Enhancement of Charge Separation and Hydrogen Evolution on

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Enhancement of Charge Separation and Hydrogen Evolution on Particulate LaTiCuSO Photocathodes by Surface Modification 5

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Jingyuan Liu, Takashi Hisatomi, Dharmapura H. K. Murthy, Miao Zhong, Mamiko Nakabayashi, Tomohiro Higashi, Yohichi Suzuki, Hiroyuki Matsuzaki, Kazuhiko Seki, Akihiro Furube, Naoya Shibata, Masao Katayama, Tsutomu Minegishi, and Kazunari Domen J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02735 • Publication Date (Web): 29 Dec 2016 Downloaded from http://pubs.acs.org on December 29, 2016

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The Journal of Physical Chemistry Letters

Enhancement of Charge Separation and Hydrogen Evolution on Particulate La5Ti2CuS5O7 Photocathodes by Surface Modification Jingyuan Liu†, Takashi Hisatomi†‡, Dharmapura H. K. Murthy§, Miao Zhong†‡, Mamiko Nakabayashi∥, Tomohiro Higashi†‡, Yohichi Suzuki§, Hiroyuki Matsuzaki§, Kazuhiko Seki§, Akihiro Furube§⊥, Naoya Shibata∥, Masao Katayama†‡, Tsutomu Minegishi†‡#, Kazunari Domen†‡* †

Department of Chemical System Engineering, School of Engineering, The University of Tokyo,

7-3-1 Hongo, Bunkyo-ku, 113-8656 Tokyo, Japan ‡

Japan Technological Research Association of Artificial Photosynthetic Chemical Process

(ARPChem), 2-11-9 Iwamotocho, Chiyoda-ku, 101-0032 Tokyo, Japan §National Institute of Advanced Industrial Science and Technology, Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, 305-8568 Ibaraki, Japan ∥

Institute of Engineering Innovation, School of Engineering, the University of Tokyo, 2-11-16

Yayoi, Bunkyo-ku, 113-8656 Tokyo, Japan ⊥

Department of Optical Science, Tokushima University, 2-1 Minamijosanjima-cho, 770-8506

Tokushima, Japan

ACS Paragon Plus Environment

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Japan Science and Technology Agency / Precursory Research for Embryonic Science and

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Technology (JST/PRESTO), Kawaguchi Center Building, 4-1-8 Honcho, Kawaguchi-shi, 3320012 Saitama, Japan Corresponding Author *Professor Kazunari Domen, E-mail: [email protected]

ABSTRACT

Particulate La5Ti2CuS5O7 (LTC) photocathodes prepared by particle transfer show a positive onset potential of 0.9 V vs. RHE for the photocathodic current in photoelectrochemical (PEC) H2 evolution. However, the low photocathodic current imposes a ceiling on the solar-tohydrogen energy conversion efficiency of PEC cells based on LTC photocathodes. To improve the photocurrent, in this work, the surface of Mg-doped LTC photocathodes was modified with TiO2, Nb2O5, and Ta2O5 by radio-frequency reactive magnetron sputtering. The photocurrent of the modified Mg-doped LTC photocathodes was doubled because these oxides formed type-II heterojunctions and extended the lifetimes of photogenerated charge carriers. The enhanced photocathodic current was attributed to hydrogen evolution at a positive potential of +0.7 V vs. RHE. This work opens up possibilities for improving PEC hydrogen evolution on particulate photocathodes based on surface oxide modifications and also highlights the importance of the band gap alignment.

TOC GRAPHICS


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KEYWORDS Water splitting, Solar hydrogen production, Oxysulfide, Oxide

Photoelectrochemical (PEC) water splitting is a promising means of generating hydrogen as a renewable energy source on a large scale, based on the use of solar radiation.1,2 Many semiconductor photoelectrodes are capable of generating high photocurrents during PEC hydrogen or oxygen evolution from water under simulated sunlight but require a sufficiently high external voltage to drive this uphill reaction.3,4 To solve this problem, p/n-PEC cells composed of a series-connected photocathode and photoanode have been designed. In this twostep excitation system, unassisted PEC water splitting can be achieved by matching the photocurrents of the photocathode and the photoanode at the same electrode potential.5-7 La5Ti2CuS5O7 (LTC)is an oxysulfide semiconductor with a band gap energy of 1.84 eV, first reported by Meignen et al. in 20048 and subsequently found to have a conduction band edge suitable for hydrogen evolution from water.9 LTC powder can be processed into photocathodes using the particle transfer method.10 In addition, p-type doping of the Ti sites in this material and


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formation of a La5Ti2Cu0.9Ag0.1S5O7solid solution (LTC0.9A0.1) had increased the photocathodic current to 0.8 mA cm-2 at 0 V vs. RHE, by approximately 20 times, since 2014 while maintaining the characteristic positive onset potential (0.9 V vs. RHE) of the photocurrent.11-14 The positive operable potential of LTC-based photocathodes during PEC hydrogen evolution is well-suited to applications in p/n-PEC cells for unassisted water splitting in conjunction with various types of photoanodes.15 However, the observed photocurrent was still far lower than the calculated maximum photocurrent of 16.8 mA cm-2 based on the band gap energy and imposed a ceiling on the solar-to-hydrogen energy conversion efficiency (STH) of the p/n PEC cells. The surface modification of compact thin film photocathodes composed of materials such as silicon,16 silicon carbide,17 metal oxides,18,19 pnictogenides20,21 and chalcogenides22 with n-type semiconducting oxide layers has been widely researched with the aim of enhancing the charge separation and improving both the photocurrent and the stability of these devices. TiO2 is one of the most popular anti-corrosive, passivation and/or charge separation layers, owing to its high chemical stability and the suitable position of its band gap. Enhancement of charge separation by heterojunctions have also been reported for photoanode systems.23-26 Despite this, the effects of surface modifications using TiO2 and similar compounds on the PEC properties of particulate photocathodes with a high degree of inhomogeneity have rarely been studied. In the present work, TiO2, Nb2O5, Ta2O5 and ZrO2were deposited on particulate Mg-doped LTC (Mg-LTC) electrodes by radio-frequency (RF) reactive magnetron sputtering. Mg-LTC electrodes modified with Nb2O5, Ta2O5 and TiO2exhibited cathodic photocurrents that approximately doubled over the potential range of 0 to 0.9 V vs. RHE compared to the value obtained from an unmodified electrode. In addition, at 0.7 V vs. RHE, Mg-LTC photocathodes modified with these oxides maintained a photocurrent attributed to PEC hydrogen evolution. Increases in the population of


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long-lived photoexcited carriers existing on the microsecond timescale in these materials demonstrated the vital role of the oxide surface modification in obtaining effective charge separation. Mg-LTC powders and photocathodes were prepared by a solid state reaction process and the particle transfer method using Au as a contact layer, respectively, as in our previous studies.12,13 Here, Mg2+ was doped into the Ti4+ sites of LTC to enhance the photocathodic response.13 Successful production of Mg-LTC was confirmed by X-ray diffraction, diffuse-reflectance spectroscopy, and scanning electron microscopy as shown in Figure S1–S3 in the Supporting Information. TiO2, Nb2O5, Ta2O5 and ZrO2 were deposited by RF reactive magnetron sputtering conducted at a constant target RF power of 70 W in a mixed Ar/O2 gas atmosphere. By controlling the sputtering time, the nominal thickness of the oxide was adjusted to 2 nm unless otherwise noted. After the deposition of the oxide, each sample was annealed in air at 473 K for one hour and subsequently modified by the photodeposition of nanoparticulate Pt. The details of the experimental procedures are provided in the Supporting Information. The presence of the oxides deposited on the Mg-LTC particles was confirmed by X-ray photoelectron spectroscopy (Figure S4 in the Supporting Information). The locations of the oxides were studied using scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS) mapping. Figure S5 in the Supporting Information shows STEM-EDS mapping images of a TiO2-deposited Mg-LTC particle as a representative case. Here, a thicker TiO2 layer of approximately 5 nm was deposited to allow for clear STEM-EDS mapping. A Ti-rich surface is evident on only one side of the rod-shaped Mg-LTC particle because the other side is shaded by the particle itself. These images demonstrate that the solid/solid junction between the Mg-LTC and the back contact metal (Au) was unaffected when


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the samples were processed into photoelectrodes. A high-resolution transmission electron microscope image of a thick-TiO2/Mg-LTC electrode after photodeposition of Pt revealed that nanoparticulate Pt was present on an almost amorphous TiO2 layer (Figure S6 in the Supporting Information), suggesting that photoexcited electrons migrated across the TiO2 layer and reduced the platinum ions. The current-potential plots obtained from Mg-LTC photocathodes modified with the various oxides during PEC hydrogen evolution under simulated sunlight irradiation (AM 1.5G)are presented in Figure 1. The photocurrents of the photocathodes modified with TiO2, Ta2O5 and Nb2O5 approximately doubled over the potential range of 0 to 0.9 V vs. RHE. Specifically, the Nb2O5-modified Mg-LTC electrode showed photocurrents of 1.4 mA cm-2 at 0 V vs. RHE and 0.4 mA cm-2 at 0.7 V vs. RHE. The enhancement of the PEC properties was not due to a change in adsorption properties of protons or hydroxyl ions because the band gap potential of LTC would follow the Nernst’s equation. The ZrO2-modified LTC electrode also generated an enhanced photocurrent from 0–0.6 V vs. RHE, although the improvement in the photocurrent was less in the case of this sample, and the photocurrent was slightly decreased within the positive potential region close to the onset of the photocathodic current. In the case of each of the oxide-modified samples, the positive onset potential of the photocathodic current was maintained. The half-cell STH for the Nb2O5-modified Mg-LTC photocathode was 0.35% at 0.5 V vs. RHE (Figure S7 in the Supporting Information), a value that is 2.3 times that obtained from the unmodified Mg-LTC electrode. Notably, the annealing of the Mg-LTC electrodes after deposition of the oxides was essential to increasing the photocurrent, suggesting the importance of the interface with the Mg-LTC and/or the crystallinity of the oxide.


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Figure 1. Current-potential plots obtained from (a) a bare Mg-LTC photocathode and photocathodes surface-modified with (b) ZrO2, (c) TiO2, (d) Ta2O5 and (e) Nb2O5. The measurements were carried out in a 0.1 M aqueous Na2SO4 solution (pH 10) under chopped simulated sunlight irradiation. The photocathodes had been modified by the photodeposition of Pt.

Typical current-time profiles of the Mg-LTC electrodes measured at 0.7 V vs. RHE are presented in Figure S8 in the Supporting Information. The initial photocurrents of the Mg-LTC photocathodes modified with Nb2O5, Ta2O5 and TiO2 were higher than that observed for the bare Mg-LTC electrode. In addition, after 30 min of measurement time, the Mg-LTC photocathodes modified with these oxides still maintained higher photocurrents than the bare photocathode. The faradaic efficiency during PEC hydrogen evolution using a Nb2O5-modified Mg-LTC photocathode was estimated to be virtually unity at 0.7 V vs. RHE (Figure 2), demonstrating that the enhanced photocurrent can be attributed to the water reduction reaction. These results show that surface modification with these oxides clearly improved the PEC hydrogen evolution on the Mg-LTC electrodes. Conversely, the ZrO2-modified Mg-LTC photocathode exhibited a lower


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photocurrent than the pristine Mg-LTC photocathode at 0.7 V vs. RHE, as expected from Figure 1.

Figure 2. Time courses of hydrogen and oxygen evolution using a Pt/Nb2O5/Mg-LTC photocathode (irradiated area: 1.26 cm2) at 0.7 V vs. RHE in a three-electrode configuration under visible light irradiation (λ > 420 nm) from a 300 W Xe lamp. The solid curves labelled with e-/2 and e-/4 show the quantities of hydrogen and oxygen expected to be generated at unity faradaic efficiency, respectively. The measurement was carried out in a 0.1 M aqueous Na2SO4 solution adjusted to pH 10. The delay for the gas detection was due to a time required for homogenization of the gaseous component in the reactor.

In previous research concerning the surface modification of thin film photocathodes with oxide overlayers, the formation of a type-II heterojunction (i.e., a heterojunction in a semiconducting material in which both the conduction band minimum and the valence band maximum are more positive than those of the photocathode material) has been known to enhance the charge separation at the interface and extend the lifetimes of photoexcited carriers. Based on


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the band gap potential of Mg-LTC (as estimated from photoelectron spectroscopy in air27) and of Nb2O5 as reported in the literature,28 it appears that Mg-LTC should be able to form a type-II heterojunction with Nb2O5, as depicted in Figure S9 in the Supporting Information. Band diagrams for the Nb2O5/Mg-LTC/Au electrode in contact with an electrolyte solution were calculated using a semiconductor device simulator (AFORS-HET) and are provided in Figure S10. Here the Mg-LTC was assumed to be an intrinsic semiconductor based on the experimentally determined Fermi level of Ga-doped LTC.29 In fact, equivalent results are obtained even when Mg-LTC is regarded as a weak p-type or n-type semiconductor, because the LTC electrode functions as a photocathode regardless of the species of the majority carrier when Au, which has a large work function, is used as the back contact material, owing to the longrange carrier transport properties of LTC.28 Additionally, the carrier concentration of Nb2O5 was estimated from Mott-Schottky plots to be 5 × 1019 cm-3 (Figure S11 in the Supporting Information) although it would be overestimated because the roughness of the oxide layer was ignored. It was found that modification with Nb2O5 did not appreciably affect the band-bending. However, because of the band offset at the interface with Mg-LTC, electron transfer from the Mg-LTC to the Nb2O5 proceeded without a potential barrier, while the migration of holes toward the surface was blocked. Ta2O530 and TiO231 have similar band positions and can form type-II heterojunctions with Mg-LTC (Figure S9 in the Supporting Information), so they are considered to act as electron-conducting, hole-blocking layers and therefore to effectively facilitate charge separation particularly near the photocurrent onset potential where charge separation based on the band-bending is hardly expected.


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Figure 3. Normalized transient decays of photogenerated charge carriers in a bare Mg-LTC/Au photocathode and photocathodes surface-modified with ZrO2, TiO2, Ta2O5 and Nb2O5, as measured under ambient conditions in air using a 532 nm pump wavelength and probing in the NIR (850–1800 nm) region.

To confirm the function of the supposed heterojunction, the lifetimes of photogenerated charge carriers were determined by microsecond transient absorption spectroscopy (TAS). As shown in Figure 3,at a 20 s delay time, the TA signal for the bare Mg-LTC photocathode (without an oxide coating) decayed to one third of the initial value, while in the case of the oxide modified Mg-LTC photocathodes the decay was only by one half. This clear enhancement in the lifetimes, and thus higher survival rates of the photogenerated carriers, indicates that the surface modification suppressed the recombination of charge carriers. As discussed previously, the formation of a type-II heterojunction favors the transfer of electrons from the Mg-LTC to the oxide and was likely responsible for the higher photocurrents that were observed. Interestingly, the lifetimes of photogenerated carriers were also extended in the ZrO2-modified Mg-LTC


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photocathode, although this effect was not anticipated based on the energetics at the interface (Figure S9 in the Supporting Information);31 the band gap of ZrO2 straddles that of Mg-LTC, such that a type-I heterojunction is formed. The photocurrent of the ZrO2-modified LTC photocathode actually decreased over the potential range of 0.6–0.9 V vs. RHE, while improving in a manner similar to that of the other oxide modified electrodes over the range of 0–0.6 V vs. RHE (Figure 1). Presumably, the ZrO2 worked to passivate surface defects on the Mg-LTC that otherwise functioned as recombination centers, and so extended the average carrier lifetime. Nevertheless, it would still be necessary to transport photoexcited electrons to the portion of the Mg-LTC surface not completely blocked by the ZrO2 and/or to accumulate a large number of electrons on the Mg-LTC surface at the heterojunction, so as to overcome the potential barrier at the conduction band edge (Figure S12 in the Supporting Information). Considering the thinness of ZrO2, it may also be proposed that electrons were transferred to defect states below the conduction band and hence a longer carrier lifetime was observed. Further investigation is therefore required to understand the complicated effect of the ZrO2 modification. In summary, Nb2O5, Ta2O5 and TiO2deposited by RF reactive magnetron sputtering were found to facilitate charge separation and thus to enhance the photocurrent of particulate Mg-LTC photocathodes.Nb2O5-modified Mg-LTC electrodes generated photocurrents of 1.4and 0.4mA cm-2 at 0 and 0.7 V vs. RHE, respectively, values that are twice those generated by the unmodified electrode. A steady photocurrent and a faradaic efficiency of unity during PEC hydrogen evolution were also confirmed at 0.7 V vs. RHE. This work suggests opportunities for improving PEC hydrogen evolution on particulate photocathodes based on surface oxide modifications and also highlights the importance of the band gap alignment. ASSOCIATED CONTENT


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Supporting Information available: Experimental details, X-ray diffraction patterns, a diffusereflectance spectrum, a scanning electron microscope image, X-ray photoelectron spectra, STEM-EDS mapping images, a high-resolution transmission electron microscope image, HCSTH curves, current-time curves, band diagrams, and Mott-Schottky plots.

ACKNOWLEDGMENT This work was financially supported by Grants-in-Aids for Scientific Research (A) (No. 16H02417), for Young Scientists (A)(No.15H05494), and for Young Scientists (B) (No. 15K17895) from the Japan Society for the Promotion of Science (JSPS), by the Artificial Photosynthesis Project of the New Energy and Industrial Technology Development Organization (NEDO),and by the CompanhiaBrasileira de Metalurgia e Mineração (CBMM). Part of this work was conducted at the Advanced Characterization Nanotechnology Platform at the University of Tokyo, supported by the Nanotechnology Platform of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors also wish to thank Mr. Atsushi Furuki of the University of Tokyo for his assistance in preparing the Mg-LTC powder. One of the authors (J. L.) gratefully acknowledges the support of a Japan Chemical Industry Association (JCIA) Fellowship. REFERENCES (1)

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TOC graphics 185x185mm (150 x 150 DPI)



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Figure 1. Current-potential plots obtained from (a) a bare Mg-LTC photocathode and photocathodes surfacemodified with (b) ZrO2, (c) TiO2, (d) Ta2O5 and (e) Nb2O5. The measurements were carried out in a 0.1 M aqueous Na2SO4 solution (pH 10) under chopped simulated sunlight irradiation. The photocathodes had been modified by the photodeposition of Pt. Figure 1 289x257mm (300 x 300 DPI)


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Figure 2. Time courses of hydrogen and oxygen evolution using a Pt/Nb2O5/Mg-LTC photocathode (irradiated area: 1.26 cm2) at 0.7 V vs. RHE in a three-electrode configuration under visible light irradiation (λ > 420 nm) from a 300 W Xe lamp. The solid curves labelled with e-/2 and e-/4 show the quantities of hydrogen and oxygen expected to be generated at unity faradaic efficiency, respectively. The measurements was carried out in a 0.1 M aqueous Na2SO4 solution adjusted to pH 10. The delay for the gas detection was due to a time required for homogenization of the gaseous component in the reactor. Figure 2 289x255mm (300 x 300 DPI)



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Figure 3. Normalized transient decays of photogenerated charge carriers in a bare Mg-LTC/Au photocathode and photocathodes surface-modified with ZrO2, TiO2, Ta2O5 and Nb2O5, as measured under ambient conditions in air using a 532 nm pump wavelength and probing in the NIR (850–1800 nm) region. Figure 3 289x255mm (300 x 300 DPI)


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Enhancement of Charge Separation and Hydrogen Evolution on

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