Nano-flower S-scheme heterojunction NiAl-LDH/MoS₂ for enhancing photocatalytic hydrogen production

Abstract

The construction of heterojunctions can effectively improve the electron transport rate and photocatalytic activity. In this work, nanoflower-like S-scheme heterojunction MoS₂/NiAl-LDH was successfully synthesized. The results indicated that the hydrogen yield of the MoS₂/NiAl-LDH composite catalyst reached 229.5 μmol in 5 hours under visible light irradiation by adjusting the pH value of the sacrificial agent and the amount of eosin. The crystal structure and morphological characteristics of the composite were analyzed by XRD, SEM and TEM. The visible light response degree and charge separation and transfer rates of photogenerated carriers of the composite were analyzed by UV-Vis DRS, PL and electrochemistry. The electronic transfer path was also analyzed by XPS. The construction of S-scheme heterojunctions can realize effective separation of electron–hole pairs and ensure that the photo-induced carriers with the strongest redox ability can be retained to participate in the photocatalytic reaction. In addition, the feasible mechanism of photocatalytic hydrogen evolution is given. This work will provide different insights into the enhancement of hydrogen evolution by heterojunctions formed by binary semiconductors.

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1 Introduction

Due to the destruction of the environment and waste of energy, the earth's resources have been seriously damaged.^1,2 Therefore, seeking a kind of green sustainable development of resources becomes particularly important.^3–5 In recent years, hydrogen has attracted wide attention because of its advantages such as cleanness, efficiency, wide range of availability and storage properties.^6–8 In 1972, Fujishima A and Honda K⁹ reported, for the first time, the decomposition of aquatic hydrogen by a TiO₂ single-crystal electrode, which opened up a path for hydrogen production by solar photolysis of water. As a precious metal, Pt plays an important role in photocatalysis, but its high price and scarce resources limit the development of Pt in the field of photocatalysis.^10–12 Therefore, it is very important to find a class of materials that can replace precious metals in reactions.

Layered double hydroxide (LDH) is a kind of two-dimensional material, in which the plate is positively charged and different anions between the layers contribute to the negative charge, and hence, charge balance can be achieved; as a result, whole LDH is not charged, neutral, and has some H₂O molecules.^13–15 The structural chemical formula is [M²⁺_1−xM³⁺_x(OH)₂]^X+ [A_x/n]ⁿ⁻·mH₂O, in which the +2 valence metal cations include Ni²⁺, Mg²⁺, Cu²⁺, and Zn²⁺, and the +3 valence metal cations include Al³⁺, Fe³⁺, Cr³⁺, and Sc³⁺.^16,17 Due to the advantages of unfixed chemical element composition, large specific surface area and good stability of LDHs, its structure is easy to adjust and control, and is widely used in photocatalysis and electrocatalysis.^18–20 However, due to the slow transfer rate of the photogenerated carriers generated by single LDH by photoexcitation, LDH itself has no obvious effect on hydrogen evolution.^21–23 Therefore, many people promote the hydrogen evolution effect by constructing heterojunctions or strengthening coupling between the two semiconductors. For example, NiAl-LDH/CdS promotes hydrogen evolution by constructing S-schemes and consuming useless electrons and holes.²⁴ CoAl-LDH/RGO nanocomposites were synthesized by a solvothermal method, which can effectively transfer and separate photogenerated charge carriers, thus enhancing the properties of the composites.²⁵ The 2D/2D heterojunction constructed by NiAl-LDH/g-C₃N₄ accelerates the separation of electron holes and promotes the hydrogen evolution effect.²⁶

In the field of photocatalysis, Mo sulfides have become the most promising Pt equators.^27–29 Among them, MoS₂ has been widely reported in the field of photocatalysis due to its extensive sources, good electrical conductivity and easy preparation.^30–32 Moreover, due to its strong absorption capacity in the visible region, narrow band gap and stable chemical properties, MoS₂ has excellent photocatalytic performance. MoS₂/Mn_0.2Cd_0.8S,³³ MoS₂/Cu₂S,³⁴ and MoS₂/g-C₃N₄³⁵ can accelerate the separation of photogenerated carriers and promote hydrogen evolution by constructing heterojunctions. In order to improve the performance of photocatalytic water decomposition, different types of heterojunctions can be constructed, such as type I, type II and type Z. The S-scheme heterojunction takes into account band bending, electrostatic interaction between semiconductors and built-in electric field, which enriches the model conditions of other heterojunctions.^36–38 The S-scheme heterojunction constructed by contacting NiAl-LDH and MoS₂ can inhibit the recombination of photo-generated carriers and also increase hydrogen production.

In this study, MoS₂ and NiAl-LDH successfully constructed the S-scheme heterojunction, which accelerated the generation and transfer of photogenerated carriers. The addition of MoS₂ broadened the absorption and utilization of available light in the composite catalyst and reduced the composite efficiency of photogenerated carriers. Therefore, the photocatalytic activity of NiAl-LDH was significantly improved.

2 Experimental section

2.1 Materials

Nickel(II) nitrate hexahydrate [(Ni(NO₃)₂·6H₂O)], aluminium nitrate nonahydrate [Al(NO₃)₃·9(H₂O)], urea (CH₄N₂O), ammonium fluoride (NH₄F), ammonium molybdate (H₈MoN₂O₄) and thiourea (CH₄N₂S) were purchased from Aladdin's official.

2.2 Method of MoS₂ preparation

MoS₂ was prepared by hydrothermal synthesis. First, 1.124 g ammonium molybdate was dissolved in 40 mL distilled water. After complete dissolution, 3.021 g thiourea was added. After ultrasonic stirring, the solution was evenly mixed and transferred to a 100 mL reactor for 10 h reaction at 180 °C. After cooling to room temperature and centrifugation, the reactor was put into an oven and dried at 80 °C for 12 h. The powder catalyst was obtained.

2.3 Method of nano flower NiAl-LDH preparation

NiAl-LDH was prepared by hydrothermal synthesis. First, 0.139 g nickel nitrate and 0.060 g aluminum nitrate were weighed and dissolved in 80 mL water. After ultrasonic stirring for 10 minutes, 0.047 g ammonium fluoride and 0.192 g urea were added, respectively. After stirring for 30 minutes, it was transferred to a 100 mL high-pressure reactor for 24 h reaction at 120 °C. After the temperature was reduced to room temperature, it was centrifuged and washed several times with deionized water and ethanol until it became neutral. The powder catalyst was obtained after drying in an oven at 80 °C for 12 h.

2.4 Preparation of nanometer flower structure catalysts

The process was carried out by physical stirring. First, a certain amount of NiAl-LDH was weighed and dissolved in 30 mL ethanol. After the mixture was evenly mixed, a certain amount of MoS₂ was added, followed by ultrasonication for half an hour and stirring for six hours. Then, they were put it in a water bath to evaporate ethanol and get a mixture of the two. Keeping the amount of NiAl-LDH (0.050 g) fixed and unchanged, different amounts of MoS₂ were added (0.100 g, 0.150 g, 0.200 g, 0.250 g, and 0.300 g) to vary the quality, and the products were named NM1, NM2, NM3, NM4, and NM5 accordingly (Scheme 1).

Scheme 1 Schematic diagram of MoS₂/NiAl-LDH synthesis.

3 Discussion of results

3.1 XRD discussion

Fig. 1(a) shows the XRD patterns of NiAl-LDH. In the XRD pattern of NiAl-LDH alone, the diffraction peak could well correspond to the standard card of NiAl-LDH (JCPDS No. 22-0452).³⁹ The diffraction peaks at 11.54°, 23.27° and 35.08° correspond one-to-one with the diffraction planes of (003), (006) and (012). Fig. 1(b) shows the XRD patterns of MoS₂ and NM1 The three diffraction peaks at 14.4°, 32.7° and 58° correspond to the diffraction surfaces of (002), (100) and (110) of MoS₂.^40,41 As can be seen from Fig. 1b, there are (003) and (012) diffraction surfaces of NiAl-LDH in the composite material. With the increase in MoS₂ loading, the diffraction peaks of the (002) crystal plane characteristic of MoS₂ gradually increased, but the diffraction peaks of NiAl-LDH became less and less obvious. This may be due to the fact that MoS₂ is loaded on the nanoflower structure of NiAl-LDH⁴² and the high proportion of MoS₂ in the composite catalyst. As a result, the crystal structure of NiAl-LDH was changed, and the diffraction peak of NiAl-LDH was covered up.

Fig. 1 XRD patterns of (a) NiAl-LDH; (b) MoS₂ and NM1; and (c) NMX (X = 1–5).

3.2 Surface analysis

3.2.1 SEM analysis. In order to investigate the morphological characteristics of the samples, field emission scanning electron microscopy (SEM) was performed on the samples. It can be seen from Fig. 2(a) that NiAl-LDH was a nanoflower structure with a diameter of about 3–4 μm. It can be seen from Fig. 2(b) that MoS₂ is an irregular nanostructure with a diameter of about 0.4–0.8 μm. Fig. 2(c) and (d) show the morphology of the composite catalyst. It can be seen that there were small nanostructures on the nanostructure of NiAl-LDH, and the sufficient contact between the two provides more paths for electron transfer. The results indicated that the composite catalyst was successfully synthesized.

Fig. 2 SEM images of (a) NiAl-LDH; (b) MoS₂; (c and d) NM3.

3.2.2 TEM analysis. TEM can better reflect the contact between the two catalysts. Fig. 3(a) shows the TEM image of NM3, and the nano-flower structure of about 3–4 μm can be observed; many MoS₂ nano-flower structures are derived at the edge of the nano-flower. The close contact between the two catalysts promotes the charge transfer, which was consistent with the structure analyzed by SEM. The lattice spacing of 0.27 nm corresponds to the (100) crystal plane of MoS₂, and 0.261 nm corresponds to the (101) crystal plane of NiAl-LDH. The EDX (S2) image can determine the types of elements in the catalyst (Fig. 3c–h), and the combined element mapping can reflect the uniform distribution of Mo, S, Ni, Al and O elements in the catalyst.

Fig. 3 Images of NM3 (a) TEM and (b) HRTEM, and (c–h) elemental mapping of NM3.

3.3 Valence state of elements

The sample was tested by XPS, and the elemental composition and valence state information of the NM3 composite catalytic material was obtained. Five elements, namely, Al, S, Mo, C and Ni, are found in the full spectrum shown in Fig. 4a. In the fine pattern of Al (Fig. 4b), the peaks at 68.2 eV and 74.2 eV correspond to Al 2p_1/2 and Al 2p_3/2 of Al³⁺,⁴³ respectively. Mo 3d (Fig. 4c) showed Mo⁴⁺ (228.2 eV and 231.4 eV) and Mo⁶⁺ (232.2 eV and 235.2 eV), and the binding energy is located at 225.5 eV due to S^2s.^44–46 The pattern of Ni 2p (Fig. 4d) mainly consists of two principal spectra located at 856.1 eV and 874 eV, which correspond to the spin–orbital pair of Ni 2p_3/2 and Ni 2p_1/2, and the two satellite peaks correspond to the divalent oxidation state of Ni. The spin energies of Ni 2p_3/2 and Ni 2p_1/2 were calculated to be 17.9 eV, which is characteristic of the Ni(OH)₂ phase in LDH.^47,48 The fine pattern of S 2p (Fig. 4f) at 161.2 eV and 162.3 eV further confirmed that the S element reacts to S²⁻ in the composite catalyst.⁴⁹ The typical peak (531.4 eV) of oxygen is shown in Fig. 4f. It can be seen from S1 that compared with the MoS₂ composite catalyst, S 2p and Mo 4d have different degrees of tendency to move towards low binding energies. Similarly, O 1s, Al 2p and Ni 2p have different degrees of tendency to move towards high binding energies compared with the NiAl-LDH composite catalyst, confirming the existence of a built-in electric field, which makes electrons move from NiAl-LDH to MoS₂.^36,50

Fig. 4 XPS images of NM3 samples: (a) full spectrum; (b) Al 2p; (c) Mo 4d; (d) Ni 2p; (e) S 2p; and (f) O 1s.

3.4 BET analysis

The specific surface area, pore volume and pore diameter of the sample were obtained by measuring the N₂ absorption desorption curve of the sample. It can be judged that NiAl-LDH, MoS₂ and NM₃ are all type IV isotherms (IUPAC classification). It can be judged from the pore diameter distribution curve (Fig. 5b) that all were mesoporous materials with H₃ type hysteresis loops. It can be seen from Table 1 that NiAl-LDH has the largest specific surface area, pore volume and pore diameter. The catalyst with a larger specific surface area can expose more active sites and promote the reaction. Compared with NiAl-LDH, the specific surface area, pore volume and pore size of NM3 are smaller, which further proves the successful combination of NiAl-LDH and MoS₂. A large number of open space structures exist in the nanoflower structure of NiAl-LDH, which enables the successful embedding of MoS₂ nanoparticles. Therefore, the specific surface area of NM3 and NiAl-LDH became smaller.

Fig. 5 (a) Adsorption–desorption isotherm of the catalyst. (b) Pore size.

Table 1 Surface area (S_BET), pore diameter (D_p), and pore volume (V_p) of MoS₂, NiAl-LDH and NM3

Samples	S BET (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore diameter (nm)
MoS2	9.431	0.048	19.504
NiAl-LDH	75.991	0.409	21.645
NM3	30.571	0.136	15.168

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3.5 Optical properties

3.5.1 Electrochemistry. In order to explore the transfer and separation efficiencies of photogenerated carriers and the electrochemical properties of the catalyst itself, transient photo-current (IT), electrochemical impedance spectra (EIS) and polarization curves (LSV) were studied. In Fig. 6(a), due to the accumulation of holes, the NM3 photocurrent intensity decreases slightly, but has the strongest photocurrent intensity, which confirms that the photogenerated carrier transfer efficiency and photocatalytic activity are significantly improved compared with MoS₂ and NiAl-LDH. At the same time, the EIS of NM3 has the smallest arc radius (Fig. 6b), which indicates that the composite catalyst is subjected to the least resistance during the transfer of photogenerated carriers. IT and LSV data showed that NM3 had a lower electron coincidence rate and a more efficient electron transfer between the two catalysts, thus increasing the production activity of H₂. In addition, the polarization curve (Fig. 6c), the lower the overpotential, the better the hydrogen production effect. Compared with NiAl-LDH and MoS₂, the overpotential of the NM3 electrode is significantly reduced, indicating that NM3 has good hydrogen production performance.

Fig. 6 NiAl-LDH, MoS₂ and NM3: (a) IT curves; (b) EIS curves; and (c) LSV curves.

3.5.2 UV-vis DRS analysis. It is an important part to evaluate the performance of catalysts to study the response degree of the catalyst to visible light. In order to study their light absorption capacity, the UV-vis DRS of NiAl-LDH, MoS₂ and NM3 were studied. As shown in the Fig. 7(a), NiAl-LDH has three absorption bands in the range of visible light, in the 200–300 nm absorption band was attributable to the ligand to the metal charge transfer (from O 2p orbit to Ni 3d_t2g) in 300–500 nm, 600–800 nm visible light range of dual-band attributed to octahedron in the structure (d–d transition of Ni²⁺).^39,51 MoS₂ and NMX showed full-spectrum absorption in the visible light range, which was caused by the black MoS₂ prepared and the large proportion of MoS₂ in the composite catalyst. In addition, the UV-vis DRS data were converted using the formula: αhν = A(hν − Eg)^n/2,⁵² where α is the absorption coefficient and hν is the photon energy, and the band gap values of NiAl-LDH and MoS₂ are 2.4 eV and 1.79 eV respectively.

Fig. 7 (a) UV-vis DRS diffuse reflectance spectra of NiAl-LDH, MoS₂, and NM3. Band gap plots of (b) NiAl-LDH and (c) MoS₂.

3.5.3 Structure of energy band. In order to ensure that H⁺ can be reduced to H₂, the Mott–Schottky (MS) plot was used to test MoS₂ and NiAl-LDH. As shown in Fig. 8(a) and (b), the CB potential relative to Ag/AgCl electrode MoS₂ and NiAl-LDH was −0.2 V and −0.92 V, respectively. The formula E_NHE = E_Ag/AgCl + 0.197 was used to transform the Ag/AgCl electrode into the standard hydrogen electrode.^52,53 It is known that the band gaps of MoS₂ and NiAl-LDH are 1.79 eV and 2.4 eV respectively, and the band structure (Fig. 8c) can be obtained.

Fig. 8 Mott–Schottky plots for (a) MoS₂ and (b) NiAl-LDH. (c) The valence band position.

3.5.4 PL analysis. In order to investigate the separation and transfer efficiency of photogenerated carriers in the hydrogen production process, the PL spectra of different catalysts were tested. Different catalysts have different peak heights. Generally speaking, the lower the peak value of PL, the better the activity of the catalyst and the lower the composite efficiency of photogenerated carriers. Fig. 9(a) shows the PL spectrum of the catalyst when the excitation wavelength is 470 nm. It can be seen that the peak value of NM3 decreases to a large extent compared with that of NiAl-LDH. Fig. 9(b) shows the comparison of peak intensities of different catalysts. It can be observed that the fluorescence quenching intensity of NM3 was the lowest, which proved that the construction of S-scheme heterojunctions can effectively avoid the problem of charge recombination between NiAl-LDH and MoS₂, thus improving the photocatalytic hydrogen evolution performance of the composite material.

Fig. 9 NiAl-LDH, MoS₂, and NMX (X = 3, 4, and 5): (a) PL spectra and (b) PL peak intensity comparison.

3.6 Production hydrogen

3.6.1 Photocatalytic activity. The catalyst was carried out in a reaction system with 10 wt% triethanolamine (TEOA) as the sacrificial agent and EY as the sensitizer. When the pH of TEOA was 10, the hydrogen production of NiAl-LDH, MoS₂ and NM3 was compared, and the hydrogen production of NM3 reached 229.5 μmol, which was 200 times and 4 times that of NiAl-LDH and MoS₂, respectively. Fig. 10(b) shows the hydrogen production of different ratios of composite catalysts in the EY sensitization system under visible light irradiation for five hours, in which NM3 showed the best hydrogen production effect under visible light irradiation. Fig. 10(c) shows the comparison of hydrogen generation of the NM3 composite catalyst in different pH values of the TEOA solution. It can be seen from the figure that the composite catalyst has the best hydrogen production effect in a mild environment (pH = 10). Excessive acid or alkali environment was not conducive to hydrogen production. Excessive alkali will reduce the content of H⁺ in the solution, which was not conducive to the hydrogen evolution reaction.⁵⁴Fig. 10(d) shows the influence of adjusting the amount of EY on hydrogen evolution. It can be seen that when the amount of EY was 24 mg, NM3 has the best effect of hydrogen production, and excessive EY will cover up the active site of the reaction and decrease the hydrogen production capacity. Fig. 10(e) shows the apparent quantum efficiency of NM3 at different wavelengths. When the wavelength is 550 nm, the best apparent quantum efficiency of the sample is 2.78%.

Fig. 10 H₂ evolution of (a) NiAl-LDH, MoS₂ and NM3; (b) NMX (X = 1–5); (c) EY affected of H₂ evolution; (d) pH (TEOA) affected of H₂ evolution; and (e) apparent quantum efficiency (AQE) of NM3.

3.6.2 Stability experiments. Whether the catalyst is stable or not is an important part of evaluating its performance. For this, we carried out a hydrogen evolution cycle test. After 5 hours of hydrogen production, the catalyst in the quartz bottle was filtered and recovered, and then the new EY and sacrificial agent were added. It can be seen that the hydrogen production effect has decreased, which may be due to the loss of catalyst in the process of extraction and filtration, resulting in a decrease in the effect. As can be seen from Fig. 11(b), the XRD pattern before and after the reaction did not change significantly. Fig. 11(c) and (d) show SEM images before and after the reaction, with no significant change in size, which could clearly display the structure of nanometer flowers. In conclusion, the composite material NM3 has good stability.

Fig. 11 (a) Cycling test. (b) XRD patterns of NM3 before and after the reaction. SEM of NM3 (c) before the reaction and (d) after the reaction.

3.7 Photocatalytic reaction mechanism

Based on the above-mentioned results, the values of CB and VB of NiAl-LDH and MoS₂ were determined, as shown in Fig. 8(a). Two kinds of semiconductors with the construction of heterojunctions can be divided into type II heterojunction or S-scheme heterojunction. Assuming that the type II heterojunction was constructed, the photogenerated electrons and holes move to the positions of NiAl-LDH and CB and VB of MoS₂ respectively, and the electrons react on CB of MoS₂ to generate H₂. On the downside, the CB of MoS₂ was very low (−0.1 eV), which does not provide sufficient driving force for H₂ production. In the process of XPS data processing, the binding energy of S 2p moves to the high and the binding energy of Ni 2p moves to the low, making the electrons move from MoS₂ to NiAl-LDH. Based on the analysis of the energy band structure and XPS data, the catalysis mechanism of the S-scheme heterojunction was proposed (Fig. 12). When the two semiconductors are in close contact, the electrons will spontaneously transfer from NiAl-LDH to MoS₂ under the influence of the coulombic force. Due to the different positive and negative characteristics of the two semiconductors, an internal electric field is formed at the interface.^55,56 Band bending is caused by the transfer and accumulation of electrons. Under the irradiation of visible light, the electrons at the MoS₂ CB combine with the holes at the NiAl-LDH VB and consume the useless electrons and holes. The hydrogen evolution process is carried out at the NiAl-LDH CB, and the reaction is carried out by the MoS₂ VB.

Fig. 12 Reaction mechanism diagram.

4 Conclusions

MoS₂ and NiAl-LDH have been successfully prepared by a hydrothermal method. The 2D/2D composite MoS₂/NiAl-LDH has been synthesized by an ordinary physical mixing method. The S-scheme heterojunction has been constructed according to the band structure and electron transfer of MoS₂ and NiAl-LDH. The close contact between MoS₂ and NiAl-LDH will cause charge transfer, and the internal electric field will greatly improve the transfer rate of photogenerated carriers. The successful coupling of the two not only accelerates the separation of photogenerated carriers, but also has the minimum resistance in the process of electron transfer, and can avoid charge recombination between NiAl-LDH and MoS₂, thus enhancing the effect of hydrogen production. Compared with MoS₂ and NiAl-LDH, the MoS₂/NiAl-LDH composite showed the highest amount of photocatalytic hydrogen production, which reached 229.5 μmol in 5 h and showed good point stability after a 20 h cycle test. Taken together, this work will provide new insights into the construction of S-type heterojunctions by photocatalysts with a 2D/2D spatial structure.

Conflicts of interest

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.

Acknowledgements

This work was financially supported by the Fundamental Research Funds for the Central Universities of North University for Nationalities (2021KJCX02), the Natural Science Foundation of Ningxia Province (2021AAC03180), and also was supported by the Xixia District Science and Technology Plan Project (XXKJ1901).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1nj04728k

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