Improved photocatalytic hydrogen evolution of CdS using earth

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Improved photocatalytic hydrogen evolution of CdS using earthabundant co-catalyst Mo2N with rod shape and large capacitance Baojun Ma, Jiawei Zhang, Keying Lin, Dekang Li, Yahui Liu, and Xu Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.9b03334 • Publication Date (Web): 12 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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ACS Sustainable Chemistry & Engineering

Improved Photocatalytic Hydrogen Evolution of CdS Using Earth-abundant Co-catalyst Mo2N with Rod Shape and Large Capacitance Baojun Ma,* Jiawei Zhang, Keying Lin, Dekang Li, Yahui Liu, XuYang State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, College of Chemistry and Chemical Engineering, Ningxia University, No. 489 Helanshan West Road, Xixia District, Yinchuan, 750021, People’s Republic of China Corresponding author: [email protected]

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Abstract: Earth-abundant catalysts (co-catalysts) for photocatalysis, photoelectrocatalysis and electrocatalysis have attracted increased attentions due to the potentially reduced cost for large scale application. Here, we firstly report and demonstrate that improving the capacitance of co-catalysts of Mo2N by modifying the morphology and orientations could produce a better photocatalytic hydrogen evolution reaction (HER) activity on CdS. Among rod, sheet, sphere shapes of Mo2N, the (111) orientated rod h-Mo2N was found to be the most efficient. The hydrogen evolution activity of h-Mo2N / CdS is 6.1 times of the Pt / CdS and 30.1 times of the bare CdS. Controlling experiment showed that the superior photocatalytic HER activity can be attributed to the higher capacitance of h-Mo2N. This study opens up a pathway to develop novel co-catalysts for photocatalyst by adjusting the capacitance through morphology controls.

Keywords: co-catalyst；capacitance；molybdenum nitride； photocatalytic H2 evolution；non-noble metal

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Introduction With the increasing of energy and environmental crisis, photocatalytic H2 production on semiconductor based photocatalysts has attracted increased attention. Generally, the efficiency and cost of a photocatalyst, which is usually composed of semiconductor and co-catalyst, are the two key obstacles for its wide application[1, 2, 3, 4, 5]. The catalytic activity of a photocatalyst can be affected by physical properties of both the semiconductor and the co-catalyst[6, 7, 8, 9, 10, 11]. However, only the effects of semiconductor and noble metals as the co-catalytst on photocatalytic activity have been extensively studied. For example, the CdS with {111}[12], ultra-thin SnS2 nanosheets with {001}[13], BiOBr nanosheets with {001}[14,15], BiVO4 with {040}[16,17,18] and TiO2 with {001}[19, 20] crystal planes have been reported for enhanced photocatalytic activities on H2 evolution, nitrogen fixation and organic pollutant degradation. Specific morphology and crystal facet of semiconductors affect the distribution and transportation of photo-excited electrons and provide more active sites, hence presenting superior photocatalytic activity. Co-catalyst, as an indispensable part of photocatalyst, can increase the photocatalytic efficiency by decreasing the activation energy of the H2 evolution reaction[21]. The properties of noble co-catalyst with special morphologies and crystal facets or the co3



catalyst loaded on specific crystal facets of semiconductor have been reported for enhanced photocatalytic activity, such as AgPt nanowires with {001}[22] crystal face, Au on TiO2 {001}[23] and Pt on TiO2 {001}[24]. While noble metals are mostly used as the co-catalysts, earthabundant non-noble metal co-catalysts[25, 26, 27, 28, 29, 30, 31] have been reported to potentially substitute the noble-metals. However, how the physical properties of non-noble metal co-catalysts affect the overall photocatalytic activities is much less investigated for an efficient and reliable photocatalytic system. Molybdenum-based compounds including sulfide, nitride, carbide, phosphide had been applied in hydrogenation and dehydrogenation process due to their similar properties of Pt[32, 33, 34, 35, 36, 37, 38]. We have reported a series of molybdenum-based compounds as efficient non-noble co-catalysts for photocatalytic H2 evolution[39, 40, 41, 42, 43] and reviewed the categories, structures, and roles of the molybdenumbased co-catalysts[44]. The junctions directly formed[45] or new species formed as bridge[46] connecting between semiconductor and co-catalyst are favorable to the transportation of the photo-excited electrons. In addition, multi-valence mixed and unstable molybdenum species also have better catalytic ability for H2 evolution[47, 48]. However, in order to get the intrinsic aspects of outstanding performance of the co-catalyst, the 4


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structure-properties relationship of this type of co-catalyst deserves further investigation. Here, we firstly synthesized three kinds of Mo2N with different morphologies: hexagonal, sheet, sphere particles, and found that the electrochemical HER activities of the different Mo2N were very similar. However, Mo2N of hexagonal rod with (111) orientation shows much higher photocatalytic HER activities when in conjunction with CdS as the light absorbing semiconductor. Controlled experiments revealed that the specific capacitance introduced by (111) orientation is one of the intrinsic factors of the co-catalyst and correlates positivity with the enhanced photocatalytic activity.

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Experimental Section Synthesis of Mo2N with different morphologies. The synthesis process of Mo2N with different morphologies includes two steps. The first step is the synthesis of MoO3 precursors with different morphologies[49]. For preparing MoO3 with hexagonal rod (hMoO3), 2.73 g ammonium heptamolybdate was completely dissolved in 10 ml of deionized water. Then, 15 mL of 67% concentrated nitric acid was added under stirring. The deposit was washed several times by deionized water and dried at 85℃ in an oven. For preparing MoO3 with sheet shape (sh-MoO3), 3mL ethylenediamine (EDA) was added to 10mL deionized water, and stirred vigorously for 20min, then 10 mL chitosan was added and stirred. Then, 20mL phosphomolybdic acid was added to the above solution and transferred into a Teflon-lined autoclave and heated at 200℃ for 24 h. The solution was filtered and washed, and then the obtained precipitate was calcined in air at 500℃ for 2h. For preparing MoO3 with sphere state (sp-MoO3), 10 mg / mL chitosan solution was added to 20 mL phosphomolybdic acid of 10 mg / mL, and the mixture was thoroughly stirred and placed in a Teflon-lined autoclave and heated at 200℃ for 24 h. The solution was centrifuged and the precipitate was collected and placed in a muffle furnace, and calcined in air at 500℃ for 2 h. The morphologies of the above MoO3 were 6


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characterized by scanning electron microscopy (SEM), as shown in Figure 1a. The second step is the nitridation of the MoO3 precursors to form Mo2N with corresponding morphologies[40]. The prepared MoO3 with different morphologies were nitrided under NH3 flow of 90 mL / min at 800℃ for 4 h (As show in the Figure S3, the Mo2N nitrided at 800 oC has the largest photocatalytic activity). After nitridation, the sample was then passivated using a mixed gas of N2 and O2 at a ratio of 99:1 for 12h at room temperature. Synthesis of Mo2N / CdS composite photocatalysts. A certain amount of Mo2N with different morphologies were dispersed in 50 mL of 0.14 moL / L Cd(NO3)2 is opropanol solution. 60mL Na2S solution of 0.14 moL / L was then added dropwise under vigorous agitation and then aged for 8 h at room temperature. The resulted precipitation along with the solution was transferred to Teflonlined stainless steel autoclave for hydrothermal treatment at 180°C for 24 h. The as-prepared composite were filtered and washed with deionized water several times and dried at 70°C for 8 h in vacuum drying oven. Characterizations. The crystal structures of MoO3 and corresponding Mo2N were determined by X-ray diffractometry (D / MAX2500, Rigaku, Japan) at 7



room temperature at a voltage of 4 kV under Cu-Kα radiation. The morphologies of samples were characterized using SEM (Hitachi, SU8020). The X-ray photoelectron spectra (XPS) data were obtained on a Thermo ESCALAB 250Xi spectrometer using Al K as the irradiation source (hν = 1486.6 eV, 150 W).The specific surface areas of Mo2N were measured by a nitrogen adsorption and desorption method on a Micromeritics ASAP-2000 Physisorption analyzer of BrunauereEmmette-Teller (BET) at 77 K. The photocatalytic activity of H2 evolution. The photocatalytic hydrogen evolution experiments were performed in a closed system (Perfect, China) of evacuation and gas circulation with a quartz cell. A 300W Xe lamp (Perfect, China) with a 420nm filter was used as visible light source. In detail, 0.1g of sample was dispersed in an aqueous solution (100 mL) containing 10mL lactic acid. The reaction system was pumped to vacuum for 30 min before irradiation. The hydrogen gas was analyzed by a gas chromatographic equipped with a 5Å molecular sieve column and TCD detector using Ar as the carrier gas. Photoelectrochemical measurements. Electrochemical and photoelectrochemical performance was studied on a CHI760D electrochemical work station (Shanghai Chenhua Instrument Co., Ltd, Shanghai, China) in a standard three-electrode 8


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system. Electrochemical impedance spectroscopies (EIS) and linear sweep voltammetry(LSV) of the samples were studied using the sample-coated FTO film as the working electrode, a Hg / HgO as a reference electrode, and a Pt wire as the counter electrode. The photoresponses (Transient photocurrent responses) were studied under 300 W Xenon lamp with a UV cutoff filter (λ > 420 nm). 0.5M Na2SO4 solution was used as the electrolyte. For the preparation of the working electrode FTO film, 20 mg of the sample was dispersed into ethanol and N, N-dimethylformamide with a volume ratio of 1:1 ( for per 1 ml of the mixture, 30 μL of Nafion D521 dispersion reagent was added as film-forming agent), then the mixture was ultrasonically treated until evenly dispersed. After that, 15 μL of the mixture was applied to the FTO glass, and the area of the working electrode was controlled by the adhesive tape to be 5 mm x 5 mm. After drying, the working electrode was transferred to a muffle furnace and calcined at 250℃ for 0.5 h to remove residual organic chemicals. Specific capacitance performances (Cyclic voltammetry, Galvanostatic charge/discharge) of the samples were carried out both in aqueous and organic solution using the sample-coated foam Ni as the working electrode and a Pt wire as the counter electrode. In aqueous 9



solution, 0.5M Na2SO4 solution was used as the electrolyte, and an Hg / HgO was used as a reference electrode. In organic solution, 0.1 M tetrabutylammonium hexafluorophosphate dissolved in acetonitrile solution was used as the electrolyte, and an Ag / AgCl was used as a reference electrode. For the preparation of the working electrode of foam Ni, 20 mg of the sample (active material), 2.5 mg activated carbon black (as conductive agent) and 2.5 mg of polyvinylidene fluoride (PVDF, as binder) was added into 1.0 ml of N-1-Methyl-2-pyrrolidinone (NMP, as the dispersant), then ground for 30 min, after that, 80 μL of the mixture was applied to foam Ni, and dried at 90℃ for 12 h, at last the working electrode was pressed at a pressure of 8 MPa for next testing. The specific capacitance is calculated according to the previous reports. ΔJ (A/g) is the difference value between anode current density and cathode current density, ΔJ (=Ja-Jc), v (V) is the scanning rate, ΔE (V) is the electrochemical window. Results and Discussion Physical properties of Mo2N. The morphologies of MoO3 samples were characterized by scanning electron microscope (SEM). As shown in Figure S1, the MoO3 with hexagonal rod (h-MoO3), sheet (sh-MoO3) and sphere (sp-MoO3) were successfully synthesized. X-ray diffraction (XRD) patterns of the three 10


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types of MoO3 reveals that the h-MoO3 corresponds to hexagonal (primitive) structure (PDF # 21-0569), while the sh-MoO3 and sp-MoO3 belong to Orthorhombic structure of Pbnm(62) (PDF # 35-0609) (Figure 1(a)). After nitridation, the morphologies of the MoO3 were maintained, as shown in the SEM images of Figure 1(b-d). The samples exhibit shapes of hexagonal rod, sheet and sphere, and were designated as h-Mo2N, shMo2N and sp-Mo2N, respectively. h-Mo2N exhibits the length of 10 to 50 μm and the diameter of 1.0 to 4.0 μm. There are some tiny burrs and holes on the surface of the rod (Figure 1(b)). sh-Mo2N are sheets with width of about 5 μm and thickness from 0.5 to 1.0 μm (Figure 1(c)), and sp-Mo2N shows sphere with a diameter of about 0.5 μm (Figure 1(d)). The bulk composition and structure were characterized by XRD (Figure 1(e)). Interestingly, the crystal structure, though different among the MoO3 precursors, becomes the same after NH3 treatment. The peak positions match with FCC Mo2N structure of Pm3m (221) (PDF # 251366, -Mo2N), which demonstrates the formation of crystallized Mo2N phase after nitridation. However, the relative peak intensities of the main peak (111) and (200) are tremendously different. The peak intensity of (111) is similar to (200) on sp-Mo2N, while the (111) peak is slightly higher than that of (200) peak on sh-Mo2N. However, h-Mo2N exhibits a 11



much stronger (111) peak than the (200) peak. Surface chemical compositions of the Mo in Mo2N were characterized by X-ray photoemission spectroscopy (XPS). Mo 3d core level spectra in the three kinds of Mo2N were compared in Figure 1(f) and Figure S2. The species of Mo in h-Mo2N are composed of Mo3+ (228.1 eV and 231.0 eV), Mo4+ (228.4 eV and 231.6 eV), Mo5+ (229.5 eV and 232.7 eV), Mo6+ (232.1 eV and 235.3 eV). The Mo 3d spectra were fitted according to literature[45]. The Mo in all the samples exhibited a mixture of Mo3+, Mo4+, Mo5+ and Mo6+, with similar contents as shown in Table S1. Photocatalytic activities of Mo2N / CdS. To investigate the co-catalytic performances of Mo2N with different morphologies, the photocatalytic activities of Mo2N / CdS, Pt / CdS, CdS and sole Mo2N are carried out in the presence of lactic solution under visible light illumination (Figure 2(a)). The sole Mo2N (for example, hMo2N) without CdS have no activity for hydrogen evolution. When Mo2N loads on CdS, all the Mo2N / CdS with Mo2N of all shapes show higher photocatalytic activities compared to bare CdS, which confirms the Mo2N is an effective co-catalyst for photocatalytic H2 evolution. The activity of h-Mo2N / CdS is highest compared to CdS loaded with Mo2N of other morphologies, and is 30.1 times of bare CdS. The co-catalytic 12


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activity of the sh-Mo2N is lower, and sp-Mo2N shows the lowest activity. The activity of h-Mo2N / CdS is 6.1 times of Pt / CdS with the optimized loading of 1 wt.% Pt. Optimized activity on h-Mo2N / CdS was obtained when the Mo2N is nitridized at 800℃ (Figure S3) and revealed strong (111) facets from XRD patterns (Figure S4), and by loading 5 wt.% of Mo2N (Figure S5). The HRTEM image of h-Mo2N / CdS is also presented in the Figure S6, and the photocatalytic activities can be maintained stable for at least 40 hours (Figure 2(b)). And there was a slight decrease of activity and no significant change of the photocatalysts h-Mo2N / CdS before and after 40 hours (Figure S7). To obtain the intrinsic co-catalytic performance difference, the activities were normalized to the specific surface area (Table 1) of Mo2N. Among the three different Mo2N, the specific surface area of sp-Mo2N is largest likely due to the smallest particle size, while that h-Mo2N is only slightly larger than sh-Mo2N probably due to the large number of defects (burrs and holes). The same activity sequence as that in Figure 2(a) was obtained (Figure 2(c)) by normalizing the activity based on surface area. Therefore the highest activity on Mo2N / CdS is an intrinsic catalytic behavior. Electrochemical characterizations of Mo2N and photocatalytic mechanism investigations. 13



To investigate the mechanism why h-Mo2N showed superior performance as a co-catalyst for CdS, we firstly studied the transient photocurrent responses of Mo2N loaded on CdS under visible light irradiation (Figure 3(a)). The bare CdS is also presented for comparison. All Mo2N / CdS showed higher photocurrent than that of CdS alone under irradiation, which further demonstrates that Mo2N are effective cocatalysts and the photo-excited electrons can transfer from CdS into the co-catalysts. In addition, the photocurrent of h-Mo2N / CdS is the largest, followed by that of sh-Mo2N / CdS and sp-Mo2N / CdS, which follows the same order of photocatalytic activity for H2 evolution. When the light is shut off, the photocurrent of the bare CdS decreases quickly. This indicates a quick recombination of photoinduced charges with solution. The Mo2N / CdS shows a slower decay of photocurrent, indicating the photo-induced charges in CdS is not immediately recombined with solution, but stored in the photocatalyst. Among the three Mo2N / CdS, the photocurrent decay of h-Mo2N / CdS is slowest, suggesting a best electron-storing ability of h-Mo2N / CdS. To prove this speculation, further electrochemical experiments were done as follows. The electron-storing abilities of Mo2N are studied by the galvanostatic charge/discharge analysis (Figure 3(b)) and double layer capacitance measurements (Figure 3(c)). The h-Mo2N shows longest 14


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charging and discharging time, followed by the sh-Mo2N and at last spMo2N. The difference of the charge/discharge time indicates that the three Mo2N have different capacitance. Figure 3(c) and Figure S8 show the specific capacitances of the three Mo2N in aqueous and organic solutions at different scan rate, respectively. The detailed cyclic voltammetry curves of Mo2N calculated for the specific capacitance in aqueous and organic solutions are shown in Figure S9 and Figure S10, respectively. The specific double layer capacitance of h-Mo2N is largest, which is 19.5 F / g in aqueous solutions, followed by sh-Mo2N (15.8 F / g) and last the sp-Mo2N (12.6 F / g). Higher double layer capacitance indicates higher electrochemically active surface area, which should be beneficial for electrochemical HER process. However, the electrochemical HER activities characterized by linear sweep voltammetry (LSV) measurement on the three Mo2N, revealed a very similar HER activities and overpotentials (Figure 3(d)). The overpotentials for HER over the three Mo2N are almost same up to -10 mA / cm2. The higher capacitance on h-Mo2N doesn’t lead to higher HER activity, indicating a less efficient charge transfer from catalyst to solution or an inferior catalytic capability of h-Mo2N. Therefore, combined with photocatalytic activity, we believe that the large capacitance mainly affects the storage of electrons. The stronger storage capacity of the electrons, the more photo-excited electrons are transferred 15



from CdS and stored in Mo2N. Therefore, the recombination of photoexcited electrons and holes of CdS is greatly reduced, leading to the increase of the photocatalytic activity. Electrochemical impedance spectra (EIS) of different Mo2N were tested to compare the interfacial charge transfer from catalyst to solution. As shown in Figure S11, at low frequency, the line of h-Mo2N is steeper compared to the others, indicating a more ideal capacitive behavior [46,47]. At high frequency, h-Mo2N exhibited the smallest semicircle, which directly suggests the lowest charge-transfer resistance between the electrode/electrolyte interfaces. Therefore, it is predicted that the comparable electrochemical HER activity is due to an inferior catalytic capability of the h-Mo2N compared to other Mo2N, i.e., sp-Mo2N and shMo2N. Based on the above analysis, the highest specific capacitance and lowest charge transfer resistance across electrode / electrolyte interface make the least catalytic non-noble metal material the best photocatalytic co-catalyst over CdS. Chen et al. [48] found that (111) oriented Mo2N could exhibit higher capacitance and conductivity, which is highly consistent with our results that our (111) orientated h-Mo2N showed higher specific capacitance and conductivity than sheet and sphere shaped structure. The higher capacitance and conductivity don’t enhance the 16


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electrochemical HER activities significantly, however, it directly contributes to a higher photocatalytic activity when loaded on CdS as the co-catalyst. Figure 4 speculates the reaction mechanism for photocatalytic H2 evolution using h-Mo2N / CdS photocatalyst. CdS is a common visible light responsive semiconductor for H2 evolution. When the light irradiates CdS, the electrons in CdS are excited and transfer to conduction band leaving holes at valence band. When the CdS is deposited by h-Mo2N, the electrons in conduction band of CdS can transfer into h-Mo2N, and then reduce the proton into H2 on surface of h-Mo2N. The holes in CdS oxidize lactic acid. Here, h-Mo2N plays the role of co-catalyst in H2 evolution. The h-Mo2N has the largest capacitance storing sufficient photo-excited electrons, and discharges slowly so as to have enough time for electrons participating reduction reaction. Therefore, the h-Mo2N / CdS shows the highest photocatalytic H2 evolution activity than Mo2N with other morphologies loaded CdS. A suitable co-catalyst for photocatalytic reaction is usually one with lower activation energy for H2 formation, i.e., better catalytic capability, such as Pt, one with atomic junction to semiconductor [6] , one with higher surface area [41], i.e., more active sites, or one forming better heterojunction with semiconductor [33]. Here, the specific capacitance is one of the intrinsic factors affecting the 17



performance of non-noble metal co-catalyst. Conclusion In summary, we firstly synthesized non-noble metal co-catalysts Mo2N with shapes of hexagonal rod, sheet and sphere, and found that hMo2N is the most efficient co-catalyst for photocatalytic H2 evolution. The photocatalytic H2 evolution activity of h-Mo2N / CdS is 30.1 times of bare CdS and 6.1 times of Pt / CdS in lactic solution under visible light irradiation. Electrochemical experiments demonstrate the largest capacitance and conductivity on (111) orientated h-Mo2N. The large capacitance to store the photo-excited electrons and facile transferring of electron to solution account for the superior co-catalytic performance of h-Mo2N. This study reveals that the performance of non-noble metal cocatalyst for photocatalytic reaction can be improved by enhancing the capacitance through controlling the morphology and crystal orientation of catalyst. Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM images, XPS spectrum, HER activity, XRD patterns, HER activity, HRTEM images, photographs of photocatalyst, CV curves of different 18


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samples, CV curves in aqueous solution, CV curves in organic solution, EIS spectra of different samples, detailed contents (%) of the Mo specials.

Conflicts of interest The authors declare that they have no competing interests. Acknowledgements This work is funded by the National First-rate Discipline Construction Project of Ningxia (Chemical Engineering and Technology), Major Innovation Projects for Building First-class Universities in China’s Western Region (ZKZD2017003) and the National Natural Science Foundation of China (NSFC,21862014). References 1. Kisch, H. Semiconductor Photocatalysis-Mechanistic and Synthetic Aspects. Angew. Chem. Int. Edit. 2013, 52, 812-847, DOI 10.1002/anie.201201200. 2. Zheng, Y.; Jiao, Y.; Jaroniec, M.; Qiao, S. Z. Advancing the Electrochemistry of the Hydrogen-Evolution Reaction through Combining Experimentand Theory. Angew. Chem. Int. Ed. 2015, 53, 216, DOI 10.1002/anie.201407031. 3. Tong, H.; Ouyang, S. X.; Bi, Y. P.; Mitsutake, N. U.; Oshikiri, Ye. J. 19



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Model Hydrodenitrogenation Catalysis: Acetonitrile Hydrogenation

on

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29



(a)

(b) hexagonal rod Intensity (a.u.)

)

sheet sphere

20

40 2 (degree)

60

(c)

(d)

d (f)

)

hexagonal rod

sheet sphere 20

40 60 2 (degree)

Envelope Counts 3+ Mo 4+ Mo 5+ Mo 6+ Mo

Intensity (a. u.)

(e) Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

238

234 230 Binding Energy / eV

226

Figure 1. (a) XRD patterns of as-synthesized MoO3 samples with different morphologies (hexagonal rod, sheet and sphere). (b-d) SEM images of Mo2N samples with different morphologies. (e) XRD patterns of Mo2N samples with different morphologies. (f) The high resolution XPS spectrum of Mo 3d in h-Mo2N. 30


(b)

(a)

8000 4000 0 0

10

20 30 Time (h)

40

)

700 600

300 200 100 0

sp-Mo2N / CdS

400

sh-Mo2N / CdS

500 h-Mo2N / CdS

-2 m -1

(c)

Evacuation Evacuation Evacuation

12000

))

Photocatalytic H2 evolution (10-6mol h

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H2 production amount (mol)

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Figure 2. (a) Photocatalytic activities of CdS loaded with different morphologies of Mo2N, Pt / CdS, CdS and sole h-Mo2N. Reaction condition: amount of the catalyst is 0.1g, aqueous lactic solution, 100 mL (10 vol%); light source, 300 W Xe lamp. (b) Time course of photocataytic H2 production over h-Mo2N / CdS. The photocatalytic system is degassed by evacuation in every 10 hours. (c)The normalized activity to specific surface area of different Mo2N co-catalysts.

31



(a) 2

Current density (A / cm )

(b) 16

h-Mo2N/CdS sh-Mo2N/CdS sp-Mo2N/CdS CdS

on off

12

sp-Mo2N sh-Mo2N h-Mo2N

8 4 0 0

50

0

100 150 200 250 300 Time / Sec

(c)

50

100

150

200

250

Time (Sec)

(d) 20 h-Mo2N sh-Mo2N sp-Mo2N

3

-1

C=15.8 F g

-1

C=19.5 F g

2

-1

C=12.6 F g

1

)

0 J (mA / cm2)

4

J (A / g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-20

h-Mo2N sh-Mo2N sp-Mo2N

-40 -60

0 0

50 100 150 200 Scan Rate (mV / s)

250

-80 -0.8

-0.6 -0.4 -0.2 E versus RHE (V)

0.0

Figure 3. Electrochemical characterizations of different Mo2N using 0.5M Na2SO4 aqueous solution as electrolyte. (a) Transient photocurrent responses of Mo2N / CdS under visible light irradiation. (b) Galvanostatic charge/discharge at a current density of 0.5A / g with the electrochemical window from -0.2 to 0.2 V. (c) Plot of the current density from the CV curves tested from aqueous system, as a function of scan rate. (d) Linear sweep voltammetry.

32


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Figure 4. The scheme of photocatalytic H2 evolution on h-Mo2N / CdS.

Mo2N

Specific surface area(m2 / g)

hexagonal rod

20.61

sheet

17.34

sphere

26.83

Table 1. Specific surface area of Mo2N with different morphologies.

33



For Table of Contents Use Only.

The rod shape Mo2N with largest capacitance shows the highest cocatalytic property for photocatalytic hydrogen evolution on CdS.

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Improved photocatalytic hydrogen evolution of CdS using earth

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