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graphene for enhanced photocatalytic H2 evolution
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A novel ultraefficient non-noble metal composite cocatalyst Mo2N/Mo2C/graphene for enhanced photocatalytic H2 evolution Baojun Ma*, Xiaoyan Wang, Keying Lin, Jie Li, Yahui Liu, Haijuan Zhan, Wanyi Liu State Key Laboratory of High-Efficiency Coal Utilization and Green Chemical Engineering, College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan, 750021, People's Republic of China
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abstract
Article history:
Non-noble metal cocatalyst is a hot research issue in photocatalytic H2 production due to
Received 5 March 2017
its low cost and potential substitution of the usually used expensive noble metal cocata-
Received in revised form
lyst. Here, we reported a new non-noble metal cocatalyst Mo2N/Mo2C/graphene (GR) for
27 April 2017
ultraefficiently enhanced photocatalytic H2 production of CdS under visible light irradia-
Accepted 30 May 2017
tion. The optimum composite 2.0 wt % Mo2N/Mo2C/GR/CdS achieves an ultrahigh H2 evo-
Available online xxx
lution rate of 4520 mmol/h/g, which is 18 times higher than that of CdS alone and 3.6 times higher than the optimum 1.0 wt % Pt/CdS. The XRD shifts and HRTEM demonstrates the
Keywords:
special structure (heterojunction) between the interfaces of Mo2N and Mo2C, which mainly
Photocatalytic
accounts for the ultraefficience of Mo2N/Mo2C/GR. This study firstly presents a novel ul-
Cocatalyst
traefficient non-noble metal cocatalyst Mo2N/Mo2C/GR and reveals the interface effects
H2 evolution
among the compositions of the cocatalyst.
Molybdenum
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Graphene Interface effect
Introduction Photocatalytic H2 production on semiconductor photocatalyst has attracted much attention for the last three decades due to the use of clean, abundant and sustainable solar energy to achieve the clean H2 energy [1e4]. In a system of photocatalytic H2 production, the cocatalyst is one of the key factor to obtain efficient H2 production [5e7]. Unfortunately, the cocatalyst is generally mainly composed of noble metals such as Pt, Ru, Pd, Au, Rh which leads to the expensive photocatalyst [8e10]. So, developing the efficient, low cost non-
noble metal cocatalyst has been a hot and indispensable research issue in the field of photocatalytic H2 production [11e18]. Recently, a series of efficient dual coupled semiconductors with structure of junctions (such as a PN junction, heterojunction, solid solution phase junction, surface phase junction, etc.) had been reported for enhanced photocatalytic activity [19e25]. The junction structure formed at the interfaces of the two semiconductors is favor of photoexcited electrons transfer and separation in the two semiconductors. Also, two noble metal cocatalysts had been found the synergistic effect on the photocatalytic H2 evolution [26e28] and
* Corresponding author. E-mail address: [email protected] (B. Ma). http://dx.doi.org/10.1016/j.ijhydene.2017.05.212 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ma B, et al., A novel ultraefficient non-noble metal composite cocatalyst Mo2N/Mo2C/graphene for enhanced photocatalytic H2 evolution, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.212
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seemed to form the materials with new structure in the reaction process. More interestingly, the Rh/Cr2O3 with core/ shell structure had been designed for efficient photocatalytic water splitting due to the efficient inhibition of reversible reaction of water splitting by the shell of Cr2O3 [29e33]. All these indicate the strategy of assembly and structure control of material with special structure for superior catalytic property is effective, whether the material is formed by two semiconductors or two cocatalysts. Molybdenum-based compounds including Molybdenum carbide, Molybdenum nitride and Molybdenum phosphide have similar properties of Pt, and extensively applied in hydrogenation and dehydrogenation process [34e38]. Recently, the author had reported Mo2C and Mo2N as the non-noble cocatalysts combined with CdS for enhanced photocatalytic H2 production under visible light irradiation [39,40]. The Mo2C/ CdS has the photocatalytic H2 evolution rate higher of 2.3 times of Pt/CdS, whereas the Mo2N/CdS has the photocatalytic H2 evolution rate similar to Pt/CdS. The structure and chemical constituent of Mo2N and Mo2C can be controlled and conversed by precise control of the preparation method to form Mo2N/ Mo2C composite [41,42]. To our knowledge, it had been not reported the relationship between structure of composite cocatalyst and catalytic property in photocatalytic H2 evolution, of which the composite cocatalysts Mo2N/Mo2C is composed of two non-noble metal cocatalysts Mo2N and Mo2C. Graphene (GR) has been a promising material with high specific surface area and superior electron mobility and combining graphene with cocatalyst to achieve highly efficient new cocatalyst in photocatalytic H2 production have been reported [15,18,43e46]. So, It is anticipating to combine Mo2N/Mo2C and graphene to form a new three component cocatalyst Mo2N/Mo2C/GR to achieve much higher photocatalytic H2 evolution rate on semiconductor. In this paper, Mo2N/Mo2C/GR is prepared by calcination of the precursor from the mixed MoCl5, carbamide and graphene oxide under N2 atmosphere. CdS semiconductor is chosen as a needle probe to evaluate the cocatalyst and the Mo2N/Mo2C/ graphene/CdS composite is prepared by a large scale method of deposition of CdS on Mo2N/Mo2C/GR. The photocatalytic activity of cocatalyst/CdS had been tested, and the structure of Mo2N/Mo2C and the interface effects among the compositions of the cocatalyst Mo2N/Mo2C/GR had been focused.
For preparation of Mo2C and Mo2N/Mo2C, all the procedures are same as that of Mo2N excepted the amount of carbamide was 3.52 g (R ¼ 8) and 0.44 g (R ¼ 1), respectively.
Preparation of graphene, Mo2N/graphene, Mo2C/graphene and Mo2N/Mo2C/graphene The graphene oxide was chosen as the precursor for preparation of graphene based non-noble metal cocatalyst. The preparation methods of Mo2N/GR, Mo2C/GR and Mo2N/Mo2C/ GR are same as that of Mo2N, Mo2C and Mo2N/Mo2C, respectively, except 0.35 g graphene oxide was firstly added in ethanol solution. For the preparation of graphene, 0.35 g graphene oxide and 0.22 g carbamide were added in 50 mL ethanol solution, and then the mixed solution was evaporated to dryness at 80 C. Last, the remaining residue was calcined at 750 C under N2 atmosphere for 4 h.
Preparation of CdS, CdS loaded with different non-noble metal cocatalysts and CdS loaded with different noble metal cocatalysts Here, all reagents (Sinopharm Chemical Reagent Co., Ltd) were analytically pure and commercially available, and used without further purification. CdS was prepared by precipitation method. Typically, 0.14 M Na2S solution (120 mL) was added dropwise in 0.14 M cadmium acetate solution (100 mL) under vigorous agitation and then aged for 12 h. Last, the deposit was filtered by distilled water and ethanol several times and then dried at 60 C for 12 h in vacuum drying oven. Non-noble metal cocatalyst/CdS was prepared by precipitation method. To take 2.0 wt % Mo2N/CdS for example, Mo2N (0.04 g) was firstly dispersed in 0.14 M cadmium acetate solution (100 mL), other preparation steps are same as that CdS. For the preparation of noble metal cocatalyst/CdS, 1.0 wt % Pt (Rh, Pd, Ru, and Au) was loaded on 0.1 g CdS through an in situ photo deposition method same as that test of photocatalytic H2 evolution. The photo deposition reaction conditions are of 10% lactic acid aqueous solution (100 mL) as holes scavenger and 300W Xe lamp (Perfect, China) with 420 nm filter (l 420 nm) as light resource. Pt (Rh, Pd, Ru, and Au) was obtained from precursor of H2PtCl6$6H2O (RhCl3$3H2O, PdCl2, RuCl3, and HAuCl4$4H2O).
Characterization of materials
Experimental Preparation of Mo2N, Mo2C and Mo2N/Mo2C Mo based compounds were synthesized by calcination of the precursor from the mixed MoCl5 (Sinopharm, china), carbamide (Sinopharm, china) and/or graphene oxide (Institute of coal chemistry, Chinese academy of sciences) under N2 atmosphere. For preparation of Mo2N, typically, 2.0 g MoCl5 was added in 50 mL ethanol solution with stirring. After MoCl5 was completely dissolved, 0.22 g carbamide (the molar ratio (R) of carbamide/Mo ¼ 0.5) was added in the solution. The mixed solution was stirred vigorously for 1 h, and then evaporated to dryness at 80 C. Last, the remaining viscous residue was calcined at 750 C under N2 atmosphere for 4 h.
The X-ray diffraction (XRD) patterns were obtained by a D/ MAX2500 diffractometer (Rigaku, Japan) with Cu Ka radiation. The UV/Vis diffuse reflection spectroscopies (UVeVis DRS) were recorded by a spectrometer (U-4100) with BaSO4 used for the corrected base line. The Raman spectra were obtained by a Horiba Jobin Yvon Lab RAM HR80 spectrograph with an Ar ion laser of 532 nm. The scanning electron microscopy (SEM) images were obtained by a scanning electron microscope (JSM7500F). The transmission electron microscopy (TEM) images were obtained by a transmission electron microscope (F20). The X-ray photoelectron spectra (XPS) were recorded by a Thermo ESCALAB 250Xi spectrometer with Al Ka radiation (hn ¼ 1486.6 eV, 150 W) and the C1s (284.8 eV) used for corrected peak.
Please cite this article in press as: Ma B, et al., A novel ultraefficient non-noble metal composite cocatalyst Mo2N/Mo2C/graphene for enhanced photocatalytic H2 evolution, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.212
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Fig. 1 e The XRD patterns of the different non-noble metal cocatalysts.
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of the reaction solutions was always kept at room temperature by using cooling water during the reaction. The amount of H2 was determined by an online gas chromatograph (5 Å molecular sieve column, TCD) using Ar gas as carrier. The apparent quantum efficiency (QE) was measured under the same photocatalytic reaction condition with irradiation light at 420 nm by using combined band-pass (Kenko) and cut-off filters (HOYA) and 300 W Xe lamp. The number of incident photons was measured by using a calibrated Si photodiode. The QE was calculated according to the following equation: QE [%] ¼ number of reacted electrons/number of incident photons 100 ¼ number of evolved H2 molecules 2/ number of incident photons 100.
Photocatalytic H2 evolution
Results and discussion The photocatalytic hydrogen evolution reactions were performed in a closed and evacuation system with a reactor of quartz and a visible-light source of 300 W Xe lamp (Perfect, China) connected to a 420 nm filter (l 420 nm). Typically, 0.1 g of the photocatalyst was suspended in a 100 mL mixed solution of 10 mL lactic acid and 90 mL deionized water using a magnetic stirrer. The suspension solutions were always degassed for 30 min to completely remove the air from the system and then irradiated by the light, and the temperature
Preparation and characterization of cocatalysts and photocatalysts Fig. 1 shows the XRD patterns of the different non-noble metal cocatalysts. The XRD patterns of GR, Mo2C and Mo2N agree with the JCPDS#26-1079, 35-0787 and 25-1366, respectively, which shows the GR, Mo2C and Mo2N are well synthesized. The XRD patterns of Mo2N/Mo2C are almost
Fig. 2 e (aed) The precise XRD patterns of different non-noble metal. cocatalysts. Please cite this article in press as: Ma B, et al., A novel ultraefficient non-noble metal composite cocatalyst Mo2N/Mo2C/graphene for enhanced photocatalytic H2 evolution, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.212
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corresponding to that of Mo2N (43.48 and 63.30 ) and Mo2C (34.27, 39.43 and 51.9 ), which demonstrates the compounds of Mo2N/Mo2C are composed of Mo2N and Mo2C. The introduction of GR into the sole Mo2N or Mo2C hardly changes XRD patterns of the corresponding cocatalyst. However, from the precise XRD patterns of the cocatalysts (Fig. 2aed), it is interesting the diffraction peaks of both Mo2N/Mo2C and Mo2N/Mo2C/GR slightly shift towards low angles corresponding to that of the sole Mo2N (Fig. 2d, 63.30 and Fig. 2c, 43.48 ) and lightly shift towards high angles corresponding to that of the sole Mo2C (Fig. 2d, 61.47 and Fig. 2a, 34.27 , Fig. 2b, 39.27 ). This means the Mo2N and Mo2C are not simple mixture in Mo2N/Mo2C and Mo2N/Mo2C/GR. There is some strong interaction at the interface between Mo2N and Mo2C. The interface structure is special, which is similar but different to Mo2N and Mo2C simultaneously. Besides, the XRD patterns of Mo2N/Mo2C/GR slight shift towards low angles corresponding to that of Mo2N/Mo2C (Figs. 1 and 2aed) after the introduction of GR into Mo2N/Mo2C, which means GR here has some strong interaction with Mo2N/ Mo2C composite. The strong interaction among the compositions in Mo2N/Mo2C and Mo2N/Mo2C/GR is reasonable according to the preparation methods of the cocatalysts which use carbamide both as carbon and nitrogen sources. Because XRD is a technique to analysis the structure of bulk materials, there is much interface with special structure between Mo2N and Mo2C according to the apparent XRD shift. So, Mo2N/Mo2C is a special and metastable structure relative to the sole Mo2N or Mo2C, and the GR easily interacts with Mo2N/Mo2C when the GR are introduced into Mo2N/Mo2C. Fig. 3 shows the Raman spectra of different cocatalysts. The Raman bands at 1592.97 and 1347.54 cm1 can be assigned to the GR [15], and the Raman bands at 990.4, 824.88 cm1 and 1592.97 cm1 can be assigned to the Mo2N [40] and Mo2C [39], respectively. The Raman spectra of Mo2N/GR, Mo2C/GR and Mo2N/Mo2C/GR show strong scattering bands of GR, which demonstrates GR is introduced into the Mo-based cocatalysts. To investigate the valence states of Mo species in Mo2N/ Mo2C/GR, the X-ray photoelectron spectra (XPS) have been carried out for the Mo 3d of Mo2N/Mo2C/GR (Fig. 4). The Mo 3d (Fig. 4a) shows the binding energy peak at 232.9 eV and the high resolution XPS (Fig. 4b) indicate the peaks of the Mo 3d5/2 (228.5 eV) and Mo 3d 3/2 (231.6 eV) are typical values for Modþ (0 < d < 4) [39,40]. Also, the species of Mo are composed of Mo4þ (229.9 and 233.2 eV) and Mo6þ (232.5 and 235.4 eV) due to the easy surface oxidation in the air [39,40].
Fig. 3 e The Raman spectra of different non-noble metal cocatalysts.
Fig. 4 e The XPS (a) and high resolution XPS spectrum for Mo 3d (b) in Mo2N/Mo2C/GR.
The morphologies of the Mo2N/Mo2C/GR are shown in Fig. 5 with the GR and Mo2N/Mo2C for comparison by the transmission electron microscopy (TEM). The GR shows the typical nanosheet morphology (Fig. 5a) with some coarse fibers of several hundred nanometers longevity and the Mo2N/Mo2C presents the particle shape (Fig. 5b) with the particle size about 10e50 nm. The Mo2N/Mo2C/GR (Fig. 5c) shows the Mo2N/Mo2C particles scattering on the GR nanosheet. The high resolution TEM of Mo2N/Mo2C/GR (Fig. 5d) clearly presents the Mo2N/Mo2C particles of 10 nm particle size combine closely with the GR fiber of the 20 nm width. The precise structure of Mo2N/Mo2C (Fig. 5e) which scatters on the GR fiber in Fig. 5d shows the close combination between Mo2N and Mo2C. The lattice fringes of Mo2N with interplanar distances of 0.228 nm is indexed to the (200) planes of Mo2N while the lattice fringes of Mo2C with interplanar distances of 0.236 nm is corresponding to the (002) planes of Mo2C, respectively. More importantly, at the interface between Mo2N and Mo2C,
Please cite this article in press as: Ma B, et al., A novel ultraefficient non-noble metal composite cocatalyst Mo2N/Mo2C/graphene for enhanced photocatalytic H2 evolution, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.212
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Fig. 6 e The UVeVis DRS spectra of CdS combined with different non-noble metal cocatalysts.
Fig. 5 e TEM images of GR (a), Mo2N/Mo2C (b), Mo2N/Mo2C/ GR (c) and (d), and HRTEM of Mo2N/Mo2C particle scattering on the GR fiber in Fig. 5d (e).
the lattice fringes become distort and can be approximately attributed to that of the Mo2N and Mo2C simultaneously. The distort lattice fringes means the interface has the special structure and Mo2N/Mo2C is not simply mixture of Mo2N and Mo2C. Mo2N/Mo2C is a composite, instead. Fig. 6 shows the UV/Vis diffuse reflection spectroscopies (UVeVis DRS) of the photocatalysts. According to onset of absorption edge of CdS (593.2 nm), the band gap of CdS is 2.09 eV. Loading cocatalysts on CdS, the photocatalysts show an enhanced photoabsorption in visible light region comparing with the bare CdS. Besides, the band edges of Mo2N/GR/CdS, Mo2C/GR/CdS, Mo2N/Mo2C/CdS and Mo2N/ Mo2C/GR/CdS apparently shift towards long wavelength comparing with the bare CdS. The Mo2N/Mo2C/GR/CdS has the absorption edge with the largest wavelength of 633.0 nm, of which corresponding to the band gap of 1.96 eV. Fig. 7 shows the TEM images of the photocatalysts. The CdS (Fig. 6d) presents small particle size of 10 nm with some
Fig. 7 e TEM images of GR/CdS (a), Mo2N/Mo2C/CdS (b), Mo2N/Mo2C/GR/CdS (c) and CdS (d).
aggregation. The GR/CdS (Fig. 7a) shows the CdS scattering on the GR sheet. The Mo2N/Mo2C/CdS (Fig. 7b) and Mo2N/Mo2C/ GR/CdS (Fig. 7c) present the similar morphology of the sole CdS (Fig. 7d) due to the low amount of cocatalysts of 2.0 wt %.
Photocatalytic H2 evolution on cocatalysts/CdS The properties of non-noble metal cocatalysts in photocatalytic H2 evolution are shown in Fig. 8a. Compared with bare CdS, all the cocatalysts enhance the photocatalytic activity of CdS significantly indicating these materials are efficient cocatalysts in photocatalytic H2 evolution. The composite Mo2N/Mo2C is superior to the sole Mo2N or Mo2C for the enhanced H2 activities of CdS. After GR are introduced, the GR-based cocatalysts including Mo2N/GR, Mo2C/GR and Mo2N/ Mo2C/GR further increase the H2 evolution of CdS compared with the corresponding cocatalysts, respectively. The Mo2N/
Please cite this article in press as: Ma B, et al., A novel ultraefficient non-noble metal composite cocatalyst Mo2N/Mo2C/graphene for enhanced photocatalytic H2 evolution, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.212
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Fig. 9 e The effect of the loading amount of Pt on the photocatalytic H2 production of CdS.
Fig. 8 e The photocatalytic H2 evolution on bare CdS and CdS loaded with non-noble metal (a) and noble metal (b) cocatalysts.
Mo2C/GR cocatalyst shows the highest enhanced activity of CdS among all non-noble mental cocatalysts. The optimum composite 2.0 wt % Mo2N/Mo2C/GR/CdS achieves an ultrahigh H2 evolution rate of 4520 mmol/h/g with the apparent quantum efficiency of 10.3% at 420 nm, which is 18 times higher than that of bare CdS. The catalytic efficiency of Mo2N/Mo2C/GR compared with the noble metal cocatalysts is shown in Fig. 8b. Here, the loading amount of noble metals is 1.0 wt % according to the optimum loading amount of Pt as the typical noble metal (Fig. 9). The Mo2N/Mo2C/GR/CdS has the H2 evolution rate 3.6 times higher than the optimum Pt/CdS (Fig. 8b). More importantly, Mo2N/Mo2C/GR is superior to all the noble metal cocatalysts chosen here indicating the ultraefficiency of the non-noble metal cocatalyst Mo2N/Mo2C/GR. The ultraefficiency of the Mo2N/Mo2C/GR may have close relationship to its structure according to the characterization analysis. When Mo2N and Mo2C combine to form Mo2N/Mo2C composite, the Mo2N and Mo2C have strong interaction to form much interface of which its structure is different from the sole Mo2N (Fig. 2d, 63.30 and Fig. 2c, 43.48 ) or Mo2C (Fig. 2d, 61.47 , Fig. 2a, 34.27 and Fig. 2b, 39.27 ) according to the XRD shifts. The HRTEM image (Fig. 5e) demonstrates the special structure at the interface (heterojunction) between Mo2N and Mo2C. So, the photocatalytic H2 evolution activity on Mo2N/Mo2C/CdS enhances compared with the Mo2N/CdS or Mo2C/CdS. Also, when the GR are introduced into Mo2N/
Mo2C, the GR easily interacts with Mo2N/Mo2C relative to the sole Mo2N or Mo2C (Figs. 1 and 2aed) due to the interface effect between GR and Mo2N/Mo2C. The interface effects among the compositions of Mo2N/Mo2C/GR (Mo2N with Mo2C, GR with Mo2N/Mo2C) may cause convenient electron transfer and distribution in the cocatalyst and lead to the superior catalytic property of the cocatalyst. Besides, the photoexcited electrons in the conduction band of the semiconductor can easily transfer to the Mo2N/Mo2C through GR due to the excellent electroconductivity of GR. So, the photocatalytic H2 evolution activity on Mo2N/Mo2C/GR/CdS enhances further and reaches the maximum among all cocatalysts loaded CdS. It is worthwhile to note the strong and wide light absorption (Fig. 6) when CdS combines with Mo2N/Mo2C/GR also accounts for the maximum activity of Mo2N/Mo2C/GR/CdS. The Scheme 1 is the proposed mechanism of photocatalytic H2 evolution reaction on Mo2N/Mo2C/GR/CdS. The Mo2N/Mo2C with special interface structure (heterojunction) combines closely on the GR surface to form the cocatalyst
Scheme 1 e The proposed mechanism of photocatalytic H2 evolution reaction on Mo2N/Mo2C/GR/CdS.
Please cite this article in press as: Ma B, et al., A novel ultraefficient non-noble metal composite cocatalyst Mo2N/Mo2C/graphene for enhanced photocatalytic H2 evolution, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.212
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Mo2N/Mo2C/GR. The CdS is deposited on the surface of GR to form the photocatalyst Mo2N/Mo2C/GR/CdS. When the CdS is excited under visible light irradiation, the photoexcited electrons leap into the conduction band of CdS, and holes are left behind in the valence band of CdS. The electrons in the conduction band of CdS transfer easily through the GR into Mo2N/ Mo2C and reduce the water into hydrogen at the surface of Mo2N/Mo2C. At the same time, the holes in the valence band of CdS can be annihilated by the oxidation of the lactic acid into lactic acid oxides. The superior catalytic property of Mo2N/Mo2C/GR due to the interface effects among the composition of the cocatalysts, the excellent electroconductivity of GR, along with the strong and wide light absorption of Mo2N/Mo2C/GR/CdS are the main reasons of the ultrahigh photocatalytic activity of Mo2N/Mo2C/GR/CdS.
Conclusion A new ultraefficient non-noble metal cocatalyst Mo2N/Mo2C/ GR was developed for enhanced photocatalytic H2 production of CdS. The Mo2N/Mo2C/GR is the most highest efficient among all Mo and GR based cocatalysts and all of noble metals cocatalysts researched here. The H2 evolution of optimum 2.0 wt % Mo2N/Mo2C/GR/CdS is 18 times higher than that of CdS alone and 3.6 times higher than the optimum 1.0 wt % Pt/ CdS. The XRD shifts and TEM images demonstrate the interface structures both between Mo2N and Mo2C, and between GR and Mo2N/Mo2C.The interface effect among the composites of Mo2N/Mo2C/GR leads to the superior catalytic H2 evolution property of the cocatalysts. Also, the excellent electroconductivity of GR and the strong and wide light absorption of Mo2N/Mo2C/GR/CdS accounts for the ultrahigh photocatalytic activity of Mo2N/Mo2C/GR/CdS. This study presents a novel ultraefficient non-noble metal cocatalyst Mo2N/Mo2C/GR and reveals the interface effect among the compositions of the cocatalyst. The findings would open a promising way to design and fabricate the efficient composite cocatalyst.
Acknowledgements This work was supported by University Research Project of Ningxia (NGY2015027), National Natural Science Foundation of China (NSFC, Grant Nos. 21263018), Project of Science and Technology of Personnel of Study Abroad (NingXia (2014) 486) and Project of One Hundred Talented People of Ningxia Province, and “Light of West China” Visiting Scholar Program of the CPC's Organization Department.
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A novel ultraefficient non-noble metal composite cocatalyst Mo2N/Mo2C/graphene for enhanced photocatalytic H2 evolution Baojun Ma*, Xiaoyan Wang, Keying Lin, Jie Li, Yahui Liu, Haijuan Zhan, Wanyi Liu State Key Laboratory of High-Efficiency Coal Utilization and Green Chemical Engineering, College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan, 750021, People's Republic of China
article info
abstract
Article history:
Non-noble metal cocatalyst is a hot research issue in photocatalytic H2 production due to
Received 5 March 2017
its low cost and potential substitution of the usually used expensive noble metal cocata-
Received in revised form
lyst. Here, we reported a new non-noble metal cocatalyst Mo2N/Mo2C/graphene (GR) for
27 April 2017
ultraefficiently enhanced photocatalytic H2 production of CdS under visible light irradia-
Accepted 30 May 2017
tion. The optimum composite 2.0 wt % Mo2N/Mo2C/GR/CdS achieves an ultrahigh H2 evo-
Available online xxx
lution rate of 4520 mmol/h/g, which is 18 times higher than that of CdS alone and 3.6 times higher than the optimum 1.0 wt % Pt/CdS. The XRD shifts and HRTEM demonstrates the
Keywords:
special structure (heterojunction) between the interfaces of Mo2N and Mo2C, which mainly
Photocatalytic
accounts for the ultraefficience of Mo2N/Mo2C/GR. This study firstly presents a novel ul-
Cocatalyst
traefficient non-noble metal cocatalyst Mo2N/Mo2C/GR and reveals the interface effects
H2 evolution
among the compositions of the cocatalyst.
Molybdenum
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Graphene Interface effect
Introduction Photocatalytic H2 production on semiconductor photocatalyst has attracted much attention for the last three decades due to the use of clean, abundant and sustainable solar energy to achieve the clean H2 energy [1e4]. In a system of photocatalytic H2 production, the cocatalyst is one of the key factor to obtain efficient H2 production [5e7]. Unfortunately, the cocatalyst is generally mainly composed of noble metals such as Pt, Ru, Pd, Au, Rh which leads to the expensive photocatalyst [8e10]. So, developing the efficient, low cost non-
noble metal cocatalyst has been a hot and indispensable research issue in the field of photocatalytic H2 production [11e18]. Recently, a series of efficient dual coupled semiconductors with structure of junctions (such as a PN junction, heterojunction, solid solution phase junction, surface phase junction, etc.) had been reported for enhanced photocatalytic activity [19e25]. The junction structure formed at the interfaces of the two semiconductors is favor of photoexcited electrons transfer and separation in the two semiconductors. Also, two noble metal cocatalysts had been found the synergistic effect on the photocatalytic H2 evolution [26e28] and
* Corresponding author. E-mail address: [email protected] (B. Ma). http://dx.doi.org/10.1016/j.ijhydene.2017.05.212 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ma B, et al., A novel ultraefficient non-noble metal composite cocatalyst Mo2N/Mo2C/graphene for enhanced photocatalytic H2 evolution, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.212
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seemed to form the materials with new structure in the reaction process. More interestingly, the Rh/Cr2O3 with core/ shell structure had been designed for efficient photocatalytic water splitting due to the efficient inhibition of reversible reaction of water splitting by the shell of Cr2O3 [29e33]. All these indicate the strategy of assembly and structure control of material with special structure for superior catalytic property is effective, whether the material is formed by two semiconductors or two cocatalysts. Molybdenum-based compounds including Molybdenum carbide, Molybdenum nitride and Molybdenum phosphide have similar properties of Pt, and extensively applied in hydrogenation and dehydrogenation process [34e38]. Recently, the author had reported Mo2C and Mo2N as the non-noble cocatalysts combined with CdS for enhanced photocatalytic H2 production under visible light irradiation [39,40]. The Mo2C/ CdS has the photocatalytic H2 evolution rate higher of 2.3 times of Pt/CdS, whereas the Mo2N/CdS has the photocatalytic H2 evolution rate similar to Pt/CdS. The structure and chemical constituent of Mo2N and Mo2C can be controlled and conversed by precise control of the preparation method to form Mo2N/ Mo2C composite [41,42]. To our knowledge, it had been not reported the relationship between structure of composite cocatalyst and catalytic property in photocatalytic H2 evolution, of which the composite cocatalysts Mo2N/Mo2C is composed of two non-noble metal cocatalysts Mo2N and Mo2C. Graphene (GR) has been a promising material with high specific surface area and superior electron mobility and combining graphene with cocatalyst to achieve highly efficient new cocatalyst in photocatalytic H2 production have been reported [15,18,43e46]. So, It is anticipating to combine Mo2N/Mo2C and graphene to form a new three component cocatalyst Mo2N/Mo2C/GR to achieve much higher photocatalytic H2 evolution rate on semiconductor. In this paper, Mo2N/Mo2C/GR is prepared by calcination of the precursor from the mixed MoCl5, carbamide and graphene oxide under N2 atmosphere. CdS semiconductor is chosen as a needle probe to evaluate the cocatalyst and the Mo2N/Mo2C/ graphene/CdS composite is prepared by a large scale method of deposition of CdS on Mo2N/Mo2C/GR. The photocatalytic activity of cocatalyst/CdS had been tested, and the structure of Mo2N/Mo2C and the interface effects among the compositions of the cocatalyst Mo2N/Mo2C/GR had been focused.
For preparation of Mo2C and Mo2N/Mo2C, all the procedures are same as that of Mo2N excepted the amount of carbamide was 3.52 g (R ¼ 8) and 0.44 g (R ¼ 1), respectively.
Preparation of graphene, Mo2N/graphene, Mo2C/graphene and Mo2N/Mo2C/graphene The graphene oxide was chosen as the precursor for preparation of graphene based non-noble metal cocatalyst. The preparation methods of Mo2N/GR, Mo2C/GR and Mo2N/Mo2C/ GR are same as that of Mo2N, Mo2C and Mo2N/Mo2C, respectively, except 0.35 g graphene oxide was firstly added in ethanol solution. For the preparation of graphene, 0.35 g graphene oxide and 0.22 g carbamide were added in 50 mL ethanol solution, and then the mixed solution was evaporated to dryness at 80 C. Last, the remaining residue was calcined at 750 C under N2 atmosphere for 4 h.
Preparation of CdS, CdS loaded with different non-noble metal cocatalysts and CdS loaded with different noble metal cocatalysts Here, all reagents (Sinopharm Chemical Reagent Co., Ltd) were analytically pure and commercially available, and used without further purification. CdS was prepared by precipitation method. Typically, 0.14 M Na2S solution (120 mL) was added dropwise in 0.14 M cadmium acetate solution (100 mL) under vigorous agitation and then aged for 12 h. Last, the deposit was filtered by distilled water and ethanol several times and then dried at 60 C for 12 h in vacuum drying oven. Non-noble metal cocatalyst/CdS was prepared by precipitation method. To take 2.0 wt % Mo2N/CdS for example, Mo2N (0.04 g) was firstly dispersed in 0.14 M cadmium acetate solution (100 mL), other preparation steps are same as that CdS. For the preparation of noble metal cocatalyst/CdS, 1.0 wt % Pt (Rh, Pd, Ru, and Au) was loaded on 0.1 g CdS through an in situ photo deposition method same as that test of photocatalytic H2 evolution. The photo deposition reaction conditions are of 10% lactic acid aqueous solution (100 mL) as holes scavenger and 300W Xe lamp (Perfect, China) with 420 nm filter (l 420 nm) as light resource. Pt (Rh, Pd, Ru, and Au) was obtained from precursor of H2PtCl6$6H2O (RhCl3$3H2O, PdCl2, RuCl3, and HAuCl4$4H2O).
Characterization of materials
Experimental Preparation of Mo2N, Mo2C and Mo2N/Mo2C Mo based compounds were synthesized by calcination of the precursor from the mixed MoCl5 (Sinopharm, china), carbamide (Sinopharm, china) and/or graphene oxide (Institute of coal chemistry, Chinese academy of sciences) under N2 atmosphere. For preparation of Mo2N, typically, 2.0 g MoCl5 was added in 50 mL ethanol solution with stirring. After MoCl5 was completely dissolved, 0.22 g carbamide (the molar ratio (R) of carbamide/Mo ¼ 0.5) was added in the solution. The mixed solution was stirred vigorously for 1 h, and then evaporated to dryness at 80 C. Last, the remaining viscous residue was calcined at 750 C under N2 atmosphere for 4 h.
The X-ray diffraction (XRD) patterns were obtained by a D/ MAX2500 diffractometer (Rigaku, Japan) with Cu Ka radiation. The UV/Vis diffuse reflection spectroscopies (UVeVis DRS) were recorded by a spectrometer (U-4100) with BaSO4 used for the corrected base line. The Raman spectra were obtained by a Horiba Jobin Yvon Lab RAM HR80 spectrograph with an Ar ion laser of 532 nm. The scanning electron microscopy (SEM) images were obtained by a scanning electron microscope (JSM7500F). The transmission electron microscopy (TEM) images were obtained by a transmission electron microscope (F20). The X-ray photoelectron spectra (XPS) were recorded by a Thermo ESCALAB 250Xi spectrometer with Al Ka radiation (hn ¼ 1486.6 eV, 150 W) and the C1s (284.8 eV) used for corrected peak.
Please cite this article in press as: Ma B, et al., A novel ultraefficient non-noble metal composite cocatalyst Mo2N/Mo2C/graphene for enhanced photocatalytic H2 evolution, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.212
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Fig. 1 e The XRD patterns of the different non-noble metal cocatalysts.
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of the reaction solutions was always kept at room temperature by using cooling water during the reaction. The amount of H2 was determined by an online gas chromatograph (5 Å molecular sieve column, TCD) using Ar gas as carrier. The apparent quantum efficiency (QE) was measured under the same photocatalytic reaction condition with irradiation light at 420 nm by using combined band-pass (Kenko) and cut-off filters (HOYA) and 300 W Xe lamp. The number of incident photons was measured by using a calibrated Si photodiode. The QE was calculated according to the following equation: QE [%] ¼ number of reacted electrons/number of incident photons 100 ¼ number of evolved H2 molecules 2/ number of incident photons 100.
Photocatalytic H2 evolution
Results and discussion The photocatalytic hydrogen evolution reactions were performed in a closed and evacuation system with a reactor of quartz and a visible-light source of 300 W Xe lamp (Perfect, China) connected to a 420 nm filter (l 420 nm). Typically, 0.1 g of the photocatalyst was suspended in a 100 mL mixed solution of 10 mL lactic acid and 90 mL deionized water using a magnetic stirrer. The suspension solutions were always degassed for 30 min to completely remove the air from the system and then irradiated by the light, and the temperature
Preparation and characterization of cocatalysts and photocatalysts Fig. 1 shows the XRD patterns of the different non-noble metal cocatalysts. The XRD patterns of GR, Mo2C and Mo2N agree with the JCPDS#26-1079, 35-0787 and 25-1366, respectively, which shows the GR, Mo2C and Mo2N are well synthesized. The XRD patterns of Mo2N/Mo2C are almost
Fig. 2 e (aed) The precise XRD patterns of different non-noble metal. cocatalysts. Please cite this article in press as: Ma B, et al., A novel ultraefficient non-noble metal composite cocatalyst Mo2N/Mo2C/graphene for enhanced photocatalytic H2 evolution, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.212
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corresponding to that of Mo2N (43.48 and 63.30 ) and Mo2C (34.27, 39.43 and 51.9 ), which demonstrates the compounds of Mo2N/Mo2C are composed of Mo2N and Mo2C. The introduction of GR into the sole Mo2N or Mo2C hardly changes XRD patterns of the corresponding cocatalyst. However, from the precise XRD patterns of the cocatalysts (Fig. 2aed), it is interesting the diffraction peaks of both Mo2N/Mo2C and Mo2N/Mo2C/GR slightly shift towards low angles corresponding to that of the sole Mo2N (Fig. 2d, 63.30 and Fig. 2c, 43.48 ) and lightly shift towards high angles corresponding to that of the sole Mo2C (Fig. 2d, 61.47 and Fig. 2a, 34.27 , Fig. 2b, 39.27 ). This means the Mo2N and Mo2C are not simple mixture in Mo2N/Mo2C and Mo2N/Mo2C/GR. There is some strong interaction at the interface between Mo2N and Mo2C. The interface structure is special, which is similar but different to Mo2N and Mo2C simultaneously. Besides, the XRD patterns of Mo2N/Mo2C/GR slight shift towards low angles corresponding to that of Mo2N/Mo2C (Figs. 1 and 2aed) after the introduction of GR into Mo2N/Mo2C, which means GR here has some strong interaction with Mo2N/ Mo2C composite. The strong interaction among the compositions in Mo2N/Mo2C and Mo2N/Mo2C/GR is reasonable according to the preparation methods of the cocatalysts which use carbamide both as carbon and nitrogen sources. Because XRD is a technique to analysis the structure of bulk materials, there is much interface with special structure between Mo2N and Mo2C according to the apparent XRD shift. So, Mo2N/Mo2C is a special and metastable structure relative to the sole Mo2N or Mo2C, and the GR easily interacts with Mo2N/Mo2C when the GR are introduced into Mo2N/Mo2C. Fig. 3 shows the Raman spectra of different cocatalysts. The Raman bands at 1592.97 and 1347.54 cm1 can be assigned to the GR [15], and the Raman bands at 990.4, 824.88 cm1 and 1592.97 cm1 can be assigned to the Mo2N [40] and Mo2C [39], respectively. The Raman spectra of Mo2N/GR, Mo2C/GR and Mo2N/Mo2C/GR show strong scattering bands of GR, which demonstrates GR is introduced into the Mo-based cocatalysts. To investigate the valence states of Mo species in Mo2N/ Mo2C/GR, the X-ray photoelectron spectra (XPS) have been carried out for the Mo 3d of Mo2N/Mo2C/GR (Fig. 4). The Mo 3d (Fig. 4a) shows the binding energy peak at 232.9 eV and the high resolution XPS (Fig. 4b) indicate the peaks of the Mo 3d5/2 (228.5 eV) and Mo 3d 3/2 (231.6 eV) are typical values for Modþ (0 < d < 4) [39,40]. Also, the species of Mo are composed of Mo4þ (229.9 and 233.2 eV) and Mo6þ (232.5 and 235.4 eV) due to the easy surface oxidation in the air [39,40].
Fig. 3 e The Raman spectra of different non-noble metal cocatalysts.
Fig. 4 e The XPS (a) and high resolution XPS spectrum for Mo 3d (b) in Mo2N/Mo2C/GR.
The morphologies of the Mo2N/Mo2C/GR are shown in Fig. 5 with the GR and Mo2N/Mo2C for comparison by the transmission electron microscopy (TEM). The GR shows the typical nanosheet morphology (Fig. 5a) with some coarse fibers of several hundred nanometers longevity and the Mo2N/Mo2C presents the particle shape (Fig. 5b) with the particle size about 10e50 nm. The Mo2N/Mo2C/GR (Fig. 5c) shows the Mo2N/Mo2C particles scattering on the GR nanosheet. The high resolution TEM of Mo2N/Mo2C/GR (Fig. 5d) clearly presents the Mo2N/Mo2C particles of 10 nm particle size combine closely with the GR fiber of the 20 nm width. The precise structure of Mo2N/Mo2C (Fig. 5e) which scatters on the GR fiber in Fig. 5d shows the close combination between Mo2N and Mo2C. The lattice fringes of Mo2N with interplanar distances of 0.228 nm is indexed to the (200) planes of Mo2N while the lattice fringes of Mo2C with interplanar distances of 0.236 nm is corresponding to the (002) planes of Mo2C, respectively. More importantly, at the interface between Mo2N and Mo2C,
Please cite this article in press as: Ma B, et al., A novel ultraefficient non-noble metal composite cocatalyst Mo2N/Mo2C/graphene for enhanced photocatalytic H2 evolution, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.212
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Fig. 6 e The UVeVis DRS spectra of CdS combined with different non-noble metal cocatalysts.
Fig. 5 e TEM images of GR (a), Mo2N/Mo2C (b), Mo2N/Mo2C/ GR (c) and (d), and HRTEM of Mo2N/Mo2C particle scattering on the GR fiber in Fig. 5d (e).
the lattice fringes become distort and can be approximately attributed to that of the Mo2N and Mo2C simultaneously. The distort lattice fringes means the interface has the special structure and Mo2N/Mo2C is not simply mixture of Mo2N and Mo2C. Mo2N/Mo2C is a composite, instead. Fig. 6 shows the UV/Vis diffuse reflection spectroscopies (UVeVis DRS) of the photocatalysts. According to onset of absorption edge of CdS (593.2 nm), the band gap of CdS is 2.09 eV. Loading cocatalysts on CdS, the photocatalysts show an enhanced photoabsorption in visible light region comparing with the bare CdS. Besides, the band edges of Mo2N/GR/CdS, Mo2C/GR/CdS, Mo2N/Mo2C/CdS and Mo2N/ Mo2C/GR/CdS apparently shift towards long wavelength comparing with the bare CdS. The Mo2N/Mo2C/GR/CdS has the absorption edge with the largest wavelength of 633.0 nm, of which corresponding to the band gap of 1.96 eV. Fig. 7 shows the TEM images of the photocatalysts. The CdS (Fig. 6d) presents small particle size of 10 nm with some
Fig. 7 e TEM images of GR/CdS (a), Mo2N/Mo2C/CdS (b), Mo2N/Mo2C/GR/CdS (c) and CdS (d).
aggregation. The GR/CdS (Fig. 7a) shows the CdS scattering on the GR sheet. The Mo2N/Mo2C/CdS (Fig. 7b) and Mo2N/Mo2C/ GR/CdS (Fig. 7c) present the similar morphology of the sole CdS (Fig. 7d) due to the low amount of cocatalysts of 2.0 wt %.
Photocatalytic H2 evolution on cocatalysts/CdS The properties of non-noble metal cocatalysts in photocatalytic H2 evolution are shown in Fig. 8a. Compared with bare CdS, all the cocatalysts enhance the photocatalytic activity of CdS significantly indicating these materials are efficient cocatalysts in photocatalytic H2 evolution. The composite Mo2N/Mo2C is superior to the sole Mo2N or Mo2C for the enhanced H2 activities of CdS. After GR are introduced, the GR-based cocatalysts including Mo2N/GR, Mo2C/GR and Mo2N/ Mo2C/GR further increase the H2 evolution of CdS compared with the corresponding cocatalysts, respectively. The Mo2N/
Please cite this article in press as: Ma B, et al., A novel ultraefficient non-noble metal composite cocatalyst Mo2N/Mo2C/graphene for enhanced photocatalytic H2 evolution, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.212
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Fig. 9 e The effect of the loading amount of Pt on the photocatalytic H2 production of CdS.
Fig. 8 e The photocatalytic H2 evolution on bare CdS and CdS loaded with non-noble metal (a) and noble metal (b) cocatalysts.
Mo2C/GR cocatalyst shows the highest enhanced activity of CdS among all non-noble mental cocatalysts. The optimum composite 2.0 wt % Mo2N/Mo2C/GR/CdS achieves an ultrahigh H2 evolution rate of 4520 mmol/h/g with the apparent quantum efficiency of 10.3% at 420 nm, which is 18 times higher than that of bare CdS. The catalytic efficiency of Mo2N/Mo2C/GR compared with the noble metal cocatalysts is shown in Fig. 8b. Here, the loading amount of noble metals is 1.0 wt % according to the optimum loading amount of Pt as the typical noble metal (Fig. 9). The Mo2N/Mo2C/GR/CdS has the H2 evolution rate 3.6 times higher than the optimum Pt/CdS (Fig. 8b). More importantly, Mo2N/Mo2C/GR is superior to all the noble metal cocatalysts chosen here indicating the ultraefficiency of the non-noble metal cocatalyst Mo2N/Mo2C/GR. The ultraefficiency of the Mo2N/Mo2C/GR may have close relationship to its structure according to the characterization analysis. When Mo2N and Mo2C combine to form Mo2N/Mo2C composite, the Mo2N and Mo2C have strong interaction to form much interface of which its structure is different from the sole Mo2N (Fig. 2d, 63.30 and Fig. 2c, 43.48 ) or Mo2C (Fig. 2d, 61.47 , Fig. 2a, 34.27 and Fig. 2b, 39.27 ) according to the XRD shifts. The HRTEM image (Fig. 5e) demonstrates the special structure at the interface (heterojunction) between Mo2N and Mo2C. So, the photocatalytic H2 evolution activity on Mo2N/Mo2C/CdS enhances compared with the Mo2N/CdS or Mo2C/CdS. Also, when the GR are introduced into Mo2N/
Mo2C, the GR easily interacts with Mo2N/Mo2C relative to the sole Mo2N or Mo2C (Figs. 1 and 2aed) due to the interface effect between GR and Mo2N/Mo2C. The interface effects among the compositions of Mo2N/Mo2C/GR (Mo2N with Mo2C, GR with Mo2N/Mo2C) may cause convenient electron transfer and distribution in the cocatalyst and lead to the superior catalytic property of the cocatalyst. Besides, the photoexcited electrons in the conduction band of the semiconductor can easily transfer to the Mo2N/Mo2C through GR due to the excellent electroconductivity of GR. So, the photocatalytic H2 evolution activity on Mo2N/Mo2C/GR/CdS enhances further and reaches the maximum among all cocatalysts loaded CdS. It is worthwhile to note the strong and wide light absorption (Fig. 6) when CdS combines with Mo2N/Mo2C/GR also accounts for the maximum activity of Mo2N/Mo2C/GR/CdS. The Scheme 1 is the proposed mechanism of photocatalytic H2 evolution reaction on Mo2N/Mo2C/GR/CdS. The Mo2N/Mo2C with special interface structure (heterojunction) combines closely on the GR surface to form the cocatalyst
Scheme 1 e The proposed mechanism of photocatalytic H2 evolution reaction on Mo2N/Mo2C/GR/CdS.
Please cite this article in press as: Ma B, et al., A novel ultraefficient non-noble metal composite cocatalyst Mo2N/Mo2C/graphene for enhanced photocatalytic H2 evolution, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.212
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Mo2N/Mo2C/GR. The CdS is deposited on the surface of GR to form the photocatalyst Mo2N/Mo2C/GR/CdS. When the CdS is excited under visible light irradiation, the photoexcited electrons leap into the conduction band of CdS, and holes are left behind in the valence band of CdS. The electrons in the conduction band of CdS transfer easily through the GR into Mo2N/ Mo2C and reduce the water into hydrogen at the surface of Mo2N/Mo2C. At the same time, the holes in the valence band of CdS can be annihilated by the oxidation of the lactic acid into lactic acid oxides. The superior catalytic property of Mo2N/Mo2C/GR due to the interface effects among the composition of the cocatalysts, the excellent electroconductivity of GR, along with the strong and wide light absorption of Mo2N/Mo2C/GR/CdS are the main reasons of the ultrahigh photocatalytic activity of Mo2N/Mo2C/GR/CdS.
Conclusion A new ultraefficient non-noble metal cocatalyst Mo2N/Mo2C/ GR was developed for enhanced photocatalytic H2 production of CdS. The Mo2N/Mo2C/GR is the most highest efficient among all Mo and GR based cocatalysts and all of noble metals cocatalysts researched here. The H2 evolution of optimum 2.0 wt % Mo2N/Mo2C/GR/CdS is 18 times higher than that of CdS alone and 3.6 times higher than the optimum 1.0 wt % Pt/ CdS. The XRD shifts and TEM images demonstrate the interface structures both between Mo2N and Mo2C, and between GR and Mo2N/Mo2C.The interface effect among the composites of Mo2N/Mo2C/GR leads to the superior catalytic H2 evolution property of the cocatalysts. Also, the excellent electroconductivity of GR and the strong and wide light absorption of Mo2N/Mo2C/GR/CdS accounts for the ultrahigh photocatalytic activity of Mo2N/Mo2C/GR/CdS. This study presents a novel ultraefficient non-noble metal cocatalyst Mo2N/Mo2C/GR and reveals the interface effect among the compositions of the cocatalyst. The findings would open a promising way to design and fabricate the efficient composite cocatalyst.
Acknowledgements This work was supported by University Research Project of Ningxia (NGY2015027), National Natural Science Foundation of China (NSFC, Grant Nos. 21263018), Project of Science and Technology of Personnel of Study Abroad (NingXia (2014) 486) and Project of One Hundred Talented People of Ningxia Province, and “Light of West China” Visiting Scholar Program of the CPC's Organization Department.
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Please cite this article in press as: Ma B, et al., A novel ultraefficient non-noble metal composite cocatalyst Mo2N/Mo2C/graphene for enhanced photocatalytic H2 evolution, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.212