Construction of MoS2/Tubular-like g-C3N4Composite Photocatalyst for Improved Visible-Light Photocatalytic Hydrogen Production from Seawater

SHI Wei-Long YANG Shuang WANG Jing-Bo LIN Xue＊, GUO Feng SHI Jun-You＊,

(1School of Material Science and Engineering,Beihua University,Jilin,Jilin 132013,China)

(2School of Material Science and Engineering,Jiangsu University of Science and Technology,Zhenjiang,Jiangsu 212003,China)

(3School of Energy and Power,Jiangsu University of Science and Technology,Zhenjiang,Jiangsu 212013,China)

Abstract:In this work,the development of efficient and stable photocatalyst has been the central topic of scientific research.Herein,MoS2nanosheets/tubular-like g-C3N4(MS/TCN)composite photocatalysts with two dimension/one dimension(2D/1D)nanostructure were fabricated through a facile solvothermal method for photocatalytic hydrogen production from seawater under visible light irradiation,exhibiting enhanced photocatalytic performance in comparison of single MS or TCN.Among of them,MS/TCN-0.5 sample(0.5%(w/w)MoS2)showed optimum H2evolution rate of 85.1 μmol·h-1and possessed outstanding stability(no significant decrease after 4 cycles).The enhanced photocatalytic activity of MS/TCN composite photocatalyst can be attributed to the fact that the loading of 2D MS,as a cocatalyst,promotes the transfer of photo-induced electron and improves the separation efficiency of electron-hole pairs of 1D TCN.

Keywords:photocatalysis;hydrogen production;carbon nitride;molybdenum disulfide

0 Introduction

With the emergence of global energy crisis and gradual deterioration of the surrounding environment,it is urgent to explore new clean energy,such as wind power generation,artificial simulation of solar photosynthesis to produce carbon-free fuel hydrogen,etc[1].In the past few decades,since the early work of Fujishima and Honda in 1972,a variety of photocatalysts,such as oxides,nitrides and sulfides,have been prepared for hydrogen production from water splitting[2-5].Although the photocatalysts have been developed,there are still some defects in the following:(i)low utilization of solar light[6-8];(ii)the complex synthesis process and high-cost precursors[9-12];(iii)the existence of toxicity and instability[5,13].Therefore,the development of suitable band gap,low cost,efficient and persistent photocatalysts is still a great challenge in the photocatalytic field.

Metal-free graphitic carbon nitride(g-C3N4)has gained extensive concern in photocatalysis field due to its charming advantages,such as proper energy band position(which straddle the redox potentials of H2O),low toxicity,good stability,available raw materials and simple synthetic method[14-20].G-C3N4displays great potential for photocatalytic H2production and organic pol-lution degradation.However,its low quantum efficiency still limits its practical activity deriving from the defects of bulk g-C3N4(low specific surface area,poor charge mobility and high recombination rate of photoexcited charge carriers)[21-23].The past few years have witnessed an increasing interest in the construction of multifarious hetero-structure photocatalysts by taking advantage of the versatile 2D platform of g-C3N4[24].Nevertheless,little attention has been paid to the investigation of other dimensions of g-C3N4in the heterostructure photocatalysts,although different morphologies(e.g.1D nanorods,2D nanosheets[23,25],3D nanospheres)of single g-C3N4have been reported for the improvement of the photocatalytic activity[26].Among of which,g-C3N4with hollow tubular nanostructure has received considerable attention as a class of 1D novel photocata-lytic materials to boost the photocatalytic activity of bulk g-C3N4because of its unique advantages,such as the low dimensionality and high surface-to-volume ratio,larger surface area and more abundant active sites,and the improvement of the light absorption and scattering[27-29].This modification of g-C3N4to improve its photocatalytic activity is still limited,the continuous exploration to build dimensional hetero-structures(DHS)with proper multifunctional material to couple with tubular-like g-C3N4using the synergism or the summation effects may realize higher photocatalytic performance deriving from the unique dimensionalitydependent advantages and reduced defects.

Up to now,several literatures have been reported that the photocatalytic performance of g-C3N4can be extremely boosted after introducing MoS2,which is attributed that MoS2possesses the excellent conductivity,strong lattice match and relatively high mobility as well as the appropriate band edge matching with g-C3N4[30-32].The recent reported MoS2/g-C3N4composites mainly include MoS2quantum dots modified g-C3N4nanosheets by thein situion exchange synthesis to produce hydrogen[33],MoS2nanosheets uniformly dispersed on the CN nanosheets using a hydrothermal deposition procedure to degrade RhB,and the MoS2nanoparticles deposited on the surface of g-C3N4nanosheets to remove MO in the aqueous solution,etc[34].So far,few literatures have been reported to explore 2D MoS2nanosheets loaded on the 1D hollow tubular-like g-C3N4to form 2D/1D heterostructure MoS2/g-C3N4composites.Furthermore,to generate hydrogen from seawater is a worthy proposition,which 97% water resource is present in the oceans in comparison with the scarcity of fresh water and the high cost of water treatment.Regrettably,the application of 2D/1D MoS2/g-C3N4composite to produce H2from seawater splitting still remains unexplored.Consequently,it is indispensable to further investigate the photocatalytic activity of 2D/1D MoS2/g-C3N4composites with respect to high activity and stabil-ity,providing competitive candidates for practical solar seawater splitting.

Scheme 1 Proposed photocatalytic mechanisms in MS/TCN composite photocatalyst

Herein,we synthesized the 2D/1D heterostructure MoS2/tubular g-C3N4nanocomposite photocatalyst by a solvothermal synthetic route,which showed excellent photocatalytic hydrogen production performance from seawater under visible-light irradiation.A series of characterizations were tested to analysis the composition,morphology and photoelectrochemical behaviors of as-prepared materials.The possible photocatalytic mechanism for the MoS2/tubular g-C3N4composites was also discussed in detail.

1 Experimental

1.1 Synthesis of photocatalysts

1.1.1 Synthesis of tubular g-C3N4

Firstly,1 g of melamine and 1.2 g of phosphoric acid were dissolved in 80 mL of deionized water,stirred in a water bath at 80℃for 1 h,and then the mixed solution was poured into a 100 mL stainless steel,heated to 180℃and reacted for 10 h.Next,the mixture was centrifuged,washed several times with deionized water,the remaining phosphorus source was removed,and dried in an oven at 60℃for further use.Finally,the as-obtained mixture solid powder was obtained by directly calcining with a 2℃·min-1heating rate and keeping at 500℃for 3 h,and then collected the product,which was labelled as TCN.

1.1.2 Synthesis of MoS2

0.2g of ammonium tetra-thiomolybdate,(NH4)2MoS4,was added to 40 mL ofN,N-dimethylformamide(DMF),stirred for half an hour,then transferred to a 50 mL stainless steel and heated at 200℃for 24 h.Finally,the MoS2(MS)powder was collected.

1.1.3 Synthesis of MoS2/tubular g-C3N4composite(MS/TCN)

Different mass ratios(0.1%,0.3%,0.5%,1%,and 2%)of MoS2/tubular g-C3N4composites were synthesized by a simple solvothermal method and the synthesis process is shown in Scheme 1.Under the condition of magnetic stirring,0.2 g of tubular g-C3N4and different mass fractions of(NH4)2MoS4(0.000 4,0.001 2,0.002 0,0.004 0,and 0.008 3 g)were added to 40 mL of DMF,stirred for half an hour,then transferred to a 50 mL stainless steel and heated in a muffle furnace at 200℃for 24 h.The as-prepared products were washed three times with ethanol and centrifuged.And the asprepared products were labelled as MS/TCN-0.1,MS/TCN-0.3,MS/TCN-0.5,MS/TCN-1,MS/TCN-2,respectively.

1.2 Characterization

X-ray diffraction(XRD)patterns of the samples were measured on a Bruker D8 Advance instrument with CuKαradiation(λ=0.154 18 nm)at the scanning rate of 7(°)·min-1from 10°to 80°with the work voltage of 40 kV and work current of 40 mA.Fourier transform infrared(FT-IR)spectra were collected on a Shimadzu IRAffinity-l spectrometer using samples embedded in potassium bromide(KBr)pellets and analyzed at the wavenumber range of 400～4 000 cm-1.Transmission electron microscopy(TEM),high-resolution TEM(HRTEM)and high-angle annular dark-field scanning trans-mission electron microscopy(HAADF-STEM)tests were carried out on a FEI Tecnai G2 F20 S-TWIN electron microscope at an accelerating voltage of 200 kV.The mapping was collected on a FEI Quanta 250 scanning electron microscopy(SEM)with energy-disperse X-ray spectroscopy(EDS)at an accelerating voltage of 15 kV.X-ray photoelectron spectroscopy(XPS)was collected on a Thermo Escalab 250Xi spectrometer with AlKα(hν=1 486.6 eV)as the excitation source.Ultraviolet visible diffuse reflectance spectra(UV-Vis DRS)were recorded on a Varian Cary 300 spectrophotometer from 200 to 800 nm(BaSO4as reference material).

1.3 Photocatalytic experiment

For photocatalytic reactions,50 mg of photocata-lyst,which dissolved in 100 mL solution(90 mL 3%(w/w)NaCl solution and 10 mL sacrificial triethanol-amine).Then,3%(w/w)Pt as co-catalyst was added by photoreduction of H2PtCl6solution to facilitate the hydrogen evolution reaction.All the above experimental steps were performed under magnetic agitation.Visible light was served by a Xenon lamp equipped with a long pass wavelength filter(λ＞420 nm).The temperature of the cooling water in the cooling circulation system was maintained at 5℃.An online GC-7920 gas chromatograph(GC)set up with a thermal conductivity detector(TCD)and 0.5 nm molecular sieve columns was employed for the evolved gas detection.N2and air were used as carrier gas.The generated gas was analyzed on a gas chromatography every 1 h.In addition,the apparent quantum efficiency(AQE)was estimated using equation as follows:

AQE=2nH2/nincidentphoton×100%

Wherenincidentphotonsis the number of incident photons andis the number of evolved H2.

2 Results and discussion

Fig.1 shows the XRD patterns of as-prepared TCN as well as MS/TCN composite materials.Two distinct peaks at 13.3°and 27.4°in TCN can be assigned to the(100)and(002)peak of graphitic carbon nitride,respectively[35-37].It is generated accepted that the(002)peak in TCN is a characteristic inter-layer stacking reflection of conjugated aromatic systems,and after loading MS nanosheets,the intensity of this(002)peak significantly decreased,which can be ascribed to the attachment of the MS nanosheets to the TCN surface[38].In addition,there was no obvious characteristic XRD peaks for MS in MS/TCN-0.5,which can be attributed to the low quantity of MS and relatively low diffraction intensity[39].

Fig.1 XRD patterns of TCN and MS/TCN-0.5 composite

To investigate the molecular structure of assynthesized TCN and MS/TCN nanocomposite,FT-IR spectra were performed and shown in Fig.2.As can be seen,the broad absorption peak 3 000～3 600 cm-1was assigned to the stretching vibrational modes of the N-H or O-H groups[40].The skeletal vibrations in the region 1 200～1 700 cm-1for aromatic CN heterocycles and the breathing vibration at 810 cm-1for triazine units can be observed distinctly[38].Considering a very limited amount of MS,no obvious change for the characteristic absorption peaks of TCN and MS/TCN-0.5 can be detected,revealing that the major chemical structures of TCN were retained in MS/TCN composite,which is consistent with the result of XRD patterns.

Fig.2 FT-IR spectra of TCN and MS/TCN-0.5

The SEM image of pure TCN is presented in Fig.3a,it is indicated that the prepared TCN was composed of regular-shaped nanotubes,and presented a plurality of stacked structures and the fluffy texture.Interestingly,when the MS nanosheets were uniformly grown on the TCN substrates,the morphology of the MS nanosheets did not change significantly,this may be due to the relatively small loading amount of MS(Fig.3b).Furthermore,in Fig.S1(Supporting information),the results of the EDX spectrum reveals that the C,N,S,and Mo elements were detected in the MS/TCN-0.5 composite,which provides solid evidence for the successful formation of the MS/TCN composite.The morphology of MS/TCN-0.5 composite was further investigated by TEM and HRTEM images.From Fig.3c,it can be find that the hollow tube diameter of TCN was 70～80 nm.As shown in the HRTEM image from Fig.3d and Fig.S2a～d MS nanosheets are uniform-ly deposited on the surface of TCN,according to the different contrast between the sections of MS nanosheets and TCN,and then identify the regions of different materials,illustrating the existence of junction/interface between TCN and MS.HAADF-STEM with the elemental mapping images were taken off for further confirmation,demonstrating that the elements of C,N,S and Mo were distributed in MS/TCN-0.5(Fig.3e,f),which provides a solid evidence for the successful formation of a MS/TCN heterostructure.Moreover,the HAADF-STEM combining four elements from Fig.S3 can clearly see that S and Mo elements are uniformly distributed on the tube walls of TCN,which can reflect the spatial orientation of the two materials.

Fig.3 SEM images of(a)TCN and(b)MS/TCN-0.5;(c)TEM,(d)HRTEM and(e)HAADF-STEM and(f)element mapping images of MS/TCN-0.5 composite

The specific bonds and chemical states of the elements in the MS/TCN photocatalyst were further investigated by XPS.The full survey spectrum in Fig.4a indicates the MS/TCN-0.5 was only composed of C,N,Mo and S elements and no impurity elements were observed.As shown in Fig.4b in the Clsspectrum,three peaks located at 284.9,285.8 and 288.3 eV were in good accordance with C-C coordination,sp3coordinated carbon bonds and N-C=N coordination,respective-ly[41].In Fig.4c,the N1speak at 398.8 eV ascribedsp2hybridized aromatic N bonded to carbon atoms(C=NC).While the other three peaks at 399.7,401.1 and 404.4 eV were attributed to the tertiary nitrogen N-(C)3groups,amino groups andπ-excitations,respective-ly[42].In Fig.4d,there are three peaks at 226.1,229.4 and 233.3 eV in high-resolution spectra of Mo3dregion,respectively.One of them was located at 225.2 eV,corresponding to S2-in the MoS2.Two distinct peaks(229.2 and 232.3 eV)were attributed to Mo3d5/2and Mo3d3/2,respectively[43].The S2pspectra of MS consist of two peaks at 161.6 and 163.5 eV,which were assigned to the S2p3/2and S2p1/2,respectively(Fig.4e)[44].

Fig.5 shows the UV-Vis diffuse reflectance(DRS)spectra of TCN and MS/TCN composites.Compared with the TCN alone,the MS/TCN composites exhibited enhanced visible-light absorption due to the strong light scattering and trapping effect from MS nanosheets[39].In addition,a significant red-shift of absorption edge can be found,which indicates that the composite photocatalyst can utilize more visible light.Furthermore,with the increase of MS loading contents,visible light absorption gradually increased,which may be due to the increase of intermolecular conjugation degree[45].Moreover,when combining MS with TCN,the color of powders changed from pale yellow into gray(top of Fig.5).

Fig.5 UV-Vis diffuse reflection spectra of TCN and MS/TCN composites

In order to evaluate the photocatalytic properties of the as-synthesized samples,triethanolamine acts as a sacrificial agent to decompose simulated seawater(3%(w/w)NaCl solution)under visible light to produce hydrogen.From the Fig.6a,the TCN exhibited photocatalytic seawater splitting performance with a hydrogen production of 32.5 μmol during the four hours of irradiation.After coupling MS nanosheets on the TCN,the photocatalytic activity of MS/TCN composite has been obvious enhanced,which is mainly due to the improved visible-light absorption and electron transfer property.Quantitatively,the H2generation rates over as-prepared samples are given in Fig.6b.Among them,when MS contents was controlled at 0.5%(w/w),the optimal photocatalytic activity of MS/TCN was obtained,and the photocatalytic H2production rate of 85.1 μmol·h-1over MS/TCN-0.5.Moreover,as the content of MS gradually increased or decreased,the hydrogen production effect shows an obvious downward trend and generally presents a normal distribution curve.This may be due to when the content of MS increases,the carrier separation efficiency decreases due to the introdu ction of excessive Mo4+,which inhibits the original number of TCN holes[45].However,if the content of MS gradually decreases,the hole oxidation ability of the overall composite material decreases with the decrease of S vacancies,which could lead to the deterioration of the hydrogen production effect[46].In addition,with the increase of MS loading,the active sites on the TCN surface could be covered,which is another reason for the decrease of hydrogen production activity[47].Fig.6c shows that photocatalytic H2evolution of bulk g-C3N4(BCN),TCN,MS/TCN-0.5 and MS/BCN-0.5 in simulated seawater under visible light.It can be clearly seen that after loading 0.5%(w/w)MS,the hydrogen production activity of BCN and TCN increased,which can further prove that the loading of MS can promote the improvement of photocatalytic activity.In addition,this result also show that the combination of 0D and 1D could present better photocatalytic activity.Table S1 lists a comparative investigation of photocatalytic hy-drogen production rates by representative reported MoS2/g-C3N4photocatalysts,indicating that MS/TCN composite photocatalyst possesses excellent photocata-lytic water splitting from seawater under visible light.Furthermore,Fig.6d shows the AQE for MS/TCN-0.5 under monochromatic light irradiation.The calculated AQE value at 420 nm came up to 2.3%,which exhibited outstanding photocatalytic activity of H2production in seawater.The stability test of MS/TCN-0.5 composite was evaluated by cyclic H2evolution experiment,and the results are depicted in Fig.6e.It is clearly observed that the MS/TCN-0.5 composite photocatalyst do not have obvious decrease for H2generation in longterm photocatalytic reaction for four cycles,verifying that MS/TCN composite photocatalyst possesses excel-lent stability for photocatalytic H2production.Fig.6f shows that XRD patterns of MS/TCN-0.5 composite before and after reaction.It can be seen that the diffraction peak strength of the sample is slightly weakened after use,which may be caused by minor damage to the surface structure during multiple washing and illumination,which weakens the crystallinity.

Fig.6 (a)H2evolution of as-prepared samples from simulated seawater(3%(w/w)NaCl solution)under visible-light irradiation(λ＞420 nm);(b)Photocatalytic H2evolution rates of MS/TCN composites with different contents of MS;(c)Photocatalytic H2evolution of BCN,TCN,MS/TCN-0.5 and MS/BCN-0.5;(d)Wavelength dependence of apparent quantum efficiency for MS/TCN-0.5 composite;(e)Cycling stability tests of photocatalytic H2 evolution over MS/TCN-0.5 sample;(f)XRD patterns of MS/TCN-0.5 photocatalyst before and after reaction

Fig.7 (a)PL spectra,and(b)time-resolved fluorescence decay curves,(c)transient photocurrent responses and(d)EIS Nyquist plots of TCN and MS/TCN-0.5

To explore the transfer mechanism of photogenerated electrons in the composite system,a series of photoelectrochemical tests were carried out.In Fig.7a,it shows that the charge-carrier transfer mechanism by the room temperature PL(photoluminescence)spectra at the excitation wavelength of 325 nm.When a cocatalyst MS was loaded,the PL of MS/TCN was drastically quenched.Obviously,the recombination of photogenerated e-h pair in TCN can be suppressed by transferring electrons to MS as the electron acceptor[48-50].For time-resolved fluorescence decay curves,a quick fluorescence decay with a short lifetime demonstrates a low possibility of the photogenerated electron-hole recombination[51].The loading of MS in MS/TCN leads to a lower probability of the recombination of photogenerated e-h pair as suggested by the decease of the fluorescence lifetime from 26.63 ns for TCN to 23.98 ns for MS/TCN-0.5(Fig.7b).And these charge separa-tion results are proved again by the following transient photocurrent response tests(Fig.7c)and electrochemical impedance spectroscopy(EIS,Fig.7d).Fig.7c makes comparison of the photocurrent response of TCN and MS/TCN-0.5 composites on a typical switch on-off cycles.Notably,MS/TCN-0.5 sample show the higher photocurrent density(about 2.0 times)than that of TCN,demonstrating the superior charge separation efficiency than TCN.Furthermore,the same results in Fig.7d are obtained by the electrochemical impedance spectroscopy(EIS).Clearly,the MS/TCN-0.5 composite exhibited a smaller radius than that of TCN,suggesting that the successful construction of MS/TCN-0.5 with defective Mo4+active sites induces the decrease of the charge transfer resistance,thus facilitating more effective charge separation and transport[52].Based on above photoelectrochemical measurements,it can be concluded that MS loading promotes the capacity of light absorption,the charge can be separated/transferred effectively through the redox cycle,thus,the photocatalytic seawater splitting process is accelerated.

According to the above analysis and experiment results,the photocatalytic mechanism of MS/TCN composite was proposed(Fig.8).Typically,the TCN as semiconductor can generate photoexcited electron-hole pairs under visible-light irradiation,and then the electrons could transfer from VB to CB,leaving holes in the VB,then the photoexcited-electrons in the CB of TCN could migrated to the MS nanosheets to form an electron receiver,while the holes left in the VB of TCN would react with sacrificial agent(TEOA)to suppress the photogenerated carriers recombination.The MS as the co-catalyst can play the role of active sites due to its lower H2evolution overpotential and strong bonding tendency with H+.MS possess the excellent electrical conductivity,which exhibit great talent in electrons acceptation and storage.At the same time,unsaturated S atoms on the edge of MS have a strong bonding tendency with H+,which can be more easily reduced the H+to generate H2in MS[53].In this work,the role of MS in composites is mainly as following two aspects:(i)enhanced the light absorption and(ii)promotion of photoinduced charge separation.A large number of literature and experimental data show that light absorption can promote the utilization of light by catalysts,thus making semiconductors absorb better and producing more photogenerated charges,thus improving the hydrogen evolution effect of composite catalysts[54].The MS/TCN composites exhibit enhanced visible-light absorption due to the strong light scattering and trapping effect from MS nanosheets,which can be confirmed by UV-Vis diffuse reflection spectra in Fig.5.In addition,the introduction of co-catalysts can effectively promote the transfer of electrons,and promote the rapid transfer of photogenerated charges produced by semiconductors under photoexcitation from the bulk phase to the surface,thus increasing the separation efficiency of photogenerated electron-hole pairs,thus achieving the effect of increasing hydrogen production efficiency[55].The lower PL intensity and short lifetime of MS/TCN in Fig.7a and b also demonstrated a low possibility of the photogenerated electron-hole recombination by the in-troduction of MS.Finally,the photocatalytic activity of MS/TCN can be greatly improved.

Fig.8 Proposed photocatalytic mechanisms in MS/TCN composite photocatalyst

3 Conclusions

In summary,a novel MS/TCN composite with 2D/1D nanostructure was successfully synthesized by a facile solvothermal route for the first time.The MS/TCN composite exhibit excellent photocatalytic activity for hydrogen production from seawater under visible light irradiation.The 2D MS could serve as a cocatalyst for the efficient capture of photogenerated electrons and active sites of 1D TCN for hydrogen generation.Our research can provide experimental data for the effective transfer of photogenerated charge by 2D/1D model and provide a new idea for the synthesis of efficient and low-cost semiconductor based photocata-lysts by environmentally friendly solar hydrogen production.

Supporting information is available at http://www.wjhxxb.cn