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Abstract
Photoelectrochemical (PEC) water splitting is one of the most promising hydrogen production methods because of its high efficiency, renewable resources and harmless by-products. Gallium nitride (GaN) is suitable for PEC water splitting because it has excellent stability in electrolyte and band gap energy which straddles the redox potential of water (Vredox = 1.23 V). These characteristics allow this material to split water stably without external bias. However, the stability of GaN is still not sufficient for practical applications. In this study, we investigated the properties of GaN photoelectrodes with aluminum oxide (Al2O3) thin film as a protection layer for increasing stability. In a long-term stability test, Al2O3-coated GaN showed more stable photocurrent than that of bare GaN. The total hydrogen production amount was also improved in Al2O3-coated samples than bare GaN. These results indicate that the Al2O3 protection layer significantly enhances stability and hydrogen production.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Today, most of the world’s energy demands depend on fossil fuels. However, a large amount of carbon dioxide from fossil fuels aggravated global warming and it became a crucial issue. It is extremely urgent to replace fossil fuels [1–4].
Hydrogen is considered one of the alternative energy. It has a high energy ratio per unit mass and harmless by-products from combustion. In addition, it has a strong possibility of utilization as an energy carrier because hydrogen is easy to transport and store. It also can be directly used as a fuel and can be used in fuel cells for production of highly efficient electric energy [4,5]. However, most of the hydrogen is produced from the reforming of fossil fuels. This means that hydrogen cannot be a perfectly clean energy source. For this reason, some alternative ways to produce hydrogen such as thermochemical and electrochemical hydrogen production methods are considered. But these methods are not always environmentally friendly and still less competitive than fossil fuel reforming. Therefore, further researches for hydrogen production methods are required. photoelectrochemical (PEC) water splitting is recognized as one of the most promising candidates to produce hydrogen. PEC water splitting is a method for directly producing hydrogen by decomposing water using sunlight. It has theoretically efficient mechanism for hydrogen production and does not cause environmental pollution [4,6,7].
Semiconductor materials used for PEC water splitting must satisfy several main criteria. To overcome the energy loss in the system, it must have a band gap energy of at least 1.7 eV, and the band edge and Fermi level of the semiconductor should straddle the redox potential of hydrogen and oxygen. Additionally, the material must be stable in the electrolyte, efficiently generate electron-hole pairs from light energy, and should have a fast charge transfer to prevent accumulation of generated minority carriers at the interface [8].
Among semiconductor materials, Gallium nitride (GaN) is the material satisfying these criteria for photoelectrolysis. Because this material contains the redox potential for water (Vredox = 1.23 V), it can split water without external bias. And it has excellent chemical resistance in electrolyte solutions. Furthermore, GaN has been studied in solid-state lighting (SSL), which indicates that it can grow crystalline epitaxial layers and electrode characteristics can be designed using some kinds of strategies like band gap engineering. So, GaN photoelectrodes could have various applications in PEC water splitting cell [9–13].
However, the practical application of GaN as a PEC cell is limited primarily due to photocorrosion in aqueous electrolytes. Photocorrosion of GaN under illumination in the PEC cell results in photoexcited holes moving to the surface of the photoelectrode and anodic oxidation occurs, as in the following reactions [14]:
As a result of this reaction, the photocorrosion of GaN photoelectrodes proceeds, and semiconductor surface damage decreases hydrogen generation efficiency. Long photoelectrode lifetimes decrease unit costs in terms of hydrogen production expense, so the long-term stability of the photoelectrode is an important evaluation factor for practical use of the PEC cell. For this purpose of high stability, some strategies have been explored. Some studies revealed that designing the surface structures of GaN photoelectrodes is partly attributed to enhance the stabilities against photocorrosion [15,16]. And selection the appropriate electrolyte such as NaCl also help to improve the stabilities of photoelectrodes [17]. Especially, introducing stable materials on the surface of photoelectrode widely have been investigated and metal oxides such as MnOx, NiOx, TiO2, and Al2O3 have all been explored [18,19]. These materials could isolate the light absorbing semiconductor from the aqueous environment by introducing a surface layer. And the protective layer must not only allow current flow across the interface but also be chemically stable under electrolyte [20].
Currently, advanced thin film fabrication can be grown by atomic layer deposition (ALD) without defect. These advances in processing make it possible to consider efficient tunnel junction photoelectrodes for photoelectrochemical water splitting [21]. In this study, we investigated the use of a thin Al2O3 layer to protect the surface of GaN in basic conditions. We coated an Al2O3 layer on a GaN photoelectrode by ALD and investigated the effect of the Al2O3 thin film on PEC properties of the GaN photoelectrode. We confirmed that the Al2O3 overlayer has a protection effect on GaN photoelectrodes and compared the PEC properties of the GaN electrodes before and after Al2O3 coating.
2. Experimental
2.1 Sample fabrication
GaN samples were fabricated using metal organic chemical vapor deposition (MOCVD). On a c-plane sapphire substrate, 2 μm of undoped GaN was grown as a buffer layer and 2 μm of n-type GaN, doped with silicon, was grown thereon. Before coating the Al2O3 layer on the GaN surface, the samples were cut into the rectangular shape, 1 x 1.5 cm2 in area, and the location for the ohmic contact with copper (Cu) wire was protected using poly methyl methacrylate (PMMA). Then, the coating process was performed using ALD. The process was conducted in a chamber at 160°C and deposition was started after exposing the sample for ~5 min to an O3 atmosphere for making the GaN surface hydrophilic. Trimethyl aluminum (TMA) was used as an aluminum source, and H2O vapor was used as an oxygen source. The Al2O3 layers were deposited to the thickness of 1 nm and 2 nm, labeled 1 nm Al2O3/GaN and 2 nm Al2O3/GaN, respectively. After ALD, the samples were annealed at 500°C for 1 h in an atmospheric environment. To confirm the thickness of the Al2O3 layer on the GaN surface, the Al2O3-coated samples were treated with a focused ion beam system (Versa 3D, FEI), and analyzed by transmission electron microscopy (TECNAI F20, FEI), and energy dispersive X-ray spectroscopy (PHOENIX, EDAX).
After deposition of Al2O3 layer, the PMMA was removed using acetone, and the ohmic contacts were formed with Cu wires using indium soldering. To preventing leakage current when these parts were immersed in the electrolyte, they were insulated with epoxy resin. The PEC characteristics were analyzed after drying in room temperature for one day to harden the epoxy resin. Figure 1 is a schematic showing the fabrication process for a sample, and the last picture is a photograph of a sample produced using the process, in the form of PEC characteristic analysis.
Fig. 1 Schematic of the process for fabricating samples for PEC property measurements (①~⑤) and photograph of the sample (⑥).
2.2 Photoelectrochemical properties
The Al2O3-coated GaN working electrodes were immersed in NaOH electrolyte at pH = 13.6. Pt wire was used as a counter electrode, and Ag/AgCl/NaCl [sodium-chloride saturated silver-chloride electrode (SSSE)] was used as a reference electrode. The electrode potential of this reference electrode is E (AgCl/Ag) = + 0.212 V. The light source was used a 500 W Xe lamp (USHIO Optical ModuleX, USHIO). The light source illuminated the samples through an ND filter (190-1700 nm, OD 50 nm, 32% transmission) to obtain conditions similar to natural light, and the light intensity was adjusted to 100 mW/cm2 using power meter measurements (NOVA2, Ophir).
PEC properties analyses including potentiostatic electrochemical impedance spectroscopy (potentiostatic EIS), cyclic voltammetry analysis, staircase linear sweep voltammetry, and chronoamperometry were measured using a pontentiostat (PARSTAT4000, Princeton Applied Research). The potentiostatic EIS and cyclic voltammetry analysis were measured using a three-electrode configuration. Staircase linear sweep voltammetry and chronoamperometry analysis were operated in a two-electrode configuration versus the Pt counter electrode. The potentiostatic EIS analysis was performed using an alternating current with a magnitude of 0.02 V at a frequency range of 100,000-1 Hz without illumination. The cyclic voltammetry was measured at a scan rate of 0.02 V/s in the range of −2.0 to + 1.0 V, and it was performed for 3 cycles in illumination. The staircase linear voltammetry was measured in the range −0.3 to + 1.6 V, with intervals of 0.1 V in illumination. The reliability of the sample was confirmed using chronoamperometry analysis for 240 min at zero bias versus Pt counter electrode.
3. Results and discussion
Figures 2(a) and 2(b) show the cross-sectional TEM images of the Al2O3 coated GaN with different thickness of Al2O3. Figure 2(a) shows the white region denotes Al2O3 layer, and dark region denotes GaN. Al2O3 layer was not successfully deposited on GaN surfaces. Al2O3 have a thickness of 1 nm and the agglomeration is observed on the GaN surface. Figure 2(b) shows that The Al2O3 have a thickness of 2 nm and uniformly coated on GaN surfaces. It indicates Al2O3 layer successfully deposited on GaN surface without agglomeration. Energy-dispersive X-ray spectroscopy (EDX) analysis confirms the existence of Ga, O, and Al elements whereas Cu and C peaks emanating from the carbon-coated TEM grid also exist (Figs. 2(c) and 2(d)). Figures 2(c) and 2(d) insets show that the composite ratio. It indicates weight percentage of 2 nm Al2O3/GaN was higher than that of 1 nm Al2O3/GaN. It shows that Al2O3 layer intentionally well grown on GaN surface. From the results, we could be thought that agglomeration was occurred in the 1 nm-thickness Al2O3 overlayer due to the insufficient ALD cycle [22].
Fig. 2 Cross-sectional TEM images of 1 nm Al2O3 overlayer (a) and 2 nm Al2O3 overlayer (b), EDX profiling data of 1 nm Al2O3 overlayer (c) and 2 nm Al2O3 overlayer (d).
In order to investigate the surface state of GaN after Al2O3 coating, potentiostatic EIS was performed. Figure 3 shows Mott–Schottky plot (M-S plot) of the impedance as a function of applied voltage, measured using potentiostatic EIS analysis. And Eq. (2) is the Mott-Schottky equation:
where ε is the dielectric constant of the semiconductor material and ε0 is the vacuum permittivity. e is the elementary charge, k is the Boltzmann constant, T is the absolute temperature, and Φsc is the potential applied to the space charge region. The Csc is capacitance and it was calculated by impedance fitting using an equivalent circuit (Fig. 3 inset). It includes a solution resistance, a double layer capacitor and a charge transfer (or polarization resistance). The double-layer capacitance is in parallel with the charge-transfer resistance. According to this equation, the flat-band potentials can be estimated by x-intercept of the trend line obtained from extrapolating on the M-S plot and the reciprocal of the slope means surface carrier concentration [23–27].Fig. 3 Mott–Schottky plot of 1 nm Al2O3/GaN, 2 nm Al2O3/GaN and reference. The equivalent circuit for impedance analysis is indicated.
In the M-S plot, the trend lines of the three samples show the same slope. It means they had the same surface carrier concentrations in the electrolyte. On the other hand, the flat-band potentials were −1.396 V in reference, −1.608 V in the v, and −1.562 V in the 2 nm Al2O3/GaN, respectively. After Al2O3 was deposited, the flat-band potential shifted −0.212 V (1 nm Al2O3/GaN) and −0.166 V (2 nm Al2O3/GaN) negatively, versus reference. It indicates Al2O3 change surface state of GaN. Formal et al. reported that Al2O3 thin film coating on hematite photoelectrodes by ALD caused a negative shift in the onset voltage over 100 mV and increasing saturated current [28]. They explained that the reason is the metal oxide layer passivates the surface state and lower the overpotential. It is agreement with our results and from these results, we presume that Al2O3 overlayer has surface state passivation effect on GaN photoelectrode.
Figure 4(a) displays the cyclic voltammetry graph. The current density is defined as the current divided by exposed area of 1cm2. The inset of Fig. 4(a) shows the current magnified Fig. 4(a) for confirming the onset voltage. The dark currents were negligible (not shown). The onset voltages were −1.333 V (Reference), −1.313 V (1 nm Al2O3) and −1.324 V (2 nm Al2O3), respectively. Generally, flat-band potentials show a similar tendency with onset voltages [29]. But in the case of Al2O3 coated GaN samples, the onset voltages did not correspond to flat-band potentials and they did not show a remarkable difference. The disagreement of onset voltages and flat-band potentials can be explained that some resistance existed at the electrode-electrolyte interface and that this resistance resulted in a large turn-on voltage. Figure 4(b) is the charge transfer resistance measured from potentiostatic EIS analysis. 1 nm Al2O3 sample shows the highest charge transfer resistance. And it decreased in the order of 2 nm Al2O3 sample and reference sample. It is because Al2O3 overlayer suppressed the charge transfer between semiconductor-electrolyte interface. Especially, in 1 nm Al2O3 sample, agglomeration makes GaN surface contact directly with electrolyte and PEC reaction was concentrated on bare GaN surface. It would cause a bottleneck effect in the charge transfer between the semiconductor–electrolyte interface and result in increasing charge transfer resistance. For 1 nm Al2O3 sample, this effect caused more overpotential and make the sample represented almost same onset voltage despite having more negative flat-band potential. Comparing the saturated photocurrent, the reference sample shows around 0.6 mA/cm−2 and it is the highest value. 1 nm Al2O3 and 2 nm Al2O3 samples are recorded around 0.5 mA/cm−2 and 2 nm Al2O3 present little higher photocurrent. The band gap energy of Al2O3 is 6.8 eV and it is a quite high value compared to that of GaN ( = 3.4 eV) [30]. Furthermore, Shi et al. reported that the transmittance of Al2O3 layer ( = 11.62 nm) from 300 nm to 800 nm wavelength indicated upper than 90% [31]. Considering these reports, the suppression of incident light absorption of the device could be negligible due to the thin thickness of 1nm and 2nm thickness. Therefore, this decrease was only caused by charge transfer resistance at the interface between GaN surface and electrolyte.
Fig. 4 (a) Third cycle of cyclic voltammetry measurement for 1 nm Al2O3/GaN, 2 nm Al2O3/GaN and reference versus Ag/AgCl/NaCl reference electrode. (b) Charge transfer resistance of 1 nm Al2O3/GaN, 2 nm Al2O3/GaN and reference versus the Ag/AgCl/NaCl reference electrode.
Figure 5(a) shows a staircase linear scan voltammetry analysis to observe the change in photocurrent density as a function of applied voltage in the two-electrode method, which is used in practical applications. This provides a potential change to the Pt counter electrode. The shape is similar to the cyclic voltammetry curve, with a slight positive shift. The graph shows a sufficiently high photocurrent at zero bias applied to the Pt counter electrode. This indicates that the GaN photoelectrode can be used as a photoelectrode in the PEC cell without external voltage.
Fig. 5 (a) Staircase linear scan voltammetry of 1 nm Al2O3/GaN, 2 nm Al2O3/GaN and reference versus Pt counter electrode. (b) Time variation in photocurrent density of for 1 nm Al2O3/GaN, 2 nm Al2O3/GaN, and reference at zero bias versus Pt counter electrode. (c) Calculated amounts of hydrogen production from the photocurrents of (b).
Figure 5(b) is a chronoamperometry analysis to verify the stability of the reference and Al2O3–coated GaN samples over time. We did not apply external bias according to the result of Fig. 5(a). It was measured for 240 min at zero bias versus a Pt counter electrode. The initial values of photocurrent density were 0.49 mA/cm2 (reference), 0.415 mA/cm2 (2 nm Al2O3) and 0.408 mA/cm2 (1 nm Al2O3), respectively. The reference sample showed the highest value and it corresponds to voltammetry tendency. But, around 30 min later, relatively rapid reduction in photocurrent was observed in the reference sample. In comparison, the Al2O3-coated samples showed almost constant photocurrent densities for 240 min. This is because Al2O3 overlayer prevents direct photocorrosion of GaN surface. These results obviously indicate that Al2O3 overlayer can improve the chemical stability of GaN photoelectrode. The cross point between the reference and Al2O3-coated samples occurred at 55 min for 1 nm Al2O3 and 43 min for 2 nm Al2O3. As the Al2O3 thickness increased, the decrease in the slope reduced and cross point presented faster. We hypothesize that this was due to an incomplete covering of the GaN surface with the 1 nm Al2O3 layer, which results in more photocorrosion than on the 2 nm Al2O3-coated sample.
We estimated amount of hydrogen production from the photocurrent in chronoamperometry. Figure. 5(c) shows the amount of hydrogen generation by integrating the photocurrent density in Fig. 5(b) according to Faraday’s law of electrolysis shown as Eq. (3):
In the equation, t is the time at which the PEC reaction proceeds, I is the photocurrent measured, and F is the Faraday constant, the value of the charge of one mole of electrons in coulombs. n is the number of electrons moving per mole of molecules, where n = 2 for hydrogen reduction [10]. Because the initial photocurrent value is higher in the reference, the initial amount of hydrogen generation is also higher in the reference. However, the difference in the slope with time made the hydrogen generation from the Al2O3-coated samples inverted with that of the reference. The cross points with the reference occurred at 123 min for 1 nm Al2O3 and 91 min for 2 nm Al2O3. After 240 min, the total hydrogen produced were 23.57 μmol/cm2 (reference), 27.18 μmol/cm2 (1 nm Al2O3), and 29.47 μmol/cm2 (2 nm Al2O3), respectively. 2 nm Al2O3 sample appeared better performance than 1 nm Al2O3 sample. It was attributed to the difference in surface morphology. From these results, the Al2O3 overlayer is confirmed that it has an effect on improving the stability and hydrogen production ability of the GaN photoelectrodes.
4. Conclusions
In this study, PEC characteristics of GaN photoelectrodes coated with an Al2O3 layer using ALD were investigated. The Al2O3 layer caused a negative shift on the flat-band potential of the Mott–Schottky plot, and the negative shift declined with increase in the thickness of the oxide layer. Unlike the flat-band potential, the cyclic voltammetry analysis indicated similar onset voltages for all three samples, and the saturated photocurrents were decreased in Al2O3 coated samples. This suggests that the Al2O3 layer lowered charge transfer of GaN photoelectrodes. When evaluating the reliability of the photoelectrode, the initial photocurrent and hydrogen production appeared the highest values in the reference, but the values of the reference sample and Al2O3 deposited samples reversed. The photocurrent and the reliability of the photoelectrode improved for the thicker Al2O3 layer. It thought to be that because the 1 nm Al2O3 left a partially bare GaN surface from fewer ALD cycles, which caused reactions on the bare site. This likely makes a bottleneck effect on the semiconductor-electrolyte interface. From these results, we concluded that introducing a material, which does not directly enhance the charge transfer of the photoelectrode, can sufficiently improve the hydrogen generating ability. This study also confirmed that the morphology of a protection layer is a critical factor for optimum photoelectrode performance.
Funding
National Research Foundation of Korea (2015R1D1A1A01058849, 2018R1A6A1A03024334).
Acknowledgments
This research was supported by the Basic Science Research Program and the Priority Research Centers Program, both through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology.
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