Optimized electronic structure and p-band centre control engineering to enhance surface absorption and inherent conductivity for accelerated hydrogen evolution over a wide pH range

Yuanzhe Wang , Dong Wang , Jiajia Gao , Xianfeng Hao , Zhiping Li , Junshuang Zhou and Faming Gao *
Key Laboratory of Applied Chemistry, College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, P. R. China. E-mail: fmgao@ysu.edu.cn

Received 21st April 2020 , Accepted 9th June 2020

First published on 9th June 2020


Numerous experiments have demonstrated that an appropriate electronic configuration can effectively activate the electrocatalytic activity. However, systematic studies on the effects of non-metallic elemental doping and its p-orbital center (εp) on electrocatalysis have not yet been carried out. Combining theoretical and experimental methods, we demonstrate an electronic configuration and p-orbital center control engineering for promoting the HER course in both acid and alkaline solutions over group VA elements doped into the inert basal plane of nanoMoS2. In acidic solutions, As-doped MoS2 has the best electrocatalytic activity. Theoretically, the calculated ΔGH of the As atom is only −0.07 eV, indicating that it has excellent catalytic performance. Furthermore, the p-orbital center under and near the Fermi level plays a significant role in the H adsorption course, and the closer the εp value is to the Fermi level, the weaker the H- non-metallic atom bond is. An appropriate εp can insure a proper strength of bond with H and further influence the catalytic activity of the HER. In alkaline solutions, P-doped MoS2 has the best electrocatalytic activity, which is due to the engineering of water dissociation sites by doping P atoms into MoS2 nanosheets. These findings pave the path to develop a rational strategy to trigger the activity of the inert basal plane of MoS2, to enhance the conductivity of inherent MoS2 towards the HER and provide a new idea that can be extended to other layered dichalcogenides.


1 Introduction

As an environmentally friendly and renewable energy source, H2 has attracted much attention as a solution to future global energy crisis.1,2 Electrolysis of water is an effective route for green, sustainable production of H2.3 To improve the high overpotential and sluggish kinetics of the hydrogen evolution reaction (HER), catalysts are needed. Pt is the most effective catalyst for the HER. However, its low natural abundance and high cost limit its widespread application.4,5 Cheap, durable, and high-performance non-Pt catalysts are highly needed for practical applications in electrochemical hydrogen production.

To date, various potential earth reserve materials have been explored for use in the electrochemical study of hydrogen (HER) catalysts.6–10 In particular, two-dimensional MoS2 is of great interest to scientists because of its high natural abundance and good stability.11 The catalytic activity of bulk MoS2 is not remarkable, but nanoMoS2 has good catalytic activity. Theory and experiments have shown that the basal plane of MoS2 is inert, and its catalytic activity is mainly derived from the active sites on its edge.12–14 When applied to electrochemical hydrogen production (HER), the low conductivity of MoS2 is also a drawback.15 In recent years, it has been found that doping can improve the conductivity of MoS2 and activate its inert base plane. By introducing different metal atoms into MoS2, the inert basal planes can be activated.16 According to the “volcano” theory, such metal atom doping can optimize the energy level of MoS2 to maximize its activity.17 There are few reports on the activation of inert basal planes by non-metal atom-doped MoS2. Using the first-principle calculations, Huang et al. predicted that the inert basal plane of MoS2 could be activated after P-doping. They synthesized P-doped 2H-MoS2, and O-doping was used to assist the P-doping process due to the difficulty of P-doping into the 2H-MoS2 structure.17 Liu et al. prepared P-doped MoS2 nanosheets with enlarged interlayer spacings, which increased the numbers of basal plane active sites and improved the conductivity.12 Our recent work shows that N-doping can effectively increase the intrinsic conductivity of the MoS2 catalyst, but it cannot activate the molybdenum disulfide inert surface.18,19

Herein, combining theoretical calculations and experimental data, we demonstrate a method to accelerate the hysteresis HER kinetics of MoS2 electrocatalysts by activating the inert basal plane and increasing its intrinsic conductivity by doping VA-group non-metallic atoms into MoS2 nanosheets. A cost-effective and facile hydrothermal method was used to prepare non-metallic atom-doped MoS2 nanoplates. In acidic solutions, 2D MoS2 nanosheets doped with different non-metallic elements possessed excellent catalytic activity compared with original MoS2, especially when As was doped into MoS2. Electrochemical oxidation of the edges leads to a significant inhibition of the catalytic activity of the original MoS2, while the catalytic activity of As-doped MoS2 nanosheets is less impacted by partial oxidation, which indicates that the basal plane is catalytic. Theoretically, the calculated ΔGH* of As is only −0.07 eV, indicating a high catalytic activity. Moreover, p-orbitals near and under the Fermi level play a key role in the adsorption course of H. The closer the level of the p-orbital centre εp is to the Fermi level, the weaker the H-doping atom bond is. In alkaline solutions, the P-doped MoS2 catalyst exhibited an excellent catalytic activity compared with other as-prepared MoS2, which was due to the engineering of water dissociation sites by doping P atoms into the MoS2 nanosheets. Furthermore, the non-metal-doped MoS2 catalyst exhibited a more rapid electron transfer rate than the original MoS2 sample.

2 Results and discussion

2.1 Sample characterization

In this work, few-layer original MoS2(MoS2), N-doped MoS2(N-MoS2), P-doped MoS2(P-MoS2) and As-doped MoS2(As-MoS2) nanosheets were successfully fabricated by an optimization hydrothermal method. An overview of the shapes of the original MoS2 (Fig. S1a, ESI), N-MoS2 (Fig. S2a, ESI), P-MoS2 (Fig. S3a, ESI) and As-MoS2 (Fig. 1a) was prepared by scanning electron microscopy (SEM). All the as-prepared products have similar petal-like shapes, consisting of thicknesses of several thin inter-connected nanoflakes. Transmission electron microscopic (TEM, Fig. 1b and Fig. S1b, S2b, S3b, ESI) images further verified that the petal-like shapes were composed of thin nanosheets. No distinct morphological changes between the four products were discovered, which indicated that N, P and As atoms were introduced and did not prominently impact the morphology of MoS2. Elemental mapping of the TEM images displays the uniform distributions of Mo, S and As over the As-MoS2 nanosheets (Fig. 1d–g). The relevant energy-dispersive X-ray (EDX) analysis further indicated that As exists in the sample, suggesting that As was successfully introduced and uniform distributed in the base plane of the product. The chemical compositions of the as-prepared N-MoS2 and P-MoS2 nanosheets were also performed by elemental mapping analyses (Fig. S2 and S3d–g, ESI). The microscopic structure of original MoS2, N-MoS2, P-MoS2 and As-MoS2 are shown in Fig. S1 (ESI) inset, Fig. S2, S3c and Fig. 1c by high-resolution transmission electron microscopy (HRTEM). N-MoS2, P-MoS2 and As-MoS2 possess similar atomic layers stacked together in the edge areas. The lattice spacings of 0.27 nm and 0.62 nm correspond to the (100) plane and (002) edge, respectively, of hexagonal MoS2.
image file: d0cp02131h-f1.tif
Fig. 1 Morphological characterization of As-MoS2 catalysts. (a) SEM, (b) TEM and (c) HRTEM images of the As-MoS2 nanosheets; (d–g) EDX elemental mapping of Mo, S, and As showing uniform distribution of the three elements.

X-ray diffraction (XRD) was used to investigate the crystalline structure of as-prepared MoS2 (Fig. 2a).20 No obvious peaks of other phases in the spectra of N-MoS2, P-MoS2 and As-MoS2, demonstrating that N, P and As are stabilized substitutions on the base S of MoS2 and maintain the original crystal structure or no detectable MoS2-based compounds, were formed. To further uncover the crystal information, Raman spectra were used to study the bonding characteristics. The Raman spectrum of pristine MoS2 is shown in Fig. 2b, and the two strong peaks observed at 378 and 402 cm−1 arise from the E12g and A1g vibrational modes of 2H phase MoS2,21 respectively. The E12g and A1g modes shift stepwise and their peak intensities were altered by N, P, and As doping in MoS2, which adjust the Mo–S modes and their vibration frequency or intensity.12 No other distinct vibration modes were found for products N-MoS2, P-MoS2 and As-MoS2, further verifying that no new phases were formed. X-ray photoelectron spectroscopy (XPS) was used to research the bonding and valence state of the as-obtained MoS2 nanoplates. For original MoS2, two sharp peaks at approximately 229.2 and 232.3 eV were observed in its Mo 3d spectrum (Fig. 2c), corresponding to Mo4+ 3d3/2 and Mo4+ 3d5/2 of 2H-MoS2,20 respectively. The high-resolution Mo 3d spectra shift to higher binding energies for the N-MoS2 samples; however, a shift to lower binding energies was observed for P-MoS2 and As-MoS2 in contrast to that of original MoS2, which was mainly due to electron transfers after N, P and As doping in MoS2. Compared with S, P and As are less electronegative, as electron donors; nevertheless, N possesses a strong electronegativity, as an electron acceptor. This was also further supported by the high-resolution S 2p XPS spectra (Fig. 2d). In the spectra, the peaks of S 2p for the N-MoS2 samples slightly shift towards higher binding energies, and the peaks of S 2p for P-MoS2 and As-MoS2 shift toward lower binding energies compared to that of original MoS2, which agrees well with our Bader charge calculations (Fig. S15–S21, ESI). The results indicated that N, P and As-doping were successful in the as-prepared samples, which agreed well with our DFT calculations. The XPS spectra of N, P and As are shown in Fig. 2e–f, respectively. The above XPS analysis was consistent with the previous elemental mapping results, verifying the chemical contents of the as-prepared nanosheets. The average specific surface areas were concluded by the Brunauer–Emmett–Teller method. The BET surface area is 28.75 m2 g−1 for As-MoS2, which is similar to that of original MoS2 (28.96 m2 g−1), N-MoS2 (29.82 m2 g−1) and P-MoS2 (29.76 m2 g−1), confirming that N, P and As have less influence on the specific surface area.


image file: d0cp02131h-f2.tif
Fig. 2 Structural characterization of the as-prepared catalysts. (a) XRD. (b) Raman patterns of the fabricated MoS2 nanosheets. (c) Mo 3d, (d) S 2p, (e) N 1s, (f) P 2p and (g) As 2p XPS spectra of the as-prepared catalysts.

2.2 Evaluating the electrocatalytic activity

To explore the impact of N, P and As dopants on the HER performance of the MoS2 system, we evaluated the comprehensive electrochemical activity of original MoS2, N-MoS2, P-MoS2 and As-MoS2 catalysts in 0.5 M H2SO4 and 1.0 M KOH solutions via a typical three-electrode system. As reference, the HER activity of commercial 20% Pt/C was also measured under the same experimental conditions.
2.2.1 Evaluating the electrocatalytic activity in acidic solutions. In the acidic solution, the As-MoS2 nanosheets exhibited a small HER overpotential of 200 mV at 10 mA cm−2. This overpotential at 10 mA cm−2 is smaller than that of the other doped nanostructure MoS2 samples and the original MoS2, which implies that the As-MoS2 sample possesses excellent catalytic performance. These results indicate that the existence of As atoms in the basal plane can dramatically improve the electrocatalytic activity.

Furthermore, the weight content of As in the as-prepared As-MoS2 samples could be modulated from 2.28% to 8.02% by adjusting the dosage of As2S2. The sample with an As doping weight content of 3.2% has the optimum activity (Fig. S5, ESI), which is in good agreement with our calculations (Fig. S23, ESI). The HER performance of As-MoS2 (overpotential at 10 mA cm−2 of 200 mV) is comparable to the most active MoS2-based HER electrocatalysts under acidic solutions reported (Table S2, ESI), indicating that As atom doping contributes to promoting the HER activity of MoS2.

To uncover the HER mechanism, the nature of the electrical conductivity and active sites of the catalysts, which are known to be pivotal for HER catalysts, were analysed. To determine the electrode kinetics of the HER on the as-prepared samples, electrochemical impedance spectroscopy measurements were utilized (Fig. 3b). It is notable that As-MoS2, N-MoS2 and P-MoS2 catalysts have charge-transfer resistance (Rct) values that are smaller than that of the original MoS2 sample, which signifies rapid charge transfer at the interface of the electrode and electrolyte and then results in a rapid reaction rate in HER kinetics.22,23 Simultaneously, the carrier densities of the samples were determined by the Mott–Schottky process to further confirm this. The curved slopes in Fig. S7 (ESI) indicate that the concentration of electrons in non-metal-atom-doped MoS2 samples is larger than that in the original MoS2 nanoplates, implying an enhancement in the electrical conductivity due to non-metal-atom doping. With the exception of the electrical conductivity of the as-prepared sample, we also concentrated on the electrochemically active surface area of the doped MoS2 nanosheets. The electrochemically active surface area of the as-obtained samples was evaluated by their electrochemical double-layer capacitance (Cdl), which can be measured by cyclic voltammetry tests.24 We estimated the capacitive current densities of the catalysts by plotting Δj = jajb at 0.255 V versus RHE as a function of scan rate as shown in Fig. S6 (ESI). The Cdl can be calculated from their linear fitted lines. We determined that the maximum capacitance value is 92 mF cm−2 for As-MoS2, which is much larger than that of the original MoS2 and reveals the highly effective electrochemical surface area of As-doped MoS2, leading to excellent catalytic performance.


image file: d0cp02131h-f3.tif
Fig. 3 Electrochemical HER measurements of different catalysts in 0.5 M H2SO4. (a) LSV polarization curves. (b) Nyquist plots of the as-prepared MoS2 nanosheets. (c) Calculated equivalent double-layer capacitance for the as-prepared MoS2 nanosheets. (d) LSV polarization curves for sample As-MoS2 before (black line) and after (red line) 3000 CV cycles. The current density as a function of time at a static overpotential of 200 mV is shown in the inset.

To evaluate the durability of the As-MoS2 catalyst in an acidic medium, linear sweep voltammetry curves of the As-MoS2 catalyst before and after 3000 cycles were obtained and are shown in Fig. 3d. The curves almost coincide with the current density, suggesting favorable durability during an electrochemical process. Furthermore, the current-dependent time relation was also conducted to evaluate the long-term stability of As-MoS2 at a static overpotential of 200 mV. Obviously, it can be observed in Fig. 3d (inset) that the current density of As-MoS2 shows no noticeable increase over 60[thin space (1/6-em)]000 s of continuous operation, confirming the outstanding long-term stability of the As-MoS2 electrode for HERs in 0.5 M H2SO4.

To obtain further insights into the improvement in the HER performance by non-metal-atom-doped MoS2 samples, comprehensive density functional theory (DFT) calculations were applied. First, the free energies of adsorbed H (ΔGH*) with different non-metallic elements substituted by S on the basal plane are presented in Fig. 4a. A ΔGH* value close to zero means excellent catalytic performance for HERs. The more positive the ΔGH* value, the weaker the hydrogen bonding, making it less susceptible to adsorption. The more negative the ΔGH* value, the stronger the hydrogen bonding, making it more difficult to desorb. Different non-metallic elements have different effects on the H bonding strength. Obviously, the free energies of adsorbed H (ΔGH*) of non-metal-atom-doped regions are closer to zero than that of the original MoS2 basal plane (1.83 eV), especially with As. However, for the As-MoS2 sample, the other S atoms on the in-plane are still inserted (Fig. S22, ESI). This indicates that As doping can break the inertia of the in-plane25 and improve the hydrogen adsorption/desorption process, which acts as a new active site. Noticeably, the free energies of adsorbed H (ΔGH*) of the As atom is only −0.07 eV, indicating that H adsorption on the surface of the MoS2 catalyst will be optimized after the incorporation of As atoms. Hence, as expected, the presence of non-metallic elements does result in additional active sites in the MoS2 base plane.


image file: d0cp02131h-f4.tif
Fig. 4 Mechanism studies using the first principles calculation under acidic conditions. (a) HER free energy diagram for non-metal sites in the basal plane of the as-prepared MoS2 nanosheets. The insets are the corresponding atomic configurations. (b) Density of states (DOS) plots of the as-prepared MoS2 nanosheets. (c) Relationship between ΔGH and εp for non-metal-atom-doped MoS2. (d) ΔGH on non-metal atoms versus the Bader charge of non-metal atoms for different structures, with the detailed data for each point shown in ESI.

The chemical nature of TMDs are largely determined by their electron structure, and the density of states (DOS) is further investigated to provide insights into different HER activities. Interestingly, when non-metallic atoms are doped into the basal plane of MoS2, significant electronic states are found at the Fermi level, suggesting that more charge carriers could be introduced to original MoS2 by non-metallic atom doping, enhancing the electronic conductivity, which can heavily accelerate the efficiency of MoS2 HER catalysts.

H* adsorption on the surface of non-metal atoms forms a non-metal–H bond. To further investigate the change in the H bonding strength of different non-metal atoms, we performed a calculation with the d band centre (εd) theory, which is commonly defined as a good descriptor of adsorbate-metal interaction.26–28 The chemical bonding energy of adsorbate-metal interaction ΔE can be defined as follows:29,30

 
image file: d0cp02131h-t1.tif(1)
According to formula (1), V is the coupling matrix factor and can be considered a constant; εs is the energy level of the adsorbate states and can be assumed to be 0 eV.

In accordance with eqn (1), the location of the d band centre can be used to predict the chemical bonding strength of the metal to the adsorbate, and the bonding capacity will be the strongest when εd is at a mimimum,31i.e., stronger adsorption. Because the density of states (DOS) near the Fermi level, which mainly originates from the d state, plays a key role in the adsorbate-metal interaction, we changed the integral domain of εd to [−1 eV, 0 eV]. For the Mo atom near the substituted non-metallic elements, the εd values of all investigated models are shown in Table S1 (ESI). We found that the εd values of the As atom are closer to the Fermi level, indicating that a stronger As–Mo bond is formed, which enhances the As atomic activity.

For the case of H interacting with non-metallic atoms, the p-orbitals of the non-metallic atoms should be taken into account in eqn (1). Similar to the case of Mo atoms, we also changed the integral domain of εp to [−1 eV, 0 eV], which is under and near the Fermi level. The free energies of adsorbed H (ΔGH*) with different non-metallic elements substituted by S on the basal plane exhibit an approximately linear trend in relation to εp. It indicates that the p-states under and near the Fermi level play a key role in the adsorbate–non-metal interaction, and the closer the εp value is to the Fermi level, the weaker the H-non-metallic atom bond is, which is contrary to the trend that was observed on the metal surfaces. This finding is consistent with Qiao's research on H* adsorption on different graphene surfaces.32 A too strong or too weak bind capacity is not advantageous for the HER, and an appropriate εp could ensure the proper strength of the bond with H and catalytic activity for the HER.

Moreover, the electronic interaction could be regarded as the coupling between the adsorbate H 1s orbital and the non-metallic atom 3p orbital, which leads to the generation of separated bonding and antibonding states, and the degree of suitable energy levels between the non-metallic atoms and the H atom determines the H non-metallic atom bonding strength.33 As the energy level of the S 3p orbital relative to the H 1s orbital is very high, the H adsorption on the in-plane of the original MoS2 is too poor, leading to an inert basal plane in the original MoS2. When a non-metal atom is doped into the MoS2 basal plane, the electron configuration will be changed by the different electronegativity (Fig. S15–S21, ESI) to counteract the energy level mismatching to enhance the H adsorption and HER performance. Different non-metallic atoms possess different capabilities to regulate the electron structure, and tuning the Bader charge leads to moderate ΔGH* (Fig. 4d) values and high HER activity.

To obtain insights into the interaction between non-metallic elements and H atoms, we also plotted the PDOS of non-metallic elements doped in Fig. S13 (ESI), in which the H atom interacts with non-metallic elements. Prior to adsorption, the projected p-orbital density of the As atom exhibits one peak at the Fermi energy. After the adsorption, the peak at the Fermi energy level disappears, indicating that this peak of states is the cause of hybridization with the H 1s orbitals.34

To experimentally verify that the introduction of the non-metal atom As can result in excess active sites in the MoS2 basal planes, we oxidized the edges of the nanosheets, which are widely regarded as the catalytically active center.35 Due to the thermodynamic stability of the MoS2 basal planes, oxidation of MoS2 nanostructures initiates at the edges and propagates into the basal plane, and thus, oxidation minimally destroys the properties of the basal plane.36 Here, MoS2 nanosheets were edge oxidized via electrochemical oxidation in an alkaline solution. It can be vividly showed that edge oxidized pristine MoS2 nanoplates exhibit a significantly inhibited activity, but the catalytic activity of the edge-oxidized As-MoS2 nanosheets was less impacted, as shown in Fig. 5. It has previously been reported that the 2H phase of MoS2 is similarly inactivated after oxidation, which is attributed to edge oxidation.36 Electrochemical oxidation of the edges leads to a significant inhibition in the catalytic performance for original MoS2, while the catalytic performance of As-doped MoS2 nanosheets is less impacted by partial oxidation, which indicates that the basal plane is catalytically active.


image file: d0cp02131h-f5.tif
Fig. 5 Mo XPS spectra of As-MoS2 before and after part-oxidized. Before (a) and after (b). (c and d) Schematic of the partial oxidation of the As-MoS2 nanosheet edges after several voltammetric cycles. (e) HER activity of MoS2 and As-MoS2, which before and after part-oxidized.
2.2.2 Evaluating the electrocatalytic activity in alkaline solutions. As we all know, MoS2 is generally accepted as a potentially promising electrocatalyst for the HER in acidic media, whereas it shows exceedingly worse catalytic activity in alkaline solutions than in acidic solutions on account of a slow water dissociation process.37,38 Combining theoretical and experimental data, we demonstrate a universal strategy to accelerate the hysteresis HER kinetics of MoS2 electrocatalysts by engineering water dissociation sites by doping non-metal atoms into MoS2 nanosheets. The P sites introduced can effectively reduce the kinetic energy barrier of the initial hydrolysis process and subsequent H2 generation, which is the rate-limiting step of the MoS2 catalyst in alkaline solutions. As a result, the P-MoS2 nanosheets exhibited a small overpotential of 200 mV at 10 mA cm−2 for HERs in alkaline media. This overpotential at 10 mA cm−2 is smaller than that of the other doped nanostructure MoS2 samples and original MoS2, which implies that the P-MoS2 sample possesses excellent catalytic performance. To test the stability of the P-MoS2 catalyst in alkaline media, linear sweep voltammetry curves of the P-MoS2 catalyst before and after 2000 cycles were obtained and are shown in Fig. S24 (ESI). The curves almost coincide with the current density, indicating excellent durability during an electrochemical process. The key reaction steps on the as-obtained MoS2 samples in alkaline HERs are researched by DFT calculations. As shown in Fig. 6b, P-MoS2 can provide the active sites for water dissociation to H* intermediates and leads to rapid HER dynamics. These results suggest that the existence of P atoms in the basal plane can effectively stimulate the electrocatalytic properties in alkaline media.
image file: d0cp02131h-f6.tif
Fig. 6 Electrochemical HER measurements of different catalysts in 1 M KOH. (a) LSV polarization curves. The results of the DFT calculations and the corresponding mechanisms of the electrocatalytic HER on the surfaces of different catalysts under alkaline conditions. (b) The corresponding free energy diagram for HERs. The insets are the corresponding atomic configurations.

3 Conclusion

In summary, we demonstrated a strategy to accelerate the hysteresis HER kinetics of MoS2 electrocatalysts by activating the inert basal plane while increasing its intrinsic conductivity by doping non-metallic atoms into MoS2 nanosheets. In acidic media, the As-doped MoS2 catalyst exhibited excellent catalytic activities compared with other as-prepared MoS2 samples. Electrochemical oxidation of the edges leads to a significant inhibition in the catalytic performance of original MoS2, while the catalytic performance of As-doped MoS2 nanosheets is less impacted by partial oxidation, which indicates that the basal plane is catalytic. Furthermore, the non-metal-doped MoS2 catalyst exhibited a rapid electron transfer rate compared to that of the pristine MoS2 samples. Theoretically, the calculated ΔGH* of As is only −0.07 eV, indicating a high catalytic performance. In alkaline media, the P-doped MoS2 catalyst exhibited excellent catalytic performance compared with that other as-prepared MoS2 samples, which was due to the engineering of the water dissociation sites by doping P atoms into MoS2 nanosheets. Our work provides a novel strategy to promote the catalytic activity of MoS2 catalysts via group VA non-metal elemental doping.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grants 21875205, 21671168) and the Natural Science Foundation of Hebei (Grant B2016203498).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d0cp02131h

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