Efficient Unassisted Overall Photocatalytic Seawater Splitting on GaN

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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Efficient Unassisted Overall Photocatalytic Seawater Splitting on GaN-Based Nanowire Arrays Xiangjiu Guan,†,‡ Faqrul Alam Chowdhury,‡ Nick Pant,‡ Liejin Guo,† Lionel Vayssieres,*,† and Zetian Mi*,‡,§ †

International Research Center for Renewable Energy, Xi’an Jiaotong University, Xi’an 710049, China Department of Electrical and Computer Engineering, McGill University, Montreal, QC H3A 0E9, Canada § Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, United States ‡

ABSTRACT: Large-scale, clean, efficient, and sustainable hydrogen production from water is one of the major goals in solar-to-fuel conversion as the sun and water represent the two most abundant and geographically balanced free resources available on earth. Considering that most of the liquid water available on the earth’s surface is present in the form of seawater, H2 generation from seawater splitting is highly desirable for large-scale practical and economical application. Herein, we report on the first demonstration of direct efficient overall solar-driven seawater splitting on p-GaN-based nanowire arrays without any external bias or sacrificial agents from various types of simulated seawater solutions. A stable solar-to-hydrogen (STH) conversion efficiency of 1.9% was obtained under concentrated irradiation, demonstrating its possible utilization for the large-scale, environmentally friendly, sustainable generation of clean solar fuel.

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INTRODUCTION The photocatalytic splitting of water is being thoroughly investigated worldwide as a potential solution to the current energy and environmental crisis (e.g., critical level of air pollution) by converting free and abundant solar energy to clean chemical energy in the form of hydrogen.1,2 During the past few decades, tremendous effort has been dedicated to the development of various new materials for pure water photocatalytic splitting, and some efficient systems have indeed been reported.3−6 However, taking into account that 93% of liquid water on the earth’s surface is contained in oceans and seas,7 hydrogen production from seawater splitting is much more desirable for practical applications to conserve very precious fresh water for agriculture, industry, and people’s need. Nevertheless, hardly any studies have been carried out on photo(electro)chemical seawater splitting,8−12 and none, so far, have been carried out on industry-friendly GaN-based materials. On average, natural seawater has a salinity of 3.1−3.8% and a pH of 7.5−8.4. It contains various ions such as Na+, K+, Mg2+, Ca2+, Cl−, Br−, SO42−, and CO32−, with NaCl being the predominant component.13 Although the oxidation of Cl− (Cl2/Cl−, 1.36 V vs NHE at pH 0) is thermodynamically more difficult than that of H2O (O2/H2O, 1.23 V vs NHE at pH 0),14 the photo-oxidation of Cl− to Cl2 involves a relatively simple two-electron redox process and is thus likely to proceed more effectively than the more complex four-electron redox process involved in the photo-oxidation of H2O to O2.15 That is, if the kinetically favorable photo-oxidation process of Cl− to © XXXX American Chemical Society

Cl2 happens in photochemical (or photoelectrochemical) seawater splitting, then the overall reaction will be improved. Therefore, it is expected that by using seawater rather than pure water, enhanced hydrogen production will be achieved and byproducts of Cl2 (chlorine) or HClO (hypochloric acid), which are commonly used disinfectants, will be obtained concomitantly. GaN is one representative of the group III nitrides that has been considered to be a viable material for overall solar water splitting.16 With its band gap straddling the redox potential of water, GaN-based materials have emerged as promising candidates for overall water splitting without external bias (unassisted). When indium (In) content is introduced, the light absorption of InGaN compounds could encompass nearly the entire solar spectrum.17 Indeed, p-type (In)GaN nanowire arrays have been reported to perform as efficient photocatalysts for the overall splitting of pure water,3,18 and the origin of their long-term performance and stability have been investigated and understood on the atomic scale.19 In this article, unassisted photocatalytic overall seawater splitting is being investigated on p-type (In)GaN nanowire arrays rather than pure water splitting. Better performance is revealed in seawater than in pure water, and the different Special Issue: Prashant V. Kamat Festschrift Received: January 25, 2018 Revised: February 14, 2018 Published: February 15, 2018 A

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Figure 1. SEM (a), TEM (b), and HRTEM images (c) and Raman spectrum (d) of an as-grown p-GaN nanowire and nanowire array on a Si wafer substrate.

CO2) was put into the chamber, and the chamber was then evacuated. The evolved gases (H2 and O2) were sampled (with an experimental accuracy of ∼10%) using a vacuum-tight syringe and analyzed by gas chromatography (Shimadzu GC8A) equipped with a molecular sieve 5A column, a thermal conductivity detector, and high-purity Ar carrier gas. Oxidation products of chlorine in the residual solution after seawater splitting were evaluated by Hach chlorine test kits (cat. no. 223101) based on standard titration methods. For seawater splitting, an aqueous solution prepared by dissolving 27.21 g of NaCl, 3.81 g of MgCl2, 1.66 g of MgSO4, 1.404 g of CaSO4, 0.577 g of K2SO4, 0.2124 g of K2CO3, and 0.08 g of MgBr2 in 1 L of distilled water was used as artificial seawater as reported in the literature as one type of natural seawater.13 For some experiments, a pure NaCl aqueous solution was used as “simulated” seawater in comparison to pure water. To set different solution pH values, 0.1 M HCl or 0.1 M NaOH was added accordingly. Rh/Cr2O3 were photodeposited on the surface of the nanowires before water splitting as a cocatalyst for unassisted overall water splitting. To further enhance the photoactivity, cobalt oxide was also deposited as a well-known oxidative cocatalyst in some of the experiments. The amount of GaN and GaN/InGaN nanowires used in water splitting experiments on Si wafer substrates with a surface area of ∼3 cm2 was estimated to be in the range of 0.38−0.48 mg. In the subsequent discussions, the activity was normalized to 1 g of photocatalyst.

reaction processes involved in the two different photocatalytic systems are being discussed in detail.

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EXPERIMENTAL METHODS p-GaN and p-GaN/InGaN nanowire arrays were grown by molecular beam epitaxy (MBE). Specific details about the growth process can be found in our previous reports.3,18 Briefly, the growth conditions involved a nitrogen (N) flow rate of ∼1.0 sccm, a forward plasma power of ∼350 W, a gallium (Ga) beam equivalent pressure (BEP) of ∼6 × 10−8 Torr, and an indium (In) BEP of ∼7 × 10−8 Torr. The growth temperature for GaN is ∼790 °C for 4 h, and the magnesium (Mg) effusion cell temperature is 265 °C, which corresponds to a Mg BEP of ∼3.1 × 10−9 Torr. The growth temperature for InGaN is in the range of 650−710 °C. For the growth of multistacked broadband GaN/InGaN nanowire arrays, the five InGaN segments and top GaN segment were also doped p-type by Mg incorporation. Scanning electron microscopy (SEM) images were recorded by an FEI Inspect F-50 field-emitting SEM. Transmission electron microscopy (TEM) images were obtained from an FEI Tecnai G2 F30 S-Twin TEM operated at an accelerating voltage of 300 kV. Raman spectra were collected at room temperature from a Horiba LabRam HR confocal Raman spectrometer equipped with a 488 nm argon ion laser. Photocatalytic overall water/seawater splitting reactions were investigated by using a 300 W xenon lamp as an outer irradiation source. The as-prepared nanowire arrays on silicon (Si) wafer substrates (surface area of ∼3 cm2) were placed on a polytetrafluoroethylene (PTFE) holder in a Pyrex chamber which was covered by a quartz lid. Distilled water/seawater purged with Ar (to remove dissolved gases such as O2 and

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RESULTS AND DISCUSSION A 45°-tilted SEM image of a typical as-grown p-GaN nanowire array on a Si wafer substrate is presented in Figure 1a, showing vertically aligned nanowires. The nanowires have a typical B

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Figure 2. (a) H2 evolution rate on as-grown p-GaN nanowire arrays in an aqueous solution of different NaCl concentrations (percentages shown refer to the enhancement of activity in NaCl solutions compared to that in pure water, i.e., 0 M NaCl). (b) Schematic illustration of the overall unassisted seawater splitting reaction on a p-GaN nanowire. (c) H2 evolution rate from aqueous NaCl solutions of different pH. (d) H2 evolution rate from pure water of different pH. The evolution rate’s experimental accuracy is ∼10%.

length of ∼600 nm and diameters of ∼50−100 nm. Figure 1b shows a TEM image of a typical single p-GaN nanowire. All nanowires are grown along the ⟨0001̅⟩ direction. The HRTEM image shown in Figure 1c demonstrates the single-crystal nature of the nanowire, with a 0.518 nm distance between adjacent lattice fringes. The nanowire consists of a c-plane (polar) top surface and m-plane (nonpolar) side walls20 with all surfaces (polar and nonpolar) being N-terminated19 for performance and stability. A typical Raman spectrum of the p-GaN nanowire is shown in Figure 1d. The local vibrational mode (LVM) at ∼655 cm−1 corresponds to the Mg−N bonds, and the appearance of the A1 longitudinal optical (LO) mode at ∼733 cm−1 indicates a weakly p-type surface due to Mg incorporation.21 We have previously demonstrated that the efficiency of the photocatalytic overall water splitting reactions could be significantly enhanced on InGaN nanowire arrays by optimizing the surface band bending by controlled Mg doping.18 Since the dominant component of natural seawater is sodium chloride (NaCl), simulated seawater splitting was initially carried out in NaCl aqueous solutions for a better analysis and understanding of the reactions involved. First, NaCl solutions with different concentrations (up to 0.5 M) were used for the seawater splitting reaction, and the results are shown in Figure 2a. Hydrogen production was significantly enhanced when NaCl solutions were used instead of pure water. Free chlorine, which might exist in the form of dissolved Cl2, HClO, and/or ClO−, was tested after the reaction and was ∼0.1 mg/L, which was quite low compared to the evolved H2/O2 gases. Thus, it was demonstrated that the Cl− oxidation process did occur during the seawater splitting reaction but only a small quantity of Cl− oxidation products remained in solution. It is worth noticing that the ratio of evolved H2 and O2 was the ideal ∼2:1 ratio and the pH of the solutions remained almost unchanged before and after the reaction. As expected, these results implied

that the chlorine oxidation reaction might actually serve as an intermediate reaction in the seawater splitting process. As depicted in Figure 2b, H2 was produced from the proton reduction by the photogenerated electrons in the conduction band of the p-GaN nanowire array; meanwhile, both H2O and Cl− could be oxidized by the photogenerated holes in the valence band. Indeed, dissolved Cl2 could react with water via the following reaction Cl 2 + H 2O → H+ + Cl− + HClO

(1)

and HClO could easily be decomposed under irradiation11 according to the following reaction: 2HClO → 2H+ + 2Cl− + O2

(2)

Seawater splitting on p-GaN nanowire arrays at different pH values was further investigated, and the results are shown in Figure 2c, during which a 0.5 M NaCl solution was used as simulated seawater whose pH was adjusted by adding HCl or NaOH. Better performance was observed at neutral pH in NaCl solution compared to either acidic or basic solutions, which was in accordance with the fact that oxygen evolution from the decomposition of ClO− or HClO is indeed favored at pH ∼6.5 according to NaCl industrial electrolysis.22 For comparison, pure water splitting at different pH values is also shown in Figure 2d. Better performance for pure water splitting was obtained at pH ∼5, which was similar to the optimized condition for GaN-ZnO photocatalysts reported by Domen’s group.23 The reduced photoactivity observed in basic solutions is mostly due to the N3− oxidation on the surface of the photocatalyst,24 which has been widely reported on nitride materials. In order to utilize sunlight energy more efficiently, p-InGaN segments were inserted into GaN nanowires, and a resulting dual-band structure was grown as an efficient visible-light-active C

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Figure 3. Scheme (a) and SEM image (b) of dual-band p-GaN/InGaN nanowire arrays grown on a Si wafer substrate. (c) H2 evolution rate (∼10% accuracy) in pure water and in 0.5 M NaCl aqueous solution under illumination equipped with a AM1.5G filter. (d) Time course of H2/O2 evolution of p-GaN/InGaN nanowire arrays from a 0.5 M NaCl solution under illumination equipped with an AM1.5G filter with (w.) and without (w.o.) the water oxidation cobalt oxide cocatalyst.

Figure 4. Comparison of H2 evolution rates (∼10% accuracy) on p-GaN/InGaN nanowire arrays in pure water, 0.5 M NaCl solution, and natural seawater under illumination equipped with (a) an AM1.5G filter and (b) a 400 nm long-pass (LP) filter. The seawater used here was the artificial seawater solution prepared according to the natural seawater composition according to ref 13 as stated in the Experimental Methods.

rate of 6.15(4) mol·g−1·h−1 was obtained, which was ∼27% higher than that of pure water splitting. Cyclic stability tests in 0.5 M NaCl solution were carried out and reported in Figure 3d, demonstrating that the p-GaN/ InGaN dual-band nanowire arrays performed steadily in simulated seawater splitting, which is in good agreement with our previous report in pure water.3 On the basis of the above-mentioned results, GaN-based materials efficiently catalyze overall seawater splitting under sunlight irradiation without any external bias (i.e., unassisted) or sacrificial agents. Direct seawater splitting was conducted by using artificial seawater in which the concentration of Cl− was ∼0.5 M. The comparison of H2 production from pure water, simulated seawater (0.5 M NaCl), and artificial natural seawater splitting on p-GaN/InGaN dual-band nanowire arrays is shown in Figure 4. The activity in artificial natural seawater was slightly reduced compared to that of simulated seawater (0.5 M NaCl aqueous solution), which is most likely due to the excess mineral composition and the slightly more basic condition (pH

photocatalyst. The nanowire-array structure is depicted in Figure 3a, in which the InGaN segments with a band gap of ∼2.46 eV correspond to an In content of ∼25%,17 and sunlight of up to 505 nm could be absorbed for a more efficient photocatalytic water splitting reaction. As shown in the SEM image in Figure 3b, vertically aligned p-GaN/InGaN nanowires could be observed, with lengths of ∼800 nm. Unassisted overall water splitting was conducted on these pGaN/InGaN nanowire arrays, and their performance in NaCl solutions as well as pure water for comparison is shown in Figure 3c. Herein, cobalt oxide was also loaded as a water oxidation cocatalyst in order to further improve the overall water splitting efficiency. Indeed, significantly enhanced unassisted overall pure water splitting was obtained. The actual mechanism of coloading and the precise nature, composition, and symmetry of the cocatalyst are currently under investigation and will be reported elsewhere. Similar to pGaN nanowire arrays, when p-GaN/InGaN dual-band nanowire arrays were used in simulated seawater, a H2 production D

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∼8).15 However, the efficiency of direct artificial natural seawater splitting (H2 ≈ 5.09(6) mol·g−1·h−1) was slightly higher than that of pure water (H2 ≈ 4.84(6) mol·g−1·h−1). The solar-to-hydrogen (STH) conversion efficiency was calculated to be ∼1.9% with an irradiation intensity of ∼27 suns. When a 400 nm long-pass (LP) filter was adopted, the H2 evolution rate was measured to be 1.77 mol·g−1·h−1, corresponding to an apparent quantum efficiency (AQE) of ∼12.2%. Both of the AQE and STH are among the highest reported for overall unassisted photocatalytic water splitting.

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CONCLUSIONS Industry-friendly GaN-based materials consisting of p-type GaN and p-GaN/InGaN nanowire arrays grown on commercial Si wafers were thoroughly investigated, for the first time, for the unassisted overall photocatalytic splitting of various types of simulated seawater (i.e., NaCl aqueous solutions of various concentrations and pH as well as artificial natural seawater containing various concentrations of different ions). Significantly enhanced H2 generation performance compared to pure water splitting was revealed and tentatively attributed to the chlorine oxidation reaction serving as a beneficial intermediate process in addition to the increase in water conductivity. While the highest amount of H2 produced was observed in neutral NaCl solutions, the much more complex natural seawater composition still performed better than pure water, revealing that the excess minerals do not interfere significantly with (photo)catalytically active sites on the surface of the nanowires. Finally, such straightforward results of efficient and stable unassisted overall photocatalytic seawater splitting on nitride materials25 do bring about the dream of generating virtually unlimited clean and sustainable energy from the most abundant, free, and geographically balanced natural resources26 available on our planet, that is, the sun and seawater, while being respectful and caring for the environment and people’s health, which is one step closer to reality2 as both GaN and Si materials are well-known processed materials and devices in industry.

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Liejin Guo: 0000-0002-3671-5628 Lionel Vayssieres: 0000-0001-5085-5806 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Emissions Reduction Alberta and by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under award no. EE0008086. L.V. and L.G. are grateful for financial support from the National Natural Science Foundation of China (grant no. 51236007). X.G. is grateful for the grant awarded by the China Scholarship Council (CSC) for his 1year visiting scholar position at McGill University, Canada under the supervision of Z.M. E

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The Journal of Physical Chemistry C Nanowires revealed by Micro-Raman Scattering and X-Ray Photoelectron Spectroscopy. Nanoscale 2014, 6, 9970−9976. (22) Karlsson, R. K. B.; Cornell, A. Selectivity between Oxygen and Chlorine Evolution in the Chlor-Alkali and Chlorate Processes. Chem. Rev. 2016, 116, 2982−3028. (23) Maeda, K.; Teramura, K.; Domen, K. Effect of Post-Calcination on Photocatalytic Activity of (Ga1‑xZnx)(N1‑xOx) Solid Solution for Overall Water Splitting under Visible Light. J. Catal. 2008, 254, 198− 204. (24) Maeda, K.; Teramura, K.; Masuda, H.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Efficient Overall Water Splitting under VisibleLight Irradiation on (Ga1‑xZnx)(N1‑xOx) Dispersed with Rh-Cr MixedOxide Nanoparticles: Effect of Reaction Conditions on Photocatalytic Activity. J. Phys. Chem. B 2006, 110, 13107−13112. (25) Su, J. Z.; Wei, Y. K.; Vayssieres, L. Stability and Performance of Sulfide-, Nitride-, and Phosphide-based Electrodes for Photocatalytic Solar Water Splitting. J. Phys. Chem. Lett. 2017, 8, 5228−5238. (26) Tachibana, Y.; Vayssieres, L.; Durrant, J. R. Artificial Photosynthesis for Solar Water-Splitting. Nat. Photonics 2012, 6, 511−518.

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