Photoelectrochemical characterization of porous Si

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International Journal of Hydrogen Energy 28 (2003) 629 – 632 www.elsevier.com/locate/ijhydene

Photoelectrochemical characterization of porous Si N.R. Mathews, P.J. Sebastian∗ , X. Mathew, V. Agarwal Centro de Investigacion en Energa-UNAM, 62580 Temixco, Morelos, Mexico Received 1 October 2001; received in revised form 1 February 2002; accepted 1 March 2002

Abstract The photoelectrochemical measurements of p-Si and porous silicon in 0:1 M H2 SO4 were done. The band gap of Si is a bit low for e7cient hydrogen production. The silicon was made porous to improve the e7ciency of hydrogen production. The porous silicon photocathodes show a substantial improvement in the hydrogen production compared to that of p-Si photocathodes. Studies were done by applying di8erent light 9ux on the porous silicon. ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. Keywords: p-Si; Porous Si; Hydrogen; Water splitting; Photocurrent

1. Introduction Hydrogen is considered as the energy carrier in a clean energy hydrogen economy. There are a lot of research activities on hydrogen generation by di8erent processes utilizing renewable energy sources. Solar energy is considered as a renewable energy source, which can be exploited for producing fuels, chemicals and power generation. There are various methods for solar hydrogen production, among that thermo-chemical production [1] and photoelectrolysis [2,3] are receiving much attention. For the electrolysis of water a theoretical thermodynamic potential di8erence of 1:23 V is needed between the cathode and anode, so that the following reaction may take place [4]: H2 O → H2 + 12 O2 ;

E = 1:23 V:

(1)

The energy corresponding to this potential di8erence is equivalent to a wavelength of 1008 nm; hence, light energy can be utilized in an electrochemical system to decompose water to produce hydrogen and oxygen. But, since water is transparent to visible light it cannot be decomposed by visible light alone. The photoelectrolysis of water using semiconductors is an interesting Eeld as in this process the semiconductor is illuminated with light and the electron– hole pairs generated within the semiconductor is used to ∗ Corresponding author. Tel.: +52-5-622-9706; fax: +52-7325-0018. E-mail address: [email protected] (P.J. Sebastian).

split water into hydrogen and oxygen, here the only energy spent is sunlight which is abundant in nature. The over voltage for hydrogen and oxygen evolution are 50 and 275 mV, respectively [5], so an electrode potential of around 1:6 V is required for the splitting of water, together with this there are internal losses of semiconductor, so all these together implies a minimum band gap of around 1:8 eV for the semiconductor [5]. Most of the semiconductor photocathodes and photoanodes are not suitable as they have either a large band gap and low e7ciency or small band gap and an external bias is needed. Moreover, the semiconductor should be stable in the electrolyte and band edges should withstand the hydrogen and oxygen evolution reactions. There are a good number of articles about the photoelectrochemical hydrogen production using silicon (Si) [6–8, and references therein]. It has been observed that the photoelectrochemical H2 evolution has signiEcantly improved after platinizing the Si surface [8]. Abruna and Bard [6] has observed that coating p-Si electrode with a thin Elm of poly (benzyl viologen) and Pt promotes the photoevolution of hydrogen. Simon and Wrighton [7] have reported that n-Si can be protected from photoanodic decomposition by a thin Elm of polyacetylene. The band gap of crystalline silicon is around 1:1 eV, which is a bit lower for the e8ective decomposition of water. The surface structure of a semiconductor highly controls its photocatalytic behavior; nanometric scale variation of the surface can vary its energy conversion e7ciency. By converting Si to porous Si, the material becomes near intrinsic and the band gap increases to around 1:6 eV

0360-3199/03/$ 30.00 ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 2 ) 0 0 1 4 2 - 8

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and along with that the e8ective area of the sample also increases that helps more e8ective water splitting. In this study Silicon and porous silicon was investigated to evaluate their possible application in a photoelectrochemical system for water splitting.

-0.005 -0.010 J (A/m2)

2. Experimental

0.000

2.1. Porous silicon preparation The wafers used for the preparation of porous silicon (Po-Si) was boron-doped crystalline silicon substrate (8 K cm), hence p-type. The rough side of the Si was coated with a thick layer of aluminum and then annealed to improve the ohmic contact. The Si was made porous by anodization in a Te9on container. The electrical connections were given from a computer controlled current source. The electrolyte was an aqueous HF/ethanol medium with 25% HF. The anodization begins when a constant current is applied between the silicon wafer and the electrolyte. The current density was maintained at 12 mA=cm2 throughout the experiment. The porosity is a linear function of the current density for a speciEc HF concentration [9]. Varying the etching time can control the thickness of the porous layer. We have estimated the porosity of the present samples as 60% in accordance with the reports [10,11]. The band gap of the samples with 72% porosity has been reported as 1:63 eV [11] and since we used Si wafers having identical physical characteristics we also expect a band gap close to this value. 2.2. Photoelectrochemical measurements For electrochemical measurements the porous silicon was cut into pieces of about 0:3 cm2 surface areas. The samples were ultrasonically washed with acetone and then with deionized water. The edges of the sample were electrically sealed using insulating glue. The electrical contact was taken from the aluminum ohmic contact using a copper wire and a conducting silver paste. Measurements were done for both Si and Po-Si. The electrochemical measurements were performed in a three-electrode electrochemical cell where porous-Si/p-Si serves as the working electrode, Pt as the counter electrode and a saturated Ag=AgCl2 (SSSC) as reference electrode. The electrolyte used was 0:1 M H2 SO4 . For the porous silicon I –V measurements were done under di8erent light intensities. The 9ux on the device was calculated using a calibrated thermopile. 3. Results and discussion The variation of photocurrent with applied voltage (I – V ) for Si and Po-Si under dark and illumination is given in Fig. 1. The light 9ux on the device was estimated as 150 mW=cm2 . From the Egure it can be observed that there

-0.015 -0.020 -0.025

Si: light Po-Si: light Po-Si: dark Si: dark

-0.030 -0.6

-0.4

-0.2

0.0

Potential (V vs. SSSC) Fig. 1. Current-potential characteristics in dark and light for Si and Po-Si. The light 9ux was 150 mW=cm2 .

is not appreciable photocurrent generation at the Si photocathode. The cathodic photocurrent is due to the H2 evolution and hence silicon as such without any modiEcation of the surface is not a potential candidate for hydrogen production. From the Egure it is clear that in the case of Po-Si there is a considerable increase in the cathodic current under illumination compared to Si. The hydrogen evolution onset potential shifts to more positive values under illumination, a shift of around 0:4 V is observed for 150 mW=cm2 illumination which is due to the photocatalysis of the semiconductor. This enhancement in cathodic current for the Po-Si can be due to: (1) increase in the band gap, approaching the optimum band gap for e7cient hydrogen production, (2) increase in the active area due to the formation of the porous layer. The exact measurement of the active area is beyond the scope of this article. Measurements were repeated many times and identical results were obtained. Fig. 2 is the variation of the H2 evolution onset point with 9ux on the Po-Si. The curves correspond to 25, 75, 125 and 150 mW=cm2 illumination from a tungsten halogen lamp. From this graph the potential corresponding to the H2 evolution was determined and plotted against the 9ux on the device (Fig. 3). Fig. 3 shows a linear dependence for hydrogen evolution potential on the level of 9ux. Similar observations have been made for the open circuit voltage in the case of photovoltaic devices [12,13]. The 9at band potential, Vfb of the Po-Si with respect to the SSSC is determined by photocurrent onset potential method. The photocurrent density (Jl ) can be written as [14] Jl2 =

2qjj0 2 2 (V − Vfb ); NA

(2)

where V is the applied potential with respect to SSSC, is the photon 9ux,  is the absorption coe7cient, NA is the acceptor concentration,  is the dielectric constant and 0 is

N.R. Mathews et al. / International Journal of Hydrogen Energy 28 (2003) 629 – 632

631

0.0

-2.0x10-7

150mW/cm^2 125 mW/cm^2 75mW/cm^2 25mW/cm^2

il (A)

-4.0x10-7

-6.0x10-7

-8.0x10-7

-1.0x10-6 -0.55

-0.50

-0.45

-0.40

-0.35

-0.30

-0.25

-0.20

Potential (V vs. SSSC) Fig. 2. I –V curves for Po-Si at di8erent light intensities.

-0.20

30 125mW/cm^2 75mW/cm^2 25mW/cm^2 20 2 2

-0.24

Jl (A/m )

-0.26

2

Voc (V vs. SSSC)

-0.22

10

-0.28

-0.30 20

40

60

80

100 120 140 160 180

0 -0.8

Flux (mW/cm2) Fig. 3. Variation of Voc for Po-Si with light 9ux on the photocathode.

-0.7

-0.6

-0.5

Potential (V vs. SSSC) Fig. 4. Graph of Jl2 (photocurrent density) against the applied potential (V ) with respect to SSSC; the 9at band potential is estimated as −0:625 V.

the permittivity of the free space. A graph of Jl2 against V will be a straight line and from the intercept and slope the Vfb and carrier concentration can be obtained. Fig. 4 is the Jl2 vs. V graph of the Po-Si in 0:1 M H2 SO4 for illumination intensities 25, 75 and 125 mW=cm2 . From the Egure the 9at band potential is estimated as −0:625 V with respect to SSSC at 300 K.

Po-Si there is a considerable increase in the cathodic current under illumination compared to the Si. The system was stable in the acidic medium even after several measurements. By applying di8erent light intensities it was found that there is a linear dependence for hydrogen evolution potential on the level of 9ux. The 9at band potential is estimated as −0:625 V with respect SSSC at 300 K.

4. Conclusion

Acknowledgements

p-type Si was made porous to increase the e7ciency of hydrogen production. It is observed that in the case of

This work was carried out as part of the CONACYT project G38618-U.

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[9] Pavesi L. Riv Nuovo Cimento 1997;20:1. [10] Canham L, editor. Properties of porous silicon. London, UK: INSPEC, 1997. [11] von Berreen J. PhD thesis, Technical University of Munich, 1997. [12] Fahrenbruch AL, Bube RH. Fundamentals of solar cells: photovoltaic energy conversion. New York: Academic Press, 1983. [13] Mathew X, Sebastian PJ, Sanchez A, Campos J. Sol Energy Mater Sol Cells 1999;59:99. [14] Yoon Ki Hyun, Seo Dong Kyun, Cho Yong Soo, Kang Dong Heon. J Appl Phys 1998;84:3954.