Platinum Nanocrystals Decorated on Defect-Rich MoS2 Nanosheets

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Platinum Nanocrystals Decorated on Defect-Rich MoS2 Nanosheets for pH-Universal Hydrogen Evolution Reaction Junfeng Xie, Li Gao, Hailong Jiang, Xiaodong Zhang, Fengcai Lei, Pin Hao, Bo Tang, and Yi Xie Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01594 • Publication Date (Web): 14 Dec 2018 Downloaded from http://pubs.acs.org on December 17, 2018

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Crystal Growth & Design

Platinum Nanocrystals Decorated on Defect-Rich MoS2 Nanosheets for pH-Universal Hydrogen Evolution Reaction Junfeng Xie,*,†,#,§ Li Gao,†,§ Hailong Jiang,‡ Xiaodong Zhang,# Fengcai Lei,†,# Pin Hao,† Bo Tang,† Yi Xie# † College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes (Ministry of Education), Institute of Molecular and Nano Science, Shandong Normal University, Jinan 250014, P. R. China. ‡ Key Laboratory for Applied Technology of Sophisticated Analytical Instrument of Shandong Province, Analysis and Test Center, Qilu University of Technology (Shandong Academy of Science), Jinan 250014, P. R. China. # Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China. Abstract Hydrogen evolution reaction (HER) has drawn substantial attention owing to its crucial role in clean energy production via water electrolysis. Developing highly efficient and electrochemically stable HER catalysts that can work under a wide range of pH values is of great importance. Herein, uniformly distributed platinum nanocrystals were decorated on the defect-rich MoS2 nanosheets (DR-MoS2) via an in-situ electrochemical dissolution-redeposition approach. With the merits of the defect-rich feature combined with the large surface area of the DR-MoS2, abundant reactive sites can be provided for adsorbing and anchoring the Pt atoms, leading to the formation of the DR-MoS2 catalyst with homogeneous decorated Pt nanocrystals. Benefitted from the synergistic effect between the DR-MoS2 and the Pt nanocrystals, the DR-MoS2-Pt composite catalyst shows remarkable HER performance in acidic and alkaline conditions, even taking precedence over the benchmarking Pt/C catalyst.

The urgent demand for renewable and sustainable fuels has prompted the exploration of advanced technologies to realize highly efficient energy conversion and clean energy production.1-2 Among various pathways for energy conversion, hydrogen evolution reaction (HER) from water electrolysis is considered to be a promising route to generate highly energetic hydrogen to solve the issues of energy crisis and environmental pollution during the past decades.3-10 In order to boost the rate of water splitting, appropriate catalysts are required for the HER process to lower down the overpotential, and among which platinum is considered to be the most active but expensive material that can catalyze HER in a wide pH range.11 Therefore, replacing Pt with highly efficient earth-abundant catalysts or lowering the usage of Pt are of high importance in achieving cost-economic hydrogen production.

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As a kind of typical two-dimensional (2D) materials, molybdenum disulfide (MoS2) based materials are regarded as effective catalysts for HER in strongly acidic electrolyte, which are deemed to be potential alternatives to Pt due to their cheapness, high earth abundance and well-defined catalytic mechanism.12-22 However, for the thermodynamically stable 2H-MoS2 phase, only the edges are active for catalyzing HER, thereby significantly restricting the active site density.14-18 Not only that, MoS2 can only exhibit HER activity in strongly acidic electrolyte, which further limits its applications in overall water splitting. Considering the high surface area and good electrochemical stability, MoS2 nanosheets may serve as effective supports for the decoration of highly catalytically active platinum species to achieve highly efficient pH-universal HER catalysis.23 Recently, the effect of platinum counter electrode in HER catalysis, especially long-term HER operation, has drawn substantial attention, which involves the electrochemical dissolution of platinum on the Pt counter electrode and subsequent re-deposition of Pt nanocrystals on the working electrode.24-29 The redox Pt dissolution-redeposition can bring in significantly enhanced HER activity of the cathode material, and further, pH-universal catalytic activity could be realized benefitted from the as-deposited Pt species, i.e., atoms, clusters or nanocrystals. Recently, Yan et al. demonstrated that Pt could be preferentially bonded on the 2H-WS2 edges via the aforementioned process owing to the lower Pt binding energy of the terminated S2− or S22− ligands with than the apical S2− sites on the thermodynamically stable basal plane.29 And further, enhanced HER activity was achieved in both acidic and alkaline electrolytes. However, the thermodynamically unfavorable edges are difficult to be preferentially exposed, therefore the low active site density significantly restricts the decoration of Pt towards high loading weight, and limiting the HER activity. Previously, our group demonstrated that engineering defect structures in the basal planes of MoS2 nanosheets can introduce additional active sites for HER, which may also provide more reactive sites for Pt decoration.14 As indicated in the HRTEM and SEM images in Figure 1A and Figure S2A, the defect-rich MoS2 nanosheets (DR-MoS2) are ~100 nm in lateral size and 2 nm in thickness. Of note, the HRTEM image clearly demonstrates the defect-rich feature of the basal surface of the as-obtained MoS2 nanosheets, where the defect sites are labelled by dashed circles. Corresponding FFT pattern of the HRTEM image reveals the quasi-hexagonal pattern with six independent arcs, which further proves the presence of rich defects on the (002) plane.14 The abundant defects may bring in enriched active sites for HER and provide more reactive sites for Pt binding. By using a three-electrode setup with a platinum gauze as the counter electrode, cyclic voltammetry (CV) was conducted, and the corresponding polarization curves at specific cycling numbers were shown in Figure 1B. As can be seen, as the cycling proceeds, the HER activity exhibits dramatic enhancement, with original onset overpotential of ~0.12 V for the pristine DR-MoS2 and almost zero overpotential for the catalyst after 500 CV cycles, demonstrating the remarkable enhancement of HER activity. Apart from the significantly reduced onset overpotential, the cathodic current at η = 0.2 V after 500 CV cycles reaches over 200 mA cm-2 for the DR-MoS2, which is approximately 12.5 times higher than the pristine catalyst, suggesting the successful decoration of platinum species on MoS2 that significantly improves the HER activity. SEM, TEM and HRTEM were performed on the DR-MoS2 catalyst after 500 CV cycles to further study the morphology and structure of the as-obtained catalyst. As shown in Figure S2B and Figure 1C, the ultrathin nanosheet morphology is retained after CV cycling, while abundant and


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homogeneously distributed nanocrystals with size of ~5 nm can be observed on the whole MoS2 nanosheets. In sharp contrast, for the defect-free MoS2 nanosheets (DF-MoS2), the Pt nanocrystals are mainly located at the edge area of the nanosheets (Figure S2D), therefore confirming the significant role of defects in realizing homogeneous Pt decoration with high loading weight. HRTEM image in Figure 1D further confirms the existence of Pt nanocrystals on the basal surface of DR-MoS2, where typical cubic structured Pt nanocrystals with d(200) of 1.96 Å can be identified.30-31 The homogeneously distributed Pt nanocrystals on the MoS2 nanosheets indicates that the defects on the basal surface are active for adsorbing and binding Pt atoms, which are similar to the edge sites.

Figure 1. (A) HRTEM image of DR-MoS2. Insets are TEM image and the FFT pattern of the HRTEM image. (B) LSV curves of the DR-MoS2 nanosheets reveal the obviously improved HER activity along with CV cycling (Pt counter electrode, H2-purged 0.5 M H2SO4). (C-D) TEM and HRTEM pictures of the DR-MoS2-Pt nanosheets.



Figure 2. (A-D) HAADF STEM image and corresponding elemental distribution of Mo, S and Pt. (E) The content of platinum in DR-MoS2 and DF-MoS2 nanosheets after various CV cycling numbers. (F) The trend of variable concentration of Pt species in the electrolyte after specific CV measurements. (G-I) XPS spectra of Mo, S and Pt. The HAADF STEM and the corresponding elemental distribution were performed to the DR-MoS2 catalyst after Pt decoration (DR-MoS2-Pt) to further investigate the distribution of Pt nanocrystals. As depicted in Figure 2A-D, molybdenum, sulfur and platinum are evenly distributed on the whole nanosheets, which stems from the presence of abundant defects that can provide binding sites for Pt decoration. Besides, as the CV cycling treatment proceeds, the content of decorated Pt species gradually increased from 0.2% for 100 CV cycles to 9.8% for 500 cycles as identified by the inductively coupled plasma (ICP) emission spectra (Figure 2E), which is consistent with the trend of HER activity as indicated in Figure 1B. Interestingly, the DF-MoS2 nanosheets exhibit limited performance enhancement during the CV treatment at the same conditions (Figure S3), and the content of decorated Pt is determined to be only 0.3% after 500 CV cycles, therefore confirming the crucial role of defects in in-situ electrochemical Pt decoration (Figure 2E). As demonstrated in previous literature, the as-decorated Pt nanocrystals are originated from the electrochemical redox Pt dissolution and redeposition process, we further carried out ICP measurements for the electrolyte with various cycling numbers (Figure 2F). As can be revealed, the concentration of Pt species in the electrolyte increases from 0.001 mg L-1 for 100 CV cycles to 0.025 mg L-1 for 500 cycles, which offers substantial Pt species for the gradually enriched Pt content. The XPS technique was conducted to understand the composition and valence state of the as-generated DR-MoS2-Pt catalyst. As shown in Figure 2G-H, both Mo and S spectra exhibit negligible difference compared with previously reported defect-rich MoS2 nanosheets, suggesting the high chemical stability of the DR-MoS2 during the CV treatment.12, 14 Besides, the Pt 4f signal in Figure 2I clearly confirms the presence of Pt0 in the catalyst, with binding energy of


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71.6 eV and 75.0 eV, respectively.29, decorated Pt nanocrystals are confirmed.

32-33

Therefore, the DR-MoS2 nanosheets with in-situ

Figure 3. (A-D) LSV curves and Tafel plots of the DR-MoS2-Pt in acidic and alkaline electrolytes. (E) Stability tests for the DR-MoS2-Pt conducted under a fixed overpotential of 0.2 V in acidic and alkaline solutions, respectively. Benefitted from the uniformly distributed Pt nanocrystals on the defect-rich MoS2 nanosheets, the as-obtained catalyst shows superior HER performance in acidic and alkaline environments. By replacing the platinum gauze with a graphite rod as the counter electrode and changing the Pt-containing electrolyte with fresh H2-purged electrolytes, LSV curves and Tafel plots were obtained (Figure 3A-D). As shown in Figure 3A, by 500 cycles pretreatment, the DR-MoS2-Pt exhibits remarkable activity for HER. The required overpotential for triggering HER is as low as 0 V, which is comparable to the commercial Pt/C catalyst with similar Pt loading. Surprisingly, the



cathodic current density of DR-MoS2-Pt is measured to be as high as 188.8 mA cm-2 at η = 0.2 V, c.a. 1.34 and 14.20 times larger than the benchmarking Pt/C and pristine DR-MoS2 catalyst, respectively. The remarkable enhancement in HER activity may arise from the synergy between platinum nanocrystals and the DR-MoS2, which can effectively enrich the catalytically active sites and endow it with large surface area and facile electron transport benefitted from the highly conductive Pt nanocrystals on the 2D DR-MoS2. Furthermore, a small Tafel slope of 40.8 mV per decade can be resulted, indicating that the DR-MoS2-Pt catalyst possesses similar reaction kinetics with Pt/C (Figure 3B). Beyond HER catalysis in acidic electrolyte, HER in alkaline condition is considered to be more attractive, especially in commercial viewpoint, since most earth-abundant catalysts for oxygen evolution can only work in strongly alkaline solution.34-48 Accordingly, we estimated the HER behavior at pH = 14 (Figure 3C). Similar to the HER behavior in acidic electrolyte, the DR-MoS2-Pt catalyst exhibits near zero HER onset overpotential in basic environment, while for the pristine DR-MoS2 nanosheets, negligible HER activity can be observed. With merits of the as-decorated Pt nanocrystals, the cathodic current of the DR-MoS2-Pt catalyst achieves 169.8 mA cm-2, which is higher than the benchmarking Pt/C (107.5 mA cm-2), further confirming its superior HER activity even in basic condition. Corresponding Tafel plots in Figure 3D further confirm the facile reaction kinetics of the DR-MoS2-Pt catalyst, where a small Tafel slope of 46.8 mV per decade can be resulted. Of note, a smaller Tafel slope is advantageous in commercial water electrolysis, since it will lead to a more obvious enhancement in HER rate at a moderately increased overpotential.12 The operational durability is also important in evaluating a commercial electrocatalyst. By applying a fixed overpotential of 0.2 V, time-dependent variations of the catalytic current density of the DR-MoS2-Pt catalyst in acidic and basic electrolytes were obtained. As shown in Figure 3E, excellent electrochemical stability can be achieved in both acidic and alkaline conditions. Typically, a 99.2% retention is determined for a 12-hour continuous HER catalysis in acidic electrolyte, while for alkaline HER catalysis, only a small performance degradation of 2.7% can be identified, therefore confirming the superior durability of the DR-MoS2-Pt catalyst. Besides, the electrochemical stability tests against CV cycling were also conducted. As demonstrated in Figure S4, nearly no fading of HER activity can be identified even for 2000 cycles, therefore verifying the high operational durability of the DR-MoS2-Pt catalyst in both acidic and alkaline environments. In summary, uniformly distributed platinum nanocrystals were decorated on the defect-rich MoS2 ultrathin nanosheets via an in-situ electrochemical approach. A redox platinum dissolution-redeposition process occurs, leading to the preferential adsorbing and anchoring of Pt atoms on the defect sites of the MoS2 nanosheets, therefore realizing homogeneous decoration of Pt nanocrystals. The as-obtained DR-MoS2-Pt composite catalyst displays remarkably promoted HER performance in acidic and alkaline environments, which even takes precedence over the benchmarking Pt/C with the same Pt content. Besides, superior operational stability of HER is also achieved in both acidic and basic electrolytes, making the DR-MoS2-Pt catalyst a potential alternative to Pt for commercial electrolytic water splitting.


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ASSOCIATED CONTENT Supporting Information The experimental section, the XRD patterns of the DR-MoS2 and DF-MoS2, the SEM images of the DR-MoS2 and DF-MoS2 before and after Pt decoration, LSV curves and XPS spectrum of the DF-MoS2 after CV cycling. This material is available free of charge via the Internet. AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] ORCID Junfeng Xie: 0000-0002-5775-3084 Yi Xie: 0000-0002-1416-5557 Author Contributions § J. X. and L. G. contributed equally. Notes The authors declare no competing financial interests.

Acknowledgements This work was supported by the National Natural Science Foundation of China (21501112, 11621063, U1632149, 21701102, U1532265 and 21331005), the Natural Science Foundation of Shandong Province (ZR2018JL009) and the Key Research Program of Frontier Sciences (QYZDY-SSW-SLH011). References 1. Dresselhaus, M. S.; Thomas, I. L., Alternative energy technologies. Nature 2001, 414, 332-337. 2. Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F., Combining theory and experiment in electrocatalysis: Insights into materials design. Science 2017, 355, 146. 3. Xie, J.; Xie, Y., Structural engineering of electrocatalysts for the hydrogen evolution reaction: Order or disorder? ChemCatChem 2015, 7, 2568-2580. 4. Turner, J. A., Sustainable hydrogen production. Science 2004, 305, 972-974. 5. Xie, J.; Xie, Y., Transition metal nitrides for electrocatalytic energy conversion: Opportunities and challenges. Chem. Eur. J. 2016, 22, 3588-3598. 6. Mallouk, T. E., Water electrolysis: Divide and conquer. Nat. Chem. 2013, 5, 362-363. 7. Xie, J.; Li, S.; Zhang, X.; Zhang, J.; Wang, R.; Zhang, H.; Pan, B.; Xie, Y., Atomically-thin molybdenum nitride nanosheets with exposed active surface sites for efficient hydrogen evolution. Chem. Sci. 2014, 5, 4615-4620. 8. Ji, Y.; Yang, L.; Ren, X.; Cui, G.; Xiong, X.; Sun, X., Nanoporous CoP3 nanowire array: acid etching preparation and application as a highly active electrocatalyst for the hydrogen evolution



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For Table of Contents Use Only

Platinum Nanocrystals Decorated on Defect-Rich MoS2 Nanosheets for pH-Universal Hydrogen Evolution Reaction Junfeng Xie,* Li Gao, Hailong Jiang, Xiaodong Zhang, Fengcai Lei, Pin Hao, Bo Tang, Yi Xie Table of contents

Synopsis Ultrafine platinum nanocrystals are decorated on the defect-rich MoS2 nanosheets via an in-situ electrochemical approach, which display highly efficient hydrogen evolution activity in both acidic and alkaline conditions.


Platinum Nanocrystals Decorated on Defect-Rich MoS2 Nanosheets

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