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Self-stabilization of zero-dimensional PdIr nanoalloys at two-dimensional manner for boosting their OER and HER performance
Journal Pre-proofs Full Length Article Self-stabilization of zero-dimensional PdIr nanoalloys at two-dimensional manner for boosting their OER and HER performance Anzhou Yang, Keying Su, Shangzhi Wang, Yingzi Wang, Xiaoyu Qiu, Wu Lei, Yawen Tang PII: DOI: Reference:
S0169-4332(20)30164-1 https://doi.org/10.1016/j.apsusc.2020.145408 APSUSC 145408
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Applied Surface Science
Received Date: Revised Date: Accepted Date:
27 November 2019 10 January 2020 14 January 2020
Please cite this article as: A. Yang, K. Su, S. Wang, Y. Wang, X. Qiu, W. Lei, Y. Tang, Self-stabilization of zerodimensional PdIr nanoalloys at two-dimensional manner for boosting their OER and HER performance, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145408
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Self-stabilization of zero-dimensional PdIr nanoalloys at two-dimensional manner for boosting their OER and HER performance Anzhou Yang a,b, Keying Su b, Shangzhi Wang b, Yingzi Wang b, Xiaoyu Qiu b,*, Wu Lei a,*, and Yawen Tang b a
School of Chemical Engineering, Nanjing University of Science and
Technology,Nanjing 210094, P. R. China b
Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative
Innovation Center of Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, P. R. China * Correspondence author: E-mail: [email protected] (X. Qiu) E-mail: [email protected] (W. Lei)
Abstract It remains a grand challenge to transform zero-dimensional (0D) noble-metal building blocks into well-defined two-dimensional (2D) architectures, which normally lacking an intrinsic driving force for anisotropic permutation in 2D manner. Herein, we report the first example of using a simple but efficient hydrothermal route to synthesize and selfstabilize ultrathin 0D PdIr NCs (2.4 nm) into a new class of freestanding 2D porous PdIr nanosheets with lateral size up to 1.5 μm (PdIr PNSs). Distinguished from previously reported 2D noble-metal nanosheets, the resultant 2D PdIr PNSs with immense surface area-to-volume ratio and high density of exposed atoms largely retain the special natures of 0D PdIr building blocks and further optimize their catalytic activity and stability at 2D manner. Electrochemical studies show that 2D PdIr PNSs deliver both excellent hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities with an HER overpotential of 40 mV and an OER overpotential of 285 mV affording 10 mA cm-2, outperforming those of 0D PdIr building blocks. The present work highlights the importance of tuning 0D nanocrystals at 2D manner for enhancing their OER and HER performance.
Keywords: PdIr alloy; Two-dimensional nanosheets; Self-stabilization; OER; HER
1. Introduction The oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are essential constituents for electrolysis of water, which are still plagued by their extremely high overpotentials due to the fourproton-coupled electron transfer process [1]. To scale up the sluggish reaction kinetic, it is urgent to develop efficient and affordable catalysts for reducing the potential and promoting the practical utilizations [2, 3]. For the present, despite great efforts have been devoted into developing nonnovel metal-based nanomaterials, noble metal based materials, such as the RuO2/IrO2 and Pt/C, are still identified as the best state-of-the-art OER and HER catalysts, respectively [4, 5]. Specifically, Ir-based nanocatalysts have attract significant attention owing to its slightly lower catalytic activity but higher stability compared to Ru [6-8]. To further improve the catalytic activity of Ir, doping with another metal has been considered as an effective way to adjust the electronic structure and strain coupling of the obtained nanohybrids [9, 10]. Alternatively, Pd, with similar electron configuration to Pt, may serve as potential dopant with Ir due to its predictable electro-catalytic activity and less dissolution than most of the non-noble metal [11, 12]. In addition, based on the unique intrinsic activity of Ir and Pd elements, further regulating and engineering their crystal facet, morphology, size, and strain are worth investigating to boost their OER and HER performance.
In views of the particle size, ultrafine 0D nanocrystals (﹤5 nm) have captured intensive attention due to their special natures, including unique geometrical structure, high-density surface atoms, and distinctive electronic feature [13]. These advantages mainly reflect in their high activity and sufficient catalytic sites for OER and HER, which hold great potential research values. However, the 0D nanocrystals with high surface energy are generally vulnerable to dissolution, agglomeration and Ostwald ripening during electrocatalysis process, thus significantly losing catalytic activity and reusability [14-16]. At this stage, self-assembly of ultrafine 0D nanocrystals into the complex multidimensional nanostructures with controllable manner could provide a promising way to adopt the good points and meanwhile avoid the shortcomings of the ultrafine 0D building blocks [17-19]. The resultant hierarchical superstructures could be typically divided into three types: 1D, 2D and 3D assemblies (as illustrated in Fig. 1). Unfortunately, the resultant 3D assemblies consisted of 0D building blocks often derived from the powerful van der Waals forces, which frequently leading to insufficient utilization of internal atoms [20]. Relatively, assembly 0D building blocks into low dimensional structures (1D and 2D) could guarantee a high percentage of surface atoms, thus offering plenty active sites and enhancing the atom utilization efficiency [21]. In particular, ultrathin 2D nanostructures are more promising for catalysis reactions because the 1D nanowires are often impaired by their
slow electron transfer rate and easy to reunite, which are adverse for mass transport and electron transport during electrocatalytic process [22-24]. Accordingly, engineering and stabilizing 0D building bolck into 2D structure could be an extraordinary strategy to guarantee the 0D atom utilization efficiency and further optimize their catalytic activities. However, transforming 0D noble metals into 2D manner faces a host of barriers in self-assembly study. Unlike transition-metal with intrinsic lamellar structures with weak interlayer interactions, the 0D noble metal nanocrystals with heavier atomic mass have a strong preference for assembling into stable, closed-packed 3D structures with lower surface energy and anisotropy [25]. Generally, 2D noble metal-based structures can only be achieved when the electrostatic interactions and hydrophobic attractions in the system balance each other [26, 27]. That is to say, it requires a high standard to search suitable solutions with multiple force balances (small positive charge, dipole moment, and directional hydrophobic attraction) to construct 2D superstructures from 0D building blocks. Additionally, the self-assembly process is always accompanied by the dissolution and coalescence of 0D building blocks, leading to the formation of rigid 2D nanostructure with smooth and intact surface [28, 29]. Such phenomenon is easy to form irreversible stackings and aggregations between adjacent 2D nanosheets during material fabrication and catalytic process, thus resulted in a detrimental loss of accessible
surface atoms and deviated from the original intention to take full advantages of 0D building blocks [30, 31]. To this end, rational design of 0D noble-metal building blocks into 2D superstructures is therefore highly demanded, yet still remains challenging. Herein, we present the preparation of a novel 2D PdIr porous nanosheet with lateral size up to 1.50 μm that consisting of horizontally assembled ultrathin 0D PdIr nanocrystals (2.4 nm) as subunits. Nanoparticles attachment and subsequent self-assembly with the assistance of poly (diallyl dimethyl ammonium chloride) (PDDA, Fig. S1, supplementary data) are the key reasons for the formation of such interesting superstructure. As expected, the 2D PdIr PNSs deliver obviously enhanced catalytic activity for both OER and HER, which require the overpotential of 285 mV for the OER and 40 mV for the HER to reach a current density of 10 mA cm-2. Such high surface area-to-volume ratio and proper particle distance between adjacent 0D PdIr building blocks could ensure the exposure of surface atoms and facilitate electron transport, which are responsible for the excellent catalytic activity and stability of 2D PdIr PNSs. 2. Experimental Section 2.1. Reagents and Chemicals Potassium tetrachloroplatinate (II) (K2PdCl4) and Iridium (III) chloride hydrate (IrCl3.xH2O) were purchased from Shanghai D&B
Biological Sci-Tech Co., Ltd. (P. R. China). Formaldehyde solution (40%) was supplied by Guangdong Guanghua Sci-Tech Co., Ltd. (Shantou, P. R. China). Poly (diallyl dimethyl ammonium chloride) (PDDA, Mw=200000350000, 20 wt% in water solution) and commercial RuO2 were purchased from Aladdin Industrial Corporation. (Shanghai, P. R. China). Commercial Pt/C was purchased from Shanghai Hesen Electric Co., Ltd. (Shanghai, China). All chemicals used in this study were of analytical reagent (AR) without any further purification. 2.2. Preparation of 0D PdIr NCs and 2D PdIr PNSs The synthesis of 0D PdIr NCs and 2D PdIr PNSs were based on hydrothermal reduction process by conducting the reaction time in the same solution. In a typical synthesis, 1 mL of 0.05 M IrCl3, 1 mL of 0.05 M K2PdCl4, and 2 mL of 0.5 M PDDA were added to 7 mL of deionized water. Then the mixture solution was kept at room temperature for 15 min under magnetic stirring. After adjusting the solution pH to 11 by using the NaOH solution (2M), 1 mL of HCHO solution (40%) was added into the mixture and stirred well. Then the mixed solution was transferred to a 25 mL Teflon-lined stainless-steel autoclave, heating at 120 °C for 1h to obtain 0D PdIr NCs and extending the reaction time to 5h to obtain 2D PdIr PNSs. After cooling down to room temperature, the obtained products were collected and purified by centrifugation at 18000 rpm for 8 min, washed
several times with DI water, and then dried at 45 °C for 8 h in a vacuum dryer. 2.3. Electrochemical measurements All electrochemical measurements were conducted using a threeelectrode system on a CHI 760D electrochemical analyzer (CH Instruments, Inc., Shanghai, China) at 25 °C. A catalyst-modified glassy carbon electrode was used as the working electrode, a graphite rod was used as the auxiliary electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. The catalyst ink was prepared by dispersing 4 mg of as-tested sample in a mixture of 0.8 mL of alcohol and 1.2 mL of deionized water via sonication for 30 min. Then 20 μL of the catalyst ink was dropped onto the clean surface of the glassy carbon electrode. After drying, 2 μL of Nafion solution (5 wt%) was coated on the surface of the modified electrode and dried again. For all samples, the same amount of catalyst loading was kept to prepare control electro-catalytic experiments. All measurements were performed in 1 M N2-saturated KOH aqueous electrolyte at a scan rate of 5 mV s −1. All potentials in this work were reported versus the reversible hydrogen electrode (RHE) by using the following equation: ERHE = ESCE + 0.059 pH + 0.242. 2.4. Instruments The morphology of products were characterized by Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM),
conducting on a JEOL JEM-2100F transmission electron microscope at an accelerating voltage of 200 kV. The chemical element analysis was measured by energy-dispersive X-ray spectrometry (EDX) coupled with SEM. The crystallinity of the samples was determined by recording X-ray diffraction (XRD) on a Model D/max-rC X-ray diffractometer operating at 40 kV and 100 mA by Cu Ka radiation source (λ=1.5406 Å). X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Thermo VG Scientific ESCALAB 250 spectrometer using Al Kα radiation. The electron binding energy was calibrated by referring to the value of C 1s peak energy of 284.6 eV. Fourier transform infrared (FT-IR) was carried out using a Nicolet 520 SXFTIR spectrometer. Ultraviolet and visible spectroscopy (UV-vis) spectra were recorded at room temperature on a Shimadzu UV3600 spectrophotometer equipped with 1.0 cm quartz cells.
3. Results and Discussion Fig. 1 An efficient hydrothermal approach was developed to synthesize the 0D PdIr NCs and transform them into 2D PdIr PNSs by conducting the reaction time (schematic illustration is displayed in Fig. 1b). In a typical protocol, IrCl3, K2PdCl4, and PDDA were mixed together to form the homogeneous mixture. After adjusting the solution pH to 11 and adding the HCHO as reducing agent, the solution was heating at 120 °C for 1h to
obtain 0D PdIr NCs, and then extending the reaction time to 5h to obtain 2D PdIr PNSs. The morphological characterizations of as-prepared 2D PdIr PNSs are displayed in Fig. 2. Typical high-angle annular dark-field scanning TEM (HAADF-STEM) images and transmission electron microscopy (TEM) show that the products are all sheet-like structure with freestanding and monodisperse states at the first glance (Fig. 2a-b). These nanosheets display an obvious rough surface with lateral size up to ≈1.5 μm. As marked by yellow circles in Fig. 2b, a mass of pores with dimeter of 25 nm can be observed within the nanosheets, which are benefiicial for mass transport during electro-catalytic process [32]. HRTEM image of the edge of an individual 2D PdIr PNSs provides further insight into the microstructure (Fig. 2c). It shows that the freestanding nanosheets are constructed by numerous building blocks of ultrathin 0D nanocrystals. When we take a closer look at the detailed structure, the 0D building blocks with an average diameter of 2.39 nm reveal theie random place and high morphology uniformity (Fig. S2, supplementary data). It is worth noting that each particle is completely preserving its original 0D structure with a proper particle distance, which against the easy dissolution and ripening during assembly process (Fig. 2d). Fig. 2 Further enlarged HRTEM image shows a mass of crystal defects of 2D PdIr PNSs (as marked by yellow circles in Fig. 2e) , which may perform
as possible catalytical active sites due to their coordinative unsaturations. The lattice spacing of 2D PdIr PNSs is measured to be 0.249 nm (Fig. 2f), which can be indexed to the (111) plane of the face-centered cubic (fcc) PdIr alloy. Fig. 2g-h show the element distribution of the products by using EDX line-scan and mapping images. Both measurements evidence the uniform distribution of the Pd and Ir elements throughout the whole nanosheets. At this stage, all the above results indicative the successful assembly of ultrafine 0D PdIr NCs at 2D manner with high surface areato-volume ratio and porosity, specifically remaining the inherent structural characteristics of ultrafine 0D building blocks. Fig. 3 From the energy-dispersive X-ray (EDX) spectrum, it verifies that the Pd/Ir atomic ratio is about 49.4: 50.6, which is almost identical to the 1:1 stoichiometry of the precursor solution, suggesting the complete reduction of metal precursors (Fig. 3a). X-ray diffraction (XRD) pattern of the 2D PdIr PNSs is shown in Fig. 3b. Three distinct diffraction peaks at 2θ = 40.4, 68.7, and 82.8° could be observed, corresponding to (111), (220), and (311) planes of PdIr, respectively (PDF No. 65-7457). FT-IR spectrum of the 2D PdIr PNSs clearly shows the stretching vibration of C-N and C-H groups, which originated from the spared PDDA on the surface of 2D PdIr PNSs (Fig. 3c). The XPS survey spectrum of 2D PdIr PNSs from 0~1200 eV further confirms the existence of PDDA, where displays a distinct
characteristic peak of N 1s (Fig. 3d). The PDDA with well water-solubility is not only propitious to the uniform coverage of as-tested catalysts on glassy carbon electrode, but also beneficial for even better activity and durability based on the synergistic contribution between soft PDDA and hard exposed PdIr lattice facets [33]. As such, it is not necessary to completely remove the PDDA. Meanwhile, the Pd/Ir atom ratio on the surface is measured to be 47.3:52.7, well matching the atom ratios analyzed from EDX pattern. From the high-resolution XPS spectra at Pd 3d and Ir 4f regions (Fig. 3e-f), it is demonstrated that both the percentage of Pd0 species and Ir0 species are in the majority, indicating that Pd2+ and Ir3+ precursors are successfully reduced to metallic states in 2D PdIr PNSs. The locations of binding energies are related to Pd0 3d (3d5/2 = 334.5 eV; 3d3/2 = 339.8 eV) and Ir0 4f (4f7/2 = 60.9 eV; 4f5/2 = 63.9 eV), respectively, which corresponding to the standard values of metallic Pd and metallic Ir species [34]. Fig. 4 We conducted a series of contrast experiments to unravel the growth and assembly mechanism of the 2D PdIr PNSs. Firstly, we elucidated the explicit role of PDDA as the research object. The products prepared in the absence of PDDA displays an irregular shape with serious agglomerations, confirming the importance of PDDA in conducting the nucleation and growth in this solution (Fig. S3, supplementary data). The PDDA with a
positively charged long-chain polyelectrolyte could readily bound the negatively charged PdCl42- to generate the PDDA-PdII complex (Fig. 4a) [35]. Fig. 4b shows the UV-vis spectrum of K2PdCl4, which exhibits typical absorption peaks at 300 and 420 nm. When PDDA was added to the K2PdCl4 solution, the characteristic absorption peak at 420 nm disappeared and the characteristic absorption peak at 300 nm showed an obviously shift, indicating the electrostatic interaction between cationic PDDA and negatively charged PdCl42-. However, form the UV-vis spectrum of IrCl3 and IrCl3+PDDA mixture, it presents nonexistent electrostatic interaction between PDDA and Ir3+ (Fig. S4, supplementary data). That is to say, the formation of 2D PdIr PNSs may be attributed to using the PDDA-PdII complex as reaction precursor. TEM images of the monocomponent products using the standard protocol were conducted to further confirm this point (Fig. S5, supplementary data). Pure Ir nanoparticles and pure Pd PNSs could be observed, well corresponding to the strong electrostatic interaction between PDDA and PdCl42-. Moreover, by changing the Pd/Ir ratio from 1:2 to 2:1 in the precursor, we get the 2D Pd2Ir1 NSs and 0D Pd1Ir2 nanoparticles (Fig. S6, supplementary data), further confirming the major interactions were derived from PDDA-PdII complex. Since the standard electrode potential of Pd2+/Pd0 (Eq = 0.951 V) is far from that of Ir3+/Ir0 (Eq = 1.156 V), we simulated the real condition to test the reduction powers of Pd2+ and Ir3+ in the existence of PDDA (Fig.
4c). Obviously, the reduction potential of Pd2+ solution (-0.195 V) and Ir3+ solution (-0.248 V) are quite similar. Such closing reduction potentials can be responsible for the co-reduction of PdCl42- and Ir3+, thus forming the PdIr alloy. At this stage, we can get the conclusion that the formation of 2D PdIr PNSs is mainly attributed to the coordination effect of PDDA-PdII complex and similar reduction potential of Pd2+ and Ir3+ in the presence of PDDA. TEM images of intermediate products were carried out to investigate the morphology evolution by collecting them at different reaction intervals. The product collected at 1 h was dominantly composed of tiny 0D NCs (Fig. 4d). At this stage, the 0D NCs were capped with positively charged PDDA, the van der Waals attractions would be weakened. When the reaction process proceeded to 2 h, these 0D NCs tended to be close to each other spontaneously (Fig. 4e). By further increasing the reaction time to 3 h (Fig. 4f), spare 2D structure could be observed with a number of dispersed 0D building blocks. At this stage, the electrostatic repulsions between positive-charged 0D NCs were able to balance each other, meanwhile preventing the 3D particle assembly in the solution. When the reaction came to 5 h (Fig. 4g), the 2D self-assembled PdIr superstructures could be achieved as a result of thermodynamic balance in this solution [36, 37]. According to the aforementioned explanations, it is reasonable to deduce that the formation of 2D PdIr PNSs might undergo two process: (1) Formation of ultrathin 0D PdIr NCs based
on the co-reduction of Pd2+ and Ir3+. (2) Generation of 2D PdIr PNSs determined by the driving forces of internal balance for the self-assembly process. Fig. 5 The HER and OER performance of 0D PdIr NCs and 2D PdIr PNSs can be systematically compared by ignoring the complication of surface contamination. Commercial Pt/C and RuO2 were also served as reference in the following discussion. From the HER polarization curves displayed in Fig. 5a, the onset overpotential and overpotential at -10 mA cm-2 were found to be 13 and 40, 25 and 260, 8 and 19 mV for 2D PdIr PNSs, 0D PdIr NCs, and Pt/C, respectively. It reveals that the HER activity of 2D PdIr PNSs is slightly lower than that of commercial 20% Pt/C but much better than 0D PdIr NCs, as well as better than some of the HER catalysts reported to date (Table S1, supplementary data). The Tafel slope of 2D PdIr PNSs was measured to be 59.2 mV per decade, which is smaller than that of 0D PdIr NCs (106.9 mV per decade), indicating the more rapid HER kinetics at 2D PdIr PNSs (Fig. 5b). The ADTs were tested to evaluate the HER durability of 2D PdIr PNSs. After 1000 CV cycles, the 2D PdIr PNSs (Fig. 5c) exhibit almost unchanged overpotential while the 0D PdIr NCs display a rapid decay (Fig. S7a, supplementary data), indicative the robustness of PdIr at 2D manner. The OER catalytic activity of 2D PdIr PNSs were assessed in the same alkaline electrolyte with the reference of
0D PdIr NCs and commercial RuO2. The 2D PdIr PNSs exhibit a remarkably higher current density and lower onset overpotential, outperforming those of the other two catalysts and most of the state-of-art OER catalysts (Table S2, supplementary data). Meanwhile, the 2D PdIr PNSs require an overpotential of only 285 mV to reach 10 mA cm-2 with a tafel plot of 76.1 mV dec−1, which is much lower than those of 0D PdIr NCs and commercial RuO2 (Fig. 5d-e), indicating the excellent electrontransfer facilitation within the 2D freestanding nanosheets. The OER durability of 2D PdIr PNSs was tested by ADTs (Fig. 5f), which shows almost unchanged overpotentials after 1000 CV cycles. As a comparison, the 0D PdIr NCs (Fig. S7b, supplementary data) show an obvious delay of activity. At this stage, we can get the conclusion that both the HER and OER stability of 2D PdIr PNSs are much better than 0D PdIr NCs. This result is attributed to the unique structure of 2D PdIr PNSs, which endow suitable void space between each 0D PdIr building blocks. As such, the 2D PdIr PNSs could hold less susceptible to dissolution, agglomeration, and Ostwald ripening compare to 0D and 1D counterparts [22, 38-40]. Moreover, the HER and OER performance of intermediate products collected at different reaction time were performed to investigate the size effect of nanosheet on catalytic performance (Fig. S8, supplementary data). The catalytic activity of intermediate products shows an obvious enhancement along with the increasment of reaction time. Overall, the 2D
PdIr PNSs, which collected at 5 h, exhibit the lowest onset overpotential and the most rapid reaction kinetics. This result could be attracted to the self-assembly process of nanosheets, which adhere PdIr building blocks to promote the electron transfer rate. The electrochemical double-layer capacitance Cdl was employed to evaluate the electrochemically active surface area of 2D PdIr PNSs (Fig. S9, supplementary data). The Cdl value was calculated to be 7.19 mF cm−2, which exhibited a large electrochemically active surface area of 2D PdIr PNSs. All these above results highlight the superior HER and OER performance of 2D PdIr PNSs, not only endow excellent activity but also exhibit superior stability against the 0D PdIr building blocks. The HER and OER performance of pure Ir NCs, pure Pd NSs, Pd1Ir2 NCs, Pd2Ir1 NSs were also measured for the component-dependent study (Fig. S10 and S11, supplementary data). The 2D PdIr PNSs show both the best HER and OER catalytic performance among all samples, including a sharp onset potential, a small overpotential reaching 10 mA cm-2, and a lower tafel slope. Fig. 6 The histograms of HER and OER onset overpotentials and overpotentials to achieve a current density of 10 mA cm-2 for different electrocatalysts are listed in Fig. 6a-b. It indicates that both the HER and OER performance herein follow the order of 2D PdIr PNSs >pure Ir NCs> Pd1Ir2 NCs> Pd2Ir1 NSs >pure Pd NSs. Due to the intrinsic superior activity
of Ir, the HER and OER activity of component-dependent products gradually shows an enhancement along with the increasing content of Ir. Combining with TEM image of each component-dependent product, the 2D porous structure is unable to be maintained while the Ir content is much excrescent than Pd, thus leading to a definite loss of catalytic activity. Accordingly, the OER and HER results are well matching the tunable composition and morphology of PdIr products, where sufficient Ir content guarantees the catalytic activity and moderate Pd content is served to maintain the 2D structures for boosting catalytic activity and stability. Furthermore, the histograms of HER and OER onset overpotentials and overpotentials to achieve a current density of 10 mA cm-2 for 2D PdIr PNSs, 0D PdIr NCs and commercial catalysts are displayed in Fig. 6c-d. Apparently, self-stabilizing 0D PdIr NCs at 2D manner can further enhance their HER and OER activity and durability. Specifically, (1) the synergy between Ir and Pd make certain contributions to the improved catalytic activities due to the modification of Ir electronic structure; (2) the unabridged 0D PdIr building blocks are well remained in 2D PdIr PNSs, which exhibit sufficient accessible active sites and high atom utilization efficiency [5, 41]; (3) distinguished from conventional 2D nanosheets with smooth and intact surface, the highly open and porous structure of 2D PdIr PNSs is advantageous to facilitate mass diffusion and transport during the HER and OER reaction, which in turn promotes the reaction kinetics [42];
(4) the self-tractive 0D PdIr building blocks with suitable void space are less susceptible to dissolution, agglomeration, and Ostwald ripening compare to 0D and 1D counterparts [43, 44].
4. Conclusion To conclude, we have demonstrated an efficient hydrothermal route to synthesize and self-stabilize ultrathin 0D PdIr NCs at 2D manner, creating a new class of freestanding 2D porous PdIr nanosheets with large surface area-to-volume ratio and enriched surface atoms. The PDDAdriven electrostatic balance is essential for such successful 2D assembly, which guarantees the atom utilization efficiency of 0D PdIr building blocks and further improve electron transport and the robustness. From the dimensional-dependent and component-dependent study for both HER and OER in alkaline medium, the 2D PdIr PNSs exhibit excellent catalytic activity and stability exceeding the 0D PdIr NCs, other component-based products and most of the reported bifunctional electrocatalysts (Tables S1 and S2, supplementary data). Such manipulation and reintegration of ultrafine 0D NCs based on electrostatic balance can offer new opportunities for constructing various 2D porous structure, which may find huge research values for widespread catalysis applications.
Acknowledgements
This work was financially supported by National Natural Science Foundation of China (21902078, 51872140, 21875112), Natural Science Foundation of Jiangsu Higher Education Institutions of China (19KJB150033), and Natural Science Foundation of Jiangsu Province (BK20171473). The authors were also grateful for the support from the National and Local Joint Engineering Research Center of Biomedical Functional Materials and a project sponsored by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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Figure Captions Fig. 1. Different assembly configurations of 0D PdIr building blocks. (a) 1D nanowires, (b) schematic illustration for the formation of 2D PdIr PNSs, and (c) 3D nanopolyhedrons. Fig. 2. Morphology representation of the 2D PdIr PNSs. (a) HAADFSTEM image, (b)-(c) Typical TEM images, (d)-(f) HRTEM images, (g) EDX line-scan profile, and (h) EDX elemental mapping images. Fig. 3. Structural and compositional examination of the 2D PdIr PNSs. (a) EDX pattern, (b) XRD pattern, (c) FT-IR spectra, (d) XPS survey spectrum of PdIr nanosheets. High-resolution XPS survey spectrum at (e) Pd 3d region, (f) Ir 4f region. Fig. 4. Mechanism understanding for the formation of 2D PdIr PNSs. (a) 3D geometrical structure of PdCl42--PDDA complex. (b) UV-vis spectra of PDDA solution, Cl4 solution, K2PdCl4 +PDDA solution, and K2PdCl4 + IrCl3 +PDDA solution. (c) Linear sweeping voltammograms of N2saturated 0.005 M K2PdCl4 + 0.01 M PDDA + 0.5 M KCl solution (blue line) and 0.005 M IrCl3 + 0.01 M PDDA + 0.5 M KCl solution (red line) at the glassy carbon electrode. Scan rate: 100 mV s-1. (d)-(g) Typical TEM images of the intermediate products collected at 1 h, 2 h, 3 h, 5 h, respectively. Fig. 5 (a) HER linear sweep voltammetry curves of the 2D PdIr PNSs, 0D PdIr NCs, and Pt/C, respectively. (b) Corresponding Tafel plots. (c) HER
polarization curves of the 2D PdIr PNSs recorded before and after 1000 cycles at a scan rate of 100 mV s−1. (d) OER linear sweep voltammetry curves of the 2D PdIr PNSs, 0D PdIr NCs, and RuO2, respectively. (e) Corresponding Tafel plots. (f) OER polarization curves of the 2D PdIr PNSs recorded before and after 1000 cycles at a scan rate of 100 mV s −1. The electrolyte used in all above conditions is 1.0 M KOH solution. Fig. 6 (a) Histograms of HER onset overpotentials and overpotentials to achieve a current density of 10 mA cm-2 for 2D PdIr PNSs, pure Ir NCs, pure Pd NSs, Pd1Ir2 NCs, and Pd2Ir1 NSs, respectively. (b) Histograms of OER onset overpotentials and overpotentials to achieve a current density of 10 mA cm-2 for 2D PdIr PNSs, pure Ir NCs, pure Pd NSs, Pd1Ir2 NCs, and Pd2Ir1 NSs, respectively. Histograms of (c) HER onset overpotentials and overpotentials to achieve a current density of 10 mA cm-2, and (d) OER onset overpotentials and overpotentials to achieve a current density of 10 mA cm-2 for 2D PdIr PNSs, 0D PdIr NCs, Pt/C, and RuO2 respectively.
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Credit Author Statement Anzhou Yang: Methodology, Formal analysis, Writing - Original Draft Keying Su: Investigation Shangzhi Wang: Investigation Yingzi Wang: Validation Xiaoyu Qiu: Conceptualization, Writing - Review & Editing, Funding acquisition Wu Lei: Resources, Project administration, Funding acquisition Yawen Tang: Supervision, Funding acquisition
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Graphical Abstract
Zero-dimensional PdIr nanoalloys self-assembed to porous nanosheets with the help of capping agent PDDA. The PdIr PNS shows excellent OER and HER catalitic activities.
Highlights 1. A simple but efficient hydrothermal route was used to synthesize and self-stabilize ultrathin 0D PdIr nanocrystals into freestanding 2D porous PdIr nanosheets (PdIr PNSs). 2. The 2D PdIr PNSs possessed large surface area-to-volume ratio and high density of exposed atoms. 3. The 2D PdIr PNSs delivered both excellent HER and OER activities and stabilities.
S0169-4332(20)30164-1 https://doi.org/10.1016/j.apsusc.2020.145408 APSUSC 145408
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Applied Surface Science
Received Date: Revised Date: Accepted Date:
27 November 2019 10 January 2020 14 January 2020
Please cite this article as: A. Yang, K. Su, S. Wang, Y. Wang, X. Qiu, W. Lei, Y. Tang, Self-stabilization of zerodimensional PdIr nanoalloys at two-dimensional manner for boosting their OER and HER performance, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145408
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Self-stabilization of zero-dimensional PdIr nanoalloys at two-dimensional manner for boosting their OER and HER performance Anzhou Yang a,b, Keying Su b, Shangzhi Wang b, Yingzi Wang b, Xiaoyu Qiu b,*, Wu Lei a,*, and Yawen Tang b a
School of Chemical Engineering, Nanjing University of Science and
Technology,Nanjing 210094, P. R. China b
Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative
Innovation Center of Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, P. R. China * Correspondence author: E-mail: [email protected] (X. Qiu) E-mail: [email protected] (W. Lei)
Abstract It remains a grand challenge to transform zero-dimensional (0D) noble-metal building blocks into well-defined two-dimensional (2D) architectures, which normally lacking an intrinsic driving force for anisotropic permutation in 2D manner. Herein, we report the first example of using a simple but efficient hydrothermal route to synthesize and selfstabilize ultrathin 0D PdIr NCs (2.4 nm) into a new class of freestanding 2D porous PdIr nanosheets with lateral size up to 1.5 μm (PdIr PNSs). Distinguished from previously reported 2D noble-metal nanosheets, the resultant 2D PdIr PNSs with immense surface area-to-volume ratio and high density of exposed atoms largely retain the special natures of 0D PdIr building blocks and further optimize their catalytic activity and stability at 2D manner. Electrochemical studies show that 2D PdIr PNSs deliver both excellent hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities with an HER overpotential of 40 mV and an OER overpotential of 285 mV affording 10 mA cm-2, outperforming those of 0D PdIr building blocks. The present work highlights the importance of tuning 0D nanocrystals at 2D manner for enhancing their OER and HER performance.
Keywords: PdIr alloy; Two-dimensional nanosheets; Self-stabilization; OER; HER
1. Introduction The oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are essential constituents for electrolysis of water, which are still plagued by their extremely high overpotentials due to the fourproton-coupled electron transfer process [1]. To scale up the sluggish reaction kinetic, it is urgent to develop efficient and affordable catalysts for reducing the potential and promoting the practical utilizations [2, 3]. For the present, despite great efforts have been devoted into developing nonnovel metal-based nanomaterials, noble metal based materials, such as the RuO2/IrO2 and Pt/C, are still identified as the best state-of-the-art OER and HER catalysts, respectively [4, 5]. Specifically, Ir-based nanocatalysts have attract significant attention owing to its slightly lower catalytic activity but higher stability compared to Ru [6-8]. To further improve the catalytic activity of Ir, doping with another metal has been considered as an effective way to adjust the electronic structure and strain coupling of the obtained nanohybrids [9, 10]. Alternatively, Pd, with similar electron configuration to Pt, may serve as potential dopant with Ir due to its predictable electro-catalytic activity and less dissolution than most of the non-noble metal [11, 12]. In addition, based on the unique intrinsic activity of Ir and Pd elements, further regulating and engineering their crystal facet, morphology, size, and strain are worth investigating to boost their OER and HER performance.
In views of the particle size, ultrafine 0D nanocrystals (﹤5 nm) have captured intensive attention due to their special natures, including unique geometrical structure, high-density surface atoms, and distinctive electronic feature [13]. These advantages mainly reflect in their high activity and sufficient catalytic sites for OER and HER, which hold great potential research values. However, the 0D nanocrystals with high surface energy are generally vulnerable to dissolution, agglomeration and Ostwald ripening during electrocatalysis process, thus significantly losing catalytic activity and reusability [14-16]. At this stage, self-assembly of ultrafine 0D nanocrystals into the complex multidimensional nanostructures with controllable manner could provide a promising way to adopt the good points and meanwhile avoid the shortcomings of the ultrafine 0D building blocks [17-19]. The resultant hierarchical superstructures could be typically divided into three types: 1D, 2D and 3D assemblies (as illustrated in Fig. 1). Unfortunately, the resultant 3D assemblies consisted of 0D building blocks often derived from the powerful van der Waals forces, which frequently leading to insufficient utilization of internal atoms [20]. Relatively, assembly 0D building blocks into low dimensional structures (1D and 2D) could guarantee a high percentage of surface atoms, thus offering plenty active sites and enhancing the atom utilization efficiency [21]. In particular, ultrathin 2D nanostructures are more promising for catalysis reactions because the 1D nanowires are often impaired by their
slow electron transfer rate and easy to reunite, which are adverse for mass transport and electron transport during electrocatalytic process [22-24]. Accordingly, engineering and stabilizing 0D building bolck into 2D structure could be an extraordinary strategy to guarantee the 0D atom utilization efficiency and further optimize their catalytic activities. However, transforming 0D noble metals into 2D manner faces a host of barriers in self-assembly study. Unlike transition-metal with intrinsic lamellar structures with weak interlayer interactions, the 0D noble metal nanocrystals with heavier atomic mass have a strong preference for assembling into stable, closed-packed 3D structures with lower surface energy and anisotropy [25]. Generally, 2D noble metal-based structures can only be achieved when the electrostatic interactions and hydrophobic attractions in the system balance each other [26, 27]. That is to say, it requires a high standard to search suitable solutions with multiple force balances (small positive charge, dipole moment, and directional hydrophobic attraction) to construct 2D superstructures from 0D building blocks. Additionally, the self-assembly process is always accompanied by the dissolution and coalescence of 0D building blocks, leading to the formation of rigid 2D nanostructure with smooth and intact surface [28, 29]. Such phenomenon is easy to form irreversible stackings and aggregations between adjacent 2D nanosheets during material fabrication and catalytic process, thus resulted in a detrimental loss of accessible
surface atoms and deviated from the original intention to take full advantages of 0D building blocks [30, 31]. To this end, rational design of 0D noble-metal building blocks into 2D superstructures is therefore highly demanded, yet still remains challenging. Herein, we present the preparation of a novel 2D PdIr porous nanosheet with lateral size up to 1.50 μm that consisting of horizontally assembled ultrathin 0D PdIr nanocrystals (2.4 nm) as subunits. Nanoparticles attachment and subsequent self-assembly with the assistance of poly (diallyl dimethyl ammonium chloride) (PDDA, Fig. S1, supplementary data) are the key reasons for the formation of such interesting superstructure. As expected, the 2D PdIr PNSs deliver obviously enhanced catalytic activity for both OER and HER, which require the overpotential of 285 mV for the OER and 40 mV for the HER to reach a current density of 10 mA cm-2. Such high surface area-to-volume ratio and proper particle distance between adjacent 0D PdIr building blocks could ensure the exposure of surface atoms and facilitate electron transport, which are responsible for the excellent catalytic activity and stability of 2D PdIr PNSs. 2. Experimental Section 2.1. Reagents and Chemicals Potassium tetrachloroplatinate (II) (K2PdCl4) and Iridium (III) chloride hydrate (IrCl3.xH2O) were purchased from Shanghai D&B
Biological Sci-Tech Co., Ltd. (P. R. China). Formaldehyde solution (40%) was supplied by Guangdong Guanghua Sci-Tech Co., Ltd. (Shantou, P. R. China). Poly (diallyl dimethyl ammonium chloride) (PDDA, Mw=200000350000, 20 wt% in water solution) and commercial RuO2 were purchased from Aladdin Industrial Corporation. (Shanghai, P. R. China). Commercial Pt/C was purchased from Shanghai Hesen Electric Co., Ltd. (Shanghai, China). All chemicals used in this study were of analytical reagent (AR) without any further purification. 2.2. Preparation of 0D PdIr NCs and 2D PdIr PNSs The synthesis of 0D PdIr NCs and 2D PdIr PNSs were based on hydrothermal reduction process by conducting the reaction time in the same solution. In a typical synthesis, 1 mL of 0.05 M IrCl3, 1 mL of 0.05 M K2PdCl4, and 2 mL of 0.5 M PDDA were added to 7 mL of deionized water. Then the mixture solution was kept at room temperature for 15 min under magnetic stirring. After adjusting the solution pH to 11 by using the NaOH solution (2M), 1 mL of HCHO solution (40%) was added into the mixture and stirred well. Then the mixed solution was transferred to a 25 mL Teflon-lined stainless-steel autoclave, heating at 120 °C for 1h to obtain 0D PdIr NCs and extending the reaction time to 5h to obtain 2D PdIr PNSs. After cooling down to room temperature, the obtained products were collected and purified by centrifugation at 18000 rpm for 8 min, washed
several times with DI water, and then dried at 45 °C for 8 h in a vacuum dryer. 2.3. Electrochemical measurements All electrochemical measurements were conducted using a threeelectrode system on a CHI 760D electrochemical analyzer (CH Instruments, Inc., Shanghai, China) at 25 °C. A catalyst-modified glassy carbon electrode was used as the working electrode, a graphite rod was used as the auxiliary electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. The catalyst ink was prepared by dispersing 4 mg of as-tested sample in a mixture of 0.8 mL of alcohol and 1.2 mL of deionized water via sonication for 30 min. Then 20 μL of the catalyst ink was dropped onto the clean surface of the glassy carbon electrode. After drying, 2 μL of Nafion solution (5 wt%) was coated on the surface of the modified electrode and dried again. For all samples, the same amount of catalyst loading was kept to prepare control electro-catalytic experiments. All measurements were performed in 1 M N2-saturated KOH aqueous electrolyte at a scan rate of 5 mV s −1. All potentials in this work were reported versus the reversible hydrogen electrode (RHE) by using the following equation: ERHE = ESCE + 0.059 pH + 0.242. 2.4. Instruments The morphology of products were characterized by Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM),
conducting on a JEOL JEM-2100F transmission electron microscope at an accelerating voltage of 200 kV. The chemical element analysis was measured by energy-dispersive X-ray spectrometry (EDX) coupled with SEM. The crystallinity of the samples was determined by recording X-ray diffraction (XRD) on a Model D/max-rC X-ray diffractometer operating at 40 kV and 100 mA by Cu Ka radiation source (λ=1.5406 Å). X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Thermo VG Scientific ESCALAB 250 spectrometer using Al Kα radiation. The electron binding energy was calibrated by referring to the value of C 1s peak energy of 284.6 eV. Fourier transform infrared (FT-IR) was carried out using a Nicolet 520 SXFTIR spectrometer. Ultraviolet and visible spectroscopy (UV-vis) spectra were recorded at room temperature on a Shimadzu UV3600 spectrophotometer equipped with 1.0 cm quartz cells.
3. Results and Discussion Fig. 1 An efficient hydrothermal approach was developed to synthesize the 0D PdIr NCs and transform them into 2D PdIr PNSs by conducting the reaction time (schematic illustration is displayed in Fig. 1b). In a typical protocol, IrCl3, K2PdCl4, and PDDA were mixed together to form the homogeneous mixture. After adjusting the solution pH to 11 and adding the HCHO as reducing agent, the solution was heating at 120 °C for 1h to
obtain 0D PdIr NCs, and then extending the reaction time to 5h to obtain 2D PdIr PNSs. The morphological characterizations of as-prepared 2D PdIr PNSs are displayed in Fig. 2. Typical high-angle annular dark-field scanning TEM (HAADF-STEM) images and transmission electron microscopy (TEM) show that the products are all sheet-like structure with freestanding and monodisperse states at the first glance (Fig. 2a-b). These nanosheets display an obvious rough surface with lateral size up to ≈1.5 μm. As marked by yellow circles in Fig. 2b, a mass of pores with dimeter of 25 nm can be observed within the nanosheets, which are benefiicial for mass transport during electro-catalytic process [32]. HRTEM image of the edge of an individual 2D PdIr PNSs provides further insight into the microstructure (Fig. 2c). It shows that the freestanding nanosheets are constructed by numerous building blocks of ultrathin 0D nanocrystals. When we take a closer look at the detailed structure, the 0D building blocks with an average diameter of 2.39 nm reveal theie random place and high morphology uniformity (Fig. S2, supplementary data). It is worth noting that each particle is completely preserving its original 0D structure with a proper particle distance, which against the easy dissolution and ripening during assembly process (Fig. 2d). Fig. 2 Further enlarged HRTEM image shows a mass of crystal defects of 2D PdIr PNSs (as marked by yellow circles in Fig. 2e) , which may perform
as possible catalytical active sites due to their coordinative unsaturations. The lattice spacing of 2D PdIr PNSs is measured to be 0.249 nm (Fig. 2f), which can be indexed to the (111) plane of the face-centered cubic (fcc) PdIr alloy. Fig. 2g-h show the element distribution of the products by using EDX line-scan and mapping images. Both measurements evidence the uniform distribution of the Pd and Ir elements throughout the whole nanosheets. At this stage, all the above results indicative the successful assembly of ultrafine 0D PdIr NCs at 2D manner with high surface areato-volume ratio and porosity, specifically remaining the inherent structural characteristics of ultrafine 0D building blocks. Fig. 3 From the energy-dispersive X-ray (EDX) spectrum, it verifies that the Pd/Ir atomic ratio is about 49.4: 50.6, which is almost identical to the 1:1 stoichiometry of the precursor solution, suggesting the complete reduction of metal precursors (Fig. 3a). X-ray diffraction (XRD) pattern of the 2D PdIr PNSs is shown in Fig. 3b. Three distinct diffraction peaks at 2θ = 40.4, 68.7, and 82.8° could be observed, corresponding to (111), (220), and (311) planes of PdIr, respectively (PDF No. 65-7457). FT-IR spectrum of the 2D PdIr PNSs clearly shows the stretching vibration of C-N and C-H groups, which originated from the spared PDDA on the surface of 2D PdIr PNSs (Fig. 3c). The XPS survey spectrum of 2D PdIr PNSs from 0~1200 eV further confirms the existence of PDDA, where displays a distinct
characteristic peak of N 1s (Fig. 3d). The PDDA with well water-solubility is not only propitious to the uniform coverage of as-tested catalysts on glassy carbon electrode, but also beneficial for even better activity and durability based on the synergistic contribution between soft PDDA and hard exposed PdIr lattice facets [33]. As such, it is not necessary to completely remove the PDDA. Meanwhile, the Pd/Ir atom ratio on the surface is measured to be 47.3:52.7, well matching the atom ratios analyzed from EDX pattern. From the high-resolution XPS spectra at Pd 3d and Ir 4f regions (Fig. 3e-f), it is demonstrated that both the percentage of Pd0 species and Ir0 species are in the majority, indicating that Pd2+ and Ir3+ precursors are successfully reduced to metallic states in 2D PdIr PNSs. The locations of binding energies are related to Pd0 3d (3d5/2 = 334.5 eV; 3d3/2 = 339.8 eV) and Ir0 4f (4f7/2 = 60.9 eV; 4f5/2 = 63.9 eV), respectively, which corresponding to the standard values of metallic Pd and metallic Ir species [34]. Fig. 4 We conducted a series of contrast experiments to unravel the growth and assembly mechanism of the 2D PdIr PNSs. Firstly, we elucidated the explicit role of PDDA as the research object. The products prepared in the absence of PDDA displays an irregular shape with serious agglomerations, confirming the importance of PDDA in conducting the nucleation and growth in this solution (Fig. S3, supplementary data). The PDDA with a
positively charged long-chain polyelectrolyte could readily bound the negatively charged PdCl42- to generate the PDDA-PdII complex (Fig. 4a) [35]. Fig. 4b shows the UV-vis spectrum of K2PdCl4, which exhibits typical absorption peaks at 300 and 420 nm. When PDDA was added to the K2PdCl4 solution, the characteristic absorption peak at 420 nm disappeared and the characteristic absorption peak at 300 nm showed an obviously shift, indicating the electrostatic interaction between cationic PDDA and negatively charged PdCl42-. However, form the UV-vis spectrum of IrCl3 and IrCl3+PDDA mixture, it presents nonexistent electrostatic interaction between PDDA and Ir3+ (Fig. S4, supplementary data). That is to say, the formation of 2D PdIr PNSs may be attributed to using the PDDA-PdII complex as reaction precursor. TEM images of the monocomponent products using the standard protocol were conducted to further confirm this point (Fig. S5, supplementary data). Pure Ir nanoparticles and pure Pd PNSs could be observed, well corresponding to the strong electrostatic interaction between PDDA and PdCl42-. Moreover, by changing the Pd/Ir ratio from 1:2 to 2:1 in the precursor, we get the 2D Pd2Ir1 NSs and 0D Pd1Ir2 nanoparticles (Fig. S6, supplementary data), further confirming the major interactions were derived from PDDA-PdII complex. Since the standard electrode potential of Pd2+/Pd0 (Eq = 0.951 V) is far from that of Ir3+/Ir0 (Eq = 1.156 V), we simulated the real condition to test the reduction powers of Pd2+ and Ir3+ in the existence of PDDA (Fig.
4c). Obviously, the reduction potential of Pd2+ solution (-0.195 V) and Ir3+ solution (-0.248 V) are quite similar. Such closing reduction potentials can be responsible for the co-reduction of PdCl42- and Ir3+, thus forming the PdIr alloy. At this stage, we can get the conclusion that the formation of 2D PdIr PNSs is mainly attributed to the coordination effect of PDDA-PdII complex and similar reduction potential of Pd2+ and Ir3+ in the presence of PDDA. TEM images of intermediate products were carried out to investigate the morphology evolution by collecting them at different reaction intervals. The product collected at 1 h was dominantly composed of tiny 0D NCs (Fig. 4d). At this stage, the 0D NCs were capped with positively charged PDDA, the van der Waals attractions would be weakened. When the reaction process proceeded to 2 h, these 0D NCs tended to be close to each other spontaneously (Fig. 4e). By further increasing the reaction time to 3 h (Fig. 4f), spare 2D structure could be observed with a number of dispersed 0D building blocks. At this stage, the electrostatic repulsions between positive-charged 0D NCs were able to balance each other, meanwhile preventing the 3D particle assembly in the solution. When the reaction came to 5 h (Fig. 4g), the 2D self-assembled PdIr superstructures could be achieved as a result of thermodynamic balance in this solution [36, 37]. According to the aforementioned explanations, it is reasonable to deduce that the formation of 2D PdIr PNSs might undergo two process: (1) Formation of ultrathin 0D PdIr NCs based
on the co-reduction of Pd2+ and Ir3+. (2) Generation of 2D PdIr PNSs determined by the driving forces of internal balance for the self-assembly process. Fig. 5 The HER and OER performance of 0D PdIr NCs and 2D PdIr PNSs can be systematically compared by ignoring the complication of surface contamination. Commercial Pt/C and RuO2 were also served as reference in the following discussion. From the HER polarization curves displayed in Fig. 5a, the onset overpotential and overpotential at -10 mA cm-2 were found to be 13 and 40, 25 and 260, 8 and 19 mV for 2D PdIr PNSs, 0D PdIr NCs, and Pt/C, respectively. It reveals that the HER activity of 2D PdIr PNSs is slightly lower than that of commercial 20% Pt/C but much better than 0D PdIr NCs, as well as better than some of the HER catalysts reported to date (Table S1, supplementary data). The Tafel slope of 2D PdIr PNSs was measured to be 59.2 mV per decade, which is smaller than that of 0D PdIr NCs (106.9 mV per decade), indicating the more rapid HER kinetics at 2D PdIr PNSs (Fig. 5b). The ADTs were tested to evaluate the HER durability of 2D PdIr PNSs. After 1000 CV cycles, the 2D PdIr PNSs (Fig. 5c) exhibit almost unchanged overpotential while the 0D PdIr NCs display a rapid decay (Fig. S7a, supplementary data), indicative the robustness of PdIr at 2D manner. The OER catalytic activity of 2D PdIr PNSs were assessed in the same alkaline electrolyte with the reference of
0D PdIr NCs and commercial RuO2. The 2D PdIr PNSs exhibit a remarkably higher current density and lower onset overpotential, outperforming those of the other two catalysts and most of the state-of-art OER catalysts (Table S2, supplementary data). Meanwhile, the 2D PdIr PNSs require an overpotential of only 285 mV to reach 10 mA cm-2 with a tafel plot of 76.1 mV dec−1, which is much lower than those of 0D PdIr NCs and commercial RuO2 (Fig. 5d-e), indicating the excellent electrontransfer facilitation within the 2D freestanding nanosheets. The OER durability of 2D PdIr PNSs was tested by ADTs (Fig. 5f), which shows almost unchanged overpotentials after 1000 CV cycles. As a comparison, the 0D PdIr NCs (Fig. S7b, supplementary data) show an obvious delay of activity. At this stage, we can get the conclusion that both the HER and OER stability of 2D PdIr PNSs are much better than 0D PdIr NCs. This result is attributed to the unique structure of 2D PdIr PNSs, which endow suitable void space between each 0D PdIr building blocks. As such, the 2D PdIr PNSs could hold less susceptible to dissolution, agglomeration, and Ostwald ripening compare to 0D and 1D counterparts [22, 38-40]. Moreover, the HER and OER performance of intermediate products collected at different reaction time were performed to investigate the size effect of nanosheet on catalytic performance (Fig. S8, supplementary data). The catalytic activity of intermediate products shows an obvious enhancement along with the increasment of reaction time. Overall, the 2D
PdIr PNSs, which collected at 5 h, exhibit the lowest onset overpotential and the most rapid reaction kinetics. This result could be attracted to the self-assembly process of nanosheets, which adhere PdIr building blocks to promote the electron transfer rate. The electrochemical double-layer capacitance Cdl was employed to evaluate the electrochemically active surface area of 2D PdIr PNSs (Fig. S9, supplementary data). The Cdl value was calculated to be 7.19 mF cm−2, which exhibited a large electrochemically active surface area of 2D PdIr PNSs. All these above results highlight the superior HER and OER performance of 2D PdIr PNSs, not only endow excellent activity but also exhibit superior stability against the 0D PdIr building blocks. The HER and OER performance of pure Ir NCs, pure Pd NSs, Pd1Ir2 NCs, Pd2Ir1 NSs were also measured for the component-dependent study (Fig. S10 and S11, supplementary data). The 2D PdIr PNSs show both the best HER and OER catalytic performance among all samples, including a sharp onset potential, a small overpotential reaching 10 mA cm-2, and a lower tafel slope. Fig. 6 The histograms of HER and OER onset overpotentials and overpotentials to achieve a current density of 10 mA cm-2 for different electrocatalysts are listed in Fig. 6a-b. It indicates that both the HER and OER performance herein follow the order of 2D PdIr PNSs >pure Ir NCs> Pd1Ir2 NCs> Pd2Ir1 NSs >pure Pd NSs. Due to the intrinsic superior activity
of Ir, the HER and OER activity of component-dependent products gradually shows an enhancement along with the increasing content of Ir. Combining with TEM image of each component-dependent product, the 2D porous structure is unable to be maintained while the Ir content is much excrescent than Pd, thus leading to a definite loss of catalytic activity. Accordingly, the OER and HER results are well matching the tunable composition and morphology of PdIr products, where sufficient Ir content guarantees the catalytic activity and moderate Pd content is served to maintain the 2D structures for boosting catalytic activity and stability. Furthermore, the histograms of HER and OER onset overpotentials and overpotentials to achieve a current density of 10 mA cm-2 for 2D PdIr PNSs, 0D PdIr NCs and commercial catalysts are displayed in Fig. 6c-d. Apparently, self-stabilizing 0D PdIr NCs at 2D manner can further enhance their HER and OER activity and durability. Specifically, (1) the synergy between Ir and Pd make certain contributions to the improved catalytic activities due to the modification of Ir electronic structure; (2) the unabridged 0D PdIr building blocks are well remained in 2D PdIr PNSs, which exhibit sufficient accessible active sites and high atom utilization efficiency [5, 41]; (3) distinguished from conventional 2D nanosheets with smooth and intact surface, the highly open and porous structure of 2D PdIr PNSs is advantageous to facilitate mass diffusion and transport during the HER and OER reaction, which in turn promotes the reaction kinetics [42];
(4) the self-tractive 0D PdIr building blocks with suitable void space are less susceptible to dissolution, agglomeration, and Ostwald ripening compare to 0D and 1D counterparts [43, 44].
4. Conclusion To conclude, we have demonstrated an efficient hydrothermal route to synthesize and self-stabilize ultrathin 0D PdIr NCs at 2D manner, creating a new class of freestanding 2D porous PdIr nanosheets with large surface area-to-volume ratio and enriched surface atoms. The PDDAdriven electrostatic balance is essential for such successful 2D assembly, which guarantees the atom utilization efficiency of 0D PdIr building blocks and further improve electron transport and the robustness. From the dimensional-dependent and component-dependent study for both HER and OER in alkaline medium, the 2D PdIr PNSs exhibit excellent catalytic activity and stability exceeding the 0D PdIr NCs, other component-based products and most of the reported bifunctional electrocatalysts (Tables S1 and S2, supplementary data). Such manipulation and reintegration of ultrafine 0D NCs based on electrostatic balance can offer new opportunities for constructing various 2D porous structure, which may find huge research values for widespread catalysis applications.
Acknowledgements
This work was financially supported by National Natural Science Foundation of China (21902078, 51872140, 21875112), Natural Science Foundation of Jiangsu Higher Education Institutions of China (19KJB150033), and Natural Science Foundation of Jiangsu Province (BK20171473). The authors were also grateful for the support from the National and Local Joint Engineering Research Center of Biomedical Functional Materials and a project sponsored by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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Figure Captions Fig. 1. Different assembly configurations of 0D PdIr building blocks. (a) 1D nanowires, (b) schematic illustration for the formation of 2D PdIr PNSs, and (c) 3D nanopolyhedrons. Fig. 2. Morphology representation of the 2D PdIr PNSs. (a) HAADFSTEM image, (b)-(c) Typical TEM images, (d)-(f) HRTEM images, (g) EDX line-scan profile, and (h) EDX elemental mapping images. Fig. 3. Structural and compositional examination of the 2D PdIr PNSs. (a) EDX pattern, (b) XRD pattern, (c) FT-IR spectra, (d) XPS survey spectrum of PdIr nanosheets. High-resolution XPS survey spectrum at (e) Pd 3d region, (f) Ir 4f region. Fig. 4. Mechanism understanding for the formation of 2D PdIr PNSs. (a) 3D geometrical structure of PdCl42--PDDA complex. (b) UV-vis spectra of PDDA solution, Cl4 solution, K2PdCl4 +PDDA solution, and K2PdCl4 + IrCl3 +PDDA solution. (c) Linear sweeping voltammograms of N2saturated 0.005 M K2PdCl4 + 0.01 M PDDA + 0.5 M KCl solution (blue line) and 0.005 M IrCl3 + 0.01 M PDDA + 0.5 M KCl solution (red line) at the glassy carbon electrode. Scan rate: 100 mV s-1. (d)-(g) Typical TEM images of the intermediate products collected at 1 h, 2 h, 3 h, 5 h, respectively. Fig. 5 (a) HER linear sweep voltammetry curves of the 2D PdIr PNSs, 0D PdIr NCs, and Pt/C, respectively. (b) Corresponding Tafel plots. (c) HER
polarization curves of the 2D PdIr PNSs recorded before and after 1000 cycles at a scan rate of 100 mV s−1. (d) OER linear sweep voltammetry curves of the 2D PdIr PNSs, 0D PdIr NCs, and RuO2, respectively. (e) Corresponding Tafel plots. (f) OER polarization curves of the 2D PdIr PNSs recorded before and after 1000 cycles at a scan rate of 100 mV s −1. The electrolyte used in all above conditions is 1.0 M KOH solution. Fig. 6 (a) Histograms of HER onset overpotentials and overpotentials to achieve a current density of 10 mA cm-2 for 2D PdIr PNSs, pure Ir NCs, pure Pd NSs, Pd1Ir2 NCs, and Pd2Ir1 NSs, respectively. (b) Histograms of OER onset overpotentials and overpotentials to achieve a current density of 10 mA cm-2 for 2D PdIr PNSs, pure Ir NCs, pure Pd NSs, Pd1Ir2 NCs, and Pd2Ir1 NSs, respectively. Histograms of (c) HER onset overpotentials and overpotentials to achieve a current density of 10 mA cm-2, and (d) OER onset overpotentials and overpotentials to achieve a current density of 10 mA cm-2 for 2D PdIr PNSs, 0D PdIr NCs, Pt/C, and RuO2 respectively.
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Credit Author Statement Anzhou Yang: Methodology, Formal analysis, Writing - Original Draft Keying Su: Investigation Shangzhi Wang: Investigation Yingzi Wang: Validation Xiaoyu Qiu: Conceptualization, Writing - Review & Editing, Funding acquisition Wu Lei: Resources, Project administration, Funding acquisition Yawen Tang: Supervision, Funding acquisition
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Graphical Abstract
Zero-dimensional PdIr nanoalloys self-assembed to porous nanosheets with the help of capping agent PDDA. The PdIr PNS shows excellent OER and HER catalitic activities.
Highlights 1. A simple but efficient hydrothermal route was used to synthesize and self-stabilize ultrathin 0D PdIr nanocrystals into freestanding 2D porous PdIr nanosheets (PdIr PNSs). 2. The 2D PdIr PNSs possessed large surface area-to-volume ratio and high density of exposed atoms. 3. The 2D PdIr PNSs delivered both excellent HER and OER activities and stabilities.