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Surface Passivation of GaN Nanowires for Enhanced Photoelectrochemical Water-Splitting Purushothaman Varadhan, Hui-Chun Fu, Davide Priante, Jose Ramon Duran Retamal, Chao Zhao, Mohamed Ebaid, Tien Khee Ng, Idris A. Ajia, Somak Mitra, Iman S Roqan, Boon S. Ooi, and Jr-Hau He Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b04559 • Publication Date (Web): 08 Feb 2017 Downloaded from http://pubs.acs.org on February 10, 2017
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Surface Passivation of GaN Nanowires for Enhanced Photoelectrochemical Water-Splitting Purushothaman Varadhan,#,$ Hui-Chun Fu,,#,$ Davide Priante,# Jose Ramon Duran Retamal,# Chao Zhao,# Mohamed Ebaid,# Tien Khee Ng,# Idirs Ajia,§ Somak Mitra,§ Iman S. Roqan,§ Boon S. Ooi,#,* and Jr-Hau He#,* #
Electrical Engineering Program, King Abdullah University of Science and Technology
(KAUST), Thuwal 23955-6900, Saudi Arabia. §
Materials Science and Engineering Program, King Abdullah University of Science and
Technology (KAUST), Thuwal 23955-6900, Saudi Arabia. $
The authors contributed equally
*
Corresponding authors:
[email protected];
[email protected] ACS Paragon Plus Environment
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ABSTRACT Hydrogen production via photoelectrochemical water-splitting is a key source of clean and sustainable energy. The use of one-dimensional nanostructures as photoelectrodes is desirable for photoelectrochemical water-splitting applications due to the ultra-large surface areas, lateral carrier extraction schemes, and superior light-harvesting capabilities. However, the unavoidable surface states of nanostructured materials create additional charge carrier trapping centers and energy barriers at the semiconductor-electrolyte interface, which severely reduce the solar-to-hydrogen conversion efficiency. In this work, we address the issue of surface states in GaN nanowire photoelectrodes by employing a simple and low-cost surface treatment method, which utilizes an organic thiol compound (i.e., 1,2-ethanedithiol). The surface treated photocathode showed an enhanced photocurrent density of -31 mA/cm2 at -0.2 V vs. RHE with an incident photon-to-current conversion efficiency of 18.3%, whereas untreated nanowires yielded only 8.1% efficiency. Furthermore, the surface passivation provides enhanced photoelectrochemical stability as surface treated nanowires retained ~80% of their initial photocurrent value and produced 8,000 µmol of gas molecules over 55 h at acidic conditions (pH ~0), whereas the untreated nanowires demonstrated only < 4 h of photoelectrochemical stability. These findings shed new light on the importance of surface passivation of nanostructured photoelectrodes for photoelectrochemical applications. KEYWORDS: III-Nitrides, nanowires, surface passivation, photoelectrochemical watersplitting, solar fuel
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Sunlight is an abundant, renewable source of energy that can be converted and stored in the form of environmental-friendly hydrogen using photoelectrochemical (PEC) water-splitting.13
To achieve a high solar-to-hydrogen conversion efficiency, nanostructured photoelectrodes are
desirable due to their ultra-high surface areas, increased light absorption, and effective charge carrier separation.4-6 However, ultra-high surface areas can be a double-edged sword in PEC water-splitting as unavoidable surface defects act as carrier trapping centers and induce surface Fermi level pinning and surface band bending (SBB),6,7 which creates an overpotential for charge carrier transport at the semiconductor-electrolyte interface that drastically reduces PEC efficiency.5-8 Such uncontrolled surface charge properties can further contribute to photocorrosion and instability under certain PEC conditions (e.g., extreme pH), thus severely limiting the practical application of this technology.
Recently, researchers have studied III-nitride nanostructures as potential candidates for PEC water-splitting applications due to their tunable bandgap, which spans nearly the entire solar spectrum and has a band-edge potential that straddles between the reduction and oxidation potential of water.9-14 Unfortunately, III-nitride nanowires (NWs) have pronounced surface states that result in surface Fermi level pinning and subsequent SBB, which leads to Shockley-ReadHall non-radiative recombination and indirect recombination related to the radial Stark effect.15,16 As a result, the PEC properties of molecular beam epitaxially (MBE) grown InGaN/GaN NW photocathodes can have a photocurrent density (Jph) as small as -2 mA/cm2 at VNHE = -0.08 V as well as a high leakage current and low turn on (Von) potential.13 Moreover, some researchers have recently raised concerns about the chemical stability of III-nitride materials in different electrolytes.17,18 For example, it has been reported that the PEC performance stability of InGaN/GaN core/shell NW photoanodes drops by 64% within 10 h under acidic conditions due
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to severe photocorrosion and dissociation of the nitride NWs.19 In addition, InGaN photoanodes undergo surface oxidation under PEC conditions to form a thin amorphous oxide that is mainly composed of Ga-O bonds, which dampen the PEC efficiency.17,18 Such small Von, low Jph, and poor PEC stability has been attributed to the surface states of III-nitride materials.17 Therefore, in order to achieve high solar-to-hydrogen conversion efficiency and long-term stability in IIInitride nanostructures for PEC water-splitting applications, it is essential to address the detrimental effects of these surface states.
In the past researchers have successfully applied the surface treatment in various semiconductors to passivate the surface states and the resulting material shows enhanced device performance including the electroluminescence and improved contact resistance.15,16,20 Sulfides are widely used to passivate the surfaces of III-V semiconductors, forming strong bonds with surface atoms in such materials as GaP, GaN, and GaAs.15,16,20-23 Among the different sulfides, ammonium sulfide ((NH4)2S) is the most commonly used inorganic compound for passivation.20 Organic sulfides, such as thioacetamide (TAM) and octadecylthiol (ODT) have also been explored to effectively passivate III-V semiconductors.21-22 GaN epitaxial layers and nanostructures have been typically passivated using various inorganic sulfides, though we recently investigated the contact resistance and enhanced emission of InGaN/GaN nanowire (NW) light emitting diodes that had been passivated with ODT.15,16 Metal-semiconductor-metal devices made using surface passivated III-V semiconductors are known to be stable in dry environments.23
However,
the
stability and
performance
of
sulfur-passivated
III-V
semiconductors under liquid/electrolyte environments has not been explored for electrochemical applications, though this information is critical for the direct conversion of solar energy into chemical fuels. Electrochemical cells operate under harsh and corrosive electrolytic
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environments at extreme pH conditions, and the migration of ions during electrochemical reactions can weaken surface passivation or protective layers. To the best of our knowledge, there are no reports on sulfur passivation of III-V semiconductors for PEC applications has been documented yet.
In this study, we developed a strategy of eroding the surface states of GaN NWs using a 1,2-ethanedithiol (EDT) surface treatment. This short carbon chain organic thiol compound significantly reduces the adverse effects of chemisorbed hydroxyl, oxide- and dangling bonds. Using optimized surface passivation conditions, EDT-treated NW (EDT-NW) photocathodes exhibited enhanced Jph, improved Von, and long-term stability as compared to the untreated-NWs. In-depth surface state analysis provides compelling experimental evidence of the suppression of hydroxyl and oxide bonds on the surface of the EDT-NWs, in addition to the significant enhancement in the carrier lifetime as shown by time-resolved optical studies, which suggests the significant removal of non-radiative recombination pathways. The results show the importance of surface states and how defect passivation can play a major role in developing better nanostructured photoelectrodes for PEC water-splitting. RESULTS AND DISCUSSION
For this work, we used a moderately doped p-GaN NW photocathode, which has been shown to have better PEC performance due to optimized band structure engineering.24 The vertically aligned Mg-doped GaN nanowires were directly grown on an n-Si substrate using radio frequency plasma-assisted molecular beam epitaxy (PA-MBE) without any external catalyst. Freshly grown GaN NW samples were then cleaned and immersed in an EDT solution for different periods of time to achieve surface passivation. Here, in this work we have used sulfur
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containing organic thiol compound, EDT for the surface passivation of GaN NWs and the discussion regarding the preference of EDT over other organic and inorganic sulfides has been provided in the Supporting Information. To fully realize the potential of GaN NWs for water reduction, a suitable catalyst is required to promote charge transfer, thereby suppressing chargecarrier recombination. As previously mentioned, NWs have a large surface area, and therefore conformal deposition of a co-catalyst is necessary to yield promising results. Pt is by far the bestknown hydrogen evolution catalyst, and therefore we employed atomic layer deposition (ALD) to achieve the conformal deposition of metal Pt over the entire NW structure (see Methods for more fabrication details).
Figure 1a shows a schematic of Pt-deposited GaN NWs on Si substrates. We characterized the structural properties of both untreated and EDT-treated NWs using scanning electron microscopy (SEM). A tilted view SEM image (Figure 1b) of the as-grown sample demonstrates that the NWs are vertically aligned to the Si substrate and have a height of ∼300 nm, a diameter of ∼40-70 nm, and an aerial density of ∼1.5 × 1010 cm-2 (inset of Figure 1b). More details about the length and diameter distribution of the GaN NWs are provided in Figure S1 in the Supporting Information. We further investigated the structural properties of GaN NWs with high-resolution transmission electron microscopy (HRTEM). Figure 1c presents an HRTEM image of an as-grown (i.e., untreated) GaN NW, which shows conformal deposition of the Pt NPs over the entire structure. HRTEM of this NW also illustrates the lattice fringes of the defect-free single crystalline GaN NW and crystalline Pt NPs, which have an average size of ~8 nm (inset of Figure 1c). The distance between two adjacent fringes is ~0.52 nm, which corresponds to the (0001) direction, confirming that the NWs were grown along the c-axis with their sidewalls being nonpolar m-planes.25 An HRTEM image (Figure 1d) of a Pt-decorated
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EDT-NW shows a thin amorphous layer covering the entire nanostructure. Moreover, TEM investigations of over 10 individual EDT-NWs confirmed the ubiquitous presence of this thin amorphous layer, which we presume to be the EDT (Figure S2). In order to understand the change in chemical properties of the GaN NWs after surface treatment, we carried out X-ray photoelectron spectroscopy (XPS) and fitted the results with Lorentzian deconvolution. Figure 2a,b shows the XPS scans of the S 2p/Ga 3s region of the untreated-NWs and EDT-NWs, respectively. Both NW samples revealed a strong peak at 160.4 eV, which can be assigned to the Ga-N bond.26 The EDT-NWs showed additional peaks at 168.3 eV and 169.5 eV, which have been attributed to the S-O bond that arises from the thiolcontaining EDT compound.26,27 We also found multiple peaks at 162.4 eV, 163.4 eV, 163.9 eV, and 164.9 eV in the EDT-NWs, which can be attributed to the C-S-H and Ga-S-C bonds and their associated doublet peaks (Figure 2b).28,29 From the doublet peaks of Ga-S-C, it is apparent that the S atoms bond to Ga rather than N, because the binding energy of Ga-S is more favorable than the formation of N-S bonds.28,29 To further confirm the S incorporation at the NW surface, we carried out S 2s scans for untreated-NWs and EDT-NWs (Figure 2c,d). The emergence of a strong peak at 232.6 eV and weak peaks at 226.2 eV and 227.5 eV in the EDT-NWs demonstrated the presence of S-O, Ga-S-C, and C-S-H bonds, respectively.30 Moreover, both untreated and EDT-treated NWs had a common peak at 237.5 eV, which we attribute to contaminants as no similar observation has been previously reported related to GaN. To gain further insight into the surface construction of the NWs, and in particular the possibility of oxide suppression, we monitored the O 1s peak (Figure 2e,f). The O 1s peak of untreated-NWs was dominated by a species at 530.9 eV, which has been previously assigned to O binding to Ga on the m-plane of GaN NWs.31,32 The side peak appearing at 531.9 eV can be attributed to Ga-O
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and weak Ga-OH bonds.33,34 In contrast, for the EDT-NWs, the peaks at 530.9 eV and 531.9 eV are considerably suppressed, and the emergence of a new peak at 537.8 eV can be assigned to SO and/or C-O bonds.26 Such a significant suppression of the Ga-OH and Ga-O signals in the O 1s spectrum clearly demonstrates that the surface treatment is effective in reducing chemisorbed OH and -O bonds on the GaN surface, which are caused by environmental conditions. All the possible bond configurations before and after EDT treatment, including Ga-N, Ga-S, Ga-S-C, and Ga-S-O, have been provided in Figure S5 of the Supporting Information. To test our hypothesis on the effective removal of chemisorbed molecules and passivation of dangling bonds by the EDT treatment, we studied the PEC properties of the GaN NW photocathodes with and without surface passivation in a custom-built quartz reactor with a three-electrode setup under 350 W AM 1.5G illumination. More details about the PEC measurements has been provided in the Methods section. The EDT treatment was carried out for different lengths of time, and the corresponding PEC results are provided in the Supporting Information (Figure S4). The best PEC results in terms of Von and current density were achieved by immersing the GaN NW photocathode in a solution of EDT for 5 min. A linear sweep voltammetry (LSV) scan under continuous illumination of the untreated-NWs and optimized EDT-NWs show the samples have a Jph of -1.85 mA/cm2 and -31 mA/cm2 at -0.2 V vs. RHE, respectively. The untreated-NWs and EDT-NWs also had a respective Von of -0.35 V and 0.10 V vs. RHE (Figure 3a,b). The poor Von of our GaN NWs as compared to the other reported photocathodes,9 has been attributed to the high resistance caused by the substrate and the PEC cell design than can be overcome with further modifications. Moreover, the EDT-NWs exhibited two other important characteristics in addition to Jph enhancement. First, the EDT-NWs had almost zero dark current density for the entire J-V scan, while there was a significant dark
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current for the untreated-NW device (Figure S3 and S4), which was caused by a shunt in the charge transport which bypasses the photoactive layer. Second, the favorable anodic shift in Von (from -0.35 V to 0.10 V vs. RHE) is important for unassisted and dual photoelectrode PEC water-splitting.10,35 The high dark current observed in the case of untreated-NW photocathode, which has a non-zero value even after many LSV cycles could be due to the fact that the photocathode has been electrocatalytically activated by pre-cathodization during the first LSV sweep. To further elucidate the performance of the EDT-NW photocathode, we measured the applied bias photon-to-current-efficiency (ABPE) under 350 W Xe-Hg illumination with an AM 1.5G filter, according to Equation 1:36
J ph (mAcm−2 ) × (1.4 − Vapp (V ) ABPE = ×100 Plight (mWcm−2 )
(1)
in which Vapp is the applied bias between the working electrode and the counter electrode, and Plight is the incident light intensity. The ABPE values for untreated and EDT-NWs were 0.13% and 1.05% at -0.5 V vs. RHE, respectively. The ABPE value for EDT-NWs was much higher than the typically reported values (
