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Energy Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. S6 — Nov. 7, 2011
  • pp: A1196–A1201
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Hydrogen gas generation using n-GaN photoelectrodes with immersed Indium Tin Oxide ohmic contacts

Shu-Yen Liu, Yu-Chuan Lin, Jhao-Cheng Ye, S. J. Tu, F. W. Huang, M. L. Lee, W. C. Lai, and J. K. Sheu  »View Author Affiliations


Optics Express, Vol. 19, Issue S6, pp. A1196-A1201 (2011)
http://dx.doi.org/10.1364/OE.19.0A1196


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Abstract

An n-GaN photoelectrochemical (PEC) cell with immersed finger-type indium tin oxide (ITO) ohmic contacts was demonstrated in the present study to enhance the hydrogen generation rate. The finger-type ITO ohmic contacts were covered with SiO2 layers to prevent the PEC cell from generating leakage current. Using a 1M NaCl electrolyte and external biases, the typical photocurrent density and gas generation rate of the n-GaN working electrodes with ITO finger contacts were found to be higher than those with Cr/Au finger contacts. The enhancement in photocurrent density or gas generation rate can be attributed to the transparent ITO contacts which allowed the introduction of relatively more photons into the GaN layer. No significant corrosion was observed in the ITO layer after the PEC process compared with the Cr/Au finger contacts which were significantly peeled from the GaN layer. These results indicate that the use of n-GaN working electrodes with finger-type ITO ohmic contacts is a promising approach for PEC cells.

© 2011 OSA

1. Introduction

2. Experiments

Si-doped GaN (n-GaN) epitaxial layers with a carrier concentration of ~1 × 1019/cm3 were grown on (0001) sapphire substrates. To clarify whether the immersed finger-type ITO ohmic contacts can lead to an enhancement in photocurrent density and hence the gas generation rate, two different n-type GaN working electrodes were designed for the PEC experiments. One was GaN working electrode with finger-type ITO ohmic contacts covered with SiO2 layer to protect the ohmic contacts from contact with the electrolyte and thus generate leakage current (labeled as PEC-1), as shown in Fig. 1(a)
Fig. 1 Schematic diagram of the photoelectrochemical cells (a) PEC1, (b) PEC2 (c) picture taken at a local area of PEC1. The inset of Fig. 1(c) shows the schematic structure of ITO/SiO2 staked layers on the GaN.
. For comparison, GaN working electrodes with ITO ohmic contacts aside from the working area (labeled as PEC-2) were also prepared, that is, the ITO ohmic contacts were not immersed in the electrolyte, as shown in Fig. 1(b). For PEC-1, the width of the ITO stripe contacts was 20 μm, and the spacing between stripes was 200 μm. The schematic diagram and the photograph of an area on the PEC-1 are shown in Fig. 1(a) and 1(c), respectively. The inset of Fig. 1(c) shows the schematic structure of the ITO ohmic contact and SiO2 protection layer stacked on the GaN layer. Further, the results for two other samples, PEC-3 and PEC-4, were demonstrated for comparison with PEC-1 and PEC-2. Similar to PEC-1, PEC-3 had an immersed finger-type ohmic contact. However, the ohmic contacts were made of a bilayer Cr/Au metal insted of ITO. The structure of PEC-4 was similar to PEC-2 but the ohmic contacts onto n-GaN were bilayer Cr/Au metal insted of ITO. Potentiostat (Autolab-PGSTAT128N) was used to supply the external bias and to measure the current density for the evaluation of the electrical properties of the PEC cells. A platinum (Pt) wire was used as the counterelectrode. A 300 W Xe lamp was used as light source, and 1 mol/L NaCl was used as the electrolyte. In this study, the PEC cells were constructed to be similar to the Hoffman apparatus for measuring the total volume of generated gases. The composition of generated gases was analyzed by a gas chromatograph (Agilent-6850). Based on the total volumes and composition of the generated gases, we thus could get the volumes of H2 and N2 gases generated from the PEC process.

3. Results and discussions

Figure 2
Fig. 2 Typical photocurrent density-bias curves of the experimental PEC cells.
shows the photocurrent density as a function of external bias voltage (VCE). VCE was the applied voltage between the n-GaN working electrode and the Pt counterelectrode. As shown in Fig. 2, the photocurrent density of PEC-1 is higher than that of PEC-2. The enhancement in photocurrent density in PEC-1 can be attributed to the distance between neighboring ITO ohmic contacts, which is small enough to increase the collection efficiency of the photo-generated electrons by the ITO ohmic contacts. In short, the photo-generated electrons can reach the ITO ohmic contacts before the electrons recombine with holes or charged defects in the GaN layer. Compared with PEC-1, the recombination of photo-generated carriers with one another or the charged defects is more likely to occur before the carriers reach the ohmic contact of PEC-2. The photocurrent densities of PEC-1 were at least 50% higher than those of PEC-2. This result can be attributed to the photo-generated electrons in PEC-1 which entail less transit time compared with PEC-2 before they reach the ohmic contacts, whereas the transit time is inversely proportional to the probability of recombination of the electrons with holes or charged defects. In PEC-2, the applied fields are difficult to distribute uniformly over the entire working area, thereby leading to a longer transit time of the electrons compared with PEC-1. Similarly, PEC-3 had a Cr/Au finger ohmic contact on the n-GaN working electrodes, so the photocurrent densities of PEC-3 were higher than those of PEC-4. Notably, the photocurrent densities of PEC-1 were slightly higher than those of PEC-3. This result can be attributed to the transparent ITO film that allowed more incident photons to reach the GaN layer compared with PEC-3 with opaque metal contacts (Cr/Au) on the n-GaN surface. To examine the stability of ITO finger contacts, the photocurrent was measured for one hour with VCE = 0.8V applied to the samples. Hydrogen gas at the Pt counter electrode was also collected. As shown in Fig. 3
Fig. 3 Photocurrent densities as a function of time when the PEC cells were biased at VCE = 0.8 V.
, the photocurrent density of PEC-1 and PEC-3 was higher than that of PEC-2 and PEC-4 because the former had immersed ITO finger-type and Cr/Au finger-type ohmic contacts on the GaN surface, respectively. The photocurrent density of PEC-1 was slightly higher than that of PEC-3. This result was consistent with the bias-dependent photocurrents, as shown in Fig. 2, and can be attributed to the transparent characteristic of the immersed ITO finger ohmic contacts. To evaluate the stability of the contacts, images taken from the surface of the working electrodes using scanning electron microscope (SEM) and optical microscope (OM) are shown in Fig. 4
Fig. 4 SEM images of the cross-section of PEC1 (a) before and (b) after photoelectrochemical measurements. (c) and (d) are the SEM images of the cross-section of PEC3 before and after photoelectrochemical measurements, respectively.
. Figures 4(a) and 4(b) correspond to PEC-1 before and after PEC measurement, respectively. The insets of Figs. 4(a) and 4(b) show the top-view of the OM images. Figures 4(c) and 4(d) correspond to PEC-3 before and after PEC measurement, respectively. The insets of Fig. 4(c) and 4(d) show the top-view of the OM images. No observable change can be seen from the ITO fingers after PEC measurements. By contrast, a part of the Cr/Au finger contacts was lifted off the n-GaN layer after the PEC measurements, as shown in the inset of Fig. 4(d). Although the photocurrent of PEC-3 was almost the same as that of PEC-1, as shown in Fig. 2 and Fig. 3, the poor stability of the contacts in the working electrodes of PEC-3 led to a reduction in the life span of the PEC cells. Based on these SEM images, the Cr/Au metal layer was peeled off the GaN layer. This result can be attributed to the relatively poor adhesion of Cr/Au on GaN. As a result, the OH- ions might diffuse through the SiO2 layer into the GaN/Cr/Au interface where the PEC process occurred. The PEC reactions at the GaN working electrode are both water oxidization and GaN decomposition, simultaneously [8

8. K. Fujii, T. Karasawa, and K. Ohkawa, “Hydrogen gas generation by splitting aqueous water using n-type GaN photoelectrode with anodic oxidation,” Jpn. J. Appl. Phys. 44(18), 543–545 (2005). [CrossRef]

]. The reactions occurred at the GaN/Cr/Au interface resulted in the Cr/Au metal contacts to peel off the GaN layer. This is probably the reason why the Cr/Au metal layer did not resist the corrosion of the electrolyte during the PEC process. However, the ITO film was firmly cohered on the GaN layer, as shown in Fig. 4(a) and 4(b), even if the ITO/n-GaN working electrodes were immersed into the NaCl electrolyte for a long time. There are two main issues in hydrogen generation by PEC water splitting. One is how to improve the hydrogen generation rate, and another one is how to prevent the photoelectrodes from the oxidation reactions. It had been revealed that the addition of CH3OH into the electrolyte could prevent the GaN from oxidation [15

15. K. Fujii, H. Nakayama, K. Sato, T. Kato, M. W. Cho, and T. Yao, “Improvement of hydrogen generation efficiency using GaN photoelectrochemical reaction in electrolytes with alcohol,” Phys. Stat. Solidi C 5(6), 2333–2335 (2008). [CrossRef]

]. In this study, we focused on the immersed finger-type ITO contacts to improve the hydrogen generation rate.

Table 1

Table 1. The Results of Solar-to-Hydrogen Conversion Efficiency, Gas Generation Rate, and Total Reaction Charge at VCE = 0.8 V

table-icon
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shows the gas generation rate and the total reaction charge of the four samples. The gas generation rates of PEC-1 and PEC-3 were both higher than those of PEC-2 and PEC-4. In addition, the gas generation rate of PEC-1 was slightly higher than that of sample PEC-3. Therefore, the gas generation rates obtained from the fabricated samples were consistent with the results on photocurrent density (shown in Fig. 3). In theory, the total charges of the electrochemical reaction are proportional to the product (i.e., hydrogen). The percentages of hydrogen in the collected gas might also be of interest. Based on the analysis of the gas chromatograph, the collected gas included 67% H2 and 33% N2. Nitrogen gas primarily came from the etching reaction that occurred at the n-GaN/electrolyte interface during the PEC process. As shown in Table 1, the gas generation rate is indeed proportional to the amount of the total reaction charge, indicating that PEC-1 and PEC-3 can generate more hydrogen gas than PEC-2 and PEC-4. However, whether the immersed ITO finger contacts in PEC-1 contributed photocurrent remains to be elucidated. Theoretically, the conduction band edge of ITO is assumed to be more positive than the reduction level of the hydrogen ion [16

16. J. E. A. M. van den Meerakker, E. A. Meulenkamp, and M. Scholten, “(Photo)electrochemical characterization of tin‐doped indium oxide,” J. Appl. Phys. 74(5), 3282 (1993). [CrossRef]

,17

17. C. G. Zosik, Handbook of Electrochemistry, 1st ed. (Elsevier, 2007) p. 342.

]. Therefore, it cannot split water under illumination without any applied electrical potential. Huang et al. reported that in the scanning of potentials ranging from −1.0 to 1.5 V, after the completion of reduction current peak, a reduction in hydrogen ions does not occur. In other words, the reduction of hydrogen ions can be observed in potentials lower than −1.0 V [18

18. C. A. Huang, K. C. Li, G. C. Tu, and W. S. Wang, “The electrochemical behavior of tin-doped indium oxide during reduction in 0.3 M hydrochloric acid,” Electrochim. Acta 48(24), 3599–3605 (2003). [CrossRef]

]. The voltages from −0.4 to 2 V were applied in the present study. Therefore, the immersed ITO finger contacts cannot generate hydrogen in this bias region. To clarify this point further, PEC cells using ITO films and 1M NaCl aqueous solution as the working electrodes and electrolyte, respectively, were also prepared for measuring the photocurrents. The ITO films with a thickness of 280 nm were deposited on a sapphire substrate. The photocurrents taken from the PEC cells were lower than those of PEC-1 and PEC-2. This result indicates that the reaction rate of oxidation in the ITO contacts was extremely low. In short, the ITO contacts in the 1M NaCl aqueous solution under bias and UV illumination were resistant to erosion.

In addition to the adhesion issue, this phenomenon shows that the ITO/n-GaN working electrodes are more stable than the Au/Cr/n-GaN electrodes. Therefore, the use of ITO films associated with GaN-based materials for the formation of working electrodes is a promising method to generate hydrogen by photoelectrolysis from water.

4. Conclusions

The photocurrent density and hydrogen generation rate of n-type GaN photoelectrodes, with and without immersed finger-type ITO (or Cr/Au) ohmic contacts, were symmetrically compared. At an external bias of 0.8 V, the n-GaN working electrodes with ITO finger contacts exhibited a 25% enhancement in gas generation rate compared with those with Cr/Au finger contacts. The marked enhancement in photocurrent can be attributed to the transparent ITO contacts which facilitated the introduction of relatively more photons into the GaN layer. Further, no significant corrosion was observed in the ITO layer after the PEC process compared with the Cr/Au finger contacts which were significantly peeled from the GaN layer. These results indicate that the use of n-GaN working electrodes with finger-type ITO contacts is a promising approach for PEC cells.

Acknowledgments

Financial support from the Bureau of Energy, Ministry of Economic Affairs of Taiwan, ROC. through grant No. 99-D0204-6 is appreciated. The authors would also like to acknowledge the National Science Council for the financial support of the research Grant Nos. 100-2112-M-006-011-MY3, 98-2221-E-218-005-MY3 and 100-3113-E-006-015.

References and links

1.

N. S. Lewis, “Light work with water,” Nature 414(6864), 589–590 (2001). [CrossRef] [PubMed]

2.

A. Fujishima and K. Honda, “Electrochemical photolysis of water at a semiconductor electrode,” Nature 238(5358), 37–38 (1972). [CrossRef] [PubMed]

3.

A. J. Nozik, “Electrode materials for photoelectrochemical devices,” J. Cryst. Growth 39(1), 200–209 (1977). [CrossRef]

4.

R. C. Kainthla and B. Zelenay, “Significant efficiency increase in self-driven photoelectrochemical cell for water photoelectrolysis,” J. Electrochem. Soc. 134(4), 841 (1987). [CrossRef]

5.

A. J. Nozik and R. Memming, “Physical chemistry of semiconductor-liquid interfaces,” J. Phys. Chem. 100(31), 13061–13078 (1996). [CrossRef]

6.

I. Waki, D. Cohen, R. Lal, U. Mishra, S. P. DenBaars, and S. Nakamura, “Direct water photoelectrolysis with patterned n-GaN,” Appl. Phys. Lett. 91(9), 093519 (2007). [CrossRef]

7.

J. Wu, W. Walukiewicz, K. M. Yu, J. W. Ager III, E. E. Haller, H. Lu, and W. J. Schaff, “Small band gap bowing in In1−xGaxN alloys,” Appl. Phys. Lett. 80(25), 4741 (2002). [CrossRef]

8.

K. Fujii, T. Karasawa, and K. Ohkawa, “Hydrogen gas generation by splitting aqueous water using n-type GaN photoelectrode with anodic oxidation,” Jpn. J. Appl. Phys. 44(18), 543–545 (2005). [CrossRef]

9.

J. Li, J. Y. Lin, and H. X. Jiang, “Direct hydrogen gas generation by using InGaN epilayers as working electrodes,” Appl. Phys. Lett. 93(16), 162107 (2008). [CrossRef]

10.

M. Ono, K. Fujii, T. Ito, Y. Iwaki, A. Hirako, T. Yao, and K. Ohkawa, “Photoelectrochemical reaction and H2 generation at zero bias optimized by carrier concentration of n-type GaN,” J. Chem. Phys. 126(5), 054708 (2007). [CrossRef] [PubMed]

11.

S. Y. Liu, J. K. Sheu, C. K. Tseng, J. C. Ye, K. H. Chang, M. L. Lee, and W. C. Lai, “Improved hydrogen gas generation rate of n-GaN photoelectrode with SiO2 Protection layer on the ohmic contacts from the electrolyte,” J. Electrochem. Soc. 157(2), B266–B268 (2010). [CrossRef]

12.

J. K. Sheu, Y. K. Su, G. C. Chi, M. J. Jou, C. M. Chang, and C. C. Liu, “Indium tin oxide ohmic contact to highly doped n-GaN,” Solid-State Electron. 43(11), 2081–2084 (1999). [CrossRef]

13.

T. Margalith, O. Buchinsky, D. A. Cohen, A. C. Abare, M. Hansen, S. P. DenBaars, and L. A. Coldren, “Indium tin oxide contacts to gallium nitride optoelectronic devices,” Appl. Phys. Lett. 74(26), 3930 (1999). [CrossRef]

14.

J. Stotter, Y. Show, S. Wang, and G. Swain, “Comparison of the Electrical, Optical, and Electrochemical Properties of Diamond and Indium Tin Oxide Thin-Film Electrodes,” Chem. Mater. 17(19), 4880–4888 (2005). [CrossRef]

15.

K. Fujii, H. Nakayama, K. Sato, T. Kato, M. W. Cho, and T. Yao, “Improvement of hydrogen generation efficiency using GaN photoelectrochemical reaction in electrolytes with alcohol,” Phys. Stat. Solidi C 5(6), 2333–2335 (2008). [CrossRef]

16.

J. E. A. M. van den Meerakker, E. A. Meulenkamp, and M. Scholten, “(Photo)electrochemical characterization of tin‐doped indium oxide,” J. Appl. Phys. 74(5), 3282 (1993). [CrossRef]

17.

C. G. Zosik, Handbook of Electrochemistry, 1st ed. (Elsevier, 2007) p. 342.

18.

C. A. Huang, K. C. Li, G. C. Tu, and W. S. Wang, “The electrochemical behavior of tin-doped indium oxide during reduction in 0.3 M hydrochloric acid,” Electrochim. Acta 48(24), 3599–3605 (2003). [CrossRef]

OCIS Codes
(310.3840) Thin films : Materials and process characterization
(310.4925) Thin films : Other properties (stress, chemical, etc.)
(310.6845) Thin films : Thin film devices and applications
(310.7005) Thin films : Transparent conductive coatings

ToC Category:
Photochemistry

History
Original Manuscript: August 8, 2011
Revised Manuscript: September 16, 2011
Manuscript Accepted: September 19, 2011
Published: October 3, 2011

Citation
Shu-Yen Liu, Yu-Chuan Lin, Jhao-Cheng Ye, S. J. Tu, F. W. Huang, M. L. Lee, W. C. Lai, and J. K. Sheu, "Hydrogen gas generation using n-GaN photoelectrodes with immersed Indium Tin Oxide ohmic contacts," Opt. Express 19, A1196-A1201 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-S6-A1196


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References

  1. N. S. Lewis, “Light work with water,” Nature414(6864), 589–590 (2001). [CrossRef] [PubMed]
  2. A. Fujishima and K. Honda, “Electrochemical photolysis of water at a semiconductor electrode,” Nature238(5358), 37–38 (1972). [CrossRef] [PubMed]
  3. A. J. Nozik, “Electrode materials for photoelectrochemical devices,” J. Cryst. Growth39(1), 200–209 (1977). [CrossRef]
  4. R. C. Kainthla and B. Zelenay, “Significant efficiency increase in self-driven photoelectrochemical cell for water photoelectrolysis,” J. Electrochem. Soc.134(4), 841 (1987). [CrossRef]
  5. A. J. Nozik and R. Memming, “Physical chemistry of semiconductor-liquid interfaces,” J. Phys. Chem.100(31), 13061–13078 (1996). [CrossRef]
  6. I. Waki, D. Cohen, R. Lal, U. Mishra, S. P. DenBaars, and S. Nakamura, “Direct water photoelectrolysis with patterned n-GaN,” Appl. Phys. Lett.91(9), 093519 (2007). [CrossRef]
  7. J. Wu, W. Walukiewicz, K. M. Yu, J. W. Ager, E. E. Haller, H. Lu, and W. J. Schaff, “Small band gap bowing in In1−xGaxN alloys,” Appl. Phys. Lett.80(25), 4741 (2002). [CrossRef]
  8. K. Fujii, T. Karasawa, and K. Ohkawa, “Hydrogen gas generation by splitting aqueous water using n-type GaN photoelectrode with anodic oxidation,” Jpn. J. Appl. Phys.44(18), 543–545 (2005). [CrossRef]
  9. J. Li, J. Y. Lin, and H. X. Jiang, “Direct hydrogen gas generation by using InGaN epilayers as working electrodes,” Appl. Phys. Lett.93(16), 162107 (2008). [CrossRef]
  10. M. Ono, K. Fujii, T. Ito, Y. Iwaki, A. Hirako, T. Yao, and K. Ohkawa, “Photoelectrochemical reaction and H2 generation at zero bias optimized by carrier concentration of n-type GaN,” J. Chem. Phys.126(5), 054708 (2007). [CrossRef] [PubMed]
  11. S. Y. Liu, J. K. Sheu, C. K. Tseng, J. C. Ye, K. H. Chang, M. L. Lee, and W. C. Lai, “Improved hydrogen gas generation rate of n-GaN photoelectrode with SiO2 Protection layer on the ohmic contacts from the electrolyte,” J. Electrochem. Soc.157(2), B266–B268 (2010). [CrossRef]
  12. J. K. Sheu, Y. K. Su, G. C. Chi, M. J. Jou, C. M. Chang, and C. C. Liu, “Indium tin oxide ohmic contact to highly doped n-GaN,” Solid-State Electron.43(11), 2081–2084 (1999). [CrossRef]
  13. T. Margalith, O. Buchinsky, D. A. Cohen, A. C. Abare, M. Hansen, S. P. DenBaars, and L. A. Coldren, “Indium tin oxide contacts to gallium nitride optoelectronic devices,” Appl. Phys. Lett.74(26), 3930 (1999). [CrossRef]
  14. J. Stotter, Y. Show, S. Wang, and G. Swain, “Comparison of the Electrical, Optical, and Electrochemical Properties of Diamond and Indium Tin Oxide Thin-Film Electrodes,” Chem. Mater.17(19), 4880–4888 (2005). [CrossRef]
  15. K. Fujii, H. Nakayama, K. Sato, T. Kato, M. W. Cho, and T. Yao, “Improvement of hydrogen generation efficiency using GaN photoelectrochemical reaction in electrolytes with alcohol,” Phys. Stat. Solidi C5(6), 2333–2335 (2008). [CrossRef]
  16. J. E. A. M. van den Meerakker, E. A. Meulenkamp, and M. Scholten, “(Photo)electrochemical characterization of tin‐doped indium oxide,” J. Appl. Phys.74(5), 3282 (1993). [CrossRef]
  17. C. G. Zosik, Handbook of Electrochemistry, 1st ed. (Elsevier, 2007) p. 342.
  18. C. A. Huang, K. C. Li, G. C. Tu, and W. S. Wang, “The electrochemical behavior of tin-doped indium oxide during reduction in 0.3 M hydrochloric acid,” Electrochim. Acta48(24), 3599–3605 (2003). [CrossRef]

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