Photoelectrochemical Hydrogen Production on InP Nanowire Arrays

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Photoelectrochemical Hydrogen Production on InP Nanowire Arrays with Molybdenum Sulfide Electrocatalysts Lu Gao, Yingchao Cui, Jia Wang, Alessandro Cavalli, Anthony Standing, Thuy Vu, Marcel Verheijen, Jos Haverkort, Erik P.A.M. Bakkers, and Peter H.L. Notten Nano Lett., Just Accepted Manuscript • Publication Date (Web): 29 May 2014 Downloaded from http://pubs.acs.org on May 29, 2014

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Photoelectrochemical Hydrogen Production on InP Nanowire Arrays with Molybdenum Sulfide Electrocatalysts Lu Gao, *,† Yingchao Cui,‡ Jia Wang,‡ Alessandro Cavalli,‡ Anthony Standing,‡ Thuy T.T. Vu,‡ Marcel A. Verheijen,‡,§ Jos E.M. Haverkort,‡ Erik P.A.M. Bakkers,*,‡,¶ and Peter H.L. Notten*,† † Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, 5600 MB Eindhoven, the Netherlands ‡ Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, the Netherlands § Philips Innovation Services Eindhoven, High Tech Campus 11, 5656AE Eindhoven, the Netherlands ¶ Kavli Institute of Nanoscience Delft, Delft University of Technology, 2600 LS Delft, the Netherlands KEYWORDS: Semiconductor nanowires, Solar fuel, Photoelectrochemistry, Hydrogen production

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Abstract:

Semiconductor nanowire arrays are expected to be advantageous for photoelectrochemical energy conversion due to their reduced materials consumption. In addition, with the nanowire geometry the length scales for light absorption and carrier separation are decoupled, which should suppress bulk recombination. Here, we use vertically-aligned ptype InP nanowire arrays, coated with noble-metal-free MoS3 nanoparticles, as the cathode for photoelectrochemical hydrogen production from water. We demonstrate a photocathode efficiency of 6.4% under Air Mass 1.5G illumination with only 3% of the surface area covered by nanowires.

Photoelectrolysis of water into hydrogen and oxygen is a promising and sustainable approach for conversion of solar energy to chemical fuels, as in photosynthesis.1-3 p-type indium phosphide (p-InP) is an attractive photocathode for hydrogen evolution due to its small band gap, appropriate band-edge potential and low surface-recombination velocity. More than 11% photocathode conversion efficiencies have been achieved with planar p-InP using noble metal electrocatalysts.4-8 However, large-scale application of these photoelectrochemical cells is limited due to the scarcity of indium and precious metal electrocatalysts. In comparison with a planar single crystal, semiconductor nanowire arrays effectively suppress the light reflection at the surface due to a graded refractive index.9 Furthermore, the subwavelength size of nanowires allow these structures to absorb light from outside their projected area thus increasing the light absorption.10 A fully functional nanowire array can be transferred into a polymer11 and back contacted, reducing the amount of indium by a factor 104 compared to a bulk electrode. In addition to saving valuable materials, the performance of the 2 ACS Paragon Plus Environment

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cells is enhanced by the wire geometry. The high aspect ratio of nanowires decouples the directions of light absorption and charge-carrier collection, and therefore increases charge separation.12 The large surface area of nanowire arrays decrease the current density and relaxes the stringent requirements for the electrocatalysts, thus the highly active precious metal catalysts can be replaced by earth-abundant catalysts with lower activity. Lastly, lattice mismatched materials can readily be grown without misfit dislocations using the nanowire geometry, enabling the fabrication of multijunction cells, which is crucial for highly-efficient water splitting devices. Recently, high efficiency solar cells based on InP nanowire arrays have been reported.13,14 Several semiconductor nanowire systems, synthesized by different methods, have been investigated by photoelectrochemistry.15-21 A few of them have been exploited for photoelectrochemical hydrogen production from water, such as Si,22-24 GaP,25 GaN26 and InGaN.27 However, even combined with noble metal catalysts, the solar conversion efficiencies for hydrogen evolution are normally limited to 2.5%, because the semiconductor materials either have too large band-gaps or inappropriate band-edge potentials as compared to the hydrogen evolution potential. In the present paper, we show, for the first time, photoelectrochemical hydrogen production from water by InP nanowire arrays. When the nanowire surface is functionalized with a noble-metal-free electrocatalyst MoS3, the conversion efficiency is significantly improved. The p-InP nanowire arrays are grown by the vapor-liquid-solid (VLS) mechanism using metalorganic vapor phase epitaxy system (MOVPE) with gold as catalyst. The gold nanoparticles with 100 nm diameter are patterned on a p-InP (111)B substrate by soft nanoimprint lithography.28 The as-grown InP nanowire arrays are approximately 1.5 µm in length, 100 nm in diameter and


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are arranged on a square lattice with center-to-center pitch of 500 nm, resulting in a 3% surface coverage by nanowires (Figure 1a). We note that the InP nanowire layer contains about 3 times less indium per unit area than a 100 nm-thick ITO film used in many photoelectrochemical cells. The reflectance of InP nanowire arrays is about 1%, which is much lower than that of planar samples (ca. 30%, see Figure S1 in the supporting information). In order to distinguish the contribution of the nanowires from that of the substrate, the substrate is insulated by SiO2 and Benzocyclobutene (BCB) photoresist, as schematically shown in Figure 2 from step (a) to (d). To improve the performance of photoelectrochemical cells, catalysts are often required to facilitate the interfacial electron transfer between semiconductors and electrolytes. Although a few noble metals are highly active for hydrogen evolution, the scarcity of these metals limits their large-scale application. A lot of effort has been devoted to replacing precious metals with earth abundant materials. Molybdenum sulfides were identified as active non-noble-metal catalysts for hydrogen evolution.29 Amorphous molybdenum sulfide MoSx films have been synthesized by chemical,30 thermal,31 electrochemical32 and photoelectrochemical33 methods. However, nanoparticles are preferable to films as they avoid excessive light absorption or reflection and preserve the desired interfacial energetics. Here we deposit MoSx nanoparticles by photochemical deposition under open-circuit condition. Under AM 1.5G illumination, the generated electrons in the conduction band of the p-InP reduce the precursor (NH4)2MoS4 at the surface to form amorphous MoSx and the generated holes in the valence band oxidize the sulfide (Figure 2 step (e)). The amount of catalyst has been optimized for all our devices to generate the maximum power. After deposition, the nanowires are characterized by Transmission Electron Microscopy (TEM) and Energy Dispersive X-ray analysis (EDX). The TEM image (Figure 1c) shows that amorphous MoSx particles (some of which are highlighted by red dashed circles) are 4 ACS Paragon Plus Environment

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randomly attached to the InP nanowire. EDX mapping of elements Mo and S in the same region (Figure 1d) confirms the composition of these particles. The composition and valence state of amorphous MoSx are further investigated by X-ray photoelectron spectroscopy (XPS) as shown in Figure 3. Commercial polycrystalline MoS2 is used as a reference compound for comparison. The binding energies of Mo 3d in the electrodeposited MoSx are the same as those in MoS2, indicating a +4 oxidation state for Mo. The S 2p spectrum of MoSx contains two doublets. The doublet at lower binding energy (161.5 and 162.7 eV) is attributed to S2- at the basal plane, which is the same as MoS2. The doublet at higher binding energy (163.1 and 164.1 eV) is attributed to S22- at the bridge site. All these assignments have been proven by the systematic study of a series of molybdenum sulfides.34 A quantification by XPS gives a Mo/S ratio of 1:3.0. From these analyses we conclude that the composition of the particles is MoS3. Three samples will be compared in the following photoelectrochemical measurements: planar p-InP; as-grown InP nanowire arrays and substrate-insulated InP nanowire arrays. All samples are studied with/without MoS3 deposited. The flatband potential (Vfb) of planar InP determined from a Mott-Schottky measurement (Figure S2 in the supporting information) is at about 1.0 V versus Reversible Hydrogen Electrode (RHE) in 1 M HClO4. The difference between Vfb and the hydrogen evolution potential defines the theoretical maximum open-circuit voltage (Vocmax) of 1.0 V, which is much larger than that of p-Si.22-24,35 The dopant density of planar InP calculated from the slope of the MottSchottky plot is 1.3×1018 cm-3, which is close to that indicated by the manufacturer (1.4-1.8×1018 cm-3). After MoS3 deposition, both the slope and the Vfb remain essentially constant, implying that the catalyst nanoparticles hardly change the interfacial energetics. Due to the different potential distributions in the nanowires and in the substrate, it is difficult to interpret the Mott-


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Schottky plots of InP nanowire arrays. However, MoS3 deposition does not change the slope and the intercept of the Mott-Schottky plot either. Figure 4 shows the current-potential (I-V) curves and photocathode conversion efficiencies of the six samples under chopped AM 1.5G illumination in 1M HClO4 electrolyte. The dark current densities of all samples are less than 1µAcm-2 in the scanned potential range. As shown in Figure 4a, the open-circuit voltage (Voc) and the short-circuit current (Isc) of the planar InP electrode under illumination are 0.20 V and 10 mAcm-2, respectively. The fill factor (ff), defined as the maximum power output divided by the product of Voc and Isc, is 0.16. After adding the catalyst, the Voc increases up to 0.55 V and the ff increases up to 0.51 (Figure 4b). The improved performance is primarily due to the good catalytic activity of MoS3 which facilitates a rapid electron transfer to the electrolyte so as to suppress recombination at the surface. As sulfides are well-known surface passivation reagents for III-V semiconductors,36 the surface states of InP may also be passivated after the catalyst deposition, resulting in an enhancement of Voc by suppressing non-radiative recombination at surface states. The passivation effect is substantiated by photoluminescence and life-time measurements at room temperature as shown in Figure S3 in the supporting information. After MoS3 deposition, the photoluminescence intensity increases by 7 times and the life-time is increased from 0.5 to 0.6 ns. Whether this passivation effect results directly from MoS3 or sulfide ions in solution during the deposition process is not clear from these results. Compared to the planar InP electrode, the substrate-insulated nanowire arrays electrode shows a higher Voc (0.45 V) but similar ff (0.19) (Figure 4c). The higher Voc of the substrate-insulated nanowire arrays electrode indicate that surface recombination is reduced compared to planar InP electrodes. After deposition of MoS3, the Voc increases slightly to 0.60 V and ff increases 6 ACS Paragon Plus Environment

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significantly to 0.44. The different improvement of Voc between planar and substrate-insulated nanowire arrays electrode after MoS3 deposition is attributed mainly to the different extent of suppression of surface recombination. It should also be noted that the amount of the catalysts may not be the same on planar as it is on nanowires, which may also contribute to the difference of Voc between the planar and nanowire electrode. The photocurrent saturates when the potential is negative enough, such that electron-hole recombination is effectively suppressed. In this situation, the saturated photocurrent density (Iph) is determined by light absorption and bulk recombination/charge separation. The theoretical limit of Iph for planar InP is 34.5 mAcm-2 assuming that each incident photon with an energy above the band gap, 1.34eV, generates one electron-hole pair and contributes to the current. Therefore, the theoretical Iph for InP nanowire arrays with 3% packaging fraction would be about 1 mAcm-2 in classical ray optics. The experimental Iph of substrate-insulated InP nanowire arrays, 18 mAcm-2 (in Figure 4d), is much larger than the theoretical Iph, indicating substantial internal light scattering and optical focusing into the InP nanowire regions, as has been observed before in nanowire solar cells13,37. The higher Isc of as-grown nanowire arrays with MoS3 (Figure 4f) compared to substrate-insulated nanowire arrays (Figure 4d) proves that the substrate/liquid junction also contributes to the photocurrent. This agrees with the simulation and experiments that 50% of the light is absorbed by the wire array13, 1% is reflected, and thus the remaining 49% of the light is absorbed by the substrate. The photocathode conversion efficiency (η) is defined according to the following equation37 η=

Vapp I

(1)

Pin


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where Pin is the power of the incident light (100 mWcm-2 under AM 1.5G illumination), Vapp the applied potential versus RHE, and I is the measured photocurrent. The photocathode efficiencies are plotted in Figure 4 (blue traces) for all samples. The maximum efficiency of as-grown InP nanowire arrays with MoS3 catalyst is 6.4% (Figure 4f), which is the highest efficiency reported for photoelectrochemical water reduction by nanowires up to now.22-27, 35 With only 3% filling fraction, the 4.7% efficiency for the substrate-insulated InP nanowire arrays with MoS3 (Figure 4d) is larger than that of the planar electrode used (Figure 4b). For planar semiconductors, electrons photogenerated at depths shorter than the Debye length can reach the surface and contribute to the current. Electrons generated at depths longer than the Debye length simply recombine (bulk recombination) with holes present in the p-type material. The high aspect ratio of nanowires decouples the directions of light absorption and charge-carrier collection. Since the electrons only have to diffuse, at maximum, the radius of nanowires, charge separation is more efficient in nanowires. In other words, the bulk recombination is suppressed compared to planar semiconductors. 34 The stability of nanowire electrodes is investigated at 0 V under continuous AM 1.5G illumination for longer time period (Figure 5). The photocurrent of as-grown InP nanowire arrays decreases to about 50% of the initial current within 1 hour. The decrease of the photocurrent probably results from the surface degradation by continuous surface charging during illumination. Transient surface charging effects are also visible at 0 V in the I-V curve under chopped illumination (Figure 4e). After MoS3 deposition, the photocurrent is quite stable and 93% of the initial photocurrent is sustained after 1 hour. The higher stability of MoS3 decorated nanowires is attributed to fast electron transfer and less surface charging current, which is consistent with the I-V curve under chopped illumination (Figure 4f). 8 ACS Paragon Plus Environment

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Finally, in order to benchmark the catalytic activity of MoS3 with Pt on p-type InP nanowires, Pt is deposited by the same approach. A 5.2% efficiency is obtained with Pt decorated nanowires (Figure S4 in the supporting information), which is lower than the 6.4% obtained with MoS3 decorated nanowires. The reason may be attributed to different properties of the semiconductor/catalyst interface for the different catalysts.38 To conclude, the combination of InP nanowires with MoS3 as catalyst is a promising system for photocatalytic reduction of water. Further improvements can be implemented by optimizing the light absorption in the nanowire array by increasing the wire diameter.


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FIGURES

Figure 1. a) 30° tilt SEM image of as-grown InP nanowire arrays. b) Cross-section SEM image of substrate-insulated InP nanowire arrays. c) TEM image of an InP nanowire after MoS3 deposition. d) MoS3 EDX mapping. The Mo-L and S-K lines are too close in the EDX spectrum to allow for separation of the contributions of the two elements. EDX mapping of elements P e) and In f).


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Figure 2. Schematic of the processes to insulate the substrate and to deposit MoS3 photochemically. a) deposition of a thin-layer of SiO2 on the InP nanowires and substrate by PECVD, b) spin-coating of thick-layer BCB on SiO2 covered InP nanowires, c) dry-etch BCB by RIE to expose the top part of the nanowires, d) wet-etch of SiO2 by HF, e) deposition of MoS3 on nanowires by illuminating the arrays with a solar simulator.


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Figure 3. X-ray Photoelectron Spectroscopy of (1) polycrystalline MoS2 and (2) amorphous MoS3 deposited on InP nanowires, in the regions of (a) Mo 3d, S 2s and (b) S 2p.


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Figure 4. The current-potential curves (black solid line) and photocathode conversion efficiencies (blue squares) of planar InP without (a) and with (b) MoS3, substrate-insulated InP nanowire arrays without (c) and with (d) MoS3, as-grown InP nanowire arrays without (e) and with (f) MoS3 in 1M HClO4 under chopped AM1.5G illumination.

Figure 5. Current stability measurements of as-grown p-type InP nanowire arrays without (red line) and with (black line) MoS3 at 0 V (vs. RHE) under AM1.5G illumination (100 mWcm-2). 13 ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information. Detailed procedures for growth, processing and testing of InP nanowire electrodes; material characterizations by SEM, TEM, PL; reflectance plots; MottSchottky Plots, PL and life time plots; The I-V curve of Pt decorated as-grown InP nanowires. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (L.G.), *E-mail: [email protected] (E.P.A.M.B.), *E-mail: [email protected] (P.H.L.N.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported in part by the Foundation for Fundamental Research on Matter (FOM), which is part of the Netherlands Organization for Scientific Research (NWO), and by Biosolar Cells program (FOM 18), which is co-financed by Dutch Ministry of Economic Affairs. Solliance, a solar energy R&D initiative of ECN, TNO, Holst, TU/e, imec and Forschungszentrum Jülich, and the Dutch province of Noord-Brabant are acknowledged for funding the TEM facility. ABBREVIATIONS


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Vfb, flatband potential; reversible hydrogen electrode; Voc, open-circuit voltage; Isc, short-circuit current; ff, fill factor; Iph, saturated photocurrent density. REFERENCES (1)

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