Interface Band Engineering Charge Transfer for 3D MoS2 Photoanode

Apr 5, 2017 - In photoelectrochemical (PEC) cells, it is a crucial issue to steer the charge flow in the electrode, including the internal movement of...
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Research Article pubs.acs.org/journal/ascecg

Interface Band Engineering Charge Transfer for 3D MoS2 Photoanode to Boost Photoelectrochemical Water Splitting Xiaoyong Xu,*,†,§ Gang Zhou,†,‡,§ Xuefeng Dong,† and Jingguo Hu*,† †

College of Physics Science and Technology, Yangzhou University, Yangzhou 225002, China Department of Physics, Nanjing University, Nanjing 210093, China

‡

S Supporting Information *

ABSTRACT: In photoelectrochemical (PEC) cells, it is a crucial issue to steer the charge flow in the electrode, including the internal movement of charge in the catalyst and the charge transfer across the catalyst−substrate interface toward the external circuit. Here, we fabricated vertically aligned MoS2 nanosheets (NSs) on carbon fiber cloth (CFC) substrates decorated without and with a Au layer as two photoanodes for PEC water splitting, whereby the interface electron transfer mediated by the embedded Au was demonstrated to contribute to photoelectrode performance. The photoexcited Au plasmon switches the interface barrier from n-type Schottky to a ptype one, making the built-in potential act in accordance with external positive potential to together drive electron transfer and charge separation at the interface. The enhanced electron-transfer dynamic at the Au-embedded interface is determined in terms of the output current, impedance, and incident photon-to-current conversion measurements, being responsible for the significantly increased PEC activity in the MoS2/Au@CFC photoelectrode. This work gains insight into the importance of engineering charge transfer across the catalyst−substrate interface in PEC electrodes. KEYWORDS: PEC cells, Transition-metal dichalcogenide, Interface charge transfer, Solar water splitting

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applications.24 Moreover, the vertical MoS2 nanosheet arrays (NSAs) have recently been fabricated on flexible carbon fiber cloth (CFC) substrates and developed as EC electrodes with multiple advantages;25,26 however, the 3D architecture based on MoS2 NSAs has not be reported for PEC electrode applications so far. In addition, for the catalytic electrodes, the active sites and the charge transport are two crucial issues that determine the working performance. Many strategies have been explored to engineer MoS2 catalysts with either more active sites or higher conductivity.27−35 For example, engineering size, morphology, and defect can preferentially expose active edge sites;27−29 semiconducting (2H)-to-(metallic) 1T phase transformation, a conducting matrix hybrid, and heteroatom incorporation can improve electric conductivity.30−35 Nevertheless, besides the properties of catalysts themselves, the interface charge transfer between catalysts and contacting layers is also a crucial issue, and especially in PEC electrodes it directly determines the photogenerated charge separation efficiency and thereby is a more crucial factor for PEC photoconversion.36 So, the ability to modulate interface charge transfer is of great importance for the design of high-performance PEC cells; however, the

INTRODUCTION Hydrogen is one of the clean and sustainable energy sources and considered as the most promising alternative for fossil fuels to solve the increasing energy crisis and environmental pollution.1−6 Electrochemical (EC) and photoelectrochemical (PEC) water splitting as two typical technologies to trigger hydrogen evolution reaction (HER) have attracted intense attention because of carbon-free emissions; meanwhile many active catalysts such as metal oxides, metal chalcogenides, and carbides have been reported for EC or PEC water splitting.7−16 Among them, two-dimensional (2D) layered transition-metal dichalcogenides (TMDs), such as MoS2, WS2, TiS2, MoSe2, and WSe2, received considerable attention owing to their excellent catalytic activities.17,18 In particular, MoS2 nanosheets (NSs) as a representative TMD have been widely used as promising electro-catalysts in EC electrodes and effective cocatalysts in photocatalysts for HER.19−21 Recently, few-layer MoS2 NSs deposited on transparent conductive substrates for PEC cell application have also been exploited, and the n-type photocurrent generation was demonstrated as an ideal photoanode material.22,23 Note that the vertical NSs-assembled threedimensional (3D) array architecture has been considered as an ideal platform that offers versatile benefits, such as preferentially exposed active sites, optimized charge transport along the 2D vector plane, enhanced light scattering, and a large electrode/electrolyte interface, for catalytic electrode © XXXX American Chemical Society

Received: November 29, 2016 Revised: February 14, 2017 Published: April 5, 2017 A

DOI: 10.1021/acssuschemeng.6b02883 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering research on interface charge transfer is rarely reported due to the focusing effort on character optimization of catalysts themselves. Herein, we fabricated vertical MoS2 NSAs on CFC substrates decorated without and with a Au layer through a facial hydrothermal method, referred to respectively as MoS2/CFC and MoS2/Au@CFC, and then they were used directly as twin photoanodes for PEC water splitting to explore the effect of interface electron-transfer modulation on the PEC activity. The embedded Au mediates the electron-transfer dynamic from MoS2 to CFC by interface band engineering, leading to the considerably enhanced photo-oxidation activity in the MoS2/ Au@CFC photoanode relative to the MoS2/CFC counterpart. This work develops a high-performance 3D MoS2 NSA-based photoanode, stressing the significance of interface band engineering charge transfer in PEC cells.

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EXPERIMENTAL SECTION

Fabrication for 3D Electrode of MoS2 NSAs. The cut CFC substrates (2 cm × 2 cm) were consecutively washed with acetone, H2SO4 (1 M), and deionized water under sonication for 2 h in each solution to thoroughly remove organic residues and surface impurities. To study the effect of the interface Au insert layer on the morphology and performance of the resultant electrodes, the clean CFC substrates were decorated with Au by using an ion sputtering high-purity Au target in N2 for 30 s. The Au-decorated CFC (Au@CFC) substrates were then annealed at 300 °C in N2 for 20 min to improve crystallization of the Au layer coated on carbon fibers. Thus, two types of CFC, i.e., with and without the decoration of a Au coating layer, were prepared as two substrates to assemble MoS2 NSs in the following hydrothermal process. The CFC and Au@CFC substrates were immersed in 25 mL of DMF solution containing 20 mg of (NH4)2MoS4 and stirred for 30 min. Then, the above solutions with immersed substrates were transferred into Teflon-lined stainless steel autoclaves and kept at 220 °C for 20 h. After the autoclaves naturally cooled down to room temperature, the substrates loaded with products were taken out from the autoclaves and washed by deionized water and ethanol successively several times and then dried in a vacuum oven at 65 °C for 12 h. The obtained MoS2 NSAs grown on CFC substrates decorated without and with a Au layer were of nearly the same mass density of about 2.3 mg cm−2, as shown in Figure S1, and they were denoted as MoS2/CFC and MoS2/Au@CFC, respectively. The schematic of the synthesis process is summarized in Figure 1a; note that such a solution-processable hydrothermal strategy is in favor of facile, green, low-cost, and large-scale production. Sample Characterization. The samples were characterized by different analytic techniques. X-ray diffraction (XRD) was carried out on a Shimadzu XRD-7000 diffractometer with Cu Kα radiation (λ = 0.15406 nm). Field emission scanning electron microscopy (FESEM) was performed on a Hitachi S-4800II field-emission scanning electron microanalyzer with an accelerating voltage of 5 kV, and the corresponding X-ray energy dispersive spectroscopy (EDS) was measured under an accelerating voltage of 20 kV. Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction (SAED) were taken on a Tecnai F30 transmission electron microscope with an acceleration voltage of 200 kV. X-ray photoelectron spectra (XPS) were recorded on an Escalab 250Xi spectrometer with Mg Kα radiation, and the binding energies were calibrated with the C 1s peak of 284.6 eV. Raman spectra were measured on a Renishaw in Via Raman microscope with an excitation laser source of 532 nm. PEC Measurement. PEC measurements were performed at room temperature on an electrochemical workstation (CHI660E) with a three-electrode configuration in a 0.1 M KH2PO4 aqueous solution with pH adjusted to 7.0. An Ag/AgCl electrode and a Pt-wire electrode were used as the reference and counter electrodes, respectively. The as-synthesized MoS2/CFC and MoS2/Au@CFC were directly used as working electrodes to compare their photoanode activities for splitting

Figure 1. (a) Schematic illustration for the synthesis process of MoS2/ Au@CFC. (b, c) FESEM and (d, e) amplified FESEM images of MoS2/CFC and MoS2/Au@CFC, and the insets in b and c are FESEM images of their carbon fibers before MoS2 growth. (f) TEM image with SAED pattern (inset) and (g) HRTEM image with locally amplified (100) phase. water. The photoelectrodes were irradiated from the front side by a 300 W Xe lamp (PLS-SXE 300C) with an AM 1.5 filter placed 10 cm away from the reaction vessel to simulate the solar illumination. The light intensities on the photoelectrodes were calibrated to 100 mW cm−2. The linear sweep voltammograms (J−V) at a scan rate of 5 mV s−1 under light illumination, the amperometric (J−t) curves under chopped light illumination at a bias voltage of 0.5 V vs Ag/AgCl electrode, and the electrochemical impedance (EI) spectra over a frequency range from 100 kHz to 0.01 Hz with an alternating-current voltage of 5 mV for the aforementioned two photoanodes were measured to investigate their PEC properties. For the investigation of PEC stability, the photocurrent retention performance over 10 000 s at a bias voltage of 0.5 V and J−V curves for 3000 cycles at a scan rate of 50 mV s−1 under light illumination were recorded. The incident photon-to-current efficiency (IPCE) measurement was performed by using several single-band-pass filters of λ ± 15 nm for 400, 420, 450, 470, 500, 530, 550, 600, 650, and 700 nm.

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RESULTS AND DISCUSSION The fabrication process of the MoS2/Au@CFC electrode is illustrated in Figure 1a. Briefly, a CFC substrate was decorated with a Au layer via ion sputtering method followed by vacuum annealing treatment. Then, vertically aligned MoS2 NSAs were B

DOI: 10.1021/acssuschemeng.6b02883 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 2. High-resolution XPS spectra in (a) Au 4f, (b) Mo 3d, and (c) S 2p regions of MoS2/Au@CFC. (d) Raman spectra of MoS2/ CFC and MoS2/Au@CFC.

CFC would induce the change of charge-transfer properties across the interface, which offers an effective modulating strategy for PEC performance from another aspect. Before loading MoS2 NSs, the FESEM images of CFC substrates decorated without and with a Au layer are shown in the insets of Figure 1b and c, respectively. The bare carbon fibers present a clean and wrinkled surface, while the decorated ones are uniformly covered with a Au surface and look from black to gold in color (Figure S1). The EDS spectra in Figure S3 illustrate that MoS2/CFC is composed of Mo, S, and C elements, whereas MoS2/Au@CFC contains an explicitly Au element besides Mo, S, and C elements, revealing the Au incorporation. The line-sweeping EDS spectra across the edge of MoS2 NSAs on a single Au@CFC fiber (Figure S3c) further support the intermediate location of Au inserted into the MoS2−CFC interface. Figure 1f and g show the TEM and HRTEM images of an individual MoS2 sheet exfoliated from Au@CFC support. The obvious ripples and corrugations observed in Figure 1f verify the ultrathin nature of NSs. The thickness of the sheet is 5−10 nm, as determined by the curled edges, corresponding to 5−10 sandwiched S−Mo−S layers. The SAED pattern (inset in Figure 1f) reveals faint diffraction rings from MoS2, indicating the quasi-amorphous structure of MoS2, consistent with the weak diffraction peaks from MoS2 in XRD patterns (Figure S4). The HRTEM image in Figure 1g exhibits the curled edges with crystal fringes of the (002) plane and the disordered basal surface with several short-range ordering domains of the (100) plane. The amplified HRTEM image for a typical ordering domain (inset in Figure 1g) reveals the quasi-periodic arrangement of atoms. Note that Xie et al. have engineered the disorder degree in MoS2 NSs to balance the benefits between active sites and conductivity for EC

synthesized on a Au-decorated CFC substrate after a hydrothermal process, and a bare CFC foam was also used as another substrate to assemble MoS2 NSAs under the same hydrothermal environment for the following comparative studies. The morphologies and microstructures of MoS2 NSs grown on CFC and Au@CFC substrates were investigated with FESEM and TEM microscopies. The FESEM images in Figures 1b and c show that large-scale NSs with a network structure uniformly grow on the CFC and Au@CFC substrates. The amplified FESEM image in Figure 1d and e reveals that the asgrown NSs are of ultrathin morphology; moreover, these NSs form the almost vertical arrays rather than assemble each other, rendering a vertically mesh-shaped structure with preferentially exposed edges and patulous pores. The cross-sectional FESEM image of NSAs anchored on CFC shown in Figure S2 reveals further the dense loading of vertical NSs with an average height of about 220 nm on single fibers. Note that the MoS2 NSAs grown on CFC and Au@CFC substrates are of similar 3D morphology, which enables some structural advantages for PEC photoelectrode application: (i) The charges could drift along the in-plane surface of each single sheet to avoid the highresistance transport along adjacent van der Waals bonded S− Mo−S layers; thus such an optimized conduction pathway would significantly improve the conductivity of MoS 2 catalysts.26 (ii) The 3D porous network structure composed of vertically aligned NSs could preferentially expose active edge sites and large surface area, which are in favor of optical absorption and redox reaction.37 (iii) The facile synthetic strategy based on cheap and flexible CFC substrates is amenable to large-scale application of PEC electrodes.25 More interestingly, the insertion of a Au layer between MoS2 and C

DOI: 10.1021/acssuschemeng.6b02883 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 3. (a) Linear sweep J−V. (b) EI curves in dark and light fields with an equivalent circuit (inset) and (c) IPCE spectra of MoS2/CFC and MoS2/Au@CFC. Interface energy-band diagrams with carrier transfer routes of MoS2/CFC and MoS2/Au@CFC in (d) dark and (e, f) light fields.

catalysis.35 The disordered structure could exploit more dangling bonds as active sites at the basal surface, and meanwhile the unique quasi-periodic arrangement in small nanodomains could partially retain the 2D electron conjugation to render fast interdomain charge transport along the basal surface. Thus, we can infer the present MoS2 NSs with quasiamorphous structure including abundant short-range ordering domains to be probably a desired platform that synergistically couples both structural and electronic benefits for PEC water splitting. The XPS characterization for MoS2/Au@CFC was performed to further confirm the element compositions and chemical states. The XPS survey spectrum shown in Figure S5 reveals the presence of Mo, S, Au, C, and O elements. The high-resolution XPS scan of the Au 4f core level in Figure 2a shows two peaks of Au 4f5/2 and Au 4f7/2 located at 87.9 and 84.3 eV, supporting the insertion of crystallized Au.38 The highresolution XPS spectra in Mo 3d and S 2p regions, as shown in Figure 2b and c, identify the dominant peaks of Mo 3d3/2, Mo 3d5/2, S 2p1/2, and S 2p3/2 located respectively at 232.1, 228.9, 163.2, and 161.8 eV, indicating the mainly chemical coordination of Mo4+ and S2− ions.39 The weak fitted peaks at 233.2 and 235.9 eV in Figure 2b correspond to oxidized Mo6+ species,26 which may be the main source leading to inplane disordering. Figure 2d shows the Raman spectra of MoS2/CFC and MoS2/Au@CFC. Two distinct peaks located at about 376.2 and 401.8 cm−1 correspond to the in-plane E12g and out-of-plane A1g vibration modes of 2H-MoS2, respectively.40 Noticeably, the present MoS2 NSs exhibit the larger width and lower intensity of the E12g peak relative to those highquality MoS2 NSs obtained though mechanical exfoliation or vapor deposition methods, which reflects the presence of an amorphous region or defect sites at the basal surface,41 being consistent with the aforementioned HRTEM observation.

Moreover, the intervention of Au insertion stiffens the overall bond stretching vibration and brings about the slight blue shift of peak positions by 1.54 cm−1, revealing the arising of strong electronic interaction between MoS2 and Au under the incident electromagnetic wave excitation.42 The PEC J−V, EI, and IPCE measurements were used to evaluate the charge transport properties combined with interface band structure analysis. As shown in Figure 3a, as the positive voltage forward sweeps in the dark field, MoS2/ CFC shows a nonlinear rising J−V curve with an onset potential of 0.41 V (Figure 3a, black), whereas MoS2/Au@CFC exhibits the reduced onset potential of 0.32 V and the increased current density in the overall voltage sweeping window (Figure 3a, blue). The shift in their onset potentials can be interpreted as a representation of the difference in interface electron transfer ability in case of their laden catalysts remaining unchanged. The difference between the work functions of CFC and MoS2 (5.0 and 4.5 eV, respectively)43−45 determines the formation of an n-type Schottky barrier (SB) at the MoS2− CFC interface to hinder electron transfer from MoS2 to CFC, as shown in the coupling interface band structure in the dark field (Figure 3d, black). After Au inserting into the MoS2−CFC interface, the redefined Fermi level decreases the SB height (SBH; Figure 3d, red), and thus a smaller onset potential and a faster current growth are observed in MoS2/Au@CFC. More interestingly, under the AM1.5 illumination, the electrons are photoexcited into the conduction band (CB) of MoS2, with the same amount of holes left behind in the valence band (VB). Due to the formation of photogenerated charges, MoS2/CFC yields a higher current density and a lower onset potential in the light field than those in the dark field (Figure 3a, red); however, these two advances resulted from light excitation are unconspicuous because the electrons still have to overcome the interfacial SB into the CFC substrate (Figure 3e). By contrast, D

DOI: 10.1021/acssuschemeng.6b02883 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 4. (a) Water splitting polarization curves, (b) J−t cycles with ON/OFF light, and (c) photoconversion efficiencies for PEC cells with different working electrodes, bare CFC, Au@CFC, MoS2/CFC, and MoS2/Au@CFC, and digital photos of O2 bubbles formed on MoS2/CFC and MoS2/ Au@CFC electrodes (inset). (d) Long-term J−t curve with polarization curves within 3000 cycles (inset) for the stability test.

MoS2/Au@CFC exhibits a considerably enhanced photocurrent density and even a spontaneous current at the zero bias potential (Figure 3a, green), which is an important feature that implies the transition of interface barrier from n-type SB to a p-type one, allowing electrons for downstream flow toward a Au@CFC substrate needless of crossing an energy barrier. The surface plasmon resonance (SPR) on phtotoexcited Au insertion could be responsible for this reconfiguration of interface band structure. When illuminated, the coherent free electrons of Au oscillate into the SPR state with a higher energy level over the CB of MoS2; thus the rebalance of Fermi levels changes the band bending direction, leading to the transition from n-type SB to a p-type one at the interface (Figure 3f). In other words, the interface band structure is adjusted via Au plasmon to steer the initial transfer of electrons from MoS2 to CFC; thus the probabilities of separation and reverse transport for photogenerated electron−hole pairs before being recombined in MoS2 are boosted to improve the photocurrent output. Of course, the “hot” electron injection from Au plasmon is also a possible donation to photoelectron output, but only such a contribution cannot cause the change of conduction nature, and thus the plasmonic Au anchored on the surface of MoS2 NSs was also found to enable the increase in photocurrent output, yet needing still certain onset potentials.22,38 In the present work, the embedded Au plasmon switches the direction of built-in electric field (or barrier) across the interface, acting as the interface electron driver to steer the n-type photocurrent even at zero bias voltage. Clearly, this control over the interface barrier (or built-in field) direction ought to be a more effective strategy to improve the PEC performance. The EI spectra in dark and light fields were studied to gain more insight into the interface charge-transfer resistance. As shown in Figure 3b, all Nyquist impedance plots display two

distinguishable curvature regions. Following the previously reported method,40 the data can be fitted to an equivalent circuit (inset in Figure 3b) consisting of constant phase elements (CPE) associated with the substrate (CPE1) and the catalyst (CPE2), and charge-transfer resistances (CTR) between substrate and catalyst (CTR1) and between catalyst and electrolyte (CTR2). The first curvature on the left in the Nyquist plots is an indicator of CTR1, where the arc diameters from MoS2/Au@CFC are smaller than those from MoS2/CFC wherever in dark and light fields; moreover the light-irradiated MoS2/Au@CFC exhibits the smallest arc diameter. This indicates that the interface electron transfer not only can be optimized by static Au but also can be further accelerated by active Au plasmon, corresponding to the controllable transfer dynamic via modulating the height and type of interface SB. Figure 3c shows the IPCE spectra of two compared electrodes (see Supporting Information for detailed calculation of IPCE values). The MoS2/Au@CFC electrode shows intrinsic superiority compared to the MoS2/CFC counterpart over the overall incident-light waveband, indicating a fortified chargeseparation dynamic at the embedded Au interface. Moreover, the most prominent improvement observed at the IPCE peak of approximately 530 nm is associated with the SPR state of Au, which further supports the role of Au plasmon in modulating the interface electron transfer. The PEC performance for water splitting was evaluated fully in terms of linear sweep J−V curves, amperometric J−t curves, photoconversion efficiencies, and durability with the calibrated potentials vs reversible hydrogen electrode (RHE; see Supporting Information for detailed calibration method). Figure 4a shows the linear sweep J−V curves of CFC, Au@ CFC, MoS2/CFC, and MoS2/Au@CFC under light illumination, where the obviously upward photocurrent curves can be E

DOI: 10.1021/acssuschemeng.6b02883 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering seen only for MoS2/CFC and MoS2/Au@CFC. This indicates that the photocatalytic activity derives from the grafted MoS2 in nature, excluding the contribution of CFC or Au@CFC substrates. However, although MoS2 is the only photoactive material, the interface between MoS2 and substrate also plays an important role in photogenerated charge separation and transport for PEC catalyses. For example, MoS2/Au@CFC yields a photocurrent of 10 mA cm−2 at 1.23 V, which is about 3.5 times higher than that from MoS2/CFC. Such a significant increase is only due to the embedded Au plasmon, which adjusts the interfacial band structure to be in accordance with the external forward bias mode, making the external and builtin potentials together drive charge transfer and separation at the interface. Figure 4b shows their corresponding J−t curves at a potential of 0.5 V under the chopped light illumination. Being consistent with J−V curves, MoS2/CFC and MoS2/Au@CFC exhibit switched ON/OFF behaviors with pulsed illumination, whereas their substrates have almost no photocurrent response. Note that the dark current in MoS2/Au@CFC is higher than that in MoS2/CFC due to the static Au-reduced SBH, while the obvious increase in photocurrent is because of the Au plasmon switched barrier from n-type SB to a p-type one. In addition, the immediate increase of photocurrent from the OFF to the ON state proves that the present photoanodes are sensitive to light illumination and efficient in the generation and separation of electron−hole pairs and electron transport to the counter electrode for water splitting. Figure 4c shows the estimated photoconversion efficiencies of the aforementioned electrodes for PEC water splitting (see the Supporting Information for detailed calculation method). MoS2/Au@CFC exhibits higher photoconversion efficiencies relative to other electrodes in the overall potential window, moreover, and achieves a maximum efficiency of 1.27% at an applied potential of 0.9 V, which surpasses the various state-of-the-art photoanodes reported recently.46,47 The insets in Figure 4c show the digital photos of the O2 bubbles over MoS2/CFC and MoS2/Au@CFC electrodes during water splitting. And one can see obviously more bubbles overflowed from the surface of the MoS2/Au@ CFC electrode, confirming directly its far superior PEC activity than that of other electrodes. Furthermore, the PEC stability of the MoS2/Au@CFC electrode was also tested in terms of cyclic J−V and long-term J−t curves. As shown in Figure 4d, the J−V curves present almost no significant change even after 3000 cycles, and the J−t curve yields a fairly stable photocurrent density; moreover, the PEC area of the MoS2/Au@CFC electrode still maintains the original morphology after stability testing (Figure S6). On the basis of the above results, the expected mechanism of enhanced PEC activity of the MoS2/Au@CFC photoanode is shown in Figure 5. When the active MoS2 NSAs are illuminated, the electron−hole pairs generate and need to be fast separated before being recombined: the electrons could transfer from the MoS2 through the substrate and external circuit toward the Pt electrode to evolve H2; the leaving holes on the MoS2 surface could be consumed to produce O2. Obviously, a crucial issue of whether the photogenerated electrons in MoS2 could be quickly extracted toward the Pt electrode strongly depends on the electron transfer dynamic at the interface between MoS2 and substrate. The photoexcited Au plasmon reconfigures the interface band structure and thus switches the barrier from n-type SB to a p-type one, leading to a favorable transition from the barrier to driver for interface electron transfer. Combining with the comprehensive analyses

Figure 5. Schematic illustrations of water splitting and interface charge-transfer modulation for the PEC cell with MoS2/Au@CFC and commercial Pt wire as the photoanode and photocathode.

for J−V, EI, and IPCE data, such an optimized electron transfer by interface band engineering is considered to be responsible for the enhanced PEC activity of the MoS2/Au@CFC electrode.

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CONCLUSION In summary, the interface charge-transfer modulating strategy in the functional photoanode made of vertical MoS2 NSAs grown on CFC substrate was proposed and realized for the enhanced PEC water splitting. The embedded Au mediating the interface band structure was exploited to promote the electron-transfer dynamic for the benefits of photogenerated charge separation and electron output. With the optimized interface electron transfer, the MoS2/Au@CFC photoelectrode shows the photocurrent up to 10 mA cm−2 at 1.23 V (vs RHE) and the maximum photoconversion efficiency up to 1.27%, yielding more five-time increases compared with the MoS2/ CFC counterpart. This work develops a highly effective 3D MoS2 photoanode, in which the importance of steering interface charge transfer is identified for PEC cells.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02883. Calculation methods for photon-to-current and photoconversion efficiencies and supplementary characterizations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 0514 87970587. Fax: +86 0514 87975467. E-mail: [email protected]. *Tel.: +86 0514 87970587. Fax: +86 0514 87975467. E-mail: [email protected]. ORCID

Xiaoyong Xu: 0000-0002-7134-2919 Author Contributions §

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acssuschemeng.6b02883 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 11574263, 11374253, and 11104240), the Open Research Fund of State Key Laboratory of Bioelectronics of Southeast University (No. I2015005), the Qing Lan Project of Jiangsu Province, and the Personnel Plan of Yangzhou University. We are grateful to Prof. Haibo Zeng for his helpful discussions and the testing center of Yangzhou university for the technical supports.

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DOI: 10.1021/acssuschemeng.6b02883 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX