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Cite this:J. Mater. Chem. C, 2020, 8, 3017

Vertically conductive MoS

₂

pyramids with a high density of active edge sites for eﬃcient hydrogen evolution†

Qingwei Zhou, âbShaoqiang Su,âPengfei Cheng, âXianbao Hu, â Xingsen Gao, âZhang Zhang *âand Jun-Ming Liu âb

Molybdenum disulfide (MoS2) has attracted tremendous interest as a noble metal-free catalyst for the hydrogen evolution reaction (HER). However, its electrocatalytic performance is currently limited by the density of active sites and poor electrical transport to these sites. Here, we report vertically conductive multi-layered MoS2pyramids with a high density of active edge sites. Conductive atomic force microscopy (c-AFM) reveals the thickness-independent vertical conductivity for the spiral MoS2 pyramids. And the active edge sites on the MoS2pyramids are confirmed through Cu electrochemical deposition. Due to the thickness-independent vertical conductivity and high density of active edge sites, the MoS2 pyramids demonstrate a highly enhanced HER performance compared to that of MoS2triangular flakes. Additionally, the multi-layered spiral pyramid structure can be extended to other transition metal dichalcogenides, and may open up various possibilities for optoelectronic and catalytic nanodevices.

Introduction

As a clean secondary energy source, hydrogen (H2) has been widely considered as one of the most promising alternatives to fossil fuels in the future to relieve the energy and environ- mental crisis.^1,2 To obtain H₂, the electrocatalytic hydrogen evolution reaction (HER) is one of the promising methods.^3,4 The electrocatalytic HER requires eﬃcient electrocatalysts to promote the kinetics of the HER on the electrode. The most active HER catalysts to date have been proven to be noble metal-based materials, but their high cost and scarcity hinder their wide application on a large scale.^5,6 Therefore, noble metal-free catalysts for the HER with comparable performance are highly needed.

Recent advances in two-dimensional (2D) materials have revealed a new class of low-dimensional systems which exhibit extraordinarily novel properties.^7–11Among them, molybdenum disulfide (MoS2) has attracted much attention as a noble metal- free HER catalyst.^12–15It has been identified both theoretically and experimentally that the plane edges of MoS2 are metallic and chemically active for the HER, while the basal plane is

semiconducting and chemically inert.^16–18However, MoS2thermo- dynamically favors the presence of its 2D basal plane and the surrounding edge sites are very limited.^19,20In order to increase its catalytic activity, more active sites have been intentionally introduced by some additional means, such as ultrasound, etching, ion bombardment, annealing, etc.^21–23 Rarely, direct growth of MoS₂with high active site density has been developed.

Moreover, the conductivity of a catalyst is another factor that has a great influence on its electrocatalytic performance.^24,25 However, MoS₂ is a van der Waals material. Generally, the vertical conductivity in a layered stack structure is poor, which critically hinders its catalytic performance. It was found that the current density for the HER decreased sharply with the increase of the number of MoS2layers, since the catalytic HER only occurs on active sites of the outermost layer and the electron needs to overcome the barrier between layers to reach the active sites of the surface layer.²⁶ Thus, although the amount of edge sites increases with the number of layers, the corresponding decrease in conductivity causes attenuated catalytic performance. Therefore, it is highly desirable to design and fabricate novel MoS2nano- structures to achieve both high conductivity and high density of exposed active edge sites for the electrocatalytic HER.

Here, vertically conductive multi-layered MoS₂ pyramids with a high density of active edge sites were realized through a chemical vapor deposition (CVD) method. Conductive atomic force microscopy (c-AFM) verified the thickness-independent vertical conductivity for the multi-layered MoS2pyramids. The catalytically active sites for the HER are confirmed to be the

aInstitute for Advanced Materials and Laboratory of Quantum Engineering and Quantum Materials, South China Academy of Advanced Optoelectronic, Guangzhou 510006, China. E-mail: [email protected]

bLaboratory of Solid State Microstructures and Innovative Center of Advanced Microstructures, Nanjing University, Nanjing, 210093, China

†Electronic supplementary information (ESI) available. See DOI: 10.1039/

c9tc05872a

Received 28th October 2019, Accepted 21st January 2020 DOI: 10.1039/c9tc05872a

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edges of each layer in the pyramidsviaelectrochemical deposition of Cu, and the HER performances of electrodes with MoS2

pyramids are further assessedviaelectrochemical characterizations. Due to the thickness-independent vertical conductivity and high density active sites, the MoS₂ pyramids demonstrate much higher HER performances than MoS₂flakes. Additionally, the multi-layered spiral pyramid structure can be extended to other transition metal dichalcogenides, expecting to open up various possibilities for optoelectronic and catalytic nanodevices.

Results and discussion

The morphologies of the MoS2pyramids were characterized by scanning electron microscopy (SEM). The top-view SEM images with diﬀerent magnifications are presented in Fig. 1a–c, and the inset is a schematic representation of a multi-layered pyramid structure with an atomic stacking MoS2 monolayer of reducing size. Fig. 1b shows a typical MoS₂pyramid with a size around 10mm, defined by the side length of the largest base. The further magnified view of a pyramid top is shown in Fig. 1c. Obviously, the atomically multi-layered structure of the MoS₂looks like a pyramid, which consists of a three-sided slope with a flat top. Each atomic plane is triangular and gradually decreases from base to top. The morphology of the MoS2

pyramid can be controlled by changing the distance between the S source and the growth substrate from 15 to 75 cm (see ESI,†Fig. S2). With farther distance from the S source, a higher density of MoS2pyramids is grown on the substrate under the same CVD conditions. And the MoS2pyramids grown on the

farthest substrate (75 cm) overlapped each other and formed a connected film beneath. Therefore, a proper substrate distance (45 cm) should ensure the high-density and the dispersity of the pyramid structures, which corresponds to the large-scale SEM image of Fig. S2d (ESI†) showing a uniform size distribution of high-density MoS₂pyramids.

The crystalline structure of the MoS₂pyramids was further elucidated by transmission electron microscopy (TEM). Fig. 1d is the top-view TEM image on the center of a multi-layered MoS₂ pyramid. The triangle edges with contrast diﬀerences stack layer- by-layer, with a gradually shrinking edge length to the center top layer as marked with dashed lines. The corresponding high- resolution TEM (HRTEM) image of Fig. 1e clearly shows two parallel edges between three MoS2layers (marked with dashed lines). The periodic atomic arrangement of each MoS2layer can be seen from the HRTEM image. By analyzing the periodic pattern of the lattice fringe image, the corresponding inter- plane spacing is 0.27 nm, which matches up with the (100) facet of MoS₂.²⁷The two plane edges are also parallel to the (100) facet of MoS₂, which is consistent with the orientation of the triangle monolayer MoS₂edges.²⁸The selected area electron diﬀraction (SAED) obtained from the same region displays trigonal structures with a single set of spot pattern features (Fig. 1f), confirm- ing that all layers have the same crystal orientation. The TEM images of two overlapped MoS2pyramids demonstrate one more set of spot patterns compared with the single MoS2pyramid (see ESI,†Fig. S3).

Two typical Raman peaks of MoS2are E¹2gand A1g modes, respectively. The layer numbers of MoS2 play a decisive role in the frequency diﬀerence between the A1g and E¹2gmodes.²⁹

Fig. 1 (a–c) Top-view SEM images with different magnifications for the MoS₂pyramids, inset in Fig. 1a is a schematic representation of a multi-layered pyramid structure for the MoS₂pyramid. (d) TEM image of the center of a typical MoS₂pyramid. (e) HRTEM image of plane edges in the MoS₂pyramid, and (f) the corresponding SAED pattern. (g) Raman spectra and (h) PL spectra of the points 1, 2 and 3 marked in the inset optical image of a MoS₂pyramid.

(i) XRD pattern, and (j) Mo 3d and S 2p XPS spectra of the as-grown MoS₂pyramids.

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Fig. 1g and h are Raman spectra and the corresponding PL spectra of a MoS2 pyramid from base to top as marked as 1, 2 and 3 in the inset optical image. The Raman spectra of the MoS₂pyramid show obvious red shift for the E¹_2gmode and blue shift for the A_1gmode from base to top, and the corresponding PL peaks show red shift, which indicates an increase in the number of layers from base to top with a multi-layered pyramid structure.

As illustrated in Fig. 1i, the XRD pattern of the as-grown MoS2pyramids confirmed their 2H phase. Strong (00l) reflec- tions illustrated the crystallinity and ordered stacking of the 2D layers in the MoS2pyramid.²⁵An estimated average of 23 MoS2

layers in each MoS2pyramid is obtained by Scherrer analysis of the peak broadening in the (002) reflection. Meanwhile, the chemical composition of the MoS2pyramid was investigatedvia XPS. Fig. 1j shows the Mo 3d and S 2p XPS spectra. The peaks at 232.27 eV and 229.11 eV correspond to the Mo 3d3/2and Mo 3d5/2, respectively, representing the Mo⁴⁺state. The small peak at 226.35 eV is identified as the S 2s peak.³⁰The peaks at 161.83 eV and 163.03 eV represent S 2p_3/2and S 2p_1/2, respectively. The XPS results indicate no obvious oxidation in the as-grown MoS₂pyramids.

The electrical properties of the MoS₂pyramid were investigated by c-AFM experiments. The as-grown MoS₂pyramid was transferred to a pre-etched Si substrate by the PS method. The surface oxide layer on the Si substrate was removed to improve the electrical conductance between the MoS2pyramids and the Si substrate. As a reference, the AFM topography image for a monolayer MoS2 flake and the corresponding c-AFM current image are shown in Fig. 2a and b. The thickness of the MoS2

flake is approximately 0.7 nm (see Fig. S4, ESI†), which is consistent with the thickness of monolayer MoS2. The electrical current image was obtained under a constant bias of 0.3 V. The conductivity of the whole MoS₂flake remains almost constant, while some areas are with lower conductivities due to surface contamination in air. Fig. 2c is the AFM topography image of a

typical MoS2pyramid. The measured height is about 13 nm, which corresponds toB20 layers of MoS2(Fig. S4, ESI†). Fig. 2d is the corresponding c-AFM current image of the pyramid, demonstrating the almost constant conductivity of the whole pyramid except for the thinner base with a lower conductivity.

In general, electron transfer between interlayers in the multi-layered MoS₂isviaelectron tunneling, and the interlayer conductivity is several orders of magnitude lower than the in-plane conductivity due to interlayer potential barriers.^31–33 With the addition of every one more layer, the exchange current density could decrease by a factor of 4.47.²⁶ However, we observed that the out-of-plane conductivity remained almost constant in the MoS2pyramids with different layers. Even the layered conductive lines in the pyramid can be distinguished with a higher magnification as shown in the magnified c-AFM current (Fig. 2f), which are consistent with the edges of each MoS2layer in the corresponding AFM topography image (Fig. 2e).

Thus, we assume that the MoS2 pyramid is a spirally layered structure, and the layers are connected together through screw dislocations. In addition, the spiral pyramid morphology can be observed in the top-view SEM images including one left-handed spiral and one right-handed spiral (see Fig. S5, ESI†). In the simplified circuit schematic (inset in Fig. 2d), the spiral morphology of the MoS₂pyramid enables a continuous pathway for charge carrier transfer from top to bottom along the metallic edges, which are connected through the spiral structure.^33–35 Therefore, the MoS2pyramid can not only provide high-density active edge sites for catalytic hydrogen adsorption, but also enable the efficient transfer of charge carriers from edges on each layer to the conductive substrate.

To characterize the hydrogen production activity, the as-grown MoS2pyramids are transferred onto glassy carbon as shown in Fig. 3(a–d), and the catalytically active sites are identified by Cu electrodeposition (Fig. 3(e–h)). The HER process is generally considered to follow the Volmer–Heyrovsky or Volmer–Tafel mechanism. First, the intermediate hydrogen is adsorbed on

Fig. 2 (a and b) AFM topography images and the corresponding c-AFM current images for a monolayer MoS₂flake, and (c and d) for a MoS₂pyramid.

Inset in (d) is a schematic illustration of charge transport along the edges of the MoS₂pyramid. (e) The magnified AFM topography image of the MoS₂ pyramid marked in (c) and (f) the corresponding c-AFM current image. The current images are obtained under a constant bias of 0.3 V.

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the active site (the Volmer step), then discharged on the pre- adsorbed hydrogen atom (the Heyrovsky step) or combines with another intermediate hydrogen to generate H2(the Tafel step).⁵ Therefore, the active sites can be identified by the elements absorbed, which have a binding free energy similar to the intermediate hydrogen. Electrochemical deposition of Cu has been demonstrated to have the same catalytically active site for the HER, which can be used to directly display the HER active site.^22,36

Edges of each layer in the MoS₂ pyramid should be the catalytically active sites for the HER. Hence, with a similar binding free energy to the intermediate hydrogen, the Cu²⁺ions are supposed to be absorbed on the edges in the MoS₂pyramid.

Fig. 3a and b are the top-view SEM images of the dispersed MoS₂pyramids before Cu electrodeposition. The Cu deposition only happened on the areas of MoS₂pyramids after Cu electrodeposition, as shown in Fig. 3e. No Cu deposition could be

recognized on the bare glassy carbon substrate. A typical MoS2

pyramid after Cu electrodeposition is revealed in Fig. 3f.

Clearly, the deposited Cu nanoparticles with a bright contrast uniformly distribute on the whole pyramid, indicating that a high density of active sites is homogeneously located on the MoS2 pyramid. Fig. 3c and d are the top-view SEM images of the overlapped MoS2 pyramids on a glassy carbon substrate, and Fig. 3g and h are the corresponding SEM images after Cu electrodeposition. Since the overlapping growth of the pyramids could block the active sites,³⁷one obvious diﬀerence is some overlapping areas on the pyramids without recognizable traces of Cu. The subsequent electrochemical characterizations also confirmed that the HER performance of the overlapped MoS₂ pyramids is not as good as that of the dispersed MoS₂ pyramids.

The HER activities are investigated in a 0.5 M H₂SO₄ electrolyte with a three-electrode apparatus. The monolayer Fig. 3 Top-view SEM images of (a and b) the dispersed MoS2pyramids and (c and d) the overlapped MoS2pyramids before Cu electrodeposition, and (e–h) the corresponding top-view SEM images after Cu electrodeposition.

Fig. 4 (a) Schematic illustration of edges in multi-layered MoS₂ pyramids as active catalytic sites for the HER. (b) Polarization curves, (c) the corresponding Tafel plots, (d) Nyquist plots and (e) the enlargement of the low resistance area for diﬀerent working electrodes, and inset is the equivalent circuit. (f) Stability test for the dispersed MoS₂pyramids before and after 1000 cycles.

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MoS2 flakes, dispersed MoS2 pyramids and overlapped MoS2

pyramids are transferred onto glassy carbon, respectively, as a working electrode (for the morphologies see Fig. S6, ESI†). As shown in the schematic diagram in Fig. 4a, compared with the monolayer MoS₂flakes, the multi-layered MoS₂ pyramids have much more catalytically active edge sites for intermediate hydrogen to be adsorbed to generate H₂. Fig. 4b shows the polarization curves of diﬀerent working electrodes. The monolayer MoS₂flake has a sluggish current response to the applied potentials, indicating that its HER performance is relatively poor. In contrast, both dispersed and overlapped MoS2 pyramids have much higher HER performances than the monolayer MoS2 flake ones. Specifically, except for the standard Pt electrode, the best performing dispersed MoS2 pyramid ones exhibit a current density of 10 mA cm ² at 393 mV, which is 100 mV lower than that of the monolayer MoS2flakes.

Tafel slope is a useful metric for evaluating catalyst performance and as an indicator of the HER mechanistic reaction processes.^38–40A small Tafel slope is preferred which indicates that the hydrogen generation rate can increase rapidly as the applied voltage increases. As plotted in Fig. 4c, the Tafel slopes of the electrodes with monolayer MoS₂ flakes, and overlapped and dispersed MoS₂ pyramids are 237 mV dec ¹, 118 mV dec ¹and 93 mV dec ¹, respectively. The Tafel slopes of the MoS2 pyramid ones are much less than that of the monolayer MoS2flakes one, due to their much higher densi- ties of active edges in the multi-layered pyramid structure.

Typically, the reduction of water to hydrogen occursviaa two- step mechanism, the first is the Volmer step with a Tafel slope of 120 mV dec ¹, and then the Heyrovsky step (40 mV dec ¹) or the Tafel step (30 mV dec ¹).^38,40 The Tafel slope of the dispersed MoS2pyramids is 93 mV dec ¹, indicating that the most likely mechanism for the HER is the Volmer–Heyrovsky mechanism.⁴¹

Electrochemical impedance spectroscopy (EIS) was also performed to investigate the electrochemical kinetics for the diﬀerent MoS₂electrodes, as shown in Fig. 4d and e. The scatters in Fig. 4e are experimental data and the solid lines are data fittings to an equivalent circuit in the inset. Table S1 in the ESI,†

shows the detailed fitting parameters of each component, in whichRct,dlis correlated to kinetics of the faradaic reaction (the reduction of H⁺to H2) across the double layer between MoS2

and the electrolyte. The smaller value ofRct,dlcorresponds to a faster reaction rate.⁴² As shown in Table S1 (ESI†), Rct,dl

significantly decreases from 1628O for the monolayer MoS2

flakes to 278.6 O and 22.8 O for the overlapped and the dispersed MoS2 pyramids, respectively.Rct,dlof the dispersed MoS2pyramids is approximately 71 times smaller than that of the monolayer MoS₂flakes, having a good correlation with its remarkably enhanced HER performance. Besides, as to the stability of the electrode with dispersed MoS₂pyramids, continuous cyclic voltammetry from 0.2 to 0.6 V (vs.RHE) before and after application of 1000 cycles was carried out in Fig. 4f.

Compared to the first cycle, the cathodic current has almost no loss after 1000 cycles, and little change in morphology after the HER process as shown in the ESI†(Fig. S7).

Conclusions

In summary, we have demonstrated the CVD growth of MoS2

pyramids in a controllable manner. The thickness-independent vertical conductivity of the multi-layered MoS₂ pyramids was confirmed by c-AFM, and the high-density of catalytically active sites on the layer edges was confirmed through Cu electrochemical deposition. Due to the thickness-independent vertical conductivity and high-density of active edge sites, the electrode with high-density dispersed MoS₂pyramids demonstrates a much enhanced HER performance than the one with monolayer MoS2

flakes. Additionally, the spiral pyramid structure can be extended to other 2D materials, and may open up various potential applica- tions for optoelectronic and catalytic nanodevices.

Experiments

Fabrication of MoS₂pyramids

The MoS₂pyramids were synthesized on silica substratesviaa CVD method in a quartz tube furnace under atmospheric pressure (Fig. S1, ESI†). The synthesis process is similar to our previous work;^43,44for details see the ESI.†

Sample characterizations

The morphology and nanostructure were characterized via SEM (ZEISS-Ultra55), TEM (JEOL JEM-2100), XRD (X’Pert PRO, PANalytical) and Raman (a42K864 Renishaw, inVia system).

The XRD patterns were measured with an X-ray wavelength of 0.154056 nm. The Raman spectra were measured with an excitation laser of 532 nm wavelength, and the laser power was set to 10%. The chemical bonding state and compositions of the samples were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi). c-AFM (Cyphert, Asylum Research) was used to characterize the surface morphologies and the electrical properties of the MoS₂pyramids.

Electrochemical measurements

The electrochemical measurements were carried out with a three-electrode apparatus in 0.5 M H2SO4 using an electrochemical station (CH Instrument, CHI660E). A silver chloride electrode and a carbon rod were used as reference and counter electrodes, respectively. The potential shift of the reference electrode was calibrated to be 0.21 Vvs.RHE. The polarization curves were obtained using linear sweep voltammetry (LSV) from 0 to 0.6 V (vs.RHE) with a scan rate of 50 mV s ¹. The curves were not corrected for any uncompensated resistance loss or any other extrinsic loss. Electrochemical impedance spectroscopy (EIS) was performed when the operating electrode was biased at 450 mV (vs.RHE), while the frequency was swept from 1 MHz to 1 Hz with an amplitude of 5 mV. The cyclic voltammetry was performed in the range from 0.2 to 0.6 V (vs.RHE) at a scan rate of 50 mV. The electrochemical deposition of Cu was performed in a 1 M CuSO4solution by LSV from 0 to 0.6 V (vs.RHE) with a scan rate of 50 mV s ¹.

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Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge financial support from the National Key Research Projects of China (2016YFA0300101, and 2015CB654602), the National Natural Science Foundation of China (51431006, and 51721001), the National Natural Science Foundation of Guangdong, China (2014A030313434, and 2016A030308019), the Pearl River Nova Program of Guangzhou (201506010019), and the Key Projects of Applied Special Funds of Guangdong Science and Technology Project (2015B090927006).

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