Superior visible light hydrogen evolution of Janus bilayer junctions via atomic-level charge flow steering

Li, Jie; Zhan, Guangming; Yu, Ying; Zhang, Lizhi

doi:10.1038/ncomms11480

Superior visible light hydrogen evolution of Janus bilayer junctions via atomic-level charge flow steering

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Published: 09 May 2016

Volume 7, article number 11480, (2016)
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Superior visible light hydrogen evolution of Janus bilayer junctions via atomic-level charge flow steering

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Jie Li^1,2,
Guangming Zhan¹,
Ying Yu² &
…
Lizhi Zhang¹

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Abstract

Although photocatalytic hydrogen evolution (PHE) is ideal for solar-to-fuel conversion, it remains challenging to construct a highly efficient PHE system by steering the charge flow in a precise manner. Here we tackle this challenge by assembling 1T MoS₂ monolayers selectively and chemically onto (Bi₁₂O₁₇) end-faces of Bi₁₂O₁₇Cl₂ monolayers to craft two-dimensional (2D) Janus (Cl₂)-(Bi₁₂O₁₇)-(MoS₂) bilayer junctions, a new 2D motif different from van der Waals heterostructure. Electrons and holes from visible light-irradiated Bi₁₂O₁₇Cl₂ are directionally separated by the internal electric field to (Bi₁₂O₁₇) and (Cl₂) end-faces, respectively. The separated electrons can further migrate to MoS₂ via Bi–S bonds formed between (Bi₁₂O₁₇) and MoS₂ monolayers. This atomic-level directional charge separation endows the Janus bilayers with ultralong carrier lifetime of 3,446 ns and hence a superior visible-light PHE rate of 33 mmol h⁻¹ g⁻¹. Our delineated Janus bilayer junctions on the basis of the oriented assembly of monolayers presents a new design concept to effectively steer the charge flow for PHE.

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Introduction

Photocatalytic hydrogen evolution (PHE) from water via solar energy and semiconductor photocatalysts provides the prospect of replacing fossil fuels with carbon-free and sustainable hydrogen energy¹. Yet so far, its realization is still a mirage because PHE efficiency is thwarted to a large extent by the undesirable electron–hole recombination arisen from the random charge flow after the electron–hole separation^{Fig. 1b–e) coupling with X-ray photoelectron spectroscopy (XPS) (Fig. 1f) and X-ray diffraction pattern (Supplementary Fig. 5 and Supplementary Note 5) identified the small and large nanosheets as MoS₂ and Bi₁₂O₁₇Cl₂, respectively. S K-edge X-ray absorption near-edge structure spectra (XANES) (Fig. 1g) revealed that the MoS₂ monolayers in the bilayer showed a distorted 1T metallic phase¹⁵. 3D topographic atomic force microscopy (AFM) images (Fig. 1h,i) and their corresponding height profiles (Fig. 1j,k) demonstrated that the average thickness values of the small-sized and large-sized nanosheets were 0.686 and 0.717 nm, respectively, well-matching with the theoretical ones of the MoS₂ and Bi₁₂O₁₇Cl₂ monolayers (Fig. 1l, Supplementary Figs 2–4 and Supplementary Note 2–4), which further evidenced the assembly of MoS₂ on Bi₁₂O₁₇Cl₂. More interestingly, we found that all the MoS₂ sheets were anchored on the same surface in Bi₁₂O₁₇Cl₂, which was further evidenced by their side-view TEM image (Fig. 1m). These observations clearly demonstrate the occurrence of an oriented assembly. As 1L-BOC has an asymmetric structure consisting of (Cl₂) and oxygen-deficient (Bi₁₂O₁₇) end-faces and their assembly might be initiated by OVs of (Bi₁₂O₁₇) end-faces, we could thus deduce that 1L-MS were anchored selectively on the (Bi₁₂O₁₇) end-faces of 1L-BOC. The aberration-corrected high-angular annular dark field scanning TEM (HAADF-STEM) image (Fig. 1n) and energy loss spectroscopy (EELS) elemental maps (Fig. 1o–s) of their cross-sectional atomic structures provided direct, atomic-resolution evidences that this oriented assembly resulted in 2D Janus bilayer junctions of (Cl₂)-(Bi₁₂O₁₇)-(MoS₂). Meanwhile, we also found that the amount of MoS₂ loaded on Bi₁₂O₁₇Cl₂ decreased with OVs quenching, and the assembly behaviour annihilated when OVs were completely removed (Supplementary Figs 5–7, Supplementary Table 1 and Supplementary Note 6 and 7), confirming that the oriented assembly of 2D Janus (Cl₂)-(Bi₁₂O₁₇)-(MoS₂) bilayer junctions was driven by the OVs on the (Bi₁₂O₁₇) end-faces of 1L-BOC.}

**Figure 1: Characterizations of the Janus bilayer junctions assembled by Bi₁₂O₁₇Cl₂ and MoS₂ monolayers (BOC-MS).**

Visible light PHE

The successful design of BOC-MS featuring Janus (Cl₂)-(Bi₁₂O₁₇)-(MoS₂) bilayer junctions allowed us to first explore their PHE activity under visible light (λ>420 nm). Visible light irradiation of the solution containing BOC-MS and ascorbic acid (organic scavengers) gave a typical gas chromatograph (GC) signal of H₂ at the retention time of 82 s (Fig. 2a), consistent with the previously reported results¹⁶. At the optimal MoS₂ loading (10 wt%, Supplementary Fig. 8a) and ascorbic acid concentration (0.3 mol l⁻¹, Supplementary Fig. 8b), BOC-MS delivered a PHE rate of 33 mmol h⁻¹ g⁻¹ (Fig. 2b) and a quantum yield of around 36% at 420 nm (Fig. 2h), representing the currently achieved state-of-the-art PHE activity among all the MoS₂-based, monolayer-based and bismuth oxyhalide-based PHE systems (Supplementary Tables 2–4). This activity could last over 100 h without significant decay and the bilayer structures were very stable (Supplementary Fig. 9 and Supplementary Note 8), indicating BOC-MS were highly active and robust for visible-light PHE. The absence of OVs signal in BOC-MS ruled out the possibility that its superior PHE activity was arisen from the OVs (Fig. 2c). During PHE, dehydroascorbic acid and its hydrated compound as the oxidized products of ascorbic acid were detected (Fig. 2d)¹⁷, indicating that ascorbic acid acted as an efficient hole scavenger to suppress the electron and hole recombination. Even using ascorbic acid as the hole scavenger and having surface OVs, 1L-BOC only exhibited a PHE rate of 0.86 mmol h⁻¹ g⁻¹, 1/38 that of BOC-MS.

This giant PHE activity enhancement by the introduction of 1L-MS ignited our interest in unravelling how 1L-MS assisted 1L-BOC to precisely steer charge flow across the assembled Janus bilayer junctions. Density functional theory calculations of 1L-MS showed its density of states resided across the Fermi level (Fig. 2e), revealing its metallic characteristic^25,26. It was found that I₁ increased ca. 17-fold from 5.23% for BOC-MS-Mix to 86.59% for BOC-MS, and its magnitude highly depended on the amounts of Bi–S bonds, confirming the electron transport from 1L-BOC to 1L-MS via the Bi–S bonds within BOC-MS (Fig. 3v). This interfacial electron transfer was ultrafast with a rate of up to 5.8 × 10⁻¹¹ s⁻¹, as calculated by (τ_BOC-MS)⁻¹−(τ_BOC-MS-Mix)⁻¹, where τ_BOC-MS and τ_BOC-MS-Mix are the average TA lifetimes for BOC-MS and BOC-MS-Mix, respectively. As a result, the electron flow from 1L-BOC to 1L-MS required only 0.68 ps (τ₁ for BOC-MS), which was inferior to the time scale (3–20 ps) required for the surface charge recombination^{1,Full size image}

Discussion

Subsequently, we quantified the efficiencies of charge transportations during the above two charge flow processes to probe the origin of the superior PHE performance of BOC-MS by using the following equations (Supplementary Methods).

In equation (1), η_abs (set as ‘1’), η_bulk, η_1L-BOC, η_interface, η_surface and η_catalysis are the efficiencies of photoabsorption, bulk charge separation of BOC-MS, bulk charge separation of 1L-BOC, interfacial charge flow from 1L-BOC to 1L-MS, surface charge separation and hydrogen-evolving catalysis, respectively. In equation (2) and equation (3), J_abs is the current density converted from the absorbed photons when assuming η_abs=1, while J_{ascorbic-acid} and J_water are the photocurrent densities measured by using ascorbic acid and water as electrolytes, respectively. The establishment of equation (2) is based on the assumption that ascorbic acid oxidation could completely suppress the surface recombination and its catalytic efficiency is 1, similar with the case that Kim et al.³⁰ used sulfite oxidation to determine the η_bulk of BiVO₄. These experiments were performed under irradiation of 420 nm monochromatic light, which gave a J_abs=0.788 mA cm⁻². For 1L-BOC, J_{ascorbic-acid} was measured to be 0.725 mA cm⁻² (Supplementary Fig. 24), so η_bulk=J_{ascorbic-acid}/J_abs=0.92. This means, through the atomic-level charge separation steering by IEF, 1L-BOC enabled 92% of photogenerated electrons and holes to be transferred from the electron–hole separation sites to its (Bi₁₂O₁₇) and (Cl₂) end-faces, respectively. Despite this ultrahigh η_bulk, the quantum yield of 1L-BOC was still as low as 0.8%, because of its ultralow η_surface (0.8%) and poor η_catalysis (10.8%; Supplementary Fig. 24d). When 1L-MS was selectively and chemically assembled on (Bi₁₂O₁₇) end-faces, η_surface and η_catalysis increased by 11.4 and 4.2 times, respectively, as this oriented assembly enabled the Janus distribution of electrons and holes in BOC-MS to prevent the surface recombination and the metallic nature of 1L-MS was highly active for the hydrogen-evolving catalysis. Moreover, another crucial factor for the remarkable quantum yield (36%) of BOC-MS lied in the ultrahigh η_interface (91%) offered by the interfacial Bi–S bonds, by which the separated electrons of 1L-BOC could quickly migrate from (Bi₁₂O₁₇) end-faces to 1L-MS, achieving atomic-level charge flow steering from 1L-BOC to 1L-MS.

In conclusion, we have designed Janus (Cl₂)-(Bi₁₂O₁₇)-(MoS₂) bilayer junctions by assembling MoS₂ monolayers selectively and chemically on (Bi₁₂O₁₇) end-faces of Bi₁₂O₁₇Cl₂ monolayers via OV chemistry. This atomic-level structural and interfacial design allowed us to steer all the charge separation, transportation and consumption at the atomic level. Electrons originating from visible light-irradiated Bi₁₂O₁₇Cl₂ were first driven by the IEF between (Cl₂) and (Bi₁₂O₁₇) end-faces to (Bi₁₂O₁₇) end-faces, and further transferred via the Bi–S bonds formed between Bi₁₂O₁₇Cl₂ and MoS₂ monolayers to MoS₂ monolayers to finally catalyse the hydrogen evolution. Meanwhile, the IEF drove holes to (Cl₂) end-faces where the organic scavenger was oxidized. Such an atomic-level steering of charge flow offered a visible-light PHE rate of 33 mmol h⁻¹ g⁻¹ with a quantum efficiency of 36% at 420 nm, superior to any reported MoS₂, or monolayer, or bismuth oxyhalide-based PHE systems. This work sheds atomic-level mechanistic insights into the flow of photogenerated electrons and holes, thus paving new ways into the exploration and design of high-performance, cost-effective photocatalysts for hydrogen evolution.

Methods

Preparation of bulk layered Bi₁₂O₁₇Cl₂ nanosheets

First, 8 mmol of BiCl₃ was dissolved in 80 ml of ethanol with vigorous stirring at ambient temperature. Next, the pH value of the obtained homogeneous solution was adjusted to 13.4 by dropwise adding NaOH (1 mol l⁻¹) solution, yielding yellow precipitations. Then these precipitations were collected, washed thoroughly with deionized water and ethanol for several times, and dried in an oven at 50 °C under vacuum. Calcination of the collected yellow powders in muffle furnace at 450 °C for 2 h produced layered Bi₁₂O₁₇Cl₂ nanosheets, which we term BOC.

Preparation of bulk layered MoS₂ nanosheets

Typically, 1.5 mmol of (NH₄)₆Mo₇O₂₄·4H₂O and 36 mmol of thiourea were added in 75 ml of distilled water at room temperature with continuous stirring to give a transparent solution. The resulting mixture solution was then poured into a 100 ml Teflon-lined stainless autoclave. The autoclave was allowed to be heated at 200 °C for 24 h under autogenous pressure, and then air cooled to room temperature. The resulting precipitates were collected, then washed with ethanol and deionized water thoroughly, and finally dried at 50 °C under vacuum. We term the obtained sample MS.

Preparation of Bi₁₂O₁₇Cl₂ and MoS₂ monolayers

Organolithium chemistry was used to exfoliate layered MoS₂ (or Bi₁₂O₁₇Cl₂) nanosheets into their single-layered counterpart. Briefly, intercalating lithium into the spaces between each neighbouring MoS₂ (or Bi₁₂O₁₇Cl₂) monolayer-unit was first conducted by immersing 0.2 g of layered MoS₂ (or 0.1 g of Bi₁₂O₁₇Cl₂) nanosheets in 8 ml (10 ml for Bi₁₂O₁₇Cl₂) of n-butyllithium (Caution: n-butyllithium is highly pyrophoric.) under argon atmosphere for 72 h at 100 °C. The suspension was then washed with hexane for several times to remove the excess of n-butyllithium. The collected nanosheets with lithium intercalation were re-dispersed in distilled water at 1 mg ml⁻¹ and sonicated at a low-power sonic bath (40 W) for 40 min to obtain exfoliated nanosheets. After centrifuging the resultant dispersions at 8,000 r.p.m. (8,000 r.p.m. for MoS₂ and 6,000 r.p.m. for Bi₁₂O₁₇Cl₂) for 10 min, the supernatants were collected, removing the unsuccessfully exfoliated nanosheets. The collected supernatants were centrifuged at 12,000 r.p.m for 10 min for several times to remove excess impurity, producing single-layered MoS₂ and Bi₁₂O₁₇Cl₂ nanosheets with yields of ∼47 and 26%, respectively. We term the as-prepared MoS₂ and Bi₁₂O₁₇Cl₂ monolayers 1L-MS and 1L-BOC, respectively.

It should be noteworthy that the surface of 1L-BOC was rich in oxygen vacancies, because, during the lithium-intercalation process, the organic ligands in n-butyllithium would interact strongly with lattice oxygen atoms on Bi₁₂O₁₇Cl₂ to form coordinate bonds, and during the ultrasonication-mediated exfoliation process, the organic ligands escaped from the interior of Bi₁₂O₁₇Cl₂, which inevitably removed a fraction of oxygen atoms from the lattice, leading to the occurrence of surface oxygen vacancies in single-layered Bi₁₂O₁₇Cl₂ nanosheets. Calcination of 1L-BOC with oxygen-vacancy concentrations of 11% in air at 200 °C for 30 min and at 300 °C for 6 h produced single-layered Bi₁₂O₁₇Cl₂ nanosheets with oxygen-vacancy concentrations of 5.3% and 0, which we call 1L-BOC-1 and 1L-BOC-2.

Assembling monolayers of Bi₁₂O₁₇Cl₂ and MoS₂

An OV-directed assembly strategy was used to assemble monolayers of MoS₂ and Bi₁₂O₁₇Cl₂. First, 10 mg of MoS₂ monolayers were dispersed in 30 ml of distilled water under ultrasonication to obtain transparent solution A. 40 mg of oxygen-deficient Bi₁₂O₁₇Cl₂ monolayers were dispersed in a three-neck flask containing 120 ml of distilled water under ultrasonication to obtain transparent solution B. Then the solution A was dropwise added to the solution B under vigorous stirring at ambient temperature to give homogeneous mixture solution. Next, the above mixture was deoxygenated by bubbling argon gas at room temperature for 60 min, subsequently refluxed at 80 °C for 2 h under a stirring rate of 200 r.p.m., and finally air cooled to room temperature. The resulting suspension were first centrifuged at 3,000 r.p.m. for 5 min to remove the aggregated precipitations, and further centrifuged at 12,000 r.p.m. for 10 min to obtain the assemble of MoS₂ and Bi₁₂O₁₇Cl₂ monolayers, which we call BOC-MS.

We treated the synthesized 1L-BOC, 1L-MS and BOC-MS by many times of water–ethanol washing and plasma treatment to eliminate the surface-adsorbed impurities. Furthermore, when measuring their heights via AFM, we diluted the concentrations of the measured solutions to a considerably low level so as to maintain the nanosheets’ surface as flat as possible, which would avoid the wrinkling or twisting.

Characterization

The powder X-ray diffraction patterns of the samples were recorded on a Bruker D8 Advance diffractometer with monochromatized Cu Kα radiation (λ=0.15418, nm). The powders were deposited on copper grids with carbon support films for electron microscopy observation. TEM and HRTEM (high-resolution TEM) observations were performed on Hitachi H-7650 and JEOL-2010F with an acceleration voltage of 200 kV. The atomic-resolution HRTEM images and EELS elemental map** images were investigated by aberration-corrected STEM and EELS carried out in a Nion UltraSTEM (Nion) microscope operating at 200 keV and equipped with a cold-field emission gun, a third-generation C3/C5 aberration corrector and a Gatan Enfinium EEL spectrometer. Energy dispersive X-ray spectroscopy were carried out using a JEM-ARM 200F Atomic-Resolution Analytical Microscope operating at an accelerating voltage of 200 kV. Elemental map**s were collected by a Gatan GIF Quantum 965 instrument. Ultraviolet–visible diffused reflectance spectra of the samples were obtained for the dry-pressed film samples with using a ultraviolet–visible spectrophotometer (UV-2550, Shimadzu, Japan) with BaSO₄ as the reflectance standard. Chemical compositions and states were analysed using XPS (Thermo Scientific ESCLAB 250**). All binding energies were referenced to the C 1 s peak (284.6 eV) arising from the adventitious carbon. Atomic concentrations were calculated by normalizing peak areas to the elemental sensitivity factor. Raman measurements were carried out by a confocal laser micro-Raman spectrometer (Thermo DXR Microscope, USA). The laser was 633 nm with a 5 mW. AFM and Kelvin probe force microscope measurements were carried out on an AFM instrument (SPM-9600, Shimadzu). The electrical resistivity study was corrected on a Keithley 4200 station with the computer-controlled four-probe technique. Photoluminescence emission and photoluminescence decay spectra were recorded at room temperature with a fluorescence spectrophotometer (Edinburgh Instruments, FLSP-920). The femtosecond-resolved TA spectra were performed using a modified ExciPro pump–probe spectrometer (CDP) in connection with an amplified femtosecond laser system (Coherent). Electron paramagnetic resonance spectra were performed on a Bruker EMX EPR Spectrometer (Billerica, MA). The Bi:O:Cl molar ratios in Bi₁₂O₁₇Cl₂ monolayers were detected by IRIS (INTREPID 2) inductively coupled plasma atomic emission spectrometry. The ¹³C NMR spectra were recorded on a Bruker AVANCE III 600M system. The Bi L-edge X-ray absorption fine structure spectroscopy was carried out at BL 14W1 beamline at the Shanghai Synchrotron Radiation Facility (SSRF) China. Surface photovoltage spectroscopy was recorded on a lock-in amplifier (SR830-DSP, Stanford Research Systems) synchronized with a light chopper (SR540).

Photocatalytic hydrogen evolution

The photocatalytic hydrogen production experiments were performed at ambient temperature and atmospheric pressure using 300 W Xe arc lamp as the light source. In all, 10 mg of photocatalyst was dispersed in 80 ml of aqueous solution containing 0.3 M ascorbic acid in a 120 ml Pyrex flask. Before irradiation, the suspensions were bubbled with nitrogen for 30 min to remove the dissolved oxygen and to ensure the anaerobic conditions of the reaction system. During the whole reaction process, the aqueous solution with photocatalyst was continuously stirred by a magnetic stirrer. The generated hydrogen gas was analysed with an online GC (C36880-14, RESTEK) equipped with a thermal conductivity detector, where Ar was used as a carrier gas.

Theoretical calculations

Theoretical calculations were performed using density functional theory as implemented in the VASP code. 3D periodic boundary conditions were used to approximate an infinite solid. Exchange-correlation effects were described through the generalized gradient approximation, within the Perdew–Burke–Ernzerhof formalism. The core electrons (Bi:(Xe), Cl:(Ne), O:(He)) were treated within the projector augmented wave method. The energy cutoff is set to be 520 eV, and the atomic positions are allowed to relax until the energy and force are <10⁻⁴ eV and 10⁻³ eV Å⁻¹, respectively.

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How to cite this article: Li, J. et al. Superior visible light hydrogen evolution of Janus bilayer junctions via atomic-level charge flow steering. Nat. Commun. 7:11480 doi: 10.1038/ncomms11480 (2016).

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Acknowledgements

This work was supported by National Natural Science Funds for Distinguished Young Scholars (Grant 21425728), National Basic Research Program of China (973 Program) (Grant 2013CB632402), National Natural Science Foundation of China (Grant 21177048, 21377044, 51472100 and 21573085) and excellent doctorial dissertation cultivation grant from Central China Normal University (2015YBZD019).

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Authors and Affiliations

Key Laboratory of Pesticide and Chemical Biology of Ministry of Education, Institute of Environmental Chemistry, College of Chemistry, Central China Normal University, Wuhan, 430079, China
Jie Li, Guangming Zhan & Lizhi Zhang
Institute of Nanoscience and Nanotechnology, College of Physical Science and Technology, Central China Normal University, Wuhan, 430079, China
Jie Li & Ying Yu

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Jie Li
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Guangming Zhan
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Ying Yu
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Lizhi Zhang
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Contributions

L.Z. and J.L. conceived the project; J.L. performed most of the experiments; G.Z. performed part of the experiments; L.Z. and J.L. co-wrote the paper. Y.Y. helped to analyse the results and took part in the revision of the manuscript. L.Z. supervised the project. All the authors discussed the results and commented on the manuscript.

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Correspondence to Lizhi Zhang.

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Supplementary information

Supplementary Information

Supplementary Figures 1–25, Supplementary Tables 1–4, Supplementary Notes 1–11, Supplementary Methods and Supplementary References. (PDF 3614 kb)

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Li, J., Zhan, G., Yu, Y. et al. Superior visible light hydrogen evolution of Janus bilayer junctions via atomic-level charge flow steering. Nat Commun 7, 11480 (2016). https://doi.org/10.1038/ncomms11480

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Received: 11 December 2015
Accepted: 31 March 2016
Published: 09 May 2016
DOI: https://doi.org/10.1038/ncomms11480
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Superior visible light hydrogen evolution of Janus bilayer junctions via atomic-level charge flow steering

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