Mo-vacancy induced high performance for photocatalytic hydrogen production over MoS2 nanosheets cocatalyst

Journal Pre-proofs Research paper Mo-vacancy induced high performance for photocatalytic hydrogen production over MoS2 nanosheets cocatalyst Wen-Jie Xie, Xuan Li, Feng-Jun Zhang PII: DOI: Reference:

S0009-2614(20)30191-3 https://doi.org/10.1016/j.cplett.2020.137276 CPLETT 137276

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Chemical Physics Letters

Received Date: Revised Date: Accepted Date:

23 December 2019 18 February 2020 24 February 2020

Please cite this article as: W-J. Xie, X. Li, F-J. Zhang, Mo-vacancy induced high performance for photocatalytic hydrogen production over MoS2 nanosheets cocatalyst, Chemical Physics Letters (2020), doi: https://doi.org/ 10.1016/j.cplett.2020.137276

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Mo-vacancy induced high performance for photocatalytic hydrogen production over MoS2 nanosheets cocatalyst Wen-Jie Xie1, Xuan Li1, Feng-Jun Zhang1,2 1

Key Laboratory of Functional Molecule Design and Interface Process, Anhui Jianzhu University, Hefei Anhui, P. R. China, 230601

2

Anhui Key Laboratory of Advanced Building Materials, Anhui Jianzhu University, Hefei Anhui, P. R. China, 230022

 Corresponding author : Zhang Feng-Jun, E-mail: [email protected]

Abstract Molybdenum disulfide (MoS2) nanosheets with Mo-vacancy were obtained through reduction with the NaBH4 solution. The amount of Mo-vacancy was explored by changing the addition of the NaBH4 solution. Moreover, the Mo-vacancy induced performance for photocatalytic hydrogen production (PHP) was also researched. PHP of MoS2-0.09/TiO2 composite material reaches 5424 μmolg-1h-1. Thus, hydrogen production rate of MoS2-0.09/TiO2 was 6.8 times than that of untreated MoS2/TiO2, which most likely resulted from the exposure of more unsaturated S atom dangling bond, serving as adsorption sites for H+ and greatly increasing the reaction sites of MoS2. Key words: Mo-vacancy; MoS2; reduction; photocatalytic hydrogen production 1. Introduction Nowadays, people are paying more attention to the development of new energy to alternatives traditional fossil fuels

1-3.

The new materials that make solar energy to 1

conversion

chemical

energy

are

particularly

eye-catching.

The

semiconductor material TiO2 has been extensively studied by scientists

traditional

4-6,

however,

the low solar energy conversion limited its application range. Due to excellent catalyst properties, MoS2 has received much attentions

7-10.

MoS2 has a layered structure in

which each layer of molybdenum atoms is sandwiched between sulfur atoms (S-Mo-S) through covalent bonds

11,12.

applications such as catalysis

This special structure has significant advantages in 13-15,

optoelectronics16, energy storage devices17, and

lubricants18. In photocatalytic reactions, MoS2 is considered to be an effective cocatalyst to replace precious metals to improve the photocatalytic properties of other semiconductors 19-21. Modification of MoS2 to expose more active sites and increase its carrier mobility has attracted the attention of more and more researchers performance of MoS2 depends on its lattice structure and morphology

22-28.

23,28.

The

A large

number of research experiments on MoS2 defects have been conducted, demonstrating that defects can alter the charge transport properties and optical properties of materials 22,25,26,29.

The introduction of defects can change the local structure of MoS2 and form a

unique chemical and electronic environment in MoS2

30.

The introduction of these

atomic defects plays an important role on the catalyst properties of MoS2 31. The impact of various defect types on the photoelectric properties of MoS2 are being studied in depth

32.

These research conclusions have very important guiding

significance for improving material quality. Feng 33 used first-principles calculations to study the structure and photoelectric properties of single-layer MoS2 with Mo-vacancy. The introduction of Mo-vacancies improves the strength of the Mo-S bond. Tsai 2

34

demonstrates that active sites can be created on the base of 2H Phase MoS2 by generating sulfur vacancies. The uncoordinated Mo atoms in the S-vacancy MoS2 introduce a gap state, which facilitates hydrogen bonding. The concentration of the Svacancy can be controlled by varying the applied desulfurization voltage. Therefore, the selection of suitable atomic defect control methods is an important part of the current research on photocatalytic material defects

35,36.

Based on the first-principles

calculation of density functional theory (DFT); Guo 37 explored various defects of MoS2 to enhance the photo catalytic activity of TiO2/MoS2 composites. There are many articles on the preparation of S-vacancy MoS2 as a cocatalyst; however, there have been few reports on the preparation of Mo-vacancies MoS2 as a cocatalyst to enhance the PHP. In this study, MoS2 was treated with NaBH4 to prepare MoS2 with rich Movacancies. The amount of Mo-vacancies was controlled by varying the addition of NaBH4. Mo-vacancy MoS2 as a cocatalyst can enhance the photocatalytic hydrogen production. And a possible PHP mechanism of the MoS2/TiO2 composite was suggested. 2. Experimental 2.1. Materials Sodium molybdate dihydrate (Na2MoO4.2H2O), tetrabutyl titanate (C16H36O4Ti), sodium borohydride (NaBH4), acetic acid (CH3COOH), thiourea (CH3CSNH2), sodium hydroxide (NaOH) and absolute ethyl alcohol (CH3CH2OH) were bought from Sinopharm Chemical Reagent Co., Ltd，. Cetyltrimethylammonium bromide (CTAB), 3

diethanolamine (C4H11NO2), and polyvinylpyrrolidone (PVP) were purchased from McLean biochemical technology co. Ltd.

(Shanghai, China). All chemicals were

analytical grade and used without further purification. 2.2. Synthesis of Mo-vacancy MoS2 Original MoS2 was prepared according to the method of literature

31.

And 0.112g of

MoS2 was added to 10mL of 20 M NaOH solution, and ultrasonic treatment was continued for 45 min to prepare solution A. The solution B was prepared by adding NaBH4 to 10 mL of 20 M NaOH solution and magnetically stirring at 50 ̊C for 30 min. The A solution was then added dropwise to the B solution for 60 min. The solution was poured into a Teflon-lined autoclave (30 mL) and kept at 150 ̊C for 10 hours, and then the black powder was washed three times with 1 mM diluted hydrochloric acid and deionized water, respectively. Then, it was placed in a vacuum drying oven and dried at 60 ̊C for 12 hours. Samples treated by NaBH4 solution with different concentrations of 0, 0.023, 0.045, 0.09, 0.23 and 0.45 mol/L were labeled as MoS2-0, MoS2-0.023, MoS2-0.045, MoS2-0.09, MoS2-0.23, and MoS2-0.45, respectively.

2.3. Synthesis of MoS2/TiO2 composites The preparation details of MoS2/TiO2 composites can be obtained from the previous work 31. Similarly, a series of MoS2 with Mo-vacancy and TiO2 composites were prepared by the same method. 2.4. Characterization: The analysis of crystalline structure were detected by X-ray diffraction Bruker D8 4

Advance diffractometer with CuKa radiation (λ=1.5406Å, 401KV, 40 mA). Raman spectra were measured using a Reflex micro-Raman spectrometer (inVia) with a 532 nm laser source. The size and morphology of the final product were investigated by scanning electron microscopy (SEM) (JOEL, JSM-7500F). And GeminiSEM 500 was used to perform elemental distribution analysis on samples. Transmission electron microscope (TEM; Hitachi-9000) at an accelerating voltage of 200 kV was also used to record the electron micrographs of photocatalysts. The ultraviolet (UV)-visible (vis) absorption spectra were performed on spectrophotometer (SolidSpec-3700, Japan) using BaSO4 as the background. The X-ray photoelectron spectra (XPS, Thermo-Fisher Escalab 250Xi) were recorded using Al Kα radiation. N2 adsorption-desorption data of the samples were obtained at 77 K using a JW-BK132F BET analyzer. And the pore size distribution was acquired from the N2-desorption branch using the BJH method. The transient photocurrent measurement uses the CHI660E (Chenhua, Shanghai) instrument as a standard three-electrode configuration. The catalyst was used as an anode, and the Pt foil was used as a cathode and Ag/AgCl was used as a reference electrode containing 1 M KOH solution. 0.5 M H2SO4 aqueous solution (starting potential of ~-0.038 V) was used as a test solution. 2.5. Photocatalytic hydrogen production The photocatalytic hydrogen production efficiency of the sample was tested by a photocatalytic hydrogen production evaluation system (AuLight, Beijing, and CELSPH2N) at room temperature. The test methods of PHP and catalyst stability were referred to our previous work 31. 5

3. Results and discussion

Fig.1 XRD patterns of MoS2-0, MoS2-0.09 and MoS2-0.45

The XRD pattern of synthesized MoS2 was shown in Fig. 1(a), all the characteristic peaks prove that MoS2 phase （ PDF No.37-1492 ） 31,38,39. Compared to MoS2, the degree of crystallization of MoS2-0.09 began to decrease. The most obvious finding of MoS2-0.45 is that the disappearance of each characteristic peak of MoS2 indicates that the crystal structure has been completely destroyed. The results (Fig S1) show that NaBH4 treatment of MoS2 can gradually erode its crystal structure.

Fig. 2 Raman spectra of MoS2 after treatment with different concentrations of NaBH4 6

Fig. 3 SEM images of (a) untreated MoS2, (b) MoS2-0.09 and (c) MoS2-0.45; TEM images of (d) untreated MoS2, (e) MoS2-0.09 and (f) MoS2-0.45; And (g) Schematic diagram of the synthesis of Mo-vacancies MoS2

The Raman spectroscopy was showed in Fig. 2. The peaks at 406 and 377 cm-1 in MoS2 represent the vibration modes of A1g and E12g, respectively. Two peaks in MoS20.045 and MoS2-0.09 indicate that a small amount of NaBH4 treatment does not destroy the integrity of the MoS2 structure. The sheet structure of MoS2 remains intact in Figs. 3 (a, b) and Figs. S2 (a, b). This is further confirmed by Figs. 3 (d,e). However, as the concentration of NaBH4 increased, the vibration peaks of E12g and A1g of MoS2-0.23 and MoS2-0.45 weakened until disappeared, indicating that the structure of MoS2 has been completely destroyed. The same conclusion is demonstrated in Fig. 3 (c, f) and (Fig S2(c, f). And by the additional Raman peaks 223, 345, 337, 490, 565, 730 cm-1 and the asymmetric vibration in the Mo4O11 40, the symmetric vibration is consistent. The synthesis schematic diagram of Mo-vacancies MoS2 is shown in Fig. 3(g). NaBH4 7

reacts with MoS2, and Mo4+ is reduced to Mo3+ and precipitates, resulting in Movacancies. Therefore, it is converted to MoS2+x after being treated with NaBH4. Excess NaBH4 destroys the structure of MoS2, and a large amount of Mo3+ forms molybdenum hydroxide under alkaline conditions. As the pH is lowered during pickling, the molybdenum hydroxide reacts with oxygen in the air to form Mo4O11. As shown in the reaction equations (1) and (2). Mo3+ + 3OH- = Mo (OH) 3

(1)

The reaction equation that occurs in contact with oxygen in the air: 8Mo (OH) 3 + 5O2 =2Mo4O11 + 12H2O

(2)

It indicates that the MoS2 nanostructure is converted to MoS2+x. This structure assures the stability of the MoS2 structure and produces a large amount of Mo-vacancies on the surface compared to the untreated MoS2 nanostructure.

Fig. 4 UV-Vis spectra of MoS2/TiO2 composites treated with different NaBH4 concentrations; (Illustration: Band gap width map after composite fitting)

The UV-vis spectra of the MoS2/TiO2 were shown in Fig.4. Treatment of the Mo8

vacancy MoS2 with NaBH4 effectively increases the absorption of ultraviolet and visible light of TiO2. Moreover, the light absorption range of the MoS2/TiO2 composite was shifted toward visible light. MoS2/TiO2 composite has a significant absorption enhancement effect at 725 nm, corresponding to its band gap width of 1.72 eV. At the same time, it was also proved that MoS2 in the composite material is a structure of a few sheets 41. Moreover, MoS2-0.09/TiO2 composites have reached a band gap width of 2.8 eV. However, the absorption strength of the MoS2-0.23/TiO2 composites was weakened, indicating that the structural integrity of MoS2 was destroyed. This demonstrates that an appropriate amount of NaBH4 treated MoS2 can effectively enhance the light absorption range of TiO2. And since MoS2 after NaBH4 treatment has more surface vacancies, these Mo-vacancies serve as nucleation sites for TiO2 growth. It can effectively improve the strength between the MoS2/TiO2 composite interfaces, so that the photogenerated carriers can be effectively separated in the photocatalytic process. Furthermore, the positions of the valence band and the conduction band can calculate with empirical formulas 42,43. As follow: ECB=X - EC – Eg/2 EVB=ECB + Eg Where EC is a constant relative to a standard hydrogen electrode, EC=4.5eV; Eg is the band gap of the semiconductor; X is the geometric mean of the absolute electronegativity of each atom in the semiconductor; According to the above formula, consult the relevant table to get the relevant data brought into the formula, and the conduction band and valence band value of sample can be calculated. ECB(MoS2-0.09/T) =-2.41, EVB(MoS2-0.09/T) =0.39; ECB(MoS2/T) =-2.51, EVB(MoS2/T) =0.49, obviously, compared with the original sample, the band gap of MoS2-0.09/T was reduced, which improves the utilization rate of photo-generated

9

carriers and reduces the probability of photo-generated electrons and holes recombining, thereby improving the efficiency of photocatalytic hydrogen production.

Table 1 Percentage of Mo and S atoms in MoS2 treated with different concentrations of NaBH4 Sample

Mo %

S%

Mo:S

MoS2

15.2

31.7

1:2.1

MoS2-0.09

9.0

22.8

1:2.5

MoS2-0.23

7.7

22.0

1:2.9

Fig. 5 XPS spectra of MoS2-0.09 and MoS2 nanosheets, (a) C 1s and (b) O 1s peaks, (c) Mo 3d S and (d) S 2p peaks

10

The XPS results for C 1s, O 1s, S 2p, and Mo 3d (Fig. 5) showed that the composition and valence state have changed greatly after MoS2 treatment with different contents of NaBH4. Calibration was performed using the binding energy of the C 1s peak at 284.8 eV in Fig. 5(a) and (Fig S3). The O1s can be deconvolved into three peaks (Fig. 5(b)) with peak positions of 531.4 eV and 532.4 eV. Corresponding to the oxygen molecules and water molecules adsorbed in MoS2, the peak value of 533.3 eV can be attributed to O-Mo-O in molybdenum oxide 44,45. The fitted peaks of untreated MoS2 at 228.7 eV and 231.9 eV represent the binding energies of Mo 3d5/2 and Mo3d3/2 in 1TMoS2. 229.4 eV and 232.6 eV are considered to be 2H-MoS2 Mo3d5/2 and Mo 3d3/2 peaks 34. As shown in Fig. 5(c), there is a significant difference between MoS2-0.09 and MoS2. Both the 1T and 2H phases of the MoS2-0.09 peak shifted to the high energy direction by 0.4 eV which indicates a shift up in binding energy. The Mo-vacancy leads to a shift up in the binding energy of the remaining atoms (Mo and S) on the surface of MoS2 after NaBH4 treatment. In addition, the MoS2-0.09 showed three new peaks of 233.5, 235.4 and 236.6 eV, representing the formation of Mo5+ and Mo6+

46.

A

phenomenon similar to the Mo spectrum was also found from the S 2p spectrum of Fig. 5d, peak shifting and producing two new peaks of 163.5 and 164.7 eV. Represents the S atom exposed after the Mo-vacancy. This phenomenon is consistent with the Raman test results. In addition, the XPS measurement results also show that when the amount of NaBH4 continuously increases, the stoichiometric ratio of Mo:S in MoS2 gradually changes from 1:2 to 1:3 (as shown in Table 1 and Fig S4), revealing that MoS2 is treated after NaBH4. A large number of Mo-vacancies are generated. However, despite these 11

Mo-vacancies, the Raman spectrum at MoS2-0.09 shows that the main features of the Raman peaks E12g and Alg are still sharp, indicating that MoS2 generally has high crystallinity but the NaBH4 treatment produces partial Mo-vacancies.

Table.2 MoS2 treated with different concentrations of NaBH4 sample

SBET (m2/g) a)

Micropore volume (10-3cm3/g)b)

Total pore volume(10-2cm3/g)c)

MoS2

8.5

2.6

2.2

MoS2-0

14.2

3.0

5.4

MoS2-0.045

42.3

7.2

5.5

MoS2-0.09

167.0

12.4

17.7

MoS2-0.23

99.6

7.8

13.0

a)

Obtained from BET method;

b) Microporous analysis by HK method (pore size＜2nm); c)

Cumulative total pore volume by BJH method;

Fig.6. (a) BET adsorption adsorption-desorption isotherms and (b) corresponding pore size 12

distribution of MoS2 treated with different concentrations of NaBH4

To further confirm change of sample structure, physical property investigation on the pore structure and size distribution was conducted (Fig. 6(a)). The results show that MoS2 treated with NaBH4 can effectively increase the specific surface area (Table 2). The untreated MoS2 specific surface area was only 8.5 m2/g. The specific surface area of the treated MoS2 increases with the concentration of NaBH4 increase. MoS2-0.09 reached a maximum specific surface area (167.0 m2/g). At the same time, the micropore volume and total pore volume also showed similar trends. MoS2-0.09 also has the largest micropore volume (12.4×10-3 cm3/g) and total pore volume (17.7×10-2 cm3/g). In addition, the pore size distribution curve of MoS2 in Fig. 6(b) also shows that the NaBH4 reduction treatment can make MoS2 have a rich microporous structure. This is advantageous for the progress of the catalytic reaction. However, as the NaBH4 concentration further increased, there was a heavy decrease in the specific surface area (99.6 m2/g), micropore volume (7.8×10-3 cm3/g) and total pore volume (13.0×10-2 cm3/g) for sample MoS2-0.23. This also demonstrates that excess NaBH4 destroys the integrity of the MoS2 structure.

13

Fig. 7 (a) Transient photocurrent of MoS2/TiO2 composite;(b) Hydrogen production efficiency of MoS2/TiO2 composites with different concentrations of NaBH4; (c) Cyclic test of photocatalytic hydrogen evolution of MoS2-0.09/TiO2 composite samples: catalyst, 0.02g; containing 20% TEOA 50 mL aqueous solution

The NaBH4 treated MoS2/TiO2 composite was used as the photoelectrochemical property of the photoelectrode evaluation electrode. As shown in Fig 7(a), the transient photocurrent response of the ITO electrode modified with MoS2/TiO2 and MoS20.09/TiO2 composites was recorded in several switching cycles under UV irradiation with a bias of -0.038 V. The MoS2-0.09/TiO2 sample showed a remarkable improvement in photocurrent density and showed the best performance, and the photocurrent density increased by 2.4 times compared with the original MoS2/TiO2 composite. It is proved that NaBH4 treatment can effectively improve the photocatalytic performance of MoS2. 14

The PHP performance of MoS2/TiO2 composites was examined with a 300W Xe lamp. As is shown in Fig. 7(b), pure TiO2 showed a low H2 evolution (176 μmolg-1 h1),

because of the rapid recombination of photogenerated carrier. However, the

hydrogen evolution rate of MoS2/TiO2 composites was 798 μmolg-1h-1, which was 4.5 times that of pure TiO2. The MoS2/TiO2 composite shows high hydrogen evolution effective, because the heterojunction was formed between TiO2 and MoS2, due to the improvement of separation efficiency of electron-hole pairs, improving the PHP efficiency. When MoS2-0 was loaded, the growth rate of H2 increased to 2460 μmolg1h-1.

When the concentration of NaBH4 was 0.045 mol/L, the hydrogen evolution rate

arrived at 4836 μmolg-1h-1, which was 6.8 times than that of MoS2/TiO2 composite. Obviously, with the increase of NaBH4 concentration, the active site of MoS2 increased significantly, and the MoS2-0.09/TiO2 achieved the maximum H2 evolution efficiency (5424 μmol-1h-1). When the amount of NaBH4 added continues to increase, there is a clear downward trend, and the efficiency of PHP is remarkably lowered. As shown in Fig. 7(c), the hydrogen production efficiency of MoS2/TiO2 composite only reduced 15% after the three times cycle test. It has been considered that the MoS2/TiO2 composite material has good stability. Furthermore, we compare the photocatalytic effect of this article with the reported work, as shown in Table 3:

15

Table 3 Comparison with reported literature results Sample

H2 Production -1 -1 (µmol g h ) N-TiO2-X@MoS2 1882 3+ MoS2/Ti -TiO2 713 MoS2/ TiO2 4300 CdS/MoS2/ Mo 4540 MoS2/ TiO2 2160 5424 This work（MoS2/ TiO2）

References 47 48 41 49 50 /

Fig. 8 Schematic diagram of photocatalytic mechanism of MoS2/TiO2 composites with Mo-vacancy

Our experimental results showed that the photocatalytic activity of MoS2 is related to the number of Mo-vacancies. A suitable number of Mo-vacancies will cause a dangling bond on the surface of the MoS2 that is adjacent to the Mo-vacancies. Therefore, a large amount of unsaturated S atoms will be generated on the surface or edge of Mo-vacancy MoS2. The PHP mechanism of Mo-vacancy MoS2/TiO2 composites is shown in Fig. 8. The abundant unsaturated S atoms in the Mo-vacancy 16

MoS2 are easily combined with hydrogen ions in water to generate sulfur-hydrogen bonds, which reduces the PHP activation energy. Thereby, the photogenerated electrons are combined with H+ in the water to achieve the purpose of efficient PHP. MoS2 with suitable amount of Mo-vacancies can reduce the recombination of photogenerated carrier effectively and strengthen interface charge transfer. The active sites of MoS2 in the surface or edge can enrich electrons, which combine with H+ to form H2. Simultaneously, photogenerated holes was consumed the sacrificial reagent. Therefore, the TiO2/MoS2 with Mo-vacancies can boost the photocatalytic efficiency. On the contrary, excessive Mo-vacancies also cause the incomplete of the MoS2 structure and limit the transmission of photogenerated electrons, thus reducing the performance of PHP. 4. Conclusion In this paper, Mo-vacancy MoS2 with high catalytic effect was successfully prepared through NaBH4 treatment. The amount of Mo-vacancies was controlled by the addition of the concentration of NaBH4. The MoS2 surface and edges after processing produced a lot of Mo-vacancy defects, so more dangling bonds of unsaturated S atoms were exposed. These unbonded S atoms can act as adsorption sites for H+, greatly increasing the reaction site of MoS2. In the photocatalytic process, Mo-vacancy defects can generate localized charge enrichment, thereby achieving more efficient PHP efficiency of Mo-vacancy MoS2/TiO2 phtocatalyst. It can open a new doorway to develop functional two-dimensional layered materials.

17

Conflicts of interest Authors declare that all authors agree to submission and no any interest.

5. Acknowledgments This work was financially supported by the Major Projects of Natural Science Research in Anhui Colleges and Universities (KJ2018ZD050), Natural Science Foundation of Anhui province (1808085ME129), Outstanding Young Talents Support Program in Colleges and Universities (gxyqZD2018056) and the College Students’ Science and Technology Innovation Foundation (2019-152).

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High lights: 1. MoS2 is treated with reductant NaBH4 to generate Mo vacancies on its surface and edges 2. The generation of Mo vacancy defects exposes more unsaturated S atom dangling bonds, it can be used as H+ adsorption sites, which greatly increases the reactive sites of MoS2. 3. The photocatalytic hydrogen efficiency of MoS2-0.09 / TiO2 nanocomposites is 6.8 times than that of untreated MoS2 / TiO2. 4. The possible photocatalytic mechanism was proposed

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Credit author statement Xie Wen-Jie：Investigation、Methodology Li Xuan：Formal analysis 、Writing - Original Draft、Data Curation Zhang Feng-Jun：Conceptualization、Resources、Supervision、Writing - Review & Editing

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