Growth of molybdenum carbide micro-islands on carbon cloth toward

Journal of

Materials Chemistry A View Article Online

PAPER

View Journal | View Issue

Published on 07 July 2015. Downloaded by Jilin University on 30/07/2015 03:48:12.

Cite this: J. Mater. Chem. A, 2015, 3, 16320

Growth of molybdenum carbide micro-islands on carbon cloth toward binder-free cathodes for efficient hydrogen evolution reaction† Meihong Fan, Hui Chen, Yuanyuan Wu, Liang-Liang Feng, Yipu Liu, Guo-Dong Li* and Xiaoxin Zou* Design and synthesis of efficient noble metal-free hydrogen evolution catalysts is of paramount importance for the practical application of water-splitting devices. Herein, we report a novel synthetic method to grow dispersed molybdenum carbide (Mo2C) micro-islands on flexible carbon cloth (CC). This method involves the controlled synthesis of a supramolecular hybrid between cetyltrimethyl ammonium cations and molybdate anions on CC, followed by simple thermal treatment of this supramolecular hybrid in Ar to form Mo2C on CC in situ. In this synthesis, the presence of cetyltrimethyl ammonium bromide is proven to be important because it effectively immobilizes molybdate ions on CC on the one hand and functions as a carbon source for the formation of Mo2C on the other. Moreover, the as-prepared Mo2C/CC composite material can serve as efficient binder-free cathodes toward the hydrogen evolution reaction (HER). The Mo2C/CC affords a current density of 10 mA cm2 at a low overpotential of 140 mV and

Received 13th May 2015 Accepted 6th July 2015

works stably in acidic media with a Faraday yield of 100%. The isolated island architecture of Mo2C ensures rich active sites to be exposed and allows the easy interaction of reactants (e.g., protons) with

DOI: 10.1039/c5ta03500g

the active sites. Also, the strong adhesion between Mo2C and carbon cloth facilitates electron transport/

www.rsc.org/MaterialsA

transfer in the composite material and is helpful for the achievement of excellent catalytic stability.

Introduction Hydrogen has long been considered as an ideal renewable and green energy carrier to cope with the imminent intractable energy issues at present. Electrolysis of water is of utmost importance and might play a primary role in future renewable energy technologies for transforming electric energy into storable chemical energy (e.g., H2).1 In this sense, exploiting water splitting electrocatalysts to produce H2 with high efficiency at low cost is desirable both for energy storage and environmental protection. Pt-group noble metals are highly active hydrogen evolution catalysts. However, they are not applicable in a large scale due to their low crustal abundance and high price.2,3 The hydrogenase and nitrogenase enzymes have shown excellent catalytic activity for the hydrogen evolution reaction (HER), in which the 3d-transition metals such as Mo, Fe and Ni are involved in the catalytic sites.4–7 Inspired by the composition/ structure of bio-enzymes, some noble metal-free hydrogen evolution catalysts with high catalytic performance have been prepared and intensively investigated recently,1 mainly

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China. E-mail: [email protected]; [email protected]; [email protected] † Electronic supplementary 10.1039/c5ta03500g

information

(ESI)

16320 | J. Mater. Chem. A, 2015, 3, 16320–16326

available.

See

DOI:

including metal carbides,8 metal phosphides,9 metal sulphides,10 metal selenides,11 metal nitrides12 and heteroatomdoped nanocarbons.13 Among carbide-based hydrogen evolution catalysts, Mo2C is a promising material and shows remarkable HER catalytic activity even in the bulk phase. Hu and co-workers rst reported commercial Mo2C microparticles acting as active HER catalysts in both acidic and alkaline media in 2012.8a Since then, dramatic efforts have been dedicated to enhance their catalytic performance. Effective strategies for improving Mo2C's catalytic activity mainly include: (i) preparing it in a nanostructured form,14–20 (ii) creating porous structure in it,21,22 and (iii) coupling it with nanocarbons (e.g., graphene and carbon nanotube).23–26 However, all the Mo2C-based catalysts mentioned above were prepared in a powdered form, and thus they had to be immobilized on a conductive substrate (e.g., glassy carbon electrode) by using polymer binders to construct a working electrode. One the one hand, polymer binders are expensive; on the other hand, their uses also might block some catalytically active sites, increase the series resistance, and thus decrease the materials' overall activity. Conversely, binder-free hydrogen evolution electrocatalysts, directly grown on good current-collecting substrates, represent an attractive electrode architecture that can effectively solve the polymer binderrelated problems. Such a concept was successfully recently applied to construct a Mo2C-based hydrogen evolution

This journal is © The Royal Society of Chemistry 2015

View Article Online

Paper


Fig. 1

Journal of Materials Chemistry A

Schematic illustration of the synthesis of micro-sized Mo2C islands via a two-step method.

electrode by two different groups,27,28 both of which used nickel foam as the current-collecting substrate. In comparison with nickel foam, carbon cloth (CC) is a more appealing substrate material, especially when the electrodes are used in acidic media.29–31 This is because CC has ne exibility, high conductivity and excellent acid resistance. But CC-supported Mo2C materials have not been synthesized, and correspondingly they have not been used as a binder-free electrode for the HER thus far. Herein, we report a novel synthetic method to grow dispersed molybdenum carbide (Mo2C) micro-islands on exible carbon cloth (CC) for the rst time. The synthesis of Mo2C/ CC is achieved via a two-step method (Fig. 1): (i) controlled synthesis of a supramolecular hybrid between cetyltrimethyl ammonium cations and molybdate anions on CC, and (ii) simple thermal treatment of this supramolecular hybrid in Ar to form Mo2C on CC in situ. Moreover, the as-prepared Mo2C/CC composite material can serve as efficient binder-free cathodes toward the HER. The excellent electrocatalytic performance of Mo2C/CC benets from the island-like morphology of Mo2C with rich catalytic sites, the intimate connection of Mo2C/CC, and high electrical conductivity.

Experimental Chemicals and reagents Ammonium molybdate was purchased from Tianjin Kaida Chemical Factory. Cetyltrimethyl ammonium bromide (CTAB) was purchased from Huishi Reagents Ltd (Shanghai, China). H2SO4 was purchased from Beijing Chemical Factory. Carbon cloth was purchased from Hesenbio Ltd (Shanghai, China). CC was pretreated by reuxing in concentrated HNO3 at 100 C for 2 hours to remove impurities on the surface, and then was cleaned with deionized water and ethanol three times and dried naturally. Molybdenum carbide (Mo2C) was purchased from Alfa-Aesar. Platinum on activated carbon (20 wt% Pt/C, Pt on an activated carbon support) was purchased from Sigma-Aldrich. Deionized water was used throughout the experiments. Synthesis of Mo2C/CC Ammonium molybdate (3.708 g) was dissolved in 30 mL distilled water, and CTAB (1.638 g) was dissolved in 180 mL


distilled water. The two solutions were then mixed under vigorous stirring, and a homogenous white emulsion is formed at once. Aer aging for 12 hours at room temperature, we discarded the supernatant solution, and transferred the le dense emulsion (30 mL) into an autoclave containing a piece of CC (1 cm 5 cm). The autoclave was sealed and maintained at 200 C for 20 h. Aer cooling down to room temperature, a CC covered with uniform white solids was obtained, and this sample was denoted as Mo-CTA/CC. Finally, the Mo-CTA/CC was placed on a quartz boat and heated at 900 C in a tube furnace for 6 h in an Ar atmosphere with a heating rate of 6 C min1. The nal product was denoted as Mo2C/CC. For comparative purpose, Mo2C particles were prepared by removing the carbon cloth from the reaction system, under otherwise similar synthetic procedures as above. This material was denoted as P-Mo2C. General characterization The structure and phase purity of the as-synthesized materials were examined with a Rigaku D/Max 2550 X-ray diffractometer ˚ Infrared spectra (FTIR) with Cu Ka radiation (l ¼ 1.5418 A). were recorded in the range of 400–4000 cm1 on a Bruker IFS 66V/S FTIR spectrometer with a resolution of 1 cm1. The morphology and microstructure were characterized with a JEOL JSM 6700F electron scanning electron microscope (SEM). Transmission electron microscopy (TEM) images were obtained with a Philips-FEI Tecnai G2S-Twin microscope equipped with a eld emission gun operating at 200 kV. Elemental analysis of possible element species in the Mo2C/CC material was conducted on a Perkin-Elmer Optima 3300 DV inductively coupled plasma atomic emission spectrometer (ICP-OES). HER measurements Electrochemical measurements were performed with a threeelectrode system CHI 660E (CH Instruments, Inc. Shanghai). A carbon rod and saturated calomel electrode (SCE) were employed as the counter electrode and reference electrode, respectively. The Mo2C/CC served as the working electrode. The electrode area exposed to the electrolyte was 0.2 cm 0.2 cm, and the rest of the electrode was carefully coated with epoxy resin. In order to evaluate the catalytic activity of the powdered sample, this sample was drop-cast onto a piece of CC with the same loading amount of Mo2C as that on Mo2C/CC (1.5 mg

J. Mater. Chem. A, 2015, 3, 16320–16326 | 16321

View Article Online



Paper

cm2), and then the resulting lm was stabilized by polymer binder Naon to form the working electrode. The potential, measured against the SCE reference electrode, was calibrated with respect to the reversible hydrogen electrode (RHE) E vs. RHE ¼ E vs. SCE + 0.26 V + 0.059 pH. LSV measurements were conducted in 0.5 M H2SO4 electrolyte with a scan rate of 50 mV s1. Tafel plots with the steady-state current density as a function of applied voltage were measured with a dwell time of 400 s. The durability of Mo2C/CC and P-Mo2C was tested by electrolysis at an overpotential of 140 mV and 180 mV for 15 h, respectively. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 100 kHz to 1 Hz. To compare the relative electrochemical active area, cyclic voltammetry measurements were carried out on the Mo2C/CC, P-Mo2C nanoparticles and commercial Mo2C powder in the range of 0.194–0.304 V vs. RHE where the currents are mostly due to the charging of the double layer, and calculated according to previous reports.32,33 Briey, the scan rate dependence of the current density at E ¼ 0.249 V vs. RHE can be depicted from the CV curves tested at different scan rates. The capacitance of the lms can be calculated from it for the slope of the DJ vs. scan rate curve, which is twice the Cdl. Though the real effective surface area of the electrode could not be calculated, it is possible to compare their relative effective surface area by comparing their slope of current density-scan rate line on the assumption that Cdl and the effective surface area are linearly proportional.

Results and discussion The Mo2C/CC material was synthesized through a two-step procedure (details of experimental procedures are provided in the Experimental section). As shown in Fig. 1, a supramolecular hybrid (Mo-CTA/CC) between cetyltrimethyl ammonium cations (CTA+) and molybdate anions on CC was rst synthesized. The resulting Mo-CTA/CC was allowed to be thermally treated at 900 C in an Ar atmosphere. This was accompanied by the transformation of Mo-CTA to Mo2C nanoparticles, leading to the formation of the Mo-CTA/CC composite material. 6CTA+ + Mo7O246 / (CTA)6(Mo7O24)Y

(1)

To study the structure of Mo-CTA/CC, Mo-CTA was prepared rst at room temperature without adding CC in the reaction system, and then the Mo-CTA was carefully characterized by powder X-ray diffraction (XRD), FT-IR spectroscopy, and inductively coupled plasma atomic emission spectroscopy (ICPOES). Mo-CTA has a different wide-angle XRD pattern from those of CTAB and (NH4)6Mo7O24, as shown in Fig. 2A. This indicates that Mo-CTA is a new compound, rather than a physical mixture of CTAB and (NH4)6Mo7O24. However, the accurate crystal structure of Mo-CTA has not yet been resolved. Elemental analysis reveals that the CTA : Mo7O24 mol ratio is close to 6 : 1 in Mo-CTA. This is in agreement with the fact that six CTA+ cations would balance one molybdate anion to make a neutral compound. Furthermore, the XRD patterns of Mo-CTA and CTAB appear very similar in the low-angle range (2q ¼ 1–8 ),

16322 | J. Mater. Chem. A, 2015, 3, 16320–16326

indicating that the packing mode of long-chained CTA components in the two samples is analogous (Fig. 2B). But the diffraction peaks of Mo-CTA located in the low angle range slightly increased about 0.2 compared to that of CTAB, probably due to the stronger interaction between CTA and molybdate. FT-IR spectra also show that Mo-CTA contains CTA components in it (Fig. 2C), because of the similarity of the IR spectra for Mo-CTA and CTAB. Based on the above results, coupled with some previous reports,34,35 the Mo-CTA should form through a simple interchange reaction between CTAB and ammonium molybdate (eqn (1) and Fig. 2D). For the immobilization of molybdate ions on CC, the presence of CTAB is proven to be important because direct interaction of CC and molybdate ions only resulted in a tiny amount of molybdate being adsorbed on CC. This also indicates that anchoring MoCTA on CC might mainly originate from the interactions of the CTA component and CC. The loading density of Mo on CC for Mo-TCA/CC is about 1.4 mg cm2. In addition to the importance of the effective immobilization of molybdate on CC, the CTA component in Mo-CTA also functions as the carbon source for the formation of Mo2C in situ, with no need to add additional carbonaceous gases such as CH4, CO, etc. Thus, a simple thermal treatment in an inert atmosphere (900 C) can directly transform Mo-CTA/CC into Mo2C/CC. Fig. 3A presents a photograph of Mo2C/CC, showing the exible feature of this material. The morphology and microstructure of Mo2C/CC were further studied by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The low-magnication SEM images (Fig. 3B and C) show that the Mo2C/CC material is composed of many island-shaped microparticles with a diameter of 0.5–1 mm, which are directly grown on the CC substrate. The more detailed structural features of Mo2C/CC are examined from the highmagnication SEM image (Fig. 2D). It is seen that the surface of

Fig. 2 (A) Wide-angle XRD patterns, (B) low-angle XRD patterns and (C) IR spectra of Mo-CTA, CTAB and (NH4)6Mo7O24. (D) Schematic illustration of the structural transformation from CTAB to Mo-CTAB.


View Article Online


Paper


a single Mo2C micro-island is rather rough, and the Mo2C micro-island actually is composed of densely packed Mo2C nanoparticle subunits. The size of Mo2C nanoparticle subunits is about 20 nm, as revealed by the TEM images (Fig. 3E). The high-resolution TEM (HRTEM) image (Fig. 2F) of Mo2C islands reveals a lattice fringe with an interplanar distance of 0.23 corresponding to the (101) crystal plane of Mo2C. The formation of Mo2C in Mo2C/CC was conrmed by powder X-ray diffraction (XRD, Fig. 4). The crystal size of Mo2C in the material was estimated by using the Scherrer formula to be around 24 nm. This result is in agreement with the TEM observation. The formation of Mo2C in Mo2C/CC was also conrmed by Raman spectroscopy (Fig. S1†). In addition, the loading amount of Mo2C on CC is found to be 1.5 mg cm2 on basis of the ICP-OES analysis. Furthermore, the contact between Mo2C micro-islands and CC is quite strong because the structure of Mo2C/CC was not destroyed even aer 30 min sonication treatment. In order to further conrm that CTA, rather than CC, is the carbon source for the formation of molybdenum carbide for the synthesis of Mo2C/CC, Mo-CTA without CC was also thermally treated in an Ar atmosphere at 900 C, and correspondingly the resulting powdered product was denoted as P-Mo2C. Fig. 5A shows the XRD patterns of P-Mo2C and a commercially available Mo2C (C-Mo2C see the SEM image in Fig. S2†). The XRD patterns of the two samples are very similar, and are well indexed to the hexagonal b-Mo2C phase (JCPDS: 35-0787). No peaks related to metallic molybdenum or molybdenum oxides are observed in the XRD pattern, demonstrating the high purity of the P-Mo2C sample. The BET surface area of P-Mo2C is about 36 m2 g1. SEM and TEM images (Fig. 5B and C) show that PMo2C consists of irregular nanoparticles with a size of 20–100 nm. A lattice fringe with d ¼ 0.23 nm (Fig. 5D) is associated with the (101) crystallographic planes of Mo2C. The TEM images (Fig. 5D and S3†) also show that there is no extra carbon around Mo2C particles in the P-Mo2C. All the above results show that

Fig. 3

Fig. 4 XRD pattern of Mo2C/CC. The XRD peak of crystalline carbon is indicated by “*”.

Mo-CTA can be thermally converted into Mo2C in situ, without the assistance of carbonaceous gases. The electrocatalytic performance of the materials was evaluated in a typical three-electrode electrochemical cell, in which a carbon rod and a saturated calomel electrode were used as the counter electrode and as the reference electrode, respectively. 0.5 M H2SO4 was used as the electrolyte. The Mo2C/CC material can serve as the working electrode directly. The electrode area exposed to the electrolyte was 0.2 cm 0.2 cm, and the rest of the electrode was carefully coated with epoxy resin. For comparative purpose, P-Mo2C, C-Mo2C and 20 wt% Pt/C were also tested. Because they are in the powered form, they have to be modied on CC to fabricate the working electrode (see Experimental section), with the assistance of a polymer binder. As shown in Fig. 6A, the Mo2C/CC material exhibits a signicant cathodic current with a small onset overpotential of 30 mV (the overpotential that the electrocatalyst needs to yield a current

(A) Digital image, (B–D) SEM images, (E) TEM image and (F) HRTEM image of Mo2C/C.


J. Mater. Chem. A, 2015, 3, 16320–16326 | 16323

View Article Online



Fig. 5 (A) XRD patterns of P-Mo2C and C-Mo2C. (B) SEM, (C) TEM, and (D) HRTEM images of P-Mo2C.

density of 1 mA cm2). In contrast, P-Mo2C and C-Mo2C exhibit the same current density at h ¼ 45 and 83 mV, respectively. Obviously, the Mo2C/CC material shows better HER catalytic activity than both P-Mo2C and C-Mo2C (their electrochemical properties are summarized in Table 1). In addition, the Mo2C/ CC material only needs an overpotential of 140 mV to achieve a current density of 10 mA cm2 (the current density expected for a 12.3% efficient solar water-splitting device). This result also

Paper

indicates that the Mo2C/CC material is among the highest active Mo2C-based HER electrocatalysts.8a,14–28 It is worth noting that it is consistently observed that the CCsupported Mo2C electrodes, including Mo2C/CC and CC-supported P-Mo2C, produce a substantial background current. A similar phenomenon was also observed in a previous report.10d In fact, we also observed this background current when using the Mo2C-free inert CC electrode, suggesting that it is primarily capacitive in origin. Fig. S4† shows polarization curves with Mo2C/CC at different scan rates (0.1–100 mV s1). From the gure, it is seen that the low scan rate can reduce the capacitive current, but do not completely eliminate the capacitive current. In order to reduce the capacitive current as much as possible, we measured steady-state current densities as a function of the applied voltages (a dwell time of 400 s). However, a stable background current is still present. Undoubtedly, the presence of background current will lead to higher false current densities than catalytic ones. Thus, subtraction of this background current allows us to obtain more clear and true analysis of catalytic activity (this method was also used previously10d), and all current density values presented here are corrected by the background currents. Fig. 6B shows the Tafel plots of Mo2C/CC, P-Mo2C and CMo2C. The linear portions of the Tafel plots are well-known to be t to the Tafel equation (h ¼ a + b log j, where h is the overpotential, j is the current density and b is the Tafel slope).36 Based on the Tafel equation, the Tafel slopes of Mo2C/CC, PMo2C and C-Mo2C are calculated to be 124, 128 and 168 mV per

Fig. 6 (A) Steady-state current densities as a function of the applied voltages after subtraction of the background current in 0.5 M H2SO4 with CMo2C, P-Mo2C, 20% Pt/C and Mo2C/CC. (B) Corresponding Tafel plots of C-Mo2C, P-Mo2C, 20% Pt/C and Mo2C/CC. (C) Current density–time (I–t) curves of P-Mo2C (h ¼ 180 mV) and Mo2C/CC (h ¼ 140 mV) for 15 h, respectively. (D) Electrochemical impedance spectroscopy (EIS) Nyquist plots of C-Mo2C, P-Mo2C and Mo2C/CC.

16324 | J. Mater. Chem. A, 2015, 3, 16320–16326


View Article Online

Paper


Table 1

Journal of Materials Chemistry A Summary of HER activities of three Mo2C-based catalysts

Catalyst

Tafel slope (mV per dec)

h at j ¼ 1 mA cm2 (mV)

h at j ¼ 10 mA cm2 (mV)

Relative surface area

C-Mo2C P-Mo2C Mo2C/CC

168 128 124

83 45 30

260 160 140

1 2.5 2.8

decade, respectively. The large Tafel slopes (>120 mV per dec.) for the three materials suggest that the electrochemical adsorption of hydrogen atoms (i.e., Volmer reaction) might be the prominent rate limiting step for the HER. Note that the Tafel slope in our case is larger than the values reported in previous Mo2C-related studies, possibly because of the relatively big Mo2C particles in the Mo2C/CC composite material. These results also indicate that further ne-controlling the crystal size of Mo2C nanoparticles on CC should be a feasible strategy to improve the catalytic activity of CC-supported Mo2C catalysts. In addition to activity, stability is also an important criterion to assess a non-Pt HER electrocatalyst. Fig. 6C depicts the current–time curve of the hydrogen evolution reaction over 15 hours at an overpotential of 140 mV in the presence of Mo2C/CC as an electrocatalyst. The result shows that aer 15 h of electrochemical reactions with Mo2C/CC, there is barely any loss of catalytic activity. There is only slight turbulence of current density due to the formation and release of H2 bubbles on the electrode surface. This result demonstrates the excellent stability of Mo2C/CC as an electrocatalyst. In comparison, the PMo2C is less stable because the current density decreases by 20% for P-Mo2C over 15 h of electrochemical reactions. The above results clearly suggest that Mo2C/CC exhibits better electrocatalytic performance than the two powdered materials (P-Mo2C and C-Mo2C). The enhanced electrocatalytic performance of the Mo2C/CC material could be due to the

following reasons: (i) the intimate contact of Mo2C microislands with CC enables good electrical transport from CC to Mo2C during the HER, and eliminates the overpotential associated with the interface of Mo2C and CC. The electrochemical impedance measurements show that Mo2C/CC has a much smaller semicircle of Nyquist plots than those of P-Mo2C and CMo2C (Fig. 6D). This indicates the rapid electron transfer (ET) and faster hydrogen evolution reaction kinetics in the presence of Mo2C/CC. (ii) The binder-free feature of Mo2C/CC can avoid the polymer binder-related drawbacks, such as blocking some catalytically active sites and increasing the series resistance. (iii) The unique island-shaped architecture of Mo2C on CC could ensure more exposed active sites. This is conrmed by the comparison of the electrochemical surface area of Mo2C/CC with those of P-Mo2C and C-Mo2C (Table 1). The electrochemical surface area of Mo2C/CC is about 2.8 times larger than that of C-Mo2C. Fig. 7 shows the comparative curves of experimentally detected H2 amount vs. the theoretical H2 amount during 140 min of the electrochemical hydrogen evolution reactions. Mo2C/ CC exhibits a stable hydrogen evolution rate of 112 mmol h1, which is very close to the theoretical hydrogen production rate. This means that Mo2C/CC exhibits nearly 100% Faradaic yield, and has excellent stability during the HER.

Conclusions In summary, we presented the growth of well-dispersed molybdenum carbide (Mo2C) micro-islands on exible carbon cloth (CC) via simple thermal treatment of the CC-supported Mo-CTA supramolecular hybrid. We also showed that the resulting Mo2C/CC material could serve as an efficient binderfree electrocatalyst for the HER. We believe that these results could encourage further research on chemical transformation of other inorganic–organic supramolecular hybrids into various novel inorganic nanomaterials with a unique composition and morphology as well as improved properties.

Acknowledgements This work was supported by the NSFC (21371070, 21401066); the National Basic Research Program of China (2013CB632403); Jilin province science and technology development projects (20140101041JC, 20130204001GX, 20150520003JH); the Graduate Innovation Fund of Jilin University (2014052).

Notes and references

Fig. 7 Hydrogen production efficiency of Mo2C/CC for HER with a current density of 3.0 mA cm2 in 0.5 M H2SO4. The exposed electrode area is about 1 cm 2 cm. The calculated H2 lines represent the expected H2 amount assuming a quantitative Faradaic yield (red line). The measured H2 lines represent the experimentally detected H2 (black line).


1 X. Zou and Y. Zhang, Chem. Soc. Rev., 2015, DOI: 10.1039/ c4cs00448e. 2 M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori and N. S. Lewis, Chem. Rev., 2010, 110, 6446–6473. 3 D. Merki and X. Hu, Energy Environ. Sci., 2011, 4, 3878–3888. 4 J. D. Evans and C. J. Pickett, Chem. Soc. Rev., 2003, 32, 268– 275.

J. Mater. Chem. A, 2015, 3, 16320–16326 | 16325

View Article Online



5 O. Einsle, F. A. Tezcan, S. L. A. Andrade, B. Schmid, M. Yoshida, J. B. Howard and D. C. Rees, Science, 2002, 297, 1696–1700. 6 D. C. Rees and J. B. Howard, Science, 2003, 300, 929–931. 7 L. C. Rai and M. Raizada, J. Gen. Appl. Microbiol., 1985, 31, 329–337. 8 (a) H. Vrubel and X. Hu, Angew. Chem., 2012, 124, 12875– 12878; (b) L. Pan, Y. Li, S. Yang, P. Liu, M. Yu and H. Yang, Chem. Commun., 2014, 50, 13135–13137; (c) H. Wu, B. Xia, L. Yu, X. Yu and X. Lou, Nat. Commun., 2015, 6, 6512–6520; (d) Y. Liu, G. Li, L. Yuan, L. Ge, H. Ding, D. Wang and X. Zou, Nanoscale, 2015, 7, 3130–3136; (e) P. Xiao, X. Ge, H. Wang, Z. Liu, A. Fisher and X. Wang, Adv. Funct. Mater., 2015, 25, 1520–1526; (f) D. V. Esposito, S. T. Hunt, A. L. Stottlemyer, K. D. Dobson and B. E. McCandless, Angew. Chem., Int. Ed., 2010, 49, 9859–9862; (g) W. Chen, J. T. Muckerman and E. Fujita, Chem. Commun., 2013, 49, 8896–8909. 9 (a) P. Liu and J. A. Rodriguez, J. Am. Chem. Soc., 2005, 127, 14871–14878; (b) J. Tian, Q. Liu, A. M. Asiri and X. Sun, J. Am. Chem. Soc., 2014, 136, 7587–7590; (c) P. Xiao, M. A. Sk, L. Thia, X. Ge, R. J. Lim, J. Wang, K. H. Lim and X. Wang, Energy Environ. Sci., 2014, 7, 2624–2629. 10 (a) 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; (b) Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong and H. Dai, J. Am. Chem. Soc., 2011, 133, 7296– 7299; (c) D. Voiry, H. Yamaguchi, J. Li, R. Silva, D. C. B. Alves, T. Fujita, M. Chen, T. Asefa, V. B. Shenoy, G. Eda and M. Chhowalla, Nat. Mater., 2013, 12, 850–855; (d) M. S. Faber, R. Dziedzic, M. A. Lukowski, N. S. Kaiser, Q. Ding and S. Jin, J. Am. Chem. Soc., 2014, 136, 10053– 10061; (e) Y. Sun, C. Liu, D. C. Grauer, J. Yano, J. R. Long, P. Yang and C. J. Chang, J. Am. Chem. Soc., 2013, 135, 17699–17702; (f) L. Feng, G. Li, Y. Liu, Y. Wu, H. Chen, Y. Wang, Y. Zou, D. Wang and X. Zou, ACS Appl. Mater. Interfaces, 2015, 7, 980–988. 11 (a) H. Tang, K. Dou, C. Kaun, Q. Kuang and S. Yang, J. Mater. Chem. A, 2014, 2, 360–364; (b) C. Tsai, K. Chan, F. A. Pedersen and J. K. Nørskov, Phys. Chem. Chem. Phys., 2014, 16, 13156– 13164; (c) D. Kong, H. Wang, Z. Lu and Y. Cui, J. Am. Chem. Soc., 2014, 136, 4897–4900; (d) H. Zhang, B. Yang, X. Wu, Z. Li, L. Lei and X. Zhang, ACS Appl. Mater. Interfaces, 2015, 7, 1772–1779. 12 (a) B. Cao, G. M. Veith, J. C. Neuefeind, R. R. Adzic and P. G. Khalifah, J. Am. Chem. Soc., 2013, 135, 19186–19192; (b) W. Chen, K. Sasaki, C. Ma, A. I. Frenkel and N. Marinkovic, Angew. Chem., Int. Ed., 2012, 51, 6131–6135. 13 (a) S. Gao, G. Li, Y. Liu, H. Chen, L. Feng, Y. Wang, M. Yang, D. Wang, S. Wang and X. Zou, Nanoscale, 2015, 7, 2306–2316; (b) X. Zou, X. Huang, A. Goswami, R. Silva, B. R. Sathe, E. Mikmekov´ a and T. Asefa, Angew. Chem., Int. Ed., 2014, 53, 4372–4376; (c) J. Deng, P. Ren, D. Deng, L. Yu, F. Yang and X. Bao, Energy Environ. Sci., 2014, 7, 1919–1923; (d) Y. Zheng, Y. Jiao, L. Li, T. Xing, Y. Chen, M. Jaroniec and

16326 | J. Mater. Chem. A, 2015, 3, 16320–16326

Paper

14 15 16 17 18 19

20 21

22 23

24 25 26

27 28 29 30

31 32 33 34 35 36

S. Qiao, ACS Nano, 2014, 8, 5290–5296; (e) Y. Zheng, Y. Jiao, Y. Zhu, L. Li, Y. Han, Y. Chen, A. Du, M. Jaroniec and S. Qiao, Nat. Commun., 2014, 5, 3783–3790. D. R. Stellwagen and J. H. Bitter, Green Chem., 2015, 17, 582– 593. N. S. Alhajri, D. H. Anjum and K. Takanabe, J. Mater. Chem. A, 2014, 2, 10548–10556. C. Ge, P. Jiang, W. Cui, Z. Pu, Z. Xing, A. M. Asiri, A. Y. Obaid, X. Sun and J. Tian, Electrochim. Acta, 2014, 134, 182–186. P. Xiao, Y. Yan, X. Ge, Z. Liu, J. Wang and X. Wang, Appl. Catal., B, 2014, 154–155, 232–237. C. Wan, Y. N. Regmi and B. M. Leonard, Angew. Chem., 2014, 126, 6525–6528. W. Chen, C. Wang, K. Sasaki, N. Marinkovic, W. Xu, J. T. Muckerman, Y. Zhu and R. R. Adzic, Energy Environ. Sci., 2013, 6, 943–951. C. Wan, N. A. Knight and B. M. Leonard, Chem. Commun., 2013, 49, 10409–10411. L. Liao, S. Wang, J. Xiao, X. Bian, Y. Zhang, M. Scanlon, X. Hu, Y. Tang, B. Liu and H. Girault, Energy Environ. Sci., 2014, 7, 387–392. X. Bian, M. D. Scanlon, S. Wang, L. Liao, Y. Tang, B. Liu and H. H. Girault, Chem. Sci., 2013, 4, 3432–3441. S. S. J. Aravind, M. Costa, V. Pereira, A. Mugweru, K. Ramanujachary and T. D. Vaden, Int. J. Hydrogen Energy, 2014, 39, 11528–11536. D. H. Youn, S. Han, J. Y. Kim, J. Y. Kim, H. Park, S. H. Choi and J. S. Lee, ACS Nano, 2014, 8, 5164–5173. W. Cui, N. Cheng, Q. Liu, C. Ge, A. M. Asiri and X. Sun, ACS Catal., 2014, 4, 2658–2661. W. Chen, S. Iyer, S. Iyer, K. Sasaki, C. Wang, Y. Zhu, J. T. Muckerman and E. Fujita, Energy Environ. Sci., 2013, 6, 1818–1826. J. Zhang, X. Meng, J. Zhao and Z. Zhu, ChemCatChem, 2014, 6, 2059–2064. K. Xiong, L. Li, L. Zhang, W. Ding, L. Peng, Y. Wang, S. Chen, S. Tan and Z. Wei, J. Mater. Chem. A, 2015, 3, 1863–1867. C. Tang, Z. Pu, Q. Liu, A. M. Asiri and X. Sun, Electrochim. Acta, 2015, 153, 508–514. Z. Li, S. Kelkar, L. Raycra, M. Garedew, J. E. Jackson, D. J. Miller and C. M. Saffron, Green Chem., 2014, 16, 844– 852. Y. Yan, B. Xia, N. Li, Z. Xu, A. Fisher and X. Wang, J. Mater. Chem. A, 2015, 3, 131–135. D. Merki, H. Vrubel, L. Rovelli, S. Fierro and X. Hu, Chem. Sci., 2012, 3, 2515–2525. X. Lu, W. Yim, B. H. R. Suryanto and C. Zhao, J. Am. Chem. Soc., 2015, 137, 2901–2907. H. Yao, X. Li, S. Liu and S. Yu, Chem. Commun., 2009, 6732– 6734. H. Li, H. Sun, W. Qi, M. Xu and L. Wu, Angew. Chem., Int. Ed., 2007, 46, 1300–1303. Y. Yan, B. Xia, Z. Xu and X. Wang, ACS Catal., 2014, 4, 1693– 1705.
