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V-MXene composite for electrocatalytic H2 evolution
Journal of Catalysis 375 (2019) 8–20
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0D/2D NiS2/V-MXene composite for electrocatalytic H2 evolution Panyong Kuang a, Min He a, Bicheng Zhu a, Jiaguo Yu a,c,⇑, Ke Fan a,⇑, Mietek Jaroniec b,⇑ a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China Department of Chemistry and Biochemistry, Kent State University, Kent, OH 44242, USA c Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia b
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 3 March 2019 Revised 26 April 2019 Accepted 17 May 2019
As a member of the family of so-called MXenes, two-dimensional vanadium carbide (V2CTx, denoted as V-MXene) used in electrocatalytic hydrogen evolution reaction (HER) is still in the stage of theoretical study without significant experimental exploration. Here, we present the theoretical and experimental studies on the intrinsic structural and electrochemical properties of V-MXene-based hybrid (NiS2/V-MXene) to verify the possibility of using it as a potential electrocatalyst for HER. For the purpose of comparison, the most popular titanium carbide (Ti3C2Tx, denoted as Ti-MXene) is studied too. As a proof of concept, NiS2/V-MXene and NiS2/Ti-MXene, are designed, prepared and experimentally explored. As expected, NiS2/V-MXene is proved to be a better electrocatalyst for HER than NiS2/Ti-MXene due to its faster electronic transfer and favorable interactions. The tight cover of NiS2 nanoparticles not only prevents V-MXene from restacking but also favors the exposure of extra active sites to accelerate the electrolysis process. Such a unique sandwich-like architecture is highly preferable to provide larger contact interface and more pathways for electron migration and mass diffusion during the electrolysis. Meanwhile, the predicted electron transfer from V-MXene to NiS2 is confirmed experimentally using X-ray photoelectron spectroscopy. This research may provide a guide for the design and development of V-MXene-based electrocatalysts for HER, and expand their potential applications. Ó 2019 Elsevier Inc. All rights reserved.
Keywords: Vanadium carbide NiS2/V-MXene Sandwich-like architecture Electrocatalyst Hydrogen evolution reaction
1. Introduction As a newly emerged class of two-dimensional (2D) materials, early transition-metal-based carbides/nitrides (MXenes) have been identified as excellent candidates for energy storage and conversion [1–4]. MXenes are generally denoted as Mn+1XnTx (n = 1, 2 or 3), where M is an early transition metal (such as V, Ti and Mo), X represents C and/or N, and Tx symbolizes the surface termination group (e.g., AO, AF or AOH) [5]. They can be obtained by removing element A (mostly Al) from a ternary parent MAX phase [6]. The abundant surface termination groups make MXenes highly hydrophilic materials with rich chemical properties comparable to other 2D materials. Besides, the multilayered structure of MXenes is beneficial for ion intercalation and electron transport, making them viable materials for a variety of potential applications [7–9]. As a result of the abundant surface chemistry and twodimensionality, MXenes with various transition metals including Ti3C2Tx, Mo2CTx [10], V4C3Tx [11] and V2CTx [12] have been already discussed in the context of electrocatalytic hydrogen evolution ⇑ Corresponding authors. E-mail addresses: [email protected] [email protected] (M. Jaroniec).
(J.
https://doi.org/10.1016/j.jcat.2019.05.019 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.
Yu),
[email protected]
(K.
Fan),
reaction (HER). In contrast to the prosperity of Ti3C2Tx (denoted as Ti-MXene), one of the most popular MXenes widely used in electro/photocatalytic HER [13,14], the theoretical and experimental studies of V2CTx (denoted as V-MXene) as a HER electrocatalyst are very rare. The development of V-MXene and related hybrids for HER is still in an infant stage as compared to the intensively studied Ti-MXene. The recent first-principle study of V-MXene shows that its hydrogen adsorption free energy is almost ideal (0 eV) after introducing transition metals that significantly attenuates the strong binding between oxygen and hydrogen [12], suggesting the potential of V-MXene as an excellent electrocatalyst for HER. More encouragingly, H2 generation during the etching process of the precursor V2AlC is another piece of evidence for potential HER application, which stimulates further research [15]. In addition, V-MXene with low formula weight holds a great promise for new-generation lithium/sodium-ion batteries and electrochemical water splitting applications [7,16]. Further investigations of V-MXene and related hybrids, both theoretical and experimental, are highly desirable to drive this exciting research area further, targeting the understanding and more efficient utilization of V-MXene for electrocatalytic water splitting. Herein, we present a combined theoretical and experimental study of the intrinsic structural and electrochemical properties of
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V-MXene-based hybrid. The density functional theory (DFT) calculations predict that V-MXene has better HER activity than Ti-MXene, the most commonly studied MXene, due to the lower hydrogen adsorption free energy and higher work function. After loading NiS2 as a model catalyst on the target MXene to form a hybrid (NiS2/MXene) and based on the DFT calculations, NiS2/V-MXene features stronger electronic transfer and interactions in comparison to those in NiS2/Ti-MXene. As a proof of concept, NiS2/V-MXene and NiS2/Ti-MXene hybrids were prepared and evaluated. As expected, NiS2/V-MXene shows higher HER activities than that of NiS2/Ti-MXene counterpart, highlighting the superiority of NiS2/V-MXene as a potential candidate for HER. Especially, the X-ray photoelectron spectroscopy (XPS) results are in excellent agreement with DFT calculations for electron migration in NiS2/V-MXene, where electrons are transferred from V-MXene to NiS2 and undergo a further step for H2 evolution favorably. These findings are meaningful and important because there are rare reports devoted to the study of V-MXene activity toward HER that combines theoretical predictions and experiment. These findings are essential for further design and development of V-MXene-based electrocatalysts for HER, and expand their applications. 2. Experimental 2.1. Materials and chemicals V2AlC and Ti3AlC2 powders were purchased from Nanjing Mission New Materials Co., Ltd. and Forsman Scientific (Beijing) Co., Ltd., respectively. Ni(NO3)26H2O (98%), hexamethylenetetramine (C6H12N4, HMT, 99%), hydrofluoric acid (HF, 40%), tetramethylammonium hydroxide (C4H13NO, TEAOH, 25% aqueous solution), S powders (99.5%) and KOH were provided by Sinopharm Chemical Reagent Co., Ltd. Nafion (5 wt%, ethanol solution) was provided by Yi Er Sheng (Kunshan) International Trade Co. Ltd. Highly oriented pyrolytic graphite (HOPG) was provided by Institute of Metal Research, Chinese Academy of Sciences.
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under high power (Ultrasonic Cell Disruptor KS-600 with output control of 55 and tune for minmum of 50) for 30 min and then transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 150 °C for 6 h. Upon the completion of hydrothermal treatment, V-MXene flakes were covered uniformly by Ni(OH)2 sheets, and the Ni(OH)2/V-MXene product was centrifuged and collected, the product was first freezed in the refrigerator over night and ultimately treated by freeze-drying for 24 h. In-situ chemical vapor deposition (CVD) method was employed to perform the sulfuration of Ni(OH)2/V-MXene. The prepared Ni(OH)2/V-MXene and S powders were put at two independent porcelain-boats with 300 mg S powders and 50 mg Ni(OH)2/V-MXene at the upstream and downstream side of the furnace, respectively; then, the furnace was allowed to anneal at 350 °C for 1 h with a ramp rate of 2 °C min 1 under Ar to obtain the NiS2/V-MXene hybrid composite. As for the synthesis of pure NiS2, similar procedure was performed without the addition of V-MXene. 2.4. Preparation of Ti-MXene flakes Ti-MXene flakes were synthesized and exfoliated through a similar method which is described elsewhere [17]. Typically, 2.0 g of Ti3AlC2 powders were added into 40 mL of HF solution and stirred at 60 °C for 24 h to partially etch the Al atoms. Upon the completion of etching, the mixed solution was centrifuged with high speed rate of 4500 rpm/min and washed with distilled water and ethanol around six times to raise the pH to almost neutral. Afterwards, the collected precipitates were redistributed into distilled water, which was then ultrasonicated under high power for 6 h to yield the isolated, large lateral sized and dispersed Ti-MXene flakes. The above obtained solution was centrifuged at high speed rate of 4500 rpm/min, the resulting precipitate was first freezed in the refrigerator over night and then treated by freezedrying for 24 h ( 40 °C). Finally, about 1.0 g Ti-MXene product was obtained. 2.5. Synthesis of NiS2/Ti-MXene hybrid composite
2.2. Preparation of exfoliated V-MXene flakes V-MXene flakes were synthesized and exfoliated through a similar method which is described elsewhere [7]. Typically, 1.0 g of V2AlC powders were added into 20 mL of HF solution and stirred at 40 °C for 48 h to partially etch the Al atoms. Upon the completion of etching, the mixed solution was centrifuged with high speed rate of 4500 rpm/min (Hettich Zentrifugen, EBA 200) and washed with distilled water and ethanol around six times to raise the pH to almost neutral. Afterwards, the collected precipitates were redistributed into distilled water, which was then ultrasonicated under high power (Ultrasonic Cell Disruptor KS-600 with output control of 55 and tune for minmum of 50) for 6 h to yield the isolated, large lateral sized and dispersed V-MXene flakes. The above obtained solution was centrifuged at high speed rate (4500 rpm/min), the resulting precipitate was first freezed in the refrigerator over night and then treated by freeze-drying for 24 h ( 40 °C). Finally, about 0.6 g V-MXene product was obtained. 2.3. Synthesis of NiS2/V-MXene hybrid composite In a typical procedure, 1.5 mmol Ni(NO3)26H2O and 1.5 mmol HMT were added into 70 mL distilled water and stirred for 10 min, the role of the HMT is to decompose in the hydrothermal process to generate OH ions, which is beneficial to the formation of Ni(OH)2. After that, different amount of V-MXene flakes (2, 10, 20 and 40 mg) was added into the solution and kept stirred vigorously for 20 min. Subsequently, the solution was ultrasonicated
The synthesis routes for Ni(OH)2/Ti-MXene and NiS2/Ti-MXene hybrid composites were similar to that of the Ni(OH)2/V-MXene and NiS2/V-MXene, except that V-MXene flakes were replaced by the same amount of Ti-MXene flakes. More detailed information regarding Computational methodology, Materials characterization, Electrode preparation, Electrochemical measurements and other tests was presented in Supplementary data. 3. Results and discussion 3.1. Hydrogen adsorption free energy and work function We first address the HER catalytic activity of V-MXene and Ti-MXene using the hydrogen adsorption free energy, DGH, as the descriptor. Until now, an ideal MXene with terminating groupfree surface has yet to be synthesized. Mixed termination groups (such as AO, AF or AOH) are usually present and randomly distributed on the surface of MXene as a result of chemical etching process, which has a significant influence on the electronic structure and electrochemical properties that should be assessed by DFT calculations too. Since MXene with a fully terminated surface are thermodynamically more favorable than those partially terminated, and their computer modelling is simpler and less expensive [18–20], the specific terminations with a full coverage of AO/AF groups on the MXene surface are considered in the present study (AOH group is not considered because it is indirectly involved in
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the primary step of the HER mechanism) [21,22]. Three positions (I, II and III; details are provided in Fig. S1) proposed for AO/AF termination groups were initially considered, and the optimal position I was chosen according to the previous research [12,23], where AO/AF are located above the hollow site of M3C3 on both sides of the MXene monolayer to maximize the coordination with M (Fig. 1a and S1). Representative 25% monolayer (ML) H coverage was considered due to the decrease of active sites with increasing the H coverage [24]. Fig. 1b–d shows the relationship between reaction coordinate and DGH. The calculated values of DGH are 0.37 and 0.45 eV for AO terminated V-MXene and Ti-MXene, respectively, suggesting the strong adsorption of H atom (Fig. 1b). In contrast, the AF terminated V-MXene and Ti-MXene show the DGH values of 2.0 and 2.8 eV, respectively (Fig. 1c). The large positive values of AF terminated MXenes provide a clear evidence of weak H adsorption as compared with H adsorption on the O-terminated counterparts. In addition, the positive DGH values of bare V-MXene and Ti-MXene also verify the poor adsorption of H atom without surface termination groups (Fig. 1d). According to the volcano-like trend between hydrogen adsorption energy and
catalytic activity, the low value of |DGH| 0 would compromise the reaction barriers, thereby achieving high HER activity [24]. A comparison of the calculated values of DGH indicates that V-MXene should outperform Ti-MXene in HER due to its lower |DGH| values as compared to those for Ti-MXene with or without surface termination groups. Nevertheless, a comparison to the typical HER catalysts such as Pt (1 1 1) and MoS2 with extremely low DGH of 0.09 and 0.08 eV [25,26], respectively, the |DGH| values of AO/AF terminated V-MXene and Ti-MXene are far greater than zero, which indicates that both V-MXene and Ti-MXene with AO/AF termination groups are not conducive for hydrogen production. These results imply that further modifications of these MXenes are necessary to enhance their intrinsic HER activity. Moreover, work function (WF) is one of the basic physical characteristics of the material that is influenced by the surface properties. As shown in Fig. 1e, the calculated WF of the AO/AF terminated MXenes are higher than that of their corresponding bare structures. For instance, the WF values of the AO terminated V-MXene and Ti-MXene are 6.26 and 5.64 eV, respectively; these values are larger than the corresponding values of 4.49 and
Fig. 1. (a) Side view of V-MXene and Ti-MXene structures. DGH of (b) AO terminated, (c) AF terminated and (d) bare V-MXene and Ti-MXene, respectively. (e) DFT calculated WF of AO, AF terminated and bare V-MXene and Ti-MXene, respectively. (f) Experimentally calculated WF of V-MXene and Ti-MXene.
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Fig. 2. (a, b) Planar-averaged electron density difference Dq(z) and (c, d) differential electron density map of NiS2/V-MXene and NiS2/Ti-MXene, respectively. The iso-surface value is 0.0005 e Å 3.
Fig. 3. (a) XRD patterns and (b) Raman spectra of the bulk V2AlC and V-MXene. (c) TEM, (d, e) HRTEM and (f) AFM images of the exfoliated V-MXene. High-resolution XPS spectra of V-MXene: (g) O 1s and (h) F 1s. (i) Water contact angle of the V-MXene film. Inset of (c): Tyndall effect of V-MXene aqueous solution and SAED pattern of V-MXene.
4.20 eV of the bare counterparts, respectively. The WF values of AF terminated V-MXene and Ti-MXene (5.13 and 4.52 eV, respectively) are between those of the AO terminated and bare MXenes.
Our calculation results are apparently consistent with the common rule that the WF values for AF terminated MXenes lie generally between the corresponding WF values for AO terminated and bare
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MXenes, especially for V-MXene and Ti-MXene [27,28]. The surface dipole effect could be a plausible explanation for the relationship between WF changes and surface termination. After AF adsorption, the out layer M injects electrons to both F atoms and the underneath X atom layer, however, the underneath X atom layer accepts more electrons than F atoms, leading to a negative dipole even though F is negatively charged. Contrarily, after AO adsorption, more electrons would be accepted by O to saturate the two unpaired p orbitals, which is greater than that by X, thus positive dipole is always induced by O [27]. Larger accumulation of electrons on O causes stronger adsorption of H in comparison to F, which is consistent with DGH that negative value represents strong hydrogen adsorption, while positive value indicates weak hydrogen adsorption [29,30]. In addition, we observe that V-MXene exhibits higher WF values as compared with those of Ti-MXene for both AO/AF terminated and bare configurations, which also agrees with previous reports [27]. Since the difficulties associated with experimental control of the MXene surface terminations, it is hard to obtain MXenes with desired surface terminations. Therefore, V-MXene and Ti-MXene with mixed termination groups were prepared and their WF values were measured (Fig. 1f and S2). Sim-
ilarly to the aforementioned specific termination groups, V-MXene with mixed termination groups exhibits higher WF (4.98 eV) than Ti-MXene (4.33 eV). The experimental finding is consistent with the calculation results. The high WF of MXenes indicates higher potential to inject electrons into semiconductors, making these MXenes good candidates for HER application. 3.2. Differential electron density calculations As a classic and widely used electrocatalyst for HER, NiS2 has been received a great attention [31–34]. For better implementation of MXene for HER, especially considering its relatively low intrinsic activity, NiS2 as a model catalyst (Fig. S3) was combined with V-MXene and Ti-MXene to form hybrid catalysts for further DFT calculations. Since HF etching results in larger AO to AF ratio and the further removal of AF groups by hydrothermal treatment (see the following sections) gives the AO surface terminated MXenes that demonstrate promise for versatile applications. The DFT calculations were carried out by using NiS2/V-MXene and NiS2/Ti-MXene with full AO termination to illustrate the chemical interaction and electron transfer ability. Fig. 2a–b presents the
Fig. 4. (a, b) SEM images of Ti3AlC2. (c) SEM and (d) TEM images of Ti-MXene. (e) XRD patterns and (f) Raman spectra of Ti3AlC2 and Ti-MXene.
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planar-averaged charge density difference along Z direction (Dq (z)) for NiS2/V-MXene and NiS2/Ti-MXene. The yellow and cyan regions represent the electron accumulation and depletion, respectively, where stronger peak intensity means more accumulated/ depleted electrons. For NiS2 the peak intensity with positive value indicates the electron accumulation, while V-MXene and Ti-MXene exhibit peak intensity with negative value, indicating the electron depletion. Notably, both peak intensities for NiS2 and V-MXene in NiS2/V-MXene are stronger than those for NiS2 and Ti-MXene in NiS2/Ti-MXene, which clearly reveals better electron transfer in NiS2/V-MXene. The differential charge density maps (Fig. 2c–d) further demonstrate the electron transfer trails, where the electrons are inclined to flow from MXene to NiS2. The calculations for NiS2/V-MXene show that the electron transfer takes place almost throughout the whole interior of V-MXene, and then electrons are injected to the surface or even interior of NiS2. By comparison, weak signals are observed for NiS2/Ti-MXene, indicating that the electron transfer is basically concentrated on the local surface of Ti-MXene and NiS2 with relatively weak transfer ability. We
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would like to point out that the smaller equilibrium distance between V-MXene and NiS2 as compared with Ti-MXene and NiS2 unambiguously proves the strong chemical interaction in NiS2/V-MXene. Furthermore, to quantify the charge transfer, Bader charge analysis was conducted and the results show that 0.63 e is transferred from V-MXene to NiS2, which is almost twice of the value from Ti-MXene to NiS2 (0.32 e ), distinctly suggesting the stronger electronic coupling effect on NiS2/V-MXene as compared with NiS2/Ti-MXene. 3.3. Characterizations of V-MXene Motivated by the foregoing discussion of the DFT calculations, next, as a proof of concept, V-MXene and NiS2/V-MXene hybrid were synthesized and employed as cathode materials for HER evaluation. Typically, V-MXene was prepared by etching the precursor V2AlC (Fig. S4) with HF solution at 40 °C for 48 h to partially remove the Al atom layers. X-ray diffraction (XRD) patterns (Fig. 3a and S5) show that the strong diffraction peaks, (0 0 2)
Fig. 5. (a) Polarization curves and (b) Nyquist plots of V-MXene and Ti-MXene electrodes. Typical CV curves of (c) V-MXene and (d) Ti-MXene electrodes with different scan rates. (e) The differences in current density (DJ = Ja Jc) plots against scan rate of V-MXene and Ti-MXene. (f) N2 adsorption-desorption isotherms of V-MXene and Ti-MXene.
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and (1 0 3), originating from the precursor V2AlC, are significantly weakened after etching treatment, validating the removal of Al layers from V2AlC. Meanwhile, a new broad peak appearing at 12.1° can be assigned to the (0 0 2) peak of V-MXene, which is downshifted from 13.5 to 12.1° as compared to that of V2AlC, indicating the increased c-lattice parameter (c-LP) from 13.2 to 14.6 Å and the enlarged interlayer spacing (d) from 6.6 to 7.3 Å (see the Supple-
mentary data for detailed calculations), which suggests the presence of termination groups and water molecules between V-MXene flakes [35]. Raman spectra in Fig. 3b show that the distinct sharp peaks of V2AlC are weakened and broadened after the etching process, attributing to the exfoliation of V-MXene and subsequently formed surface functional groups that can affect phonon dispersion of V-MXene [3,36]. Interestingly, the scanning
Fig. 6. Schematic illustration of the synthetic process of NiS2/V-MXene. Step 1: HF etching and ultrasonic exfoliation. Step 2: Electrostatic adsorption of Ni2+. Step 3: Hydrothermal reaction. Step 4: In-situ CVD treatment.
Fig. 7. (a) Polarization curves, (b) overpotential at 10 mA cm 2 and (c) Nyquist plots obtained on NiS2 and NiS2/V-MXene electrocatalysts with varied amount of V-MXene. (d) ICP-AES result of the NiS2/V-MXene sample with V-MXene amount of 13 wt%.
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electron microscopy (SEM) image (Fig. S6) shows accordion-like V-MXene with a large lateral size, greatly increasing adsorption of ions and contact area with electrolyte. The transmission electron microscope (TEM) image of V-MXene flakes exfoliated by ultrasonication (Fig. 3c) shows a thin-layer feature with a flat surface. In general, the resulting V-MXene aqueous suspension is stable and does not aggregate owing to the negative zeta potential of V-MXene flakes (Fig. S7), which ultimately displays a clear Tyndall effect (inset of Fig. 3c). Selected area electron diffraction (SAED) pattern of V-MXene (inset of Fig. 3c) suggests the preservation of the hexagonal basal structure derived from its parent phase after HF treatment. High-resolution TEM (HRTEM) image in Fig. 3d clearly reveals the estimated interplanar distance of V-MXene of about 0.73 nm, which is consistent with XRD results. In addition, mild etching unintentionally brings a low concentration of defects with more exposed possible active sites (Fig. 3e), which also improves the conductivity [37]. Fig. 3f presents the atomic force microscopy (AFM) image of a piece of typical exfoliated V-MXene flake, showing a large lateral size with an ultrathin thickness of about 1.2 nm. The inductively coupled plasma atomic emission spectroscopy (ICP-AES) shows 51.4 wt% of Al atoms from the precursor V2AlC persisting in V-MXene after HF etching (Fig. S8), including some Al-containing impurities such as AlF3 and Al2O3; so it can be concluded that more than 48.6% of V2AlC has been converted into V-MXene. The XPS results confirm the presence of O and F elements (ratio of O to F: 83/17), suggesting the AO and AF terminated surface and the formation of V-O and V-F bonds after Al removal (Fig. 3g–h and Fig. S9). The rich amount of termination groups on the V-MXene surface could endow a good hydrophilic behavior with a small water contact angle of 32° (Fig. 3i). As a control, Ti-MXene was successfully prepared as well and the related characterizations are presented in Fig. 4 and S10–S11.
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The SEM image shows accordion-like morphology composed of thin layered Ti-MXene flakes, which is further confirmed by the TEM and AFM images. As for the XRD patterns, the diffraction peak for (1 0 4) plane of Ti3AlC2, which is located at 39°, is absent in the pattern of Ti-MXene, suggesting the successful removal of Al atom layers by etching. In addition, the (0 0 2) peak of Ti-MXene compared to that of Ti3AlC2 has shifted from 9.5 to 6.3°, indicating the increased c-LP from 18.6 to 28.0 Å, and enlarged interlayer spacing from 9.3 to 14.0 Å. On the other hand, the greatly decreased and broadened sharp peaks of Ti3AlC2 in Raman spectra further confirm the chemical etching of Al atom layers. 3.4. Content optimization of V-MXene In agreement with our theoretical predictions, the performed experiments show that V-MXene exhibits higher HER activity with lower charge transfer resistance than that of Ti-MXene (Fig. 5a–b). Likewise, the larger specific surface area and more accessible catalytic sites of V-MXene should also be responsible for its relatively higher HER activity (Fig. 5c–f). However, the activity of both is not satisfying, as expected by the DFT calculations in Fig. 1. Therefore, V-MXene and Ti-MXene were further coupled with the model catalyst NiS2 and employed as HER hybrid electrocatalysts. The related characterizations and synthetic process (take NiS2/V-MXene as an example) are presented in Fig. 6. Prior to the synthesis, V-MXene flakes are fabricated by selectively etching out the Al atom layers in V2AlC phase with HF and subsequently exfoliation in aqueous solution under intense ultrasonic. The presence of negatively charged AO and AF groups on the surface of V-MXene enables the electrostatic adsorption of Ni2+. Afterwards, Ni(OH)2 would formed easily under basic conditions created by the hydrolysis of HMT (C6H12N4 + 10H2O ? 6H2CO + 4NH+4 + 4OH ) [38]. Finally, Ni(OH)2
Fig. 8. (a) SEM image of Ni(OH)2/V-MXene. (b, c) SEM images, (d) XRD pattern, (e) TEM image, (f) HRTEM image and (g-l) elemental mapping images of the NiS2/V-MXene.
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sheets were thoroughly converted to NiS2 NPs through in-situ CVD treatment. The amount of V-MXene in NiS2/V-MXene was optimized to be 13 wt% (Fig. 7). Further characterizations and experiments were carried out for NiS2/V-MXene (13 wt%) catalyst (for convenience, it is still defined as NiS2/V-MXene in the following sections, unless noted otherwise). 3.5. Characterizations of NiS2/V-MXene Fig. 8a shows the SEM image of Ni(OH)2/V-MXene and Fig. 8b–c present the SEM images of NiS2/V-MXene obtained from Ni(OH)2/V-MXene via sufficient sulfuration treatment, which reveals the surface uniformly covered by NiS2 nanoparticles (NPs) and the well-maintained large lateral size of V-MXene flakes. The tight cover of NiS2 NPs not only prevents V-MXene flakes from restacking but also favors the exposure of extra active sites to accelerate the electrolysis process. As can be seen from the XRD pattern of NiS2/V-MXene (Fig. 8d), no other impurity peaks except from the peaks derived from NiS2 and V-MXene can be observed, suggesting the full sulfuration of hydroxide. The disappearance of (0 0 2) peak from V-MXene in XRD pattern of NiS2/V-MXene implies that the restacking of flakes could be efficiently inhibited by NiS2 NPs grafting on their surface. TEM analysis confirms the successful anchoring of NiS2 NPs that are tightly attached to V-MXene surface, forming a sandwich-like structure (Fig. 8e). Such a unique architecture is highly preferable to provide larger contact interface and more pathways for electron migration and mass diffusion during the electrolysis. The HRTEM images (Fig. 8f) demonstrate the distinguished contrast between NiS2 and V-MXene,
where the lattice fringe of 0.284 nm is attributed to the (2 0 0) plane of NiS2. In addition, the SAED pattern of NiS2/V-MXene reveals the polycrystalline feature of NiS2 (Fig. S12a). The elemental mapping images and energy-dispersive X-ray (EDX) spectrum show uniform distribution of Ni, S, V, C and O elements (Fig. 8g–l and S12b). These results verify the successful preparation of NiS2/V-MXene hybrid electrocatalyst. In addition, Fig. 9 shows the SEM, TEM, HRTEM and elemental mapping images of NiS2/TiMXene, suggesting the successful preparation of counterpart as well. 3.6. XPS analysis The elemental composition and chemical environment in NiS2/V-MXene sample is further confirmed by XPS, as shown in Fig. 10. The Ni 2p XPS spectrum (Fig. 10a) shows two peaks at 854.0 and 857.3 eV for Ni 2p3/2 with an intense satellite peak at 860.8 eV, which are characteristic for Ni2+ [39–41]. For the S 2p XPS spectrum (Fig. 10b), two peaks located at 162.7 and 164.1 eV, attributing to S 2p3/2 and S 2p1/2, consistent with the previous reports [42–45]. The V 2p XPS spectrum (Fig. 10c) shows vanadium in V2+ and V4+ oxidation states, attributing to the slight surface oxidation of V-MXene flakes [46–49]. The C 1s XPS spectrum in Fig. 10d can be fitted by four peaks at 283.2, 284.7, 285.8 and 288.7 eV, assigned to the bonds of VAC, CAC, CAO and C@O [50–52], respectively. Moreover, the O 1s XPS spectrum (Fig. 10e) shows three peaks located at 530.1, 532.1 and 533.4 eV, attributed to the bonds of VAO, adsorbed O and H2O, respectively [53–55]. The signal of F 1s is negligible (Fig. S13), suggesting the
Fig. 9. (a, b) SEM, (c) TEM, (d) HRTEM and (e) EDS mapping images of NiS2/Ti-MXene.
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Fig. 10. The XPS spectra of NiS2/V-MXene. (a) Ni 2p, (b) S 2p, (c) V 2p, (d) C 1s and (e) O 1s. The merged XPS spectra of (f) V 2p and (g) Ni 2p for NiS2/V-MXene, NiS2 and VMXene. (h) Proposed electron transfer illustration of NiS2/V-MXene.
replacement of AF groups by oxygen-containing groups under alkaline conditions created by hydrolysis of hexamethylenetetramine during synthesis. Particularly, as displayed in Fig. 10f–g and Table S1, the V 2p XPS spectrum of NiS2/V-MXene exhibits peak shift to higher binding energy as compared with that for bare V-MXene, whereas the opposite shift direction is observed for Ni 2p XPS spectrum of NiS2/V-MXene as compared with the bare NiS2. These results suggest the prominent chemical and electronic interaction in NiS2/V-MXene, where electron transfer takes place from V-MXene to NiS2 (Fig. 10h), which is in very good agreement with the aforementioned DFT calculations. 3.7. HER activity evaluation Next, we evaluated the electrocatalytic HER activity of NiS2/V-MXene on a pre-cleaned glassy carbon electrode (GCE) in 1.0 M KOH electrolyte. NiS2/Ti-MXene, V-MXene, Ti-MXene and bare NiS2 (Fig. S14) were used for comparison. As shown in Fig. 11a, NiS2/V-MXene exhibits much smaller overpotential of 179 mV to achieve 10 mA cm 2 catalytic current density (g10) than NiS2/Ti-MXene, NiS2, V-MXene and Ti-MXene. The pronounced HER activity of NiS2/V-MXene electrocatalyst can be further
verified by its smaller Tafel slope, lower onset overpotential (the overpotential at catalytic current density of 1 mA cm 2) and larger |J300| value (the absolute current density at 300 mV overpotential) as compared with those of other counterparts (Fig. 11b and S15). The small Tafel slope value of 85 mV dec 1 indicates that NiS2/V-MXene electrocatalyst catalyze the HER process through a Volmer-Heyrovsky mechanism. On the other hand, the HER activity of NiS2/V-MXene compares favorably with those of other state-of-the-art electrocatalysts that shows outstanding catalytic performance and reaction kinetics (Fig. S16 and Table S2). Nyquist plots demonstrate greatly enhanced electron transfer between NiS2 and V-MXene owning to better chemical interaction and conductivity between NiS2 NPs and V-MXene (Fig. 11c). Additionally, up to 95% Faradaic efficiency of HER is achieved by NiS2/V-MXene, revealing its high selectivity for H2 evolution in HER process (Fig. 11d). Thanks to the unique and kinetically favorable structure, NiS2/V-MXene shows satisfactory long-term durability when biased galvanostatically at 10 mA cm 2. The polarization curve shows negligible variation as compared with that tested prior to the evaluation of stability (Fig. 11e). Moreover, larger electrochemical active surface area and specific surface area with more accessible active sites (Fig. 11f and
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Fig. 11. (a) Polarization curves, (b) Tafel slopes and (c) Nyquist plots of NiS2/V-MXene, NiS2/Ti-MXene, NiS2, V-MXene and Ti-MXene electrodes. (d) HER Faradaic efficiency of NiS2/V-MXene and NiS2/Ti-MXene. (e) Polarization curves of NiS2/V-MXene before and after long-term HER tests. Inset is the long-term durability test. (f) The differences in current density (DJ = Ja Jc) plots against scan rate of NiS2/V-MXene and NiS2/Ti-MXene.
Fig. 12a–d) of NiS2/V-MXene catalyst also guarantee the higher extrinsic catalytic activity towards HER process. The turnover frequency (TOF) and current density based on the mass activity (Jactive mass) further demonstrate the enhanced HER activity of NiS2/V-MXene as compared to NiS2/Ti-MXene (Fig. 12e–f). With the excellent activity and durability, NiS2/V-MXene holds the promise as a new-type HER catalyst. To sum up, the profound synergistic effects between nanoparticle-MXene, including restrained flakes stacking, strengthened interfacial coupling and promoted electron transfer, are important attributes of the optimized HER activities of NiS2/V-MXene hybrid. 4. Conclusions In summary, we have shown that DFT calculations can be used to predict the HER activity of V-MXene and related hybrid and
further prove the predictions through experimental evaluations. Specifically, both DFT calculations and subsequent experimental results show that both V-MXene and its related hybrid NiS2/V-MXene are better electrocatalysts for HER than the Ti-MXene and NiS2/Ti-MXene counterparts. The experimental results are in excellent agreement with theoretical predictions. Lower |DGH| and higher WF of V-MXene and more advantageous electronic transfer and interaction in NiS2/V-MXene are responsible for higher HER activity. Most importantly, the calculations reported here were performed prior to the experiments. In other words, this study represents an unusual case that theory accurately predicts the electrocatalytic activity of V-MXene and related hybrid. The results presented here can help to enrich our understanding about possibility of using V-MXene-based hybrid as potential electrocatalyst for HER and further expand its applications to the energy conversion/storage and other fields.
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Fig. 12. Typical CV curves of (a) NiS2/V-MXene and (b) NiS2/Ti-MXene with different scan rates. N2 adsorption-desorption isotherms of (c) NiS2/V-MXene and (d) NiS2/Ti-MXene. (e) The TOF and (f) Jactive mass curves for NiS2/V-MXene, NiS2/Ti-MXene and NiS2, respectively.
Declaration of Competing Interest The authors declare no competing financial interest.
References [1] [2] [3] [4]
Acknowledgements [5]
This study was partially supported by National Natural Science Foundation of China (U1705251, 21433007 and 51772234), National Key Research and Development Program of China (2018YFB1502001), Innovative Research Funds of SKLWUT (2017-ZD-4), and National Postdoctoral Program for Innovative Talents (BX20180231).
Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2019.05.019.
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0D/2D NiS2/V-MXene composite for electrocatalytic H2 evolution Panyong Kuang a, Min He a, Bicheng Zhu a, Jiaguo Yu a,c,⇑, Ke Fan a,⇑, Mietek Jaroniec b,⇑ a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China Department of Chemistry and Biochemistry, Kent State University, Kent, OH 44242, USA c Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia b
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 3 March 2019 Revised 26 April 2019 Accepted 17 May 2019
As a member of the family of so-called MXenes, two-dimensional vanadium carbide (V2CTx, denoted as V-MXene) used in electrocatalytic hydrogen evolution reaction (HER) is still in the stage of theoretical study without significant experimental exploration. Here, we present the theoretical and experimental studies on the intrinsic structural and electrochemical properties of V-MXene-based hybrid (NiS2/V-MXene) to verify the possibility of using it as a potential electrocatalyst for HER. For the purpose of comparison, the most popular titanium carbide (Ti3C2Tx, denoted as Ti-MXene) is studied too. As a proof of concept, NiS2/V-MXene and NiS2/Ti-MXene, are designed, prepared and experimentally explored. As expected, NiS2/V-MXene is proved to be a better electrocatalyst for HER than NiS2/Ti-MXene due to its faster electronic transfer and favorable interactions. The tight cover of NiS2 nanoparticles not only prevents V-MXene from restacking but also favors the exposure of extra active sites to accelerate the electrolysis process. Such a unique sandwich-like architecture is highly preferable to provide larger contact interface and more pathways for electron migration and mass diffusion during the electrolysis. Meanwhile, the predicted electron transfer from V-MXene to NiS2 is confirmed experimentally using X-ray photoelectron spectroscopy. This research may provide a guide for the design and development of V-MXene-based electrocatalysts for HER, and expand their potential applications. Ó 2019 Elsevier Inc. All rights reserved.
Keywords: Vanadium carbide NiS2/V-MXene Sandwich-like architecture Electrocatalyst Hydrogen evolution reaction
1. Introduction As a newly emerged class of two-dimensional (2D) materials, early transition-metal-based carbides/nitrides (MXenes) have been identified as excellent candidates for energy storage and conversion [1–4]. MXenes are generally denoted as Mn+1XnTx (n = 1, 2 or 3), where M is an early transition metal (such as V, Ti and Mo), X represents C and/or N, and Tx symbolizes the surface termination group (e.g., AO, AF or AOH) [5]. They can be obtained by removing element A (mostly Al) from a ternary parent MAX phase [6]. The abundant surface termination groups make MXenes highly hydrophilic materials with rich chemical properties comparable to other 2D materials. Besides, the multilayered structure of MXenes is beneficial for ion intercalation and electron transport, making them viable materials for a variety of potential applications [7–9]. As a result of the abundant surface chemistry and twodimensionality, MXenes with various transition metals including Ti3C2Tx, Mo2CTx [10], V4C3Tx [11] and V2CTx [12] have been already discussed in the context of electrocatalytic hydrogen evolution ⇑ Corresponding authors. E-mail addresses: [email protected] [email protected] (M. Jaroniec).
(J.
https://doi.org/10.1016/j.jcat.2019.05.019 0021-9517/Ó 2019 Elsevier Inc. All rights reserved.
Yu),
[email protected]
(K.
Fan),
reaction (HER). In contrast to the prosperity of Ti3C2Tx (denoted as Ti-MXene), one of the most popular MXenes widely used in electro/photocatalytic HER [13,14], the theoretical and experimental studies of V2CTx (denoted as V-MXene) as a HER electrocatalyst are very rare. The development of V-MXene and related hybrids for HER is still in an infant stage as compared to the intensively studied Ti-MXene. The recent first-principle study of V-MXene shows that its hydrogen adsorption free energy is almost ideal (0 eV) after introducing transition metals that significantly attenuates the strong binding between oxygen and hydrogen [12], suggesting the potential of V-MXene as an excellent electrocatalyst for HER. More encouragingly, H2 generation during the etching process of the precursor V2AlC is another piece of evidence for potential HER application, which stimulates further research [15]. In addition, V-MXene with low formula weight holds a great promise for new-generation lithium/sodium-ion batteries and electrochemical water splitting applications [7,16]. Further investigations of V-MXene and related hybrids, both theoretical and experimental, are highly desirable to drive this exciting research area further, targeting the understanding and more efficient utilization of V-MXene for electrocatalytic water splitting. Herein, we present a combined theoretical and experimental study of the intrinsic structural and electrochemical properties of
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V-MXene-based hybrid. The density functional theory (DFT) calculations predict that V-MXene has better HER activity than Ti-MXene, the most commonly studied MXene, due to the lower hydrogen adsorption free energy and higher work function. After loading NiS2 as a model catalyst on the target MXene to form a hybrid (NiS2/MXene) and based on the DFT calculations, NiS2/V-MXene features stronger electronic transfer and interactions in comparison to those in NiS2/Ti-MXene. As a proof of concept, NiS2/V-MXene and NiS2/Ti-MXene hybrids were prepared and evaluated. As expected, NiS2/V-MXene shows higher HER activities than that of NiS2/Ti-MXene counterpart, highlighting the superiority of NiS2/V-MXene as a potential candidate for HER. Especially, the X-ray photoelectron spectroscopy (XPS) results are in excellent agreement with DFT calculations for electron migration in NiS2/V-MXene, where electrons are transferred from V-MXene to NiS2 and undergo a further step for H2 evolution favorably. These findings are meaningful and important because there are rare reports devoted to the study of V-MXene activity toward HER that combines theoretical predictions and experiment. These findings are essential for further design and development of V-MXene-based electrocatalysts for HER, and expand their applications. 2. Experimental 2.1. Materials and chemicals V2AlC and Ti3AlC2 powders were purchased from Nanjing Mission New Materials Co., Ltd. and Forsman Scientific (Beijing) Co., Ltd., respectively. Ni(NO3)26H2O (98%), hexamethylenetetramine (C6H12N4, HMT, 99%), hydrofluoric acid (HF, 40%), tetramethylammonium hydroxide (C4H13NO, TEAOH, 25% aqueous solution), S powders (99.5%) and KOH were provided by Sinopharm Chemical Reagent Co., Ltd. Nafion (5 wt%, ethanol solution) was provided by Yi Er Sheng (Kunshan) International Trade Co. Ltd. Highly oriented pyrolytic graphite (HOPG) was provided by Institute of Metal Research, Chinese Academy of Sciences.
9
under high power (Ultrasonic Cell Disruptor KS-600 with output control of 55 and tune for minmum of 50) for 30 min and then transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 150 °C for 6 h. Upon the completion of hydrothermal treatment, V-MXene flakes were covered uniformly by Ni(OH)2 sheets, and the Ni(OH)2/V-MXene product was centrifuged and collected, the product was first freezed in the refrigerator over night and ultimately treated by freeze-drying for 24 h. In-situ chemical vapor deposition (CVD) method was employed to perform the sulfuration of Ni(OH)2/V-MXene. The prepared Ni(OH)2/V-MXene and S powders were put at two independent porcelain-boats with 300 mg S powders and 50 mg Ni(OH)2/V-MXene at the upstream and downstream side of the furnace, respectively; then, the furnace was allowed to anneal at 350 °C for 1 h with a ramp rate of 2 °C min 1 under Ar to obtain the NiS2/V-MXene hybrid composite. As for the synthesis of pure NiS2, similar procedure was performed without the addition of V-MXene. 2.4. Preparation of Ti-MXene flakes Ti-MXene flakes were synthesized and exfoliated through a similar method which is described elsewhere [17]. Typically, 2.0 g of Ti3AlC2 powders were added into 40 mL of HF solution and stirred at 60 °C for 24 h to partially etch the Al atoms. Upon the completion of etching, the mixed solution was centrifuged with high speed rate of 4500 rpm/min and washed with distilled water and ethanol around six times to raise the pH to almost neutral. Afterwards, the collected precipitates were redistributed into distilled water, which was then ultrasonicated under high power for 6 h to yield the isolated, large lateral sized and dispersed Ti-MXene flakes. The above obtained solution was centrifuged at high speed rate of 4500 rpm/min, the resulting precipitate was first freezed in the refrigerator over night and then treated by freezedrying for 24 h ( 40 °C). Finally, about 1.0 g Ti-MXene product was obtained. 2.5. Synthesis of NiS2/Ti-MXene hybrid composite
2.2. Preparation of exfoliated V-MXene flakes V-MXene flakes were synthesized and exfoliated through a similar method which is described elsewhere [7]. Typically, 1.0 g of V2AlC powders were added into 20 mL of HF solution and stirred at 40 °C for 48 h to partially etch the Al atoms. Upon the completion of etching, the mixed solution was centrifuged with high speed rate of 4500 rpm/min (Hettich Zentrifugen, EBA 200) and washed with distilled water and ethanol around six times to raise the pH to almost neutral. Afterwards, the collected precipitates were redistributed into distilled water, which was then ultrasonicated under high power (Ultrasonic Cell Disruptor KS-600 with output control of 55 and tune for minmum of 50) for 6 h to yield the isolated, large lateral sized and dispersed V-MXene flakes. The above obtained solution was centrifuged at high speed rate (4500 rpm/min), the resulting precipitate was first freezed in the refrigerator over night and then treated by freeze-drying for 24 h ( 40 °C). Finally, about 0.6 g V-MXene product was obtained. 2.3. Synthesis of NiS2/V-MXene hybrid composite In a typical procedure, 1.5 mmol Ni(NO3)26H2O and 1.5 mmol HMT were added into 70 mL distilled water and stirred for 10 min, the role of the HMT is to decompose in the hydrothermal process to generate OH ions, which is beneficial to the formation of Ni(OH)2. After that, different amount of V-MXene flakes (2, 10, 20 and 40 mg) was added into the solution and kept stirred vigorously for 20 min. Subsequently, the solution was ultrasonicated
The synthesis routes for Ni(OH)2/Ti-MXene and NiS2/Ti-MXene hybrid composites were similar to that of the Ni(OH)2/V-MXene and NiS2/V-MXene, except that V-MXene flakes were replaced by the same amount of Ti-MXene flakes. More detailed information regarding Computational methodology, Materials characterization, Electrode preparation, Electrochemical measurements and other tests was presented in Supplementary data. 3. Results and discussion 3.1. Hydrogen adsorption free energy and work function We first address the HER catalytic activity of V-MXene and Ti-MXene using the hydrogen adsorption free energy, DGH, as the descriptor. Until now, an ideal MXene with terminating groupfree surface has yet to be synthesized. Mixed termination groups (such as AO, AF or AOH) are usually present and randomly distributed on the surface of MXene as a result of chemical etching process, which has a significant influence on the electronic structure and electrochemical properties that should be assessed by DFT calculations too. Since MXene with a fully terminated surface are thermodynamically more favorable than those partially terminated, and their computer modelling is simpler and less expensive [18–20], the specific terminations with a full coverage of AO/AF groups on the MXene surface are considered in the present study (AOH group is not considered because it is indirectly involved in
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the primary step of the HER mechanism) [21,22]. Three positions (I, II and III; details are provided in Fig. S1) proposed for AO/AF termination groups were initially considered, and the optimal position I was chosen according to the previous research [12,23], where AO/AF are located above the hollow site of M3C3 on both sides of the MXene monolayer to maximize the coordination with M (Fig. 1a and S1). Representative 25% monolayer (ML) H coverage was considered due to the decrease of active sites with increasing the H coverage [24]. Fig. 1b–d shows the relationship between reaction coordinate and DGH. The calculated values of DGH are 0.37 and 0.45 eV for AO terminated V-MXene and Ti-MXene, respectively, suggesting the strong adsorption of H atom (Fig. 1b). In contrast, the AF terminated V-MXene and Ti-MXene show the DGH values of 2.0 and 2.8 eV, respectively (Fig. 1c). The large positive values of AF terminated MXenes provide a clear evidence of weak H adsorption as compared with H adsorption on the O-terminated counterparts. In addition, the positive DGH values of bare V-MXene and Ti-MXene also verify the poor adsorption of H atom without surface termination groups (Fig. 1d). According to the volcano-like trend between hydrogen adsorption energy and
catalytic activity, the low value of |DGH| 0 would compromise the reaction barriers, thereby achieving high HER activity [24]. A comparison of the calculated values of DGH indicates that V-MXene should outperform Ti-MXene in HER due to its lower |DGH| values as compared to those for Ti-MXene with or without surface termination groups. Nevertheless, a comparison to the typical HER catalysts such as Pt (1 1 1) and MoS2 with extremely low DGH of 0.09 and 0.08 eV [25,26], respectively, the |DGH| values of AO/AF terminated V-MXene and Ti-MXene are far greater than zero, which indicates that both V-MXene and Ti-MXene with AO/AF termination groups are not conducive for hydrogen production. These results imply that further modifications of these MXenes are necessary to enhance their intrinsic HER activity. Moreover, work function (WF) is one of the basic physical characteristics of the material that is influenced by the surface properties. As shown in Fig. 1e, the calculated WF of the AO/AF terminated MXenes are higher than that of their corresponding bare structures. For instance, the WF values of the AO terminated V-MXene and Ti-MXene are 6.26 and 5.64 eV, respectively; these values are larger than the corresponding values of 4.49 and
Fig. 1. (a) Side view of V-MXene and Ti-MXene structures. DGH of (b) AO terminated, (c) AF terminated and (d) bare V-MXene and Ti-MXene, respectively. (e) DFT calculated WF of AO, AF terminated and bare V-MXene and Ti-MXene, respectively. (f) Experimentally calculated WF of V-MXene and Ti-MXene.
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Fig. 2. (a, b) Planar-averaged electron density difference Dq(z) and (c, d) differential electron density map of NiS2/V-MXene and NiS2/Ti-MXene, respectively. The iso-surface value is 0.0005 e Å 3.
Fig. 3. (a) XRD patterns and (b) Raman spectra of the bulk V2AlC and V-MXene. (c) TEM, (d, e) HRTEM and (f) AFM images of the exfoliated V-MXene. High-resolution XPS spectra of V-MXene: (g) O 1s and (h) F 1s. (i) Water contact angle of the V-MXene film. Inset of (c): Tyndall effect of V-MXene aqueous solution and SAED pattern of V-MXene.
4.20 eV of the bare counterparts, respectively. The WF values of AF terminated V-MXene and Ti-MXene (5.13 and 4.52 eV, respectively) are between those of the AO terminated and bare MXenes.
Our calculation results are apparently consistent with the common rule that the WF values for AF terminated MXenes lie generally between the corresponding WF values for AO terminated and bare
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MXenes, especially for V-MXene and Ti-MXene [27,28]. The surface dipole effect could be a plausible explanation for the relationship between WF changes and surface termination. After AF adsorption, the out layer M injects electrons to both F atoms and the underneath X atom layer, however, the underneath X atom layer accepts more electrons than F atoms, leading to a negative dipole even though F is negatively charged. Contrarily, after AO adsorption, more electrons would be accepted by O to saturate the two unpaired p orbitals, which is greater than that by X, thus positive dipole is always induced by O [27]. Larger accumulation of electrons on O causes stronger adsorption of H in comparison to F, which is consistent with DGH that negative value represents strong hydrogen adsorption, while positive value indicates weak hydrogen adsorption [29,30]. In addition, we observe that V-MXene exhibits higher WF values as compared with those of Ti-MXene for both AO/AF terminated and bare configurations, which also agrees with previous reports [27]. Since the difficulties associated with experimental control of the MXene surface terminations, it is hard to obtain MXenes with desired surface terminations. Therefore, V-MXene and Ti-MXene with mixed termination groups were prepared and their WF values were measured (Fig. 1f and S2). Sim-
ilarly to the aforementioned specific termination groups, V-MXene with mixed termination groups exhibits higher WF (4.98 eV) than Ti-MXene (4.33 eV). The experimental finding is consistent with the calculation results. The high WF of MXenes indicates higher potential to inject electrons into semiconductors, making these MXenes good candidates for HER application. 3.2. Differential electron density calculations As a classic and widely used electrocatalyst for HER, NiS2 has been received a great attention [31–34]. For better implementation of MXene for HER, especially considering its relatively low intrinsic activity, NiS2 as a model catalyst (Fig. S3) was combined with V-MXene and Ti-MXene to form hybrid catalysts for further DFT calculations. Since HF etching results in larger AO to AF ratio and the further removal of AF groups by hydrothermal treatment (see the following sections) gives the AO surface terminated MXenes that demonstrate promise for versatile applications. The DFT calculations were carried out by using NiS2/V-MXene and NiS2/Ti-MXene with full AO termination to illustrate the chemical interaction and electron transfer ability. Fig. 2a–b presents the
Fig. 4. (a, b) SEM images of Ti3AlC2. (c) SEM and (d) TEM images of Ti-MXene. (e) XRD patterns and (f) Raman spectra of Ti3AlC2 and Ti-MXene.
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planar-averaged charge density difference along Z direction (Dq (z)) for NiS2/V-MXene and NiS2/Ti-MXene. The yellow and cyan regions represent the electron accumulation and depletion, respectively, where stronger peak intensity means more accumulated/ depleted electrons. For NiS2 the peak intensity with positive value indicates the electron accumulation, while V-MXene and Ti-MXene exhibit peak intensity with negative value, indicating the electron depletion. Notably, both peak intensities for NiS2 and V-MXene in NiS2/V-MXene are stronger than those for NiS2 and Ti-MXene in NiS2/Ti-MXene, which clearly reveals better electron transfer in NiS2/V-MXene. The differential charge density maps (Fig. 2c–d) further demonstrate the electron transfer trails, where the electrons are inclined to flow from MXene to NiS2. The calculations for NiS2/V-MXene show that the electron transfer takes place almost throughout the whole interior of V-MXene, and then electrons are injected to the surface or even interior of NiS2. By comparison, weak signals are observed for NiS2/Ti-MXene, indicating that the electron transfer is basically concentrated on the local surface of Ti-MXene and NiS2 with relatively weak transfer ability. We
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would like to point out that the smaller equilibrium distance between V-MXene and NiS2 as compared with Ti-MXene and NiS2 unambiguously proves the strong chemical interaction in NiS2/V-MXene. Furthermore, to quantify the charge transfer, Bader charge analysis was conducted and the results show that 0.63 e is transferred from V-MXene to NiS2, which is almost twice of the value from Ti-MXene to NiS2 (0.32 e ), distinctly suggesting the stronger electronic coupling effect on NiS2/V-MXene as compared with NiS2/Ti-MXene. 3.3. Characterizations of V-MXene Motivated by the foregoing discussion of the DFT calculations, next, as a proof of concept, V-MXene and NiS2/V-MXene hybrid were synthesized and employed as cathode materials for HER evaluation. Typically, V-MXene was prepared by etching the precursor V2AlC (Fig. S4) with HF solution at 40 °C for 48 h to partially remove the Al atom layers. X-ray diffraction (XRD) patterns (Fig. 3a and S5) show that the strong diffraction peaks, (0 0 2)
Fig. 5. (a) Polarization curves and (b) Nyquist plots of V-MXene and Ti-MXene electrodes. Typical CV curves of (c) V-MXene and (d) Ti-MXene electrodes with different scan rates. (e) The differences in current density (DJ = Ja Jc) plots against scan rate of V-MXene and Ti-MXene. (f) N2 adsorption-desorption isotherms of V-MXene and Ti-MXene.
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and (1 0 3), originating from the precursor V2AlC, are significantly weakened after etching treatment, validating the removal of Al layers from V2AlC. Meanwhile, a new broad peak appearing at 12.1° can be assigned to the (0 0 2) peak of V-MXene, which is downshifted from 13.5 to 12.1° as compared to that of V2AlC, indicating the increased c-lattice parameter (c-LP) from 13.2 to 14.6 Å and the enlarged interlayer spacing (d) from 6.6 to 7.3 Å (see the Supple-
mentary data for detailed calculations), which suggests the presence of termination groups and water molecules between V-MXene flakes [35]. Raman spectra in Fig. 3b show that the distinct sharp peaks of V2AlC are weakened and broadened after the etching process, attributing to the exfoliation of V-MXene and subsequently formed surface functional groups that can affect phonon dispersion of V-MXene [3,36]. Interestingly, the scanning
Fig. 6. Schematic illustration of the synthetic process of NiS2/V-MXene. Step 1: HF etching and ultrasonic exfoliation. Step 2: Electrostatic adsorption of Ni2+. Step 3: Hydrothermal reaction. Step 4: In-situ CVD treatment.
Fig. 7. (a) Polarization curves, (b) overpotential at 10 mA cm 2 and (c) Nyquist plots obtained on NiS2 and NiS2/V-MXene electrocatalysts with varied amount of V-MXene. (d) ICP-AES result of the NiS2/V-MXene sample with V-MXene amount of 13 wt%.
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electron microscopy (SEM) image (Fig. S6) shows accordion-like V-MXene with a large lateral size, greatly increasing adsorption of ions and contact area with electrolyte. The transmission electron microscope (TEM) image of V-MXene flakes exfoliated by ultrasonication (Fig. 3c) shows a thin-layer feature with a flat surface. In general, the resulting V-MXene aqueous suspension is stable and does not aggregate owing to the negative zeta potential of V-MXene flakes (Fig. S7), which ultimately displays a clear Tyndall effect (inset of Fig. 3c). Selected area electron diffraction (SAED) pattern of V-MXene (inset of Fig. 3c) suggests the preservation of the hexagonal basal structure derived from its parent phase after HF treatment. High-resolution TEM (HRTEM) image in Fig. 3d clearly reveals the estimated interplanar distance of V-MXene of about 0.73 nm, which is consistent with XRD results. In addition, mild etching unintentionally brings a low concentration of defects with more exposed possible active sites (Fig. 3e), which also improves the conductivity [37]. Fig. 3f presents the atomic force microscopy (AFM) image of a piece of typical exfoliated V-MXene flake, showing a large lateral size with an ultrathin thickness of about 1.2 nm. The inductively coupled plasma atomic emission spectroscopy (ICP-AES) shows 51.4 wt% of Al atoms from the precursor V2AlC persisting in V-MXene after HF etching (Fig. S8), including some Al-containing impurities such as AlF3 and Al2O3; so it can be concluded that more than 48.6% of V2AlC has been converted into V-MXene. The XPS results confirm the presence of O and F elements (ratio of O to F: 83/17), suggesting the AO and AF terminated surface and the formation of V-O and V-F bonds after Al removal (Fig. 3g–h and Fig. S9). The rich amount of termination groups on the V-MXene surface could endow a good hydrophilic behavior with a small water contact angle of 32° (Fig. 3i). As a control, Ti-MXene was successfully prepared as well and the related characterizations are presented in Fig. 4 and S10–S11.
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The SEM image shows accordion-like morphology composed of thin layered Ti-MXene flakes, which is further confirmed by the TEM and AFM images. As for the XRD patterns, the diffraction peak for (1 0 4) plane of Ti3AlC2, which is located at 39°, is absent in the pattern of Ti-MXene, suggesting the successful removal of Al atom layers by etching. In addition, the (0 0 2) peak of Ti-MXene compared to that of Ti3AlC2 has shifted from 9.5 to 6.3°, indicating the increased c-LP from 18.6 to 28.0 Å, and enlarged interlayer spacing from 9.3 to 14.0 Å. On the other hand, the greatly decreased and broadened sharp peaks of Ti3AlC2 in Raman spectra further confirm the chemical etching of Al atom layers. 3.4. Content optimization of V-MXene In agreement with our theoretical predictions, the performed experiments show that V-MXene exhibits higher HER activity with lower charge transfer resistance than that of Ti-MXene (Fig. 5a–b). Likewise, the larger specific surface area and more accessible catalytic sites of V-MXene should also be responsible for its relatively higher HER activity (Fig. 5c–f). However, the activity of both is not satisfying, as expected by the DFT calculations in Fig. 1. Therefore, V-MXene and Ti-MXene were further coupled with the model catalyst NiS2 and employed as HER hybrid electrocatalysts. The related characterizations and synthetic process (take NiS2/V-MXene as an example) are presented in Fig. 6. Prior to the synthesis, V-MXene flakes are fabricated by selectively etching out the Al atom layers in V2AlC phase with HF and subsequently exfoliation in aqueous solution under intense ultrasonic. The presence of negatively charged AO and AF groups on the surface of V-MXene enables the electrostatic adsorption of Ni2+. Afterwards, Ni(OH)2 would formed easily under basic conditions created by the hydrolysis of HMT (C6H12N4 + 10H2O ? 6H2CO + 4NH+4 + 4OH ) [38]. Finally, Ni(OH)2
Fig. 8. (a) SEM image of Ni(OH)2/V-MXene. (b, c) SEM images, (d) XRD pattern, (e) TEM image, (f) HRTEM image and (g-l) elemental mapping images of the NiS2/V-MXene.
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sheets were thoroughly converted to NiS2 NPs through in-situ CVD treatment. The amount of V-MXene in NiS2/V-MXene was optimized to be 13 wt% (Fig. 7). Further characterizations and experiments were carried out for NiS2/V-MXene (13 wt%) catalyst (for convenience, it is still defined as NiS2/V-MXene in the following sections, unless noted otherwise). 3.5. Characterizations of NiS2/V-MXene Fig. 8a shows the SEM image of Ni(OH)2/V-MXene and Fig. 8b–c present the SEM images of NiS2/V-MXene obtained from Ni(OH)2/V-MXene via sufficient sulfuration treatment, which reveals the surface uniformly covered by NiS2 nanoparticles (NPs) and the well-maintained large lateral size of V-MXene flakes. The tight cover of NiS2 NPs not only prevents V-MXene flakes from restacking but also favors the exposure of extra active sites to accelerate the electrolysis process. As can be seen from the XRD pattern of NiS2/V-MXene (Fig. 8d), no other impurity peaks except from the peaks derived from NiS2 and V-MXene can be observed, suggesting the full sulfuration of hydroxide. The disappearance of (0 0 2) peak from V-MXene in XRD pattern of NiS2/V-MXene implies that the restacking of flakes could be efficiently inhibited by NiS2 NPs grafting on their surface. TEM analysis confirms the successful anchoring of NiS2 NPs that are tightly attached to V-MXene surface, forming a sandwich-like structure (Fig. 8e). Such a unique architecture is highly preferable to provide larger contact interface and more pathways for electron migration and mass diffusion during the electrolysis. The HRTEM images (Fig. 8f) demonstrate the distinguished contrast between NiS2 and V-MXene,
where the lattice fringe of 0.284 nm is attributed to the (2 0 0) plane of NiS2. In addition, the SAED pattern of NiS2/V-MXene reveals the polycrystalline feature of NiS2 (Fig. S12a). The elemental mapping images and energy-dispersive X-ray (EDX) spectrum show uniform distribution of Ni, S, V, C and O elements (Fig. 8g–l and S12b). These results verify the successful preparation of NiS2/V-MXene hybrid electrocatalyst. In addition, Fig. 9 shows the SEM, TEM, HRTEM and elemental mapping images of NiS2/TiMXene, suggesting the successful preparation of counterpart as well. 3.6. XPS analysis The elemental composition and chemical environment in NiS2/V-MXene sample is further confirmed by XPS, as shown in Fig. 10. The Ni 2p XPS spectrum (Fig. 10a) shows two peaks at 854.0 and 857.3 eV for Ni 2p3/2 with an intense satellite peak at 860.8 eV, which are characteristic for Ni2+ [39–41]. For the S 2p XPS spectrum (Fig. 10b), two peaks located at 162.7 and 164.1 eV, attributing to S 2p3/2 and S 2p1/2, consistent with the previous reports [42–45]. The V 2p XPS spectrum (Fig. 10c) shows vanadium in V2+ and V4+ oxidation states, attributing to the slight surface oxidation of V-MXene flakes [46–49]. The C 1s XPS spectrum in Fig. 10d can be fitted by four peaks at 283.2, 284.7, 285.8 and 288.7 eV, assigned to the bonds of VAC, CAC, CAO and C@O [50–52], respectively. Moreover, the O 1s XPS spectrum (Fig. 10e) shows three peaks located at 530.1, 532.1 and 533.4 eV, attributed to the bonds of VAO, adsorbed O and H2O, respectively [53–55]. The signal of F 1s is negligible (Fig. S13), suggesting the
Fig. 9. (a, b) SEM, (c) TEM, (d) HRTEM and (e) EDS mapping images of NiS2/Ti-MXene.
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Fig. 10. The XPS spectra of NiS2/V-MXene. (a) Ni 2p, (b) S 2p, (c) V 2p, (d) C 1s and (e) O 1s. The merged XPS spectra of (f) V 2p and (g) Ni 2p for NiS2/V-MXene, NiS2 and VMXene. (h) Proposed electron transfer illustration of NiS2/V-MXene.
replacement of AF groups by oxygen-containing groups under alkaline conditions created by hydrolysis of hexamethylenetetramine during synthesis. Particularly, as displayed in Fig. 10f–g and Table S1, the V 2p XPS spectrum of NiS2/V-MXene exhibits peak shift to higher binding energy as compared with that for bare V-MXene, whereas the opposite shift direction is observed for Ni 2p XPS spectrum of NiS2/V-MXene as compared with the bare NiS2. These results suggest the prominent chemical and electronic interaction in NiS2/V-MXene, where electron transfer takes place from V-MXene to NiS2 (Fig. 10h), which is in very good agreement with the aforementioned DFT calculations. 3.7. HER activity evaluation Next, we evaluated the electrocatalytic HER activity of NiS2/V-MXene on a pre-cleaned glassy carbon electrode (GCE) in 1.0 M KOH electrolyte. NiS2/Ti-MXene, V-MXene, Ti-MXene and bare NiS2 (Fig. S14) were used for comparison. As shown in Fig. 11a, NiS2/V-MXene exhibits much smaller overpotential of 179 mV to achieve 10 mA cm 2 catalytic current density (g10) than NiS2/Ti-MXene, NiS2, V-MXene and Ti-MXene. The pronounced HER activity of NiS2/V-MXene electrocatalyst can be further
verified by its smaller Tafel slope, lower onset overpotential (the overpotential at catalytic current density of 1 mA cm 2) and larger |J300| value (the absolute current density at 300 mV overpotential) as compared with those of other counterparts (Fig. 11b and S15). The small Tafel slope value of 85 mV dec 1 indicates that NiS2/V-MXene electrocatalyst catalyze the HER process through a Volmer-Heyrovsky mechanism. On the other hand, the HER activity of NiS2/V-MXene compares favorably with those of other state-of-the-art electrocatalysts that shows outstanding catalytic performance and reaction kinetics (Fig. S16 and Table S2). Nyquist plots demonstrate greatly enhanced electron transfer between NiS2 and V-MXene owning to better chemical interaction and conductivity between NiS2 NPs and V-MXene (Fig. 11c). Additionally, up to 95% Faradaic efficiency of HER is achieved by NiS2/V-MXene, revealing its high selectivity for H2 evolution in HER process (Fig. 11d). Thanks to the unique and kinetically favorable structure, NiS2/V-MXene shows satisfactory long-term durability when biased galvanostatically at 10 mA cm 2. The polarization curve shows negligible variation as compared with that tested prior to the evaluation of stability (Fig. 11e). Moreover, larger electrochemical active surface area and specific surface area with more accessible active sites (Fig. 11f and
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Fig. 11. (a) Polarization curves, (b) Tafel slopes and (c) Nyquist plots of NiS2/V-MXene, NiS2/Ti-MXene, NiS2, V-MXene and Ti-MXene electrodes. (d) HER Faradaic efficiency of NiS2/V-MXene and NiS2/Ti-MXene. (e) Polarization curves of NiS2/V-MXene before and after long-term HER tests. Inset is the long-term durability test. (f) The differences in current density (DJ = Ja Jc) plots against scan rate of NiS2/V-MXene and NiS2/Ti-MXene.
Fig. 12a–d) of NiS2/V-MXene catalyst also guarantee the higher extrinsic catalytic activity towards HER process. The turnover frequency (TOF) and current density based on the mass activity (Jactive mass) further demonstrate the enhanced HER activity of NiS2/V-MXene as compared to NiS2/Ti-MXene (Fig. 12e–f). With the excellent activity and durability, NiS2/V-MXene holds the promise as a new-type HER catalyst. To sum up, the profound synergistic effects between nanoparticle-MXene, including restrained flakes stacking, strengthened interfacial coupling and promoted electron transfer, are important attributes of the optimized HER activities of NiS2/V-MXene hybrid. 4. Conclusions In summary, we have shown that DFT calculations can be used to predict the HER activity of V-MXene and related hybrid and
further prove the predictions through experimental evaluations. Specifically, both DFT calculations and subsequent experimental results show that both V-MXene and its related hybrid NiS2/V-MXene are better electrocatalysts for HER than the Ti-MXene and NiS2/Ti-MXene counterparts. The experimental results are in excellent agreement with theoretical predictions. Lower |DGH| and higher WF of V-MXene and more advantageous electronic transfer and interaction in NiS2/V-MXene are responsible for higher HER activity. Most importantly, the calculations reported here were performed prior to the experiments. In other words, this study represents an unusual case that theory accurately predicts the electrocatalytic activity of V-MXene and related hybrid. The results presented here can help to enrich our understanding about possibility of using V-MXene-based hybrid as potential electrocatalyst for HER and further expand its applications to the energy conversion/storage and other fields.
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Fig. 12. Typical CV curves of (a) NiS2/V-MXene and (b) NiS2/Ti-MXene with different scan rates. N2 adsorption-desorption isotherms of (c) NiS2/V-MXene and (d) NiS2/Ti-MXene. (e) The TOF and (f) Jactive mass curves for NiS2/V-MXene, NiS2/Ti-MXene and NiS2, respectively.
Declaration of Competing Interest The authors declare no competing financial interest.
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Acknowledgements [5]
This study was partially supported by National Natural Science Foundation of China (U1705251, 21433007 and 51772234), National Key Research and Development Program of China (2018YFB1502001), Innovative Research Funds of SKLWUT (2017-ZD-4), and National Postdoctoral Program for Innovative Talents (BX20180231).
Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2019.05.019.
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