Vertically aligned oxygen-doped molybdenum disulfide nanosheets grown on carbon cloth realizing robust hydrogen evolution reaction

Abstract

The catalytic activity of an electrocatalyst is determined by the density of active sites and the electric conductivity, namely, the density of electrically connected active sites. In this work, elemental incorporation, disorder engineering and material hybridization were applied to molybdenum disulfide (MoS₂) simultaneously to realize a high-level synergistic optimization for both active sites and electric conductivity, achieving highly efficient hydrogen-evolving performance finally. Benefitting from the synergistic optimization, the vertically aligned oxygen-doped MoS₂/carbon cloth catalyst shows an ultralow onset overpotential of 90 mV to initiate the HER process, and an extremely high catalytic current of 225 mA cm⁻² was measured at an overpotential of 300 mV. Not only that, superior stability was also achieved, making this novel catalyst promising for practical applications such as electrolytic water splitting and a co-catalyst for photocatalytic/photoelectrochemical hydrogen production. The synergistic optimization strategy reported in this work would shed light on the systematic design of highly efficient electrocatalysts in the future.

---

Introduction

Exploring efficient catalysts for the electrochemical hydrogen evolution reaction (HER) has become more and more imperative owing to the urgent demand for clean energy to face the emerging energy crisis and subsequent environmental issues.^1–4 To date, increasing the density of active sites and improving the electric conductivity of hydrogen-evolving catalysts have been considered as two efficient pathways to optimize the electrocatalytic HER activity, which can also be regarded integratedly as increasing the electrically connected active sites that are effective to the HER.^5–9 Thus, developing a systematic strategy for synergistic structural and electronic modulations is in urgent demand to achieve excellent performance towards hydrogen-evolving electrocatalysis, which stimulates the research passion of chemists and materials scientists. Among a large variety of as-explored HER catalysts, molybdenum disulfide (MoS₂) has been considered as one of the most classical candidates to replace highly efficient noble metal catalysts due to its low price, earth abundance, high efficiency and definite catalytic mechanism, for which plenty of strategies have been readily involved to improve its catalytic activity, including elemental doping, crystal facet engineering, thickness control, interface engineering and hybridization with highly conductive materials.^10–28 While for further improving the HER activity of MoS₂-based catalysts, synergistic optimization with beneficial active site density as well as fast electron transport is urgently required. Unfortunately, the exposure of highly active edge sites of semiconducting MoS₂ often results in the damage or degradation of the two-dimensional electron conjugation system that guarantees the intralayered electron transport, thus leading to a lower density of “effective” active sites, namely, the electrically connected active sites, that can catalyze the HER process. Owing to such a contradiction between the crystal structure and the electronic structure of MoS₂, optimizing the relationship between the active sites and the electric conductivity synergistically still remains a challenge, and would be a key route to achieve the enhancement of HER catalytic efficiency.

During the past few years, highly conductive carbonaceous materials have been considered as a kind of effective support to load nanosized catalysts with high exposure of active sites.^29–34 Benefitting from the freestanding skeleton with high electric conductivity, rich active sites can be effectively involved in catalytic processes, finally leading to a significant enhancement of the catalytic efficiency. Following this idea, lots of efforts have been made to realize the enhancement of the electric conductivity of catalysts by introducing carbonaceous supports, and thus boost the catalytic reactions.^29–34 However, for these hybrid catalysts, simultaneously enriching the thermodynamically unstable active edge sites and improving the intrinsic conductivity of the catalytically active MoS₂ component are rarely reported. Recently, the authors have explored the oxygen-incorporated MoS₂ ultrathin nanosheets (OMS) with controllable disorder engineering, in which the oxygen dopants and the unique disordered structure can lead to the effective synergistic optimization of both active sites and electric conductivity.²² Unfortunately, the semiconducting nature of the MoS₂ nanosheet catalyst still significantly impedes the electron transport during the HER process. To overcome this obstacle, we propose that highly conductive carbon cloth (CC) could be involved as a freestanding support to act as an electron “superhighway” to facilitate the electron transport and further boost HER catalysis. In this work, a highly efficient and freestanding MoS₂-based electrode was prepared by growing structurally optimized oxygen-incorporated MoS₂ ultrathin nanosheets on highly conductive carbon cloth. The optimized OMS possesses balanced active sites as well as intralayered electric conductivity, while the carbon cloth can efficiently transfer the electrons during the HER catalysis. Benefitting from the synergistic optimization strategy, the oxygen-incorporated MoS₂ ultrathin nanosheets/carbon cloth (OMS/CC) hybrid catalyst shows tremendous enhancement in HER performance, which achieves an ultralow onset overpotential of 90 mV and a small Tafel slope of 58 mV per decade, accompanied by a high cathodic current density (225 mA cm⁻² at an overpotential of 300 mV). Furthermore, a superior stability was also achieved, for which negligible degradation of catalytic current was observed even after 2000 cyclic voltammetry (CV) cycles or 12-hour continuous HER operation, making the hybridized catalyst a promising alternative for noble metals in practical water splitting. The synergistic optimization strategy in this work will shed light on the design of highly active catalysts for electrocatalysis in energy-related fields.

Experimental

Synthesis of OMS/CC

Typically, 1/7 mmol (NH₄)₆Mo₇O₂₄·4H₂O (177 mg, 1 mmol Mo) and 5 mmol thiourea (381 mg) were dissolved in 40 mL distilled water under vigorous stirring to form a homogeneous solution. After stirring for 30 min, the solution was transferred into a 50 mL Teflon-lined stainless steel autoclave, a piece of hydrophilic carbon cloth with a size of 1 × 3 cm was placed in, and maintained at 180 °C for 18 h. Then the reaction system was cooled down to room temperature naturally. The as-obtained products were rinsed with distilled water and ethanol repeatedly, and dried at 60 °C under vacuum overnight. The OMS/CC catalysts with different disorder degrees were synthesized by controlling the synthesis temperature ranging from 140 °C to 200 °C.

Characterization

X-ray diffraction (XRD) was performed on a Philips X'Pert Pro Super diffractometer with Cu K_α radiation (λ = 1.54178 Å). The scanning electron microscopy (SEM) images were taken on a JEOL JSM-6700F SEM. Transmission electron microscopy (TEM) was carried out on a JEM-2100F field emission electron microscope at an acceleration voltage of 200 kV. The high-resolution TEM (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and corresponding energy-dispersive spectroscopy (EDS) mapping analyses were performed on a JEOL JEM-ARF200F TEM/STEM with a spherical aberration corrector. X-ray photoelectron spectra (XPS) were acquired on an ESCALAB MK II with Mg K_α as the excitation source.

Electrochemical measurements

All the electrochemical measurements were performed in a three-electrode system on an electrochemical workstation (CHI660D). Typically, linear sweep voltammetry with a scan rate of 5 mV s⁻¹ was conducted in 0.5 M H₂SO₄ (sparged with pure H₂) by using the OMS/CC freestanding catalyst as the working electrode, the Ag/AgCl (in 3 M KCl solution) electrode as the reference electrode, and a graphite rod (Alfa Aesar, 99.9995%) as the counter electrode. For the measurements of powdery catalyst, 4 mg OMS and 30 μL Nafion solution (Sigma Aldrich, 5 wt%) were dispersed in 1 mL water–isopropanol solution with a volume ratio of 3 : 1 by sonicating for 1 h to form a homogeneous ink. Then 5 μL of the ink (containing 20 μg of catalyst) was loaded onto a glassy carbon electrode with 3 mm diameter (loading 0.285 mg cm⁻²) as the working electrode. Cyclic voltammetry (CV) was conducted between −0.3 and 0.2 V vs. RHE at 50 mV s⁻¹ to investigate the cycling stability. The Nyquist plots were obtained with frequencies ranging from 100 kHz to 0.1 Hz at an overpotential of 250 mV. All the potentials were calibrated to a reversible hydrogen electrode (RHE).

Results and discussion

The synergistically optimized catalyst was prepared via a one-step route of in situ growth of oxygen-incorporated MoS₂ ultrathin nanosheets on carbon cloth with hydrophilic characteristics (see the Experimental section for details). Of note is that unlike the synthesis of freestanding oxygen-incorporated MoS₂ nanosheets, the concentration of reactants in synthesizing OMS/CC nanohybrids is much lower to avoid the accumulation of OMS and obtain a vertically aligned morphology. At an optimized temperature of 180 °C, OMS with optimized disorder engineering can be intimately grown on a CC support. As shown in the inset of Fig. 1A, the as-obtained OMS/CC catalyst is of freestanding nature, which can be directly used as an electrode for electrochemical water splitting. The coadjacent carbon fibers offer a robust and porous support for the OMS, and the high electric conductivity of the carbon cloth would provide an ideal electron transport pathway for the electrocatalytic HER process. In order to investigate the structural information of the product, X-ray diffraction (XRD) analysis was applied, from which the as-obtained XRD pattern indicates that the product is of high crystallinity (Fig. 1A). Notably, the XRD pattern is different from the standard pattern of 2H-MoS₂ (JCPDS card no. 73-1508), of which two new peaks emerge in the low-angle region with the corresponding d spacings of 9.50 Å and 4.75 Å, respectively. The diploid relationship of the d spacings confirms the formation of a lamellar structure with an enlarged interlayer spacing of 9.5 Å, which matches well with the value of disorder-engineered MoS₂ nanosheets.²² Furthermore, two broadened peaks in the high-angle region (32° and 57°) can be well indexed to the (100) and (110) planes of the pristine 2H-MoS₂, indicating a similar atomic arrangement along the basal planes. It is noteworthy that the absence of high-index diffraction peaks reveals the short-range disordering characteristics of the hybrid catalyst, which can provide more active sites for HER catalysis. Scanning electron microscopy (SEM) was utilized to investigate the morphological information of the hybridized product. As can be seen from Fig. 1B, the product is a robust intertexture wove by carbon fibers with interconnected microstructure. The interconnected carbon-based skeleton can ensure a robust and highly conductive feature, which enables the product to be used as a freestanding hydrogen-evolving electrode without binders. High resolution SEM images revealed that the oxygen incorporated MoS₂ nanosheets are of high density, and in particular, are vertically grown on the carbon fibers (Fig. 1B and C). With the consideration of the fact that the active sites of MoS₂ are located at the edges, abundant active edge sites can be preferentially exposed benefitting from this unique morphology, thus facilitating the electrochemical hydrogen evolution reaction.^35–37

Fig. 1 (A) XRD pattern of the OMS/CC freestanding catalyst accompanied by the standard pattern of 2H-MoS₂. Inset shows a digital photograph of the OMS/CC freestanding catalyst. (B) Low-resolution SEM image showing the cross-linked feature of the hybrid catalyst. (C–D) SEM images in high resolution clearly revealed the surface structure of OMS/CC with vertically aligned ultrathin nanosheets.

Transmission electron microscopy (TEM) was utilized to further investigate the morphological information of the ultrathin nanosheets grown on a CC support. As depicted in Fig. 2A, the nanosheets stripped from the carbon cloth are in a typical nanosheet morphology, with a uniform size of approximately 200 nm. Of note is that the TEM image highlights the ultrathin nature of the nanosheets, which is beneficial to the electrochemical processes due to the feasible ion permeation or gas release.^38–41 The high-resolution TEM (HRTEM) image of an individual ultrathin nanosheet highlights the disordered structure in the basal plane (Fig. 2B). Detailed analyses suggest that the disordered structure of the nanosheets is built by tiny hexagonal MoS₂ nanodomains with a quasi-periodic alignment. This unique alignment can ensure the relatively high electron transport along the nanosheets due to the partial maintenance of the two-dimensional (2D) electron conjugation system. The inset of Fig. 2B shows the fast Fourier transform (FFT) pattern transformed from the HRTEM image, further confirming the quasi-periodic hexagonal alignment of nanodomains with the disordered structure. The disordered structure with quasi-periodic features can not only provide abundant unsaturated sulfur atoms to catalyze the HER, but also facilitates the electron transport on the nanoscale to further enhance the HER activity. A cross-sectional HRTEM image was obtained to investigate the thickness of the OMS on the carbon cloth. As shown in Fig. 2C, a typical layered structure can be clearly observed from the crystal fringe of OMS, from which a uniform interplanar spacing of 0.95 nm can be identified, which agrees well with the result from XRD analysis. The enlarged interplanar spacing compared with the traditional 2H-MoS₂ may arise from the unique disordered structure which significantly reduces the van der Waals interaction between layers. In addition, the thickness of the nanosheet on the CC skeleton is ∼5 nm, which corresponds to 5 MoS₂ layers, indicating the ultrathin feature of the nanosheets.

Fig. 2 (A) TEM image of the oxygen-incorporated MoS₂ ultrathin nanosheets stripped from the carbon cloth support. (B) HRTEM image of the OMS, verifying the disordered structure. Inset depicts the FFT pattern with the feature of six individual diffraction arcs, confirming the quasi-periodic alignment of the MoS₂ nanodomains. (C) Cross-sectional HRTEM image of the curled fringe of an OMS nanosheet giving the interlayer spacing of 0.95 nm. (D) HAADF-STEM image of an individual oxygen-incorporated MoS₂ ultrathin nanosheet grown on carbon cloth. (E–G) Corresponding EDS mapping images indicate that molybdenum, sulfur and oxygen are homogeneously distributed on the whole nanosheet.

Of note is that low-temperature synthesis is not only prone to lead to the formation of the disordered structure, but is also responsible for the possible elemental incorporation owing to the incomplete reaction between reactants. In order to survey the chemical composition of the product, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and corresponding energy-dispersive spectroscopy (EDS) mapping analyses were performed (Fig. 2D–G). As can be seen, the nanosheets are comprised of molybdenum, sulfur and oxygen, and the uniform distribution of these three elements clearly confirms the oxygen incorporation in MoS₂ nanosheets. The atomic ratio of Mo : S is identified to be 1 : 2.06, which can be attributed to the highly disordered structure that can expose additional unsaturated sulfur atoms. The unsaturated sulfur atoms can act as the active sites for HER catalysis. Furthermore, the oxygen incorporation in this hybrid catalyst also benefits the HER performance. As demonstrated in previous literature, both theoretical and experimental results indicated that oxygen incorporation in MoS₂ can lead to the decease of bandgap and achieve a fast electron transport to boost HER catalysis.²² Hence, oxygen incorporation in the OMS/CC hybrid catalyst can facilitate the intralayered electron transport. From an universal viewpoint, when combining the beneficial elemental incorporation with the high electric conductivity gained from the carbonaceous electron transport “superhighway”, the overall electric conductivity can be maximally optimized, thus leading to the highest density of “effective” active sites for the HER. Furthermore, the disordered structure provides abundant active sites that can be electrically connected, finally achieving a maximum optimization of active sites and electric conductivity synergistically.

X-ray photoelectron spectroscopy (XPS) was performed to further investigate the chemical state of the hybrid catalyst. As shown in Fig. 3A, two characteristic peaks located at 229.1 eV and 232.2 eV arising from Mo 3d_5/2 and Mo 3d_3/2 orbitals can be identified, suggesting the dominance of Mo^IV in the OMS/CC hybrid catalyst.²⁰ Besides, the S 2p region (Fig. 3B) exhibits a single doublet with the 2p_3/2 peak at 161.7 eV, which is consistent with the −2 oxidation state of sulfur,²⁰ thus confirming the MoS₂ structure. Detailed compositional analysis reveals that the atomic ratio of Mo : S is 1 : 2.05, which is consistent with the result from EDS analysis. It is worth noting that, although the oxygen atoms doped in the MoS₂ lattice seem to replace the sulfur atoms which would cause the increase of Mo : S ratio, the highly disordered structure can offer enough edge sites that can expose additional sulfur atoms, leading to rich sulfur atoms making the Mo : S ratio exceeding the stoichiometric atom ratio of 1 : 2 and thus providing abundant active sulfur sites for the HER. In addition, the oxygen incorporation in MoS₂ was also identified. As shown in Fig. 3C, the signal of oxygen can be divided into two independent peaks. In detail, the O 1s peak located at 530.1 eV corresponds to the binding energy of oxygen in Mo^IV–O bonds,⁴² thus verifying the successful oxygen incorporation rather than surface oxidation, while the peak located at 531.9 eV can be attributed to the adsorbed water molecules.²² Hence, the oxygen-incorporated MoS₂ nanosheets on the carbon cloth were identified.

Fig. 3 XPS data of the OMS/CC hybrid catalyst. (A) The binding energies of molybdenum show that no oxidation of Mo^IV occurs. (B) The spectra of sulfur indicate that the chemical state is −2. (C) The binding energies of oxygen suggest that two kinds of chemical states exist in the sample, which correspond to the oxygen in Mo^IV–O bonds and adsorbed water molecules.

In order to verify our hypothesis on the synergistic optimization strategy, electrochemical measurements were carried out in 0.5 M H₂SO₄ solution. As shown in Fig. 4A, the polarization curve of OMS/CC suggests an extremely high HER activity, from which an ultralow onset overpotential of 90 mV can be determined, which is the best record for MoS₂-based HER catalysts up to now^{47,51,56–62} and also close to the theoretical value.¹⁹ As a sharp comparison, the OMS without hybridization of carbon cloth shows an onset overpotential of 120 mV, confirming the significant role of the conductive carbon skeleton in HER catalysis. Of note is that the ultralow onset overpotential also takes precedence over a variety of MoS₂-based HER catalysts,^{32,34,43–55} including many newly reported hybrid catalysts coupled with carbonaceous materials (Fig. 5), suggesting that the ultrahigh HER activity of OMS/CC is not only determined by the hybridization with highly conductive materials, but also benefits from the excellent intrinsic catalytic behavior of the OMS that guarantees the fast intralayered conductivity as well as the abundant active sites.

Fig. 4 (A) Polarization curves of OMS/CC, OMS, and bare carbon cloth. Inset: Polarization curves near the HER onset. A low overpotential (120 mV) is observed to drive a 10 mA cm⁻² catalytic current for the OMS/CC, which is much lower than that of the OMS catalyst (193 mV). (B) Corresponding Tafel plots of the OMS/CC hybrid catalyst, OMS catalyst, the bare carbon cloth as well as the 5% Pt/C benchmark catalyst.

Fig. 5 Performance comparison of various MoS₂-based HER catalysts, including the OMS/CC hybrid catalyst in this work, MoS₂/carbon nanofibers (CNF),⁴³ amorphous MoS_x/carbon paper (CP),⁴⁴ MoS₂ nanosheets/carbon fiber cloth (CFC),⁴⁵ MoS₂/MoO₂ composites,⁴⁶ 1T-MoS₂ nanosheets,⁴⁷ MoS₂ supported on reduced graphene oxide-modified carbon nanotube/polyimide (PI/CNT-rGO) film,⁴⁸ MoS₂ nanoparticles/graphene,³² amorphous MoS_x/graphene/Ni foam,³⁴ MoSSe alloy nanoflakes,⁴⁹ MoS₂/graphite hybrids,⁵⁰ core–shell MoO₃/MoS₂ nanowires,⁵¹ monolayer MoS₂ quantum dots,⁵² MoS₂ nanosheets/CC,⁵³ MoS₂ nanosheets/rGO,⁵⁴ and MoS₂ nanodots.⁵⁵

Besides, after the hybridization with carbon cloth, the cathodic current density is remarkably improved. An ultrahigh current density of 225 mA cm⁻² was detected at an overpotential of 300 mV, whereas only 124 mA cm⁻² of catalytic current was gained for bare OMS without hybridization, showing an 1.8 fold enhancement in HER activity. The overpotential required to drive a 10 mA cm⁻² catalytic current is an important criterion to evaluate an advanced HER electrocatalyst, which could be regarded as overpotential to trigger an obvious HER catalysis. As shown in the inset of Fig. 4A, the enlarged LSV curves near the onset region demonstrate that the OMS/CC hybrid catalyst requires as low as 120 mV to drive a 10 mA cm⁻² cathodic current (with extraction of the background current); while for the bare OMS catalyst, the overpotential required to achieve the same current density is 193 mV, confirming the significant improvement of HER activity via material hybridization. Of note is that the current density of the OMS/CC hybrid catalyst before reaching the HER onset is larger than those materials without hybridization, which can be attributed to the large double-layer capacitance of the carbon cloth. Moreover, the catalytic current density at a given potential of the OMS/CC hybrid catalyst is also sound when comparing with recent reported results (Fig. 5). At an overpotential of 200 mV, the current density of the OMS/CC hybrid catalyst shows an ultrahigh current density of 78 mA cm⁻², which is 1.8–52 times higher than a large variety of MoS₂-based HER catalysts, demonstrating the superior HER activity of the optimized hybrid catalyst. Tafel plots were obtained to investigate the kinetic information during HER catalysis. As can be seen from Fig. 4B, the Tafel slope of OMS/CC of 58 mV per decade is comparable to that of the bare OMS, suggesting the same catalytic mechanism of the HER process. Of note is that this value is among the best records of MoS₂-based catalysts, demonstrating the facile kinetics during HER catalysis.^{47,51,56–62}

The remarkable enhancement of cathodic current can be attributed to the high electric conductivity and abundant active sites, which guarantees the fast electron transport, and finally, enriches the “effective” active sites. In order to understand the role of the MoS₂ structure in HER activity in the OMS/CC hybrid system, the catalysts synthesized at various temperatures are tested, which possess different effective active site densities, that is, the lower the temperature conditions, the more the number of unsaturated sulfur atoms (active sites). As demonstrated in Fig. 6A, the catalyst prepared at 180 °C displays the highest HER activity, which may be attributed to its optimized active sites and intrinsic conductivity. Nyquist plots were applied to survey the electric conductivity of the OMS/CC hybrid catalyst and bare OMS (Fig. 6B), from which one can observe that after hybridizing with CC, the resistivity of the catalyst decreases obviously, giving direct and solid evidence of the fact that the interconnected carbon fibers can facilitate the electron transport, enrich the density of the electrically connected active sites, and finally enhance the HER activity.

Fig. 6 (A) Polarization curves of OMS/CC synthesized at various temperatures. (B) Nyquist plots indicating the enhanced conductivity after hybridization with carbon cloth.

Except for the HER activity, long-period stability is also a key criterion to evaluate an electrocatalyst. In order to investigate the stability of OMS/CC, a long-term CV test was conducted. As shown in Fig. 7A, only slight degradation of the catalytic current density can be revealed even after 2000 CV cycles. A high retention of 97% in the current density at an overpotential of 300 mV can be achieved, suggesting the excellent operational stability of the as-prepared OMS/CC hybrid catalyst. For commercial utilization in electrolytic water splitting, continuous operation under a static overpotential is more meaningful. As demonstrated in Fig. 7B, the cathodic current density under a static overpotential of 200 mV shows negligible changes even after 12-hour continuous operation, giving solid evidence of the superior electrochemical stability of the OMS/CC hybrid catalyst. The good operational stability of OMS/CC may arise from the intimate hybridization between OMS and CC. The superior HER activity combined with the excellent stability and freestanding feature makes the novel OMS/CC hybrid catalyst a promising candidate of noble metals for practical water splitting.

Fig. 7 Stability tests of the OMS/CC hybrid catalyst. (A) IV curves of OMS/CC for the first cycle and after 2000 cycles, respectively. (B) Chronoamperometry data (j–t) of the OMS/CC hybrid catalyst at a constant overpotential of 200 mV, demonstrating the excellent durability of the catalysts.

Conclusions

In this work, oxygen-doped MoS₂ ultrathin nanosheets with a high density of active sites were vertically grown on a highly conductive carbon cloth skeleton, and a synergistic optimization of active sites and electric conductivity was successfully achieved, realizing remarkably improved HER performance. In order to enrich the active sites, disorder engineering of MoS₂ nanosheets was performed, while for enhancing the electric conductivity, oxygen incorporation as well as hybridization with carbonaceous materials were carried out, finally realizing the maximum optimization of the HER catalyst. After the synergistic optimization, the vertically aligned oxygen-doped MoS₂/carbon cloth catalyst exhibits an ultralow onset overpotential and an ultrahigh catalytic current density accompanied by superior operational stability, making it the best among the MoS₂-based HER catalysts to date. The unique synergistic optimization strategy reported in this work can not only be used to improve the performance of various electrocatalysts, but can also provide the opportunity for future design of novel catalysts.

Acknowledgements

This work was financially supported by the 973 Program (2013CB933800), the National Natural Science Foundation of China (21501112, 21535004, 21227005, 21390411), and the Natural Science Foundation of Shandong Province (ZR2014BQ007).

Notes and references

1. M. S. Dresselhaus and I. L. Thomas, Nature, 2001, 414, 332–337.
2. J. K. Norskov and C. H. Christensen, Science, 2006, 312, 1322–1323.
3. J. A. Turner, Science, 2004, 305, 972–974.
4. C. G. Morales-Guio, L.-A. Stern and X. Hu, Chem. Soc. Rev., 2014, 43, 6555–6569.
5. J. Xie and Y. Xie, Chem. – Eur. J., 2016, 22, 3588–3598.
6. H. Li, C. Tsai, A. L. Koh, L. Cai, A. W. Contryman, A. H. Fragapane, J. Zhao, H. S. Han, H. C. Manoharan, F. Abild-Pedersen, J. K. Norskov and X. Zheng, Nat. Mater., 2016, 15, 48–53.
7. J. Xie and Y. Xie, ChemCatChem, 2015, 7, 2568–2580.
8. Y. Yan, B. Xia, X. Ge, Z. Liu, J.-Y. Wang and X. Wang, ACS Appl. Mater. Interfaces, 2013, 5, 12794–12798.
9. J. Xie, S. Li, X. Zhang, J. Zhang, R. Wang, H. Zhang, B. Pan and Y. Xie, Chem. Sci., 2014, 5, 4615–4620.
10. R. Ye, P. del Angel-Vicente, Y. Liu, M. J. Arellano-Jimenez, Z. Peng, T. Wang, Y. Li, B. I. Yakobson, S.-H. Wei, M. J. Yacaman and J. M. Tour, Adv. Mater., 2016, 28, 1427–1432.
11. F. Lai, Y.-E. Miao, Y. Huang, Y. Zhang and T. Liu, ACS Appl. Mater. Interfaces, 2016, 8, 3558–3566.
12. A. W. Maijenburg, M. Regis, A. N. Hattori, H. Tanaka, K.-S. Choi and J. E. ten Elshof, ACS Appl. Mater. Interfaces, 2014, 6, 2003–2010.
13. Z. Xing, Q. Liu, A. M. Asiri and X. Sun, Adv. Mater., 2014, 26, 5702–5707.
14. N. Liu, Y. Guo, X. Yang, H. Lin, L. Yang, Z. Shi, Z. Zhong, S. Wang, Y. Tang and Q. Gao, ACS Appl. Mater. Interfaces, 2015, 7, 23741–23749.
15. J. Xie, H. Zhang, S. Li, R. Wang, X. Sun, M. Zhou, J. Zhou, X. W. Lou and Y. Xie, Adv. Mater., 2013, 25, 5807–5813.
16. W. Cui, Q. Liu, Z. Xing, A. M. Asiri, K. A. Alamry and X. Sun, Appl. Catal., B, 2015, 164, 144–150.
17. W. Cui, N. Cheng, Q. Liu, C. Ge, A. M. Asiri and X. Sun, ACS Catal., 2014, 4, 2658–2661.
18. X. Dai, K. Du, Z. Li, M. Liu, Y. Ma, H. Sun, X. Zhang and Y. Yang, ACS Appl. Mater. Interfaces, 2015, 7, 27242–27253.
19. B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jørgensen, J. H. Nielsen, S. Horch, I. Chorkendorff and J. K. Nørskov, J. Am. Chem. Soc., 2005, 127, 5308–5309.
20. J. Kibsgaard, Z. Chen, B. N. Reinecke and T. F. Jaramillo, Nat. Mater., 2012, 11, 963–969.
21. T. F. Jaramillo, K. P. Jørgensen, J. Bonde, J. H. Nielsen, S. Horch and I. Chorkendorff, Science, 2007, 317, 100–102.
22. J. Xie, J. Zhang, S. Li, F. Grote, X. Zhang, H. Zhang, R. Wang, Y. Lei, B. Pan and Y. Xie, J. Am. Chem. Soc., 2013, 135, 17881–17888.
23. X. Sun, J. Dai, Y. Guo, C. Wu, F. Hu, J. Zhao, X. Zeng and Y. Xie, Nanoscale, 2014, 6, 8359–8367.
24. Y. Yang, H. Fei, G. Ruan, C. Xiang and J. M. Tour, Adv. Mater., 2014, 26, 8163–8168.
25. Z. Lu, W. Zhu, X. Yu, H. Zhang, Y. Li, X. Sun, X. Wang, H. Wang, J. Wang, J. Luo, X. Lei and L. Jiang, Adv. Mater., 2014, 26, 2683–2687.
26. Y. Tan, P. Liu, L. Chen, W. Cong, Y. Ito, J. Han, X. Guo, Z. Tang, T. Fujita, A. Hirata and M. W. Chen, Adv. Mater., 2014, 26, 8023–8028.
27. Y. Yan, B. Xia, X. Qi, H. Wang, R. Xu, J.-Y. Wang, H. Zhang and X. Wang, Chem. Commun., 2013, 49, 4884–4886.
28. Y. Yan, X. Ge, Z. Liu, J.-Y. Wang, J.-M. Lee and X. Wang, Nanoscale, 2013, 5, 7768–7771.
29. Y. Zhao, X. Xie, J. Zhang, H. Liu, H.-J. Ahn, K. Sun and G. Wang, Chem. – Eur. J., 2015, 21, 15908–15913.
30. P. Li, Z. Yang, J. Shen, H. Nie, Q. Cai, L. Li, M. Ge, C. Gu, X. a. Chen, K. Yang, L. Zhang, Y. Chen and S. Huang, ACS Appl. Mater. Interfaces, 2016, 8, 3543–3550.
31. X.-Y. Yu, H. Hu, Y. Wang, H. Chen and X. W. Lou, Angew. Chem., Int. Ed., 2015, 54, 7395–7398.
32. Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong and H. Dai, J. Am. Chem. Soc., 2011, 133, 7296–7299.
33. D. J. Li, U. N. Maiti, J. Lim, D. S. Choi, W. J. Lee, Y. Oh, G. Y. Lee and S. O. Kim, Nano Lett., 2014, 14, 1228–1233.
34. Y.-H. Chang, C.-T. Lin, T.-Y. Chen, C.-L. Hsu, Y.-H. Lee, W. Zhang, K.-H. Wei and L.-J. Li, Adv. Mater., 2013, 25, 756–760.
35. H. Wang, D. Kong, P. Johanes, J. J. Cha, G. Zheng, K. Yan, N. Liu and Y. Cui, Nano Lett., 2013, 13, 3426–3433.
36. D. Kong, H. Wang, J. J. Cha, M. Pasta, K. J. Koski, J. Yao and Y. Cui, Nano Lett., 2013, 13, 1341–1347.
37. H. Wang, Z. Lu, S. Xu, D. Kong, J. J. Cha, G. Zheng, P.-C. Hsu, K. Yan, D. Bradshaw, F. B. Prinz and Y. Cui, Proc. Natl. Acad. Sci. U. S. A., 2013, 110, 19701–19706.
38. M. Xu, T. Liang, M. Shi and H. Chen, Chem. Rev., 2013, 113, 3766–3798.
39. X. Zhang and Y. Xie, Chem. Soc. Rev., 2013, 42, 8187–8199.
40. J. Xie, X. Sun, N. Zhang, K. Xu, M. Zhou and Y. Xie, Nano Energy, 2013, 2, 65–74.
41. J. Xie, R. Wang, J. Bao, X. Zhang, H. Zhang, S. Li and Y. Xie, Inorg. Chem. Front., 2014, 1, 751–756.
42. Y. Sun, X. Hu, W. Luo and Y. Huang, ACS Nano, 2011, 5, 7100–7107.
43. X. Guo, G.-l. Cao, F. Ding, X. Li, S. Zhen, Y.-f. Xue, Y.-m. Yan, T. Liu and K.-n. Sun, J. Mater. Chem. A, 2015, 3, 5041–5046.
44. A. B. Laursen, P. C. K. Vesborg and I. Chorkendorff, Chem. Commun., 2013, 49, 4965–4967.
45. H. Yu, X. Yu, Y. Chen, S. Zhang, P. Gao and C. Li, Nanoscale, 2015, 7, 8731–8738.
46. L. Yang, W. Zhou, D. Hou, K. Zhou, G. Li, Z. Tang, L. Li and S. Chen, Nanoscale, 2015, 7, 5203–5208.
47. D. Voiry, M. Salehi, R. Silva, T. Fujita, M. Chen, T. Asefa, V. B. Shenoy, G. Eda and M. Chhowalla, Nano Lett., 2013, 13, 6222–6227.
48. Y. Jiang, X. Li, S. Yu, L. Jia, X. Zhao and C. Wang, Adv. Funct. Mater., 2015, 25, 2693–2700.
49. Q. Gong, L. Cheng, C. Liu, M. Zhang, Q. Feng, H. Ye, M. Zeng, L. Xie, Z. Liu and Y. Li, ACS Catal., 2015, 5, 2213–2219.
50. X. Zheng, J. Xu, K. Yan, H. Wang, Z. Wang and S. Yang, Chem. Mater., 2014, 26, 2344–2353.
51. Z. Chen, D. Cummins, B. N. Reinecke, E. Clark, M. K. Sunkara and T. F. Jaramillo, Nano Lett., 2011, 11, 4168–4175.
52. X. Ren, L. Pang, Y. Zhang, X. Ren, H. Fan and S. Liu, J. Mater. Chem. A, 2015, 3, 10693–10697.
53. Y. Yan, B. Xia, N. Li, Z. Xu, A. Fisher and X. Wang, J. Mater. Chem. A, 2015, 3, 131–135.
54. Z. H. Deng, L. Li, W. Ding, K. Xiong and Z. D. Wei, Chem. Commun., 2015, 51, 1893–1896.
55. J. Benson, M. Li, S. Wang, P. Wang and P. Papakonstantinou, ACS Appl. Mater. Interfaces, 2015, 7, 14113–14122.
56. J. D. Benck, Z. Chen, L. Y. Kuritzky, A. J. Forman and T. F. Jaramillo, ACS Catal., 2012, 2, 1916–1923.
57. L. Liao, J. Zhu, X. Bian, L. Zhu, M. D. Scanlon, H. H. Girault and B. Liu, Adv. Funct. Mater., 2013, 23, 5326–5333.
58. Z. Lu, H. Zhang, W. Zhu, X. Yu, Y. Kuang, Z. Chang, X. Lei and X. Sun, Chem. Commun., 2013, 49, 7516–7518.
59. D. Merki, S. Fierro, H. Vrubel and X. Hu, Chem. Sci., 2011, 2, 1262–1267.
60. D. Merki and X. Hu, Energy Environ. Sci., 2011, 4, 3878–3888.
61. H. Vrubel, D. Merki and X. Hu, Energy Environ. Sci., 2012, 5, 6136–6144.
62. M. A. Lukowski, A. S. Daniel, F. Meng, A. Forticaux, L. Li and S. Jin, J. Am. Chem. Soc., 2013, 135, 10274–10277.

---

Footnote

† These authors contributed equally to this work.

---