Improved corrosion resistance of GaN electrodes in NaCl electrolyte for photoelectrochemical hydrogen generation

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 4 4 3 3 e1 4 4 3 9

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Improved corrosion resistance of GaN electrodes in NaCl electrolyte for photoelectrochemical hydrogen generation Ding-Hsiun Tu a, Hsin-Chieh Wang b, Po-Sheng Wang a, Wei-Chao Cheng c, Kuei-Hsien Chen b,**, Chih-I. Wu a,*, Surojit Chattopadhyay d,**, Li-Chyong Chen e a

Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 106, Taiwan Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan c Department of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan d Institute of Biophotonics, National Yang Ming University, Taipei, Taiwan e Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan b

article info

abstract

Article history:

A significant improvement in the stability of high-quality GaN films, for photoelectro

Received 13 June 2013

chemical hydrogen generation, has been demonstrated using near neutral NaCl(aq) elec-

Received in revised form

trolyte instead of conventional acidic HCl(aq). The experimental results conclude that the

19 August 2013

as-grown surface oxide passivates the surface from corrosion and, therefore, leads to a

Accepted 23 August 2013

higher photocurrent. Our result paves the way for the future development of stable

Available online 1 October 2013

hydrogen generation with abundant sea water and high-efficiency IIIeV compound semiconductors.

Keywords:

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

GaN Hydrogen generation Water splitting Stability Sea water

1.

Introduction

Photoelectrochemical (PEC) water splitting, possibly one of the future solutions for generating clean and renewable hydrogen gas from water, has received great attention in recent years. Since the first demonstration from Honda and Fujishima [1] more than four decades ago, much effort has

been devoted to develop a high-efficiency, high-stability and low-cost PEC system. For example, John Turner et al. achieved high solar-to-hydrogen efficiency by using GaInP2 and GaAs composite photoelectrodes [2]. Although IIIeV compound semiconductors are usually chemically stable in harsh environments, such high-efficiency photoelectrodes cannot endure long-term operation since etching is unavoidable

* Corresponding author. Tel.: þ886 939603591; fax: þ886 233663683. ** Corresponding authors. E-mail addresses: [email protected] (K.-H. Chen), [email protected], [email protected] (C.-I. Wu), sur@ym. edu.tw (S. Chattopadhyay). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.08.095

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during the reaction [3]. Therefore researchers have turned to oxide materials for their higher stability [4e6]. However, lower efficiency has always been a problem with highly stable oxides: for example, alpha-Fe2O3 [7,8]. For high band-gap oxide materials, the efficiency is inherently low according to theoretical predictions. As a result, researchers have started to tune the band gap by various methods [9e14], and to focus on enhancing the stability of some high-efficiency photoelectrodes [15e18]. GaN, a robust optoelectronic material [19] with a direct band gap and suitable band-edge potentials, has been considered a promising material for water splitting [20]. Fujii et al. have demonstrated the PEC water splitting properties of both n- and p-type GaN [21,22]. Some studies showed that the efficiency of GaN can be enhanced by crystallographic etching or metal nanoparticle decoration [23,24]. Although the band gap of GaN is relatively high and the theoretical solar-to-hydrogen efficiency is less than 1%, the extension to InGaN with a tunable band gap by incorporation of indium enables nitride-based materials to remain the focus of PEC hydrogen generation [25e31]. On the other hand, the electrolyte is another key component in the PEC system. In PEC reactions, whether for experiments or for real hydrogen production, the pure electrolyte always plays an important role. Researchers utilize acid or base solutions for the non-sacrificial reagent reaction, which needs plenty of fresh water. However, the total usable freshwater supply for ecosystems and humans is only less than 1 percent of all freshwater resources [32]. According to the United Nations World Water Development Report [33], fresh water will become “blue gold” in the near future, which means the cost of hydrogen from water splitting will rise year by year even if the efficiency and stability of photoelectrodes both reach the standards of the D.O.E (U.S. Department of Energy). This problem can be solved if a highly efficient and stable reaction can be realized in sea water. However, in the literature, the study of PEC hydrogen generation with sea water or NaCl solutions is still rare [34]. In this study, we demonstrate a stable n-GaN PEC hydrogen generation process in a near neutral NaCl solution and compare the results with the conventional 1 M HCl solution, which has usually been used to get a higher photocurrent with n-GaN. To analyze the reason for the enhanced stability in detail, X-ray photoemission spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS) are conducted. The result paves the way for the future development of stable hydrogen generation with abundant sea water and high-efficiency IIIeV compound semiconductors.

2.

Experimental methods

2.1.

Photoelectrochemical measurements

Commercial c-plane crystalline n-GaN wafers with carrier concentrations of 2  1016, 2  1017, and 2  1018 cm3, grown on sapphire with thicknesses around 6 mm (TDI, USA) were used as the electrodes. The ohmic contact was fabricated with 30 nm Ti/100 nm Au by e-beam deposition. Silver wires were attached to this metal contact by silver glue and the exposed metal contacts and silver wires were isolated from possible

electrolyte exposure using epoxy resin (Supporting information S9). A conventional three-electrode PEC setup was employed for the IeV measurement. The reference and counter electrodes were Ag/AgCl and Pt, respectively (Supporting information S10). The applied voltage and photocurrent were controlled and measured by a Solartron 1470E. A Xe lamp (150 W, OSRAM, Germany) was used for the 100 mW/cm2 incident light power density. The stability measurements at zero bias for 4 h were conducted in a twoelectrode system with Pt as the counter electrode.

2.2.

Characterization

After these stability measurements, the used electrolytes were diluted to 1 L and analyzed by ICP-MS (Thermo X Series II) to determine the dissolved Ga3þ content. From the detected quantity of Ga3þ, we could obtain the amount of charge resulting from sample degradation by a simple stoichiometric relationship. The Si concentration of the electrolyte was not measured, since at these doping concentrations (1016e1018 cm3) it would be below the detection limit for our ICP-MS measurement. To analyze the causes of the enhanced stability, XPS (ESCA VG Scientific Theta Probe [UK] with Al Ka X-ray excitation) and EIS were conducted. Then MotteSchottky plots were derived from the EIS measurements within a frequency range of 0.020e20 kHz with a 10 mV bias. The SEM images of the samples after 4 h of PEC stability measurement were taken by a FESEM F6700 (JEOL, Japan).

3.

Results and discussion

3.1.

The IeV measurement and stability results

The temporal stability of the photocurrent in the GaNelectrolyte system was studied for different carrier concentrations of GaN in 1 M NaCl(aq) and HCl(aq). Fig. 1 shows a comparison of the photocurrent stability in HCl(aq) and NaCl(aq) electrolytes, and the latter clearly reveals an enhanced photocurrent, with subsequent saturation for all n-

Fig. 1 e Photocurrent, at 0 bias voltage, as a function of time measured on n-type c-GaN samples with carrier concentrations of 1016, 1017 and 1018 cmL3 in a twoelectrode system (with Pt counter electrode).

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GaN electrodes. On the other hand, the measurements in 1 M HCl(aq) show relatively flat or even decreasing characteristics over time, similar to the results reported in earlier works [35]. The increase in carrier concentration in n-GaN from 1016 to 1018 cm3 results in a decreased photocurrent value at the initial stage in both electrolytes. This is due to higher recombination loss of the photogenerated carriers at defect sites created by the dopant [36]. However, the sample with highest carrier concentration (1018 cm3) shows the maximum increase of w70% from the initial photocurrent value after 4 h of reaction. The basic IeV data for all the samples in HCl and NaCl electrolyte is shown in Supporting information (S1). Based on the two-electrode measurement, the projected efficiency values can be computed according to the formula by M. Ono et al. [36]. In this study, we use Erev ¼ 1.4 V for the standard Cl2/Cl reduction potential, and the efficiencies calculated from the zero bias current of 1016, 1017, and 1018 cm3 GaN samples after 4 h of PEC reaction in NaCl are 0.53, 0.52 and 0.418%, respectively. Among the samples with different carrier concentrations, the 1016 cm3 sample demonstrates the best stability (Fig. 1). The initial (up to several minutes) photocurrents are almost

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identical for the samples with the same carrier concentrations studied in either HCl(aq) or NaCl(aq) electrolytes. Although the proton concentration in 1 M HCl(aq) is much higher, which can help the reduction reaction, the rate-limiting step for the overall hydrogen generation is the oxidation process. Therefore, as long as the oxidation reaction rates are similar, photocurrent generation will be similar. Surprisingly, in NaCl(aq), the photocurrents increase with time, eventually reaching saturation after about 4 h, and stay at the same level for at least w9 h (Supporting information S2). As the facet and morphology will also affect the water splitting efficiency [23], we used scanning electron microscopy (SEM) to observe the surface morphology after PEC stability measurement.

3.2.

The SEM and XPS analysis of the GaN photo-anode

From the SEM images (Fig. 2), we can clearly see that in both electrolytes, the number of etched pores increases as the sample carrier concentration increases [37]. However, the density of pores is lower and the shapes of the etched pits are predominantly triangular with the NaCl (Fig. 2(e) and (f)) solution. In NaCl, both surface oxidation and etching happen

Fig. 2 e Top-view SEM images of GaN surfaces after 4 h of PEC measurement in (aec) 1 M HCl(aq), and (def) 1 M NaCl(aq), for sample carrier concentrations of 1016, 1017 and 1018 cmL3, respectively. The arrows show the etching centers with triangular etching pits. The non-defect hexagonal islands are shown by the red boxes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 3 e The Ga 2p XPS spectra of the sample surfaces of (a) pristine GaN, GaN after 4 h of PEC in (b) 1 M HCl(aq) and (c) 1 M NaCl(aq); the dashed brown lines are the fitted curves. The spectra are shifted vertically for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

simultaneously, whereas only the etching process occurs in HCl due to the dissolution of the oxide in the extreme pH environment [38]. The etching of GaN should start from the structure defect and grain boundary, i.e., the space between colliding GaN islands during growth [39], which is why triangular pits are formed. On the other hand, oxidation happens mainly on the hexagonal islands of GaN [0001]. The carrier concentration increase leads to more defect sites and therefore generates more etching pits with larger size, as can be seen in Fig. 2(d) and (f). With more etched pores, the surface area for the PEC reaction increases and more photocurrent is generated with the passage of time in NaCl. As a consequence, the increase in percentage of the photocurrent is higher with higher carrier concentration. Images of the surfaces of the GaN electrodes after 5 h of stability testing in HCl and NaCl can be found in Supporting information (S3). The chemical reaction at the GaN electrodes was investigated via XPS. Fig. 3(a) shows the Ga 2p3 peak of pristine n-type

GaN (3  1016 cm3), near 1118.28 eV, whereas Fig. 3(b) and (c) shows the same peaks for n-GaN after 4 h of PEC measurement in 1 M HCl(aq) and 1 M NaCl(aq), respectively. There is no obvious shift of the Ga 2p peak for the sample in 1 M HCl(aq) (Fig. 3(b)) compared to that of pristine GaN (Fig. 3(a)). But the same Ga 2p peak shifts to higher binding energy, w1118.63 eV, after 4 h of PEC reaction in 1 M NaCl(aq) (Fig. 3(c)). This is an indication of GaN surface oxidation in 1 M NaCl(aq) which is not observed for GaN immersed in HCl (Fig. 3(b)). To monitor the oxidation/etching reaction in detail, a depth profile using XPS was taken to check for chemical modification of the sample post-PEC reaction. Fig. 4 shows the XPS depth profile of the sample after 4 h of PEC stability measurement in 1 M NaCl(aq). The Ga 2p peak shifts from a higher binding energy of w1118.63 eV, corresponding to gallium oxide, to w1117.13 eV, corresponding to Ga 2p in bulk GaN, after about 110 s of Arþ etching [40]. After dry etching for w110 s, the peak stabilizes at 1117.13 eV and stops shifting any further, which indicates a fully gallium nitride bulk. Considering that the etching rate is around 0.16  A/s, the thickness of the oxide layer is around 2 nm and does not grow after 90 min of PEC reaction (Supporting information S4). The sample in 1 M HCl(aq) does not show such oxidation layers. To identify the origin of the oxidation mechanisms, the N1s core level was also monitored in addition to the Ga 2p, for pristine GaN and those reacted in HCl and NaCl, using XPS. Atomic percentages of Ga, N, O, and N/Ga ratio are listed in Table S5. It can be observed that the post-PEC reaction oxygen content in GaN is much higher in NaCl than in the other two, indicating stronger oxidation under illumination. In HCl, however, etching of the oxidized GaN occurs due to the acidic environment. The pristine and HClreacted (3 h) samples show similar oxygen content on the surface. In the pristine sample, the O1s signal originates from the surface oxide. Although before XPS measurement we remove the surface oxide with 37% HCl, further oxidation cannot be avoided in ambient conditions prior to sample loading into the XPS chamber. For the same reason, GaN samples after several hours of reaction in HCl also develop some surface oxides under ambient conditions before being loaded into the chamber. The NaCl-reacted sample (3 h) has lower N/Ga ratios than pristine and HClreacted samples because the surface contains more oxygen due to oxidation.

3.3.

Fig. 4 e XPS depth profiles of the Ga 2p spectrum for the GaN surfaces after 4 h of PEC in 1 M NaCl(aq). The legend indicates different ArD etching times from 0 to 170 s. Spectra are shifted vertically for clarity.

The gallium ion concentration analysis by ICP-MS

In Table 1, we tabulate the dissolved Ga3þ concentration and the charges resulting from degradation of GaN as percentages of the total charges, as obtained from inductively coupled plasma mass spectrometry (ICP-MS) measurements. The method of estimating the percentage of charges in the electrolyte is provided in Supporting information (S6). Evidently, there are more Ga ions detected in the 1 M HCl(aq) electrolyte, confirming that the electrode degradation is more severe in HCl. The dissolved Ga3þ content in the 1 M NaCl(aq) is 100 times lower e sometimes below the detection limit e than in the 1 M HCl(aq) case. In both solutions, most of the photo-generated holes (hþ) at the GaN surface oxidize the chlorides (Cl) in the solution [41] whereas the reduction of protons (hþ) occurs

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Table 1 e Detected Ga3D concentrations, using ICP-MS, in the electrolyte after 4 h of PEC stability measurement of n-type c-GaN/sapphire thin films, with different carrier concentrations, in HCl and NaCl electrolyte. Carrier concentration (cm3) 1016 1017 1018 a

3þ

Detected Ga

concentration

a

1 M HCl(aq)

1 M NaCl(aq)

1.39 ppb (1.72%) 1.86 ppb (2.20%) 1.42 ppb (3.28%)

<0.01 ppb (<0.02%) <0.01 ppb (<0.02%) <0.01 ppb (<0.02%)

Percentage of charges from degradation.

at the Pt surface. The gallium oxide forms simultaneously because a small fraction of the holes react oxidatively with the non-defective host, which is known as the photo-oxidation reaction of GaN [42]. In the 1 M HCl solution, the gallium oxide dissolves due to the low-pH environment and the GaN underneath is therefore revealed, which leads to a continuing etching of GaN [38]. Clearly, this etching of the non-defective GaN is inhibited in NaCl because of the near neutral pH value. The oxide acts as a protective layer that improves the stability of GaN in 1 M NaCl(aq). Meanwhile the photocurrent can be enhanced due to the efficient surface passivation effect [42]. The ICP-MS data corroborates the information obtained from the SEM images that find a large pore distribution on the GaN sample surface after the PEC reaction in 1 M HCl(aq). It is also in agreement with the XPS findings that in NaCl(aq) the Ga atoms stay in the oxide layer rather than dissolving in the solution.

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3.4. The MotteSchottky measurements and the band diagram Having confirmed the chemical difference of the GaN surfaces in both electrolytes after the PEC measurements, electronic properties were studied via MotteSchottky plots, 1/C2 vs. V, where C is the capacitance and V is the voltage. Fig. 5 shows the MotteSchottky plot for GaN (1017 cm3 sample) in the two electrolytes before and after the PEC reactions. The slope of the MotteSchottky plot (Fig. 5(a) and (b)) is inversely proportional to the sample surface carrier concentration and the intercept on the V-axis can be considered as the flat-band potential. From the plots, a consistent increase in surface carrier concentration is observed post-PEC reaction in both electrolytes, with rather similar slopes in Fig. 5(a) and (b). But in 1 M NaCl(aq), the flat-band potential shows a positive shift from 0.9 to 0.19 V, in contrast with the case of 1 M HCl(aq), where the flat-band potential changes only slightly from 0.61 to 0.57 V. For reduced band bending, although the driving force for the transfer of the photo-generated holes would be weakened, the recombination rate of the electronehole pairs at the surface is reduced due to the efficient surface passivation by oxides, which results in a larger photocurrent [42]. Simultaneously, the coherently-grown gallium oxide itself can act as a surface photo-catalyst, and hence it is advantageous for charge transfer at the solid/liquid interface [43,44] and hinders further corrosion. No significant changes in the flat-band potential and surface-band alignment of GaN film are observed within the HCl electrolyte (Fig. 5(a) and (c)). This behavior is consistent for all the GaN samples with different carrier concentrations (Supporting information S7) studied in

Fig. 5 e MotteSchottky plots and schematics of band structure of GaN (with carrier concentration 1017 cmL3) before and after 4 h PEC stability measurement in (a) 1 M HCl(aq) and (b) 1 M NaCl(aq). (c) and (d) illustrate the implications of the MotteSchottky results (in a and b) on the GaN band structures.

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HCl and NaCl. The change in the flat-band potential before and after the PEC reaction is always small in HCl but considerably larger in NaCl for all the samples studied (Supporting information S8). For example, the shifts in the flat-band potential from before to after PEC in NaCl for the samples with 1016, 1017 and 1018 cm3 doping concentrations are 0.43, 0.71 and 0.53 V, respectively. The corresponding shifts in the flatband potential never exceed 0.04 V for the HCl electrolyte. This interface model explains the photocurrent stability data and corrosion resistance of GaN in NaCl electrolyte.

4.

Conclusion

In conclusion, a significant improvement in the photoelectrochemical stability and corrosion resistance of highquality GaN films for photoelectrochemical hydrogen generation applications has been demonstrated using near neutral NaCl(aq) electrolyte instead of the conventional acidic HCl(aq). The observation is true for different carrier concentrations of the GaN films. Direct microscopic images show large-scale random PEC etching in HCl but lesser preferential etching of the defect sites in the GaN film with the 1 M NaCl(aq) electrolyte. Electrolyte analysis, to determine the dissolved Ga concentration, corroborates the SEM result: a smaller amount of dissolved Ga, and hence corrosion, is found in the 1 M NaCl(aq) electrolyte. The greater stability in NaCl(aq) electrolyte may be attributed to a surface oxide layer, extending as much as 2 nm below the surface, which prevents the large-scale EC etching of the GaN film as indicated by the XPS and MotteSchottky plots. An interfacial energy diagram is proposed to give insight into the causes of this corrosion resistance in NaCl electrolyte. This strategy of stabilization with near neutral sea water as an NaCl electrolyte can be applied to other materials, especially IIIeV compound semiconductors.

Acknowledgment We thank the Core Facility Center, Office of Research and Development, TMU for the help of the ICP-MS measurement.

Appendix A. Supplementary data Additional information about PEC measurement setup and results, long tern stability test, SEM, XPS analysis, MotteSchottky plots and flat-band potentials are included. This material is available online at http://dx.doi.org/10.1016/j. ijhydene.2013.08.095.

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