Applied Science and Convergence Technology 2022; 31(2): 51-55
Published online March 30, 2022
Copyright © The Korean Vacuum Society.
aDepartment of Electronic and Information Materials Engineering, Division of Advanced Materials Engineering and Research Center of Advanced Materials Development, Jeonbuk National University, Jeonju 54896, Republic of Korea
bDivision of Advanced Materials Engineering, Jeonbuk National University, Jeonju 54896, Republic of Korea
cElectronics and Telecommunications Research Institute, Daejeon 34129, Republic of Korea
dDepartment of Physics, Kunsan National University, Kunsan 54150, Republic of Korea
eDepartment of Physics, Kangwon National University, Chuncheon 24341, Republic of Korea
Correspondence to:E-mail: firstname.lastname@example.org
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We report improved photoelectrochemical water splitting (PEC-WS) using GaN nanowires (NWs) with reverse-mesa structures (RMNWs) formed on Si(111) as a photoanode material. The GaN-RMNW photoanode exhibited a current density of 2.62 mA/cm2 and an applied photonto-current efficiency of 1.65% at 0.6 V versus a reversible hydrogen electrode. These values are considerably higher than those (1.16 mA/cm2 and 1.24%) of the photoanode based on GaN NWs with uniform hexagonal-pillar structures. The improved PEC-WS using the GaN-RMNW photoanode is attributed to the increase in the number of carriers participating in the PEC-WS reaction. The increase in the effective carriers is primarily due to the high crystallinity of the GaN RMNWs and the increase in the absorption rate of the incident light by the reverse-mesa structures. In addition, the energy-band structure between the GaN RMNWs and Si(111) promotes the separation of photogenerated carriers. Consequently, it reduces carrier recombination inside the photoanode, thereby enabling a high-performance PEC-WS.
Keywords: Photoelectrochemical water splitting, GaN nanowire, Reverse-mesa structure, High crystallinity, Photoanode
Photoelectrochemical water splitting (PEC-WS) is a promising method for providing clean and sustainable hydrogen energy [1–3]. Recently, metal oxides (MOs) with large energy band gaps have been actively used as photoanode materials owing to their nontoxicity and cost-effectiveness [4,5]. For example, Seabold
For a PEC-WS system, a semiconductor-based photoelectrode is considered a promising approach because it absorbs solar light and promotes redox reactions [10,11]. Currently, photoanodes based on III-nitride material systems are receiving considerable attention, owing to their wide and tunable energy bandgap, high carrier mobility, and chemical stability [12–14]. Most importantly, the energy band structure of GaN is suitable for water redox reactions . GaN nanowires (NWs) are a promising photoanode material for highperformance PEC-WS, owing to their large surface-to-volume ratio, fast mobility, and spatial separation of photogenerated carriers [16–18]. However, to date, the PEC-WS performance of GaN NWs is insufficient for practical applications. For example, Bae
In this study, we propose an improved PEC-WS using highcrystalline GaN NWs with reverse-mesa structures (RMNWs) as the photoanode material by increasing the number of photogenerated carriers participating in the reaction with the electrolyte. To increase the number of effective carriers, the absorption rate of the incident light was improved by confining more light inside the photoanode using the GaN-RMNW structure. The shape of the GaN NWs was manipulated from a hexagonal pillar to a reverse mesa by varying the ratio of N flux to Ga flux (V/III ratio). The structural properties of the GaN NWs were analyzed using field-emission scanning electron microscopy (FE-SEM) and double-crystal X-ray diffraction (DCXRD) measurements. Photoluminescence (PL) spectroscopy and UV-visible spectrophotometry were used to investigate the optical properties of the GaN NWs. PEC-WS using GaN-NW photoanodes was characterized in a 0.5-M H2S4 solution.
The GaN NWs used as photoanodes were grown on Si(111) using plasma-assisted molecular-beam epitaxy. As a first step of growing the NWs, the oxide layer naturally formed on the Si(111) substrate was eliminated by annealing in a chamber at a temperature of 900°C. Subsequently, a SiN
Structural characterization of the GaN NWs was conducted using FE-SEM (SU-70, Hitachi) and DCXRD (Max-2500, Rigaku). To investigate the optical properties of the GaN NWs, PL spectroscopy using a diode-pumped solid-state laser with a wavelength of 266 nm and a UV-visible spectrophotometer (UV-2550, Shimadzu) with a fixed slit size of 5 nm were performed. For the absorbance measurements of the GaN-NW samples, the baseline was first set using two Si substrates. Then, one of the Si substrates was replaced with the GaN-NW sample. The relative number of carriers that participated in the PEC-WS reaction was measured using an incident photon-to-electron conversion efficiency (IPCE) system (K3100, McScience). All PEC-WS measurements were conducted using a potentiostat (Reference-3000, Gamry Instruments Inc.) in a three-electrode configuration composed of a Pt counter electrode, Ag/AgCl reference electrode, and GaN-NW working electrode as the photoanode. A 0.5-M H2SO4 solution with a pH value of 0.3 was used as the electrolyte. According to the Nernst equation, the measured voltage was converted into a potential versus RHE . A xenon lamp (MAX-303, McScience) with a power density of 100 mW/cm2 was employed as the light source. The photoelectrochemical properties of the GaN-NW photoanodes were recorded by sweeping the voltage from 0 to 1.2 V versus RHE.
The cross-sectional FE-SEM images of the (a) Ref-NW, (b) RMNW1, (c) RMNW2, and (d) RMNW3 samples are shown in Fig. 1. The insets show the plan-view FE-SEM images. The average heights (diameters) of the Ref-NW, RMNW1, RMNW2, and RMNW3 samples were measured to be 332.8±15.9 (66.8±9.6), 304.6±17.8 (71.3±5.7), 177.5±40.7 (74.8±16.1), and 98.3±47.3 nm (57.4±11.6 nm), respectively; these measurements are summarized in Fig. 1(e). Because the width of the NWs varies with the vertical position, the average diameters of the GaN-NW samples were measured at half the maximum height. The NWs were approximated as cylindrical, and their volumes were calculated using the diameters and heights obtained above. The volumes of the single NWs for the Ref-NW, RMNW1, RMNW2, and RMNW3 samples were 1.166×106, 1.216×106, 0.780×106, and 0.254×106 nm3, respectively. The difference in the NW volume between Ref-NW and the RMNW samples was less than 4% and was negligible considering the calculation error in the structural dimensions. The spatial densities of the NWs for the Ref-NW, RMNW1, RMNW2, and RMNW3 samples were measured to be 4.2×109, 5.0×109, 5.4×109, and 6.2×109/cm2, respectively. An increase in the spatial density of the GaN NWs was clearly observed in the plan-view FE-SEM images as the V/III ratio increased. Moreover, the shape of the GaN NWs changed from hexagonal pillars to reverse mesas as the V/III ratio increased. These results can be explained by the N-blocking effect . Under N-rich conditions, the number of Ga atoms migrating towards the upper region of the GaN NWs is restricted. Thus, the Ga atoms that cannot migrate toward the upper region of the GaN NWs form nucleation seeds on the Si(111).
Figure 2(a) shows the DCXRD rocking curves of the Ref-NW, RMNW1, RMNW2, and RMNW3 samples. The two strong peaks observed at 28.32 and 34.5° in the DCXRD curves correspond to Si(111) and GaN(0002), respectively . An asymmetric property was observed in the peak corresponding to Si(111), which is related to the lattice mismatch between SiN
Figure 3(a) shows a schematic of the PEC-WS cell with a three-electrode configuration in 0.5-M H2SO4 electrolyte. Figures 3(b) and 3(c) show the current density of the GaN-NW photoanodes as a function of voltage versus RHE under dark and illuminated conditions, respectively. Under dark conditions, the Ref-NW, RMNW1, RMNW2, and RMNW3 photoanodes exhibited current densities of 100, 400, 1, and 5 nA/cm2 at 0.6 V versus RHE, respectively. For the illuminated condition, the Ref-NW, RMNW1, RMNW2, and RMNW3 photoanodes exhibited current densities of 1.16, 2.62, 0.15, and 0.37 mA/cm2 at 0.6 V versus RHE, respectively. From this result, we can estimate that the photoanodes in this study exhibit a better performance than those of previous studies [19,37]. For example, Bae
As described earlier, the improved results obtained by the PEC-WS with the GaN-RMNW photoanode is mainly attributed to the increase in the degree of light absorption, owing to the reverse-mesa structure of the GaN NWs. Figure 4 shows a three-dimensional schematic and the energy-band structure of the GaN-RMNW photoanode to explain the working mechanism of the improved PEC-WS performance. Because the difference in the NW volume between the Ref-NW and RMNW1 samples was negligibly small, it was assumed that the effect of the difference on the volume of the PEC-WS was negligible. That is, under illuminated conditions, the amount of light absorbed directly by the GaN NWs and GaN RMNWs is considered to be almost the same, owing to their similar volumes. However, for the RMNW1 photoanode, the probability of absorbing light reflected from the Si substrate increases, owing to the structural characteristics of GaN RMNWs compared to those of GaN NWs with a uniform hexagonal pillar structure. As shown in Fig. 4, the GaN RMNWs can absorb not only directly incident light, but also light reflected by the surface of the Si(111) substrate. This increase in the degree of light confinement and consequent absorption enables the GaN RMNWs to generate more carriers, thereby contributing to the PEC-WS reaction. Furthermore, the energy-band structure between Si(111) and the GaN RMNWs promotes the separation of photogenerated carriers and reduces the carrier-recombination rate inside the photoanode, enabling an improved PEC-WS.
PEC-WS using GaN RMNWs as the photoanode was significantly influenced by the degree of light absorption. The wavelengthdependent absorption of the GaN-NW photoanodes was investigated to understand the influence of light absorption on PEC-WS. Figure 5(a) shows the absorption spectra of the Ref-NW, RMNW1, RMNW2, and RMNW3 photoanodes measured at wavelengths ranging from 360 to 700 nm. The degree of light absorption of the GaNRMNW photoanodes is considerably higher than that of the Ref-NW photoanode, which indicates that the GaN-RMNW photoanode can absorb more light than the GaN NWs with a hexagonal pillar structure, as explained earlier. Figure 5(b) shows the IPCE spectra of the GaN-NW photoanodes as a function of wavelength, ranging from 360 to 700 nm. The IPCE values of the Ref-NW, RMNW1, RMNW2, and RMNW3 photoanodes were measured to be 43.4, 55.4, 29.2, and 15.7%, respectively, at a wavelength of 360 nm, which corresponds to the energy bandgap of GaN. Above 360 nm, there were no significant changes in the IPCE curves of the photoanodes. The IPCE value of the RMNW1 photoanode was higher than that of the Ref-NW photoanode, which is attributed to the reverse-mesa structure, which allows for an increase in the amount of light absorption. However, for the RMNW2 and RMNW3 photoanodes, the IPCE values were significantly lower than those of the other anodes. This resulted from an increase in the degree of influence of the interface between GaN and SiN
In summary, we demonstrated an improved PEC-WS using GaN RMNWs as a photoanode. The GaN-RMNW photoanode exhibited a current density of 2.62 mA/cm2 and ABPE of 1.65% at 0.6 V versus RHE, which are improved results compared to those of previous studies. The improvement in PEC-WS using the GaN-RMNW photoanode was attributed to the increase in the number of carriers participating in the PEC-WS reaction, owing to the formation of high-crystalline GaN RMNWs. In addition, the degree of light absorption at the GaNRMNW photoanode was significantly higher than that of the GaN NWs with a uniform hexagonal pillar structure, owing to an increase in the confinement of incident light by the reverse-mesa structure. The energy band structure between the GaN RMNWs and Si(111) induced the separation of photogenerated carriers, and consequently, reduced the carrier recombination rate inside the photoanode.
This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2018R1D1A1B07043442).
The authors declare no conflicts of interest.