the photovoltaic solar cell. The microcrystalline silicon film was stably adherent to the glassy carbon substrate after completing the photoelectrochemical measurements in these highly acidic solutions. The open-circuit photovoltage was 0.47-0.49 V. This is higher than the 0.3 V value obtained for the microcrystalline silicon thin film electrode covered with a continuous 1.5-nm-thick Pt layer, which was deposited using the electron-beam evaporation method. These results clearly indicate that the Pt-nanoparticlemodified microcrystalline silicon thin film electrodes work by using the same mechanism as the Pt-nanoparticle-modified single-crystalline n-Si electrodes, which work as ideal semiconductor photoelectrodes for generating high photovoltage and stable photocurrent described in previous sections 1.2 and 2.3.1. The reduction of redox electrolyte concentration increased the short-circuit photocurrent density to 9.1 from 4.2 mA cm-2 (Fig. 15, solid line). This increase is caused by a decrease in the visible light absorption of the triiodide (I3-) ion in the redox solution. The increased photocurrent raised open-circuit photovoltage to 0.49 V,

**3.2 Solar to chemical conversion (solar hydrogen production) via hydrogen iodide** 


*U* / V vs. Pt-counterelectrode in HBr Fig. 16. Photocurrent density versus potential (*j-U*) curve (Solid line) for the Pt-nanoparticle modified microcrystalline Si thin film electrode measured in the hydroiodic acid and iodine mixture solution of the anode compartment of the two-compartment cell for solar to chemical conversion (solar hydrogen production, Fig. 1b). Dashed line: Pt electrode measured in the anode compartment of the two-compartment cell instead of the Si photoelectrode. Electrolyte solutions: anode compartment: 3.0 M HI/0.002 M I2; cathode compartment: 3.0 M HBr.

The Pt-nanoparticle modified microcrystalline Si thin film electrode were used for solar to chemical conversion via the photoelectrochemical decomposition of hydrogen iodide to iodine and hydrogen gas as the multicrystalline Si photoelectrodes (section 2.4). For the photoelectrochemical decomposition of hydrogen iodide, a two-compartment cell was used

0

2

4

6

*j* / mA cm-2

8

10

and thus the photovoltaic conversion efficiency reached 2.7%.

We deposited the Pt nanoparticles on the microcrystalline silicon surface using electroless displacement deposition as for the multicrystalline Si photoelectrodes (section 2.1). Figure 14 shows an scanning electron microscopic (SEM) image of the microcrystalline silicon film's surface after immersion in the Pt deposition solution for 120 s. Platinum nanoparticles of 3- 200 nm in size and 1.5 x 1010 cm-2 in particle density were scattered on the film. The size and distribution density of Pt particles varied with the deposition conditions, such as oxide layer formation on the films before deposition and the immersion time of films in the deposition solution. The distribution density is much higher than that for a single-crystalline n-Si wafer, but the changing behaviors of the size and distribution density are similar to those of the single crystalline (Yae et al., 2007c, 2008).

Fig. 14. Scanning electron microscopic image of Pt-nanoparticle modified microcrystalline Si thin film surface.

Fig. 15. Photocurrent density versus potential (*j-U*) curves for photovoltaic photoelectrochemical solar cell equipped with the Pt-nanoparticle modified microcrystalline Si photoelectrode measured in the 7.6 mol dm-3 (M) hydroiodic acid (HI)/0.05 M iodine (I2) (dashed line) and 3.0 M HI/0.002 M I2 (solid line) redox solutions.

We deposited the Pt nanoparticles on the microcrystalline silicon surface using electroless displacement deposition as for the multicrystalline Si photoelectrodes (section 2.1). Figure 14 shows an scanning electron microscopic (SEM) image of the microcrystalline silicon film's surface after immersion in the Pt deposition solution for 120 s. Platinum nanoparticles of 3- 200 nm in size and 1.5 x 1010 cm-2 in particle density were scattered on the film. The size and distribution density of Pt particles varied with the deposition conditions, such as oxide layer formation on the films before deposition and the immersion time of films in the deposition solution. The distribution density is much higher than that for a single-crystalline n-Si wafer, but the changing behaviors of the size and distribution density are similar to those of

Fig. 14. Scanning electron microscopic image of Pt-nanoparticle modified microcrystalline

300 nm


*U* / V vs. Pt-counterelectrode

photoelectrochemical solar cell equipped with the Pt-nanoparticle modified microcrystalline Si photoelectrode measured in the 7.6 mol dm-3 (M) hydroiodic acid (HI)/0.05 M iodine (I2)

Fig. 15. Photocurrent density versus potential (*j-U*) curves for photovoltaic

(dashed line) and 3.0 M HI/0.002 M I2 (solid line) redox solutions.

0

2

4

6

8

*j* / mAcm-2

10

the single crystalline (Yae et al., 2007c, 2008).

Si thin film surface.

For the photovoltaic photoelectrochemical solar cell (Fig. 1a), the Pt-nanoparticle-modified microcrystalline silicon thin film electrode and Pt-plate counterelectrode were immersed in a hydroiodic acid and iodine redox electrolyte solution as for the multicrystalline Si photoelectrodes (section 2.3). Figure 15 shows the photocurrent density versus potential (*j-U*) curves for the photovoltaic solar cell. The microcrystalline silicon film was stably adherent to the glassy carbon substrate after completing the photoelectrochemical measurements in these highly acidic solutions. The open-circuit photovoltage was 0.47-0.49 V. This is higher than the 0.3 V value obtained for the microcrystalline silicon thin film electrode covered with a continuous 1.5-nm-thick Pt layer, which was deposited using the electron-beam evaporation method. These results clearly indicate that the Pt-nanoparticlemodified microcrystalline silicon thin film electrodes work by using the same mechanism as the Pt-nanoparticle-modified single-crystalline n-Si electrodes, which work as ideal semiconductor photoelectrodes for generating high photovoltage and stable photocurrent described in previous sections 1.2 and 2.3.1. The reduction of redox electrolyte concentration increased the short-circuit photocurrent density to 9.1 from 4.2 mA cm-2 (Fig. 15, solid line). This increase is caused by a decrease in the visible light absorption of the triiodide (I3-) ion in the redox solution. The increased photocurrent raised open-circuit photovoltage to 0.49 V, and thus the photovoltaic conversion efficiency reached 2.7%.

### **3.2 Solar to chemical conversion (solar hydrogen production) via hydrogen iodide decomposition**

The Pt-nanoparticle modified microcrystalline Si thin film electrode were used for solar to chemical conversion via the photoelectrochemical decomposition of hydrogen iodide to iodine and hydrogen gas as the multicrystalline Si photoelectrodes (section 2.4). For the photoelectrochemical decomposition of hydrogen iodide, a two-compartment cell was used (Fig. 1b and 2).

Fig. 16. Photocurrent density versus potential (*j-U*) curve (Solid line) for the Pt-nanoparticle modified microcrystalline Si thin film electrode measured in the hydroiodic acid and iodine mixture solution of the anode compartment of the two-compartment cell for solar to chemical conversion (solar hydrogen production, Fig. 1b). Dashed line: Pt electrode measured in the anode compartment of the two-compartment cell instead of the Si photoelectrode. Electrolyte solutions: anode compartment: 3.0 M HI/0.002 M I2; cathode compartment: 3.0 M HBr.

Solar to Chemical Conversion


titanium dioxide photoelectrochemical cell.

0

0.05

*j* / mAcm-2

0.1

Using Metal Nanoparticle Modified Low-Cost Silicon Photoelectrode 249

The titanium dioxide photoanode was prepared as follows. Transparent conductive tin oxide (SnO2)-coated glass plates were used as substrates. Titanium dioxide powder (P-25, average crystallite size: 21 nm) was ground with nitric acid, acetyl acetone, surfactant (Triton X-100), and water in a mortar. The obtained paste was coated on the substrate and dried. The titanium dioxide-nanoparticle film was heated in air at 500°C for three hours. The titanium dioxide electrode was prepared by connecting a copper wire to the bare part of the conductive tin oxide film with silver paste and covering it with insulating epoxy resin.


Fig. 18. Photocurrent density versus potential (*j-U*) curve for the titanium dioxide photoelectrode


photoelectrochemical solar cell equipped with a Pt-nanoparticle-modified microcrystalline Si electrode in the redox solution under simulated solar light illumination through the

Fig. 19. Photocurrent density versus potential (*j-U*) curve for the photovoltaic

*U* / V vs. Pt-counterelectrode

0

1

2

3

*j* / mAcm-2

4

5

in a perchloric acid aqueous solution under chopped simulated solar illumination.

*U* / V vs. Ag/AgCl

The solid line in Fig. 16 shows the photocurrent density versus potential (*j-U*) curve for the Pt-nanoparticle-modified microcrystalline Si thin film electrode measured in the hydroiodic acid and iodine mixture solution of the anode compartment of the two-compartment cell. The potential of the electrode was measured versus the Pt counterelectrode in the hydrobromic acid solution of the cathode compartment. In the short-circuit condition under the simulated solar illumination, we obtained a shirt-circuit photocurent density of 6.8 mA cm-2, the solution color on the photoelectrode surface darkened, and gas evolution occurred at the Pt cathode surface. These results clearly show that the photoelectrochemical solar cell equipped with the Pt-nanoparticle-modified microcrystalline Si thin film electrode can decompose hydrogen iodide into hydrogen gas and iodine with no external bias with 2.3% of solar-to-chemical conversion efficiency.

### **3.3 Hydrogen production via solar water splitting using multi-photon system**

A multi-photon system equipped with the microcrystalline Si thin film and titanium dioxide (TiO2) photoelectrodes in series (Fig. 17) was prepared based on a work in literature using a dye-sensitization-photovoltaic cell and a tungsten trioxide (WO3) photoanode (Grätzel, 1999). A titanium dioxide photoanode and a Pt cathode (counterelectrode) were immersed in a perchloric acid (HClO4) aqueous solution in a quartz cell. A photovoltaic photoelectrochemical solar cell equipped with the Pt-nanoparticle-modified microcrystalline Si electrode (section 3.1) was connected to the titanium dioxide photoanode and Pt cathode in series. Simulated solar light irradiated to the titanium dioxide photoelectrode. The titanium dioxide, which has a 3-eV energy band gap, absorbs the short-wavelength part (UV) of the solar light. The long-wavelength part of the solar light transmitted by the titanium dioxide and quartz cell reaches the Pt-nanoparticle-modified microcrystalline Si thin-film of the photovoltaic photoelectrochemical solar cell. The photovoltaic cell applies bias between the titanium dioxide photoanode and the Pt cathode in a perchloric acid aqueous solution for splitting water to hydrogen and oxygen.

Fig. 17. Schematic illustration of multi-photon system equipped with titanium dioxide and microcrystalline Si photoelectrodes for solar water splitting.

The solid line in Fig. 16 shows the photocurrent density versus potential (*j-U*) curve for the Pt-nanoparticle-modified microcrystalline Si thin film electrode measured in the hydroiodic acid and iodine mixture solution of the anode compartment of the two-compartment cell. The potential of the electrode was measured versus the Pt counterelectrode in the hydrobromic acid solution of the cathode compartment. In the short-circuit condition under the simulated solar illumination, we obtained a shirt-circuit photocurent density of 6.8 mA cm-2, the solution color on the photoelectrode surface darkened, and gas evolution occurred at the Pt cathode surface. These results clearly show that the photoelectrochemical solar cell equipped with the Pt-nanoparticle-modified microcrystalline Si thin film electrode can decompose hydrogen iodide into hydrogen gas and iodine with no external bias with 2.3%

**3.3 Hydrogen production via solar water splitting using multi-photon system** 

aqueous solution for splitting water to hydrogen and oxygen.

microcrystalline Si photoelectrodes for solar water splitting.

O2 H2

H2O

Light

A multi-photon system equipped with the microcrystalline Si thin film and titanium dioxide (TiO2) photoelectrodes in series (Fig. 17) was prepared based on a work in literature using a dye-sensitization-photovoltaic cell and a tungsten trioxide (WO3) photoanode (Grätzel, 1999). A titanium dioxide photoanode and a Pt cathode (counterelectrode) were immersed in a perchloric acid (HClO4) aqueous solution in a quartz cell. A photovoltaic photoelectrochemical solar cell equipped with the Pt-nanoparticle-modified microcrystalline Si electrode (section 3.1) was connected to the titanium dioxide photoanode and Pt cathode in series. Simulated solar light irradiated to the titanium dioxide photoelectrode. The titanium dioxide, which has a 3-eV energy band gap, absorbs the short-wavelength part (UV) of the solar light. The long-wavelength part of the solar light transmitted by the titanium dioxide and quartz cell reaches the Pt-nanoparticle-modified microcrystalline Si thin-film of the photovoltaic photoelectrochemical solar cell. The photovoltaic cell applies bias between the titanium dioxide photoanode and the Pt cathode in a perchloric acid

Fig. 17. Schematic illustration of multi-photon system equipped with titanium dioxide and

2H+

e-

TiO Pt Pt <sup>2</sup> n-Si Light

TiO2 Pt c-

2

HI/I2 aq.

Si:H

HClO4 aq. HI -I

e-

of solar-to-chemical conversion efficiency.

The titanium dioxide photoanode was prepared as follows. Transparent conductive tin oxide (SnO2)-coated glass plates were used as substrates. Titanium dioxide powder (P-25, average crystallite size: 21 nm) was ground with nitric acid, acetyl acetone, surfactant (Triton X-100), and water in a mortar. The obtained paste was coated on the substrate and dried. The titanium dioxide-nanoparticle film was heated in air at 500°C for three hours. The titanium dioxide electrode was prepared by connecting a copper wire to the bare part of the conductive tin oxide film with silver paste and covering it with insulating epoxy resin.

Fig. 18. Photocurrent density versus potential (*j-U*) curve for the titanium dioxide photoelectrode in a perchloric acid aqueous solution under chopped simulated solar illumination.

Fig. 19. Photocurrent density versus potential (*j-U*) curve for the photovoltaic photoelectrochemical solar cell equipped with a Pt-nanoparticle-modified microcrystalline Si electrode in the redox solution under simulated solar light illumination through the titanium dioxide photoelectrochemical cell.

Solar to Chemical Conversion

**4. Conclusion** 

**5. Acknowledgment** 

al., 2007a, copyright Elsevier (2007).

Using Metal Nanoparticle Modified Low-Cost Silicon Photoelectrode 251

Fig. 21. Short-circuit photocurrent density (*j*) as a function of time (*t*) for the multi-photon

Multicrystalline silicon wafers and microcrystalline silicon thin films, which are common and prospective low-cost semiconductor materials for solar cells, respectively, were successfully applied to produce solar hydrogen via photodecomposition of hydrogen iodide and solar water splitting. These photoelectrochemical solar cells have the following advantages: 1) simple fabrication of a cell by immersing the electrode in an electrolyte solution; 2) there is no need for a p-type semiconductor or a transparent conducting layer; and 3) direct solar-to-chemical conversion (fuel production). Modification of silicon surface with platinum nanopartilces by electroless displacement deposition and porous layer formation by metal-particle-assisted hydrofluoric acid etching improve solar cell characteristics. The solar-to-chemical conversion efficiency reached 5% for the photodecomposition of hydrogen iodide, and hydrogen gas

evolution was obtained by the solar water splitting with no input of external electricity.

The author is grateful to Prof. H. Matsuda, Dr. N. Fukumuro (University of Hyogo), Dr. S. Ogawa, Prof. N. Yoshida, Prof. S. Nonomura (Gifu University), Mr. S. Sakamoto (Nippon Oikos Co., Ltd.), and Prof. Y. Nakato (Osaka University) for co-work and valuable discussions. The author would like to thank the students who collaborated: H. Miyasako, T. Kobayashi, K. Suzuki, and A. Onaka. The author is grateful to Prof. Y. Uraoka of Nara Institure of Science and Technology for the simulation of the solar water splitting using the multi-photon system. The present work was partly supported by the following programs: Grants-in-Aid for Scientific Research (C) from the JSPS (17560638, 20560676, and 23560875), Grants-in-Aid for education and research from Hyogo Prefecture through the University of Hyogo, Core Research for Evolutional Science and Technology (CREST) from the Japan Science and Technology Agency (JST), and Research for Promoting Technological Seeds from JST. The author wishes to thank Nippon Sheet Glass Co., Ltd. for donating transparent conductive tin oxide coated glass plates. Figures 15 and 16 were reprinted from ref. Yae et

system of Fig. 20 under simulated solar illumination.

Figure 18 shows the photocurrent density versus potential (*j-U*) curve for the titanium dioxide photoelectrode in a perchloric acid aqueous solution under simulated solar illumination. The dissolved oxygen in the solution was eliminated by using argon gas flow into the solution before the measurement. The anodic photocurrent starts to generate at -0.14 V vs. Ag/AgCl. This onset potential is more positive than -0.24 V vs. Ag/AgCl for hydrogen evolution, and thus this electrode cannot split water into hydrogen and oxygen without external bias. Figure 19 shows the photocurrent density versus potential (*j-U*) curve for the photovoltaic photoelectrochemical solar cell equipped with a Pt-nanoparticle-modified microcrystalline Si electrode in the redox solution under simulated solar light illumination through the titanium dioxide photoelectrochemical cell. The shirt-circuit photocurrent density was decreased from 5.3 mA cm-2 for the cell under direct solar light illumination to 2.6 mA cm-2 by light attenuation with the titanium dioxide cell. The multi-photon system (Fig. 17) using the same titanium dioxide and Pt-nanoparticle-modified microcrystalline Si electrodes as those in Figs. 18 and 19 indicated the photocurrent density versus potential (*j-U*) curve of Fig. 20. This system generated anodic photocurrent at a potential that was more negative than -0.24 V vs. Ag/AgCl for hydrogen evolution. Figure 21 shows that steady photocurrent was obtained for the multiphoton system in the short-circuit condition (Fig. 17). Tiny gas bubble formed on the Pt cathode during measurement under the short-circuit condition. These results show that this multi-photon system can split water into hydrogen and oxygen with no external bias with solar light. Since two photoelectrodes of titanium dioxide and Pt-nanoparticle-modified microcrystalline Si were connected in series, photovoltage was the sum of the two electrodes' values and photocurrent was the lower of the two electrodes' values. Therefore, the photocurrent density for water splitting was determined by that of the titanium dioxide electrode and very low. The photocurrent density, and thus hydrogen production by solar water splitting, is expected to increase by using a semiconductor with a narrower band gap, such as tungsten trioxide, instead of titanium dioxide. The theoretical simulation obtained 8 mA cm-2 of shirt-circuit photocurrent density, that is, 10% of solar-to-chemical conversion efficiency for solar water splitting for the tungsten trioxide and Si multi-photon system.

Fig. 20. Photocurrent density versus potential (*j-U*) curve for the multi-photon system (Fig. 17) using the same titanium dioxide thin film and Pt-nanoparticle modified microcrystalline Si photoelectrodes and electrolyte solutions as those in Figs. 18 and 19 under simulated solar light illumination.

Fig. 21. Short-circuit photocurrent density (*j*) as a function of time (*t*) for the multi-photon system of Fig. 20 under simulated solar illumination.
