Surface Confinement Etching and Polarization Matter: A New

Jul 10, 2017 - One of the important objectives in fuel-cell technology is to improve the activity and reduce the loading of Pt for hydrogen-evolution ...
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Surface Confinement Etching and Polarization Matter: A New Approach To Prepare Ultrathin PtAgCo Nanosheets for HydrogenEvolution Reactions Azhar Mahmood, Haifeng Lin, Nanhong Xie, and Xun Wang* Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing, 100084, China S Supporting Information *

ABSTRACT: One of the important objectives in fuel-cell technology is to improve the activity and reduce the loading of Pt for hydrogen-evolution electrocatalysis. Here, an oxidative etching strategy of stacking faults is developed to prepare PtAgCo nanosheets by element-specific anisotropic growth. Sophisticated use of defects in crystal growth allows tailoring the morphology and interfacial polarization to improve catalytic performance of nanosheets for the hydrogen-evolution reaction. Systematic studies reveal that the presence of the stacking faults may be the knob for the formation of nanosheets. In particular, the chemical composition of nanosheets is potentially the key for altering the hydrogen-evolution reaction. As a result, the PtAgCo-II ultrathin nanosheets possess useful HER properties, achieving a current density up to 705 mA cm−2 at a potential of −400 mV.

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INTRODUCTION With the depletion of fossil fuels, increasing global warming issues, and clean energy demands, hydrogen (H2) is being regarded as a highly efficient, clean, and environmentally friendly renewable energy source. Among various H2 generation techniques, the electrocatalytic hydrogen-evolution reaction (HER) is considered to be one of the most effective and promising techniques.1−6 With regard to the HER process, effective electrocatalysts with low overpotential and higher current density are needed. To date, platinum (Pt) is still considered the most active catalyst for HER. Unfortunately, the natural scarcity and high cost of Pt is an impediment against its widespread application in HER. Toward this end, the ultimate goal for the HER process is the designing to achieve higher active sites catalysts with low Pt contents.7−9 An alternative strategy to solve this problem is to alloy Pt with a less expensive metal by an engineering process to obtain the desirable exposed crystal facets at the surface of electrocatalysts.5 Therefore, fascinating shape evolution with enhanced catalytic properties is one key factor for the continuously growing research of Pt-based electrocatalysts.3,5 The enhanced catalysis of Pt-based multimetallic nanocrystals is attributed to the synergetic factors which mainly include strain effect, electronic effect, geometric effect, surface polarization, and other interfacial effects.9−12 Among the various shapes of Ptbased nanocrystals,13 the ultrathin two-dimensional (2D) nanosheets (NSs) shape is more promising, yet it has remained under-researched. Synthesis of ultrathin 2D nanosheets is still an enormous challenge. Owing to the distinctive ultrathin 2D © 2017 American Chemical Society

shapes with single or few atomic layers thickness and abundant active sites and high density of unsaturated atoms, NS in the sense of morphology may provide better catalytic activity.10,15 Along with conventional applications, NSs are considered promising for perpendicular magnetic recording media due to their ultrathin 2D features.15 Among alloy nanostructures, late transition metals such as Ni, Fe, and Co are used to form electrocatalytically active alloys with Pt.16,17 For example, in the case of the HER, Xiong et al. have demonstrated that the interfacial polarization between Pt and the other metals may induce the accumulation of negative charges on the Pt surface and facilitate the HER process.9,10 Considering the shape evolution factors, when the nuclei have grown to a critical size, they may evolve into seeds with different twin structures. Such structures may include multiply twinned, singly twinned, or stacking-fault-lined structures, which will determine the final products.15,18 We have already been successful in synthesizing 2D, PtAg ultrathin NSs through an oxidative etching of twin nanocrystals, where defects have turned into powerful means for the growth and final morphology of PtAg NSs. Due to this, PtAg NSs contained 1/3(422) reflections that were parallel to the basal (111) planes13 which confirmed the occurrence of stacking faults11 and further etching process deformation in the shape of PtAg NSs. It could be deduced that there is a driving force that stabilizes other shapes. One possible reason for such Received: April 18, 2017 Revised: July 5, 2017 Published: July 10, 2017 6329

DOI: 10.1021/acs.chemmater.7b01598 Chem. Mater. 2017, 29, 6329−6335

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Chemistry of Materials

played a crucial role. It is well-documented that the generating rate of nuclei can be controlled by reducing agents.19,20 We also noticed that the average size and shape of the PtAg NSs obtained in the presence of ascorbic acid (Figure 2a) are different if compared with studies, which did not utilize ascorbic acid (Figure S1), at the same experimental conditions. Thus, within the current study reported here, we carried out the synthesis of NSs without reducing agent and increased the reaction temperature and time compared to our previously reported work.13 The average size of PtAgCo NSs is slightly smaller compared to PtAg NSs. Therefore, ascorbic acid, reaction time, and temperature are differentiating factors of the current study compared to previous studies. The structure of the nanosheets is characterized by transmission electron microscopy (TEM), high-angle annular dark-field scanning TEM (HAADF-STEM), high-resolution TEM (HRTEM), and energy-dispersive X-ray (EDX) mapping images. This confirms the formation of NSs under the given conditions. As shown in Figure 1a, PtAgCo NSs have been successfully synthesized. The

deviation is the entry into kinetic control, instead of thermodynamic control, by the synthesis of PtAg NSs that has already been proven.13 The main question remains as how to get further control over NSs without changing the oxidative environment? In our reaction system, the oxidative etching is the main factor for the formation of PtAg nanosheets through twinned seeds. In general, defects zones of PtAg nanostructures are more susceptible to attack by active species.13 The results of the present synthesis have given us an insight into a novel method for constructing PtAgCo NSs with desirable performance, utilizing an oxidative etching growth strategy of staking faults. We have demonstrated this concept by measuring the HER of NSs. The clear advantage of this strategy is that Pt based alloy NSs activity can be tuned to higher values by alloying Pt with Co at the edges. The activity trend observed of HER demonstrates the beneficial effects of tuning edges and morphology of NSs, which can affect interfacial polarization coordination and strain of surface atoms, performing HER catalysis.9,10,12

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EXPERIMENTAL SECTION

Synthesis of PtAg Nanosheets. Ultrathin PtAg nanosheets synthesis was carried out by two-step method. First, a mixture of 50 mg of tris and 200 mg of PVP was dissolved in 2 mL of HCHO solution and transferred to Teflon-lined stainless steel autoclave which was heated at 200 °C for 3 h. A gel-like material was obtained, after the washing and centrifugation in acetone. Second, a homogeneous solution of 0.01 mmol of Pt(acac)2, 0.01 mmol of AgNO3, 0.06 g of KI, and 0.03 g of ascorbic acid was prepared in 2 mL of formamide solvent and poured into a 12 mL Teflon-lined stainless steel autoclave along with the gel-ike material which was prepared in the first step; the autoclave was then kept in the oven at 130 °C for 3 h. The final product was obtained after washing with acetone. Synthesis of PtAgCo Nanosheets. PtAgCo nanosheets synthesis was also carried out by a two-step method, except that Co(acac)2 precursor was added in the second step along with Pt(acac)2, and AgNO3 without reducing agent (i.e., ascorbic acid) for trimetallic nanosheets. A mixture of 50 mg of tris and 200 mg of PVP was dissolved in 2 mL of HCHO solution and transferred to Teflon-lined stainless steel autoclave. The autoclave was then heated at 200 °C for 3 h. A gel-like material was obtained, after the washing and centrifugation in acetone. Second, a homogeneous solution of 0.01 mmol of Pt(acac)2, 0.01 mmol of AgNO3, and Co(acac)2 along 0.06 g of KI was prepared in 2 mL of formamide solvent and poured into a 12 mL Teflon-lined stainless steel autoclave. The gel-like material which was prepared in the first step was also added. The autoclave was then kept in the oven at 150 °C for 6 h. The final product was obtained and washed with acetone. It is important to note that, for the preparation of trimetallic nanosheets with different concentrations of Co, we vary the cobalt concentrations (0.005, 0.01, 0.015 mmol). Synthesis of PtCo and AgCo Nanostructures. The synthesis procedure and experimental conditions for PtCo and AgCo nanostructures were identical to those applied to the PtAgCo nanosheets. Synthesis of PtAgCu Tetrapods and PtAgFe Multipods. The synthesis procedure and experimental conditions for PtAgCu tetrapods and PtAgFe multipods were identical to those applied to PtAgCo nanosheets, except that, in the second step for tetrapods and multipods, we added Cu(acac)2 and Fe(acac)2 precursor instead of Co(acac)2 precursor along with Pt(acac)2 and AgNO3.

Figure 1. (a) TEM, (b) HRTEM, (c) HAADF-STEM, line-scanning, and corresponding EDX mapping images of PtAgCo nanosheets.

HRTEM image taken for trimetallic NSs shows lattice spacings of 0.229 nm, which is matching well with the calculated value of {111}-planes (Figure 1b). While for PtAg NSs, the HRTEM and SAED pattern image showed lattice fringes with an interplanar spacing of 0.240 nm (Figure 2b,c), respectively, the result corresponded to 1/3(422) fringes of face-centered cubic (fcc). The observation of 1/ 3(422) fringes indicates that PtAg NSs have (111) as basal planes.11 The reasons for unique (111) stacking faults in PtAg NSs are: (i) positive heat of mixing of Pt−Ag alloy,21 (ii) different reduction kinetics of Ptn+ and Ag+ precursors,22 and (iii) large lattice mismatch (4.2%) that create miscibility gap between Pt and Ag alloy.23 As verified by an electrochemical Co-striping method, the exposed surface of PtAg NSs indeed has (111) as basal planes24−26 (Figure S2). The sophisticated use of defects allows the Co to be introduced and modify the surface electronic structure of as-prepared PtAgCo NSs. The result indicates that the following unique characteristics of PtAg NSs are inherited: (i) enhanced activity for HER, (ii) minimized Pt content by alloying with less expensive metal (Co), and (iii) edges of PtAgCo NSs becoming more thick, if compared to PtAg NSs due to the incorporation of Co.

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RESULTS AND DISCUSSION For a typical synthesis of PtAgCo NSs, the previous methodology was partly modified, and it was observed that reducing agent (ascorbic acid) and reaction temperature have 6330

DOI: 10.1021/acs.chemmater.7b01598 Chem. Mater. 2017, 29, 6329−6335

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Chemistry of Materials

Figure 2. (a) TEM, (b) HRTEM, and (c) SAED pattern of a single PtAg nanosheet (shown above), and (d) HAADF-STEM, linescanning, and corresponding EDX mapping images of PtAg nanosheets.

Interestingly, the edge thickness of PtAgCo NSs can be controlled by simply altering the ratio of Co to Pt and Ag precursors in our synthetic scheme. With increasing the amount of Co from 0.005 to 0.015 mmol, as determined by inductively coupled plasma mass spectroscopy (ICP-MS, Table S1), we have been able to tune the edge thickness of NSs. We name this series of ternary alloy NSs as PtAgCo-I, PtAgCo-II, and PtAgCo-III, where I, II, and III stand for 0.005, 0.01, and 0.015 mmol, respectively (Figure 3b−d). Further, the incorporation of Co has been proven by the vibrating sample magnetometer (VSM) technique (Figure 3e). Different from the PtAg NSs, due to the increasing concentration of Co, PtAgCo NSs are more ferromagnetic (Figure S3) due to the strong d-electron interaction between Co and Pt. The fcc structure of PtAgCo NSs is supported by their X-ray diffraction (XRD) pattern (Figure S4). Further, the chemical structure of nanosheets is characterized by X-ray photoelectron spectroscopy (XPS) to enable investigating the surface composition and oxidation states of elements (Figure S23). Upon achieving the controllable Co-doped synthesis of trimetallic NSs, we are in position to investigate why Co can be selectively deposited on the edges of NSs rather than in the center. To better understand the formation process of these unusual PtAgCo NSs, the intermediate products collected at different reaction durations were investigated by TEM and ICP-MS. We first performed experiments of PtAg NSs. The product obtained after 3 and 6 h reaction time at 150 °C was aggregated NSs as compared with product obtained after the same reaction time at 130 °C in the presence of ascorbic acid (Figure S5). It became apparent that, due to the presence of stacking faults in PtAg NSs (Figure 2b), the oxidative etching process has continued further and shapes of NSs have deformed. We assume that aggregation may be due to the ultrathin nature of NSs and the required seeds due to oxidative etching of Ag were not provided by the remaining precursors to maintain the lateral size of NSs. Therefore, in order to provide the required seeds, we have introduced Co, and interestingly, ultrathin NSs were obtained at 150 °C in the absence of ascorbic acid (Figure 1a). Furthermore, in the presence of

Figure 3. (a−d) TEM images of PtAg and PtAgCo nanostructures prepared with different molar ratios of Pt(acac)2/AgNO3/Co(acac)2: (a) 0.01:0.01, (b) 0.01:0.01:0.005, (c) 0.01:0.01:0.01, (d) 0.01:0.01: 0.015, where (e) the magnetic field dependence of magnetization (M− H) curve at 300 K for PtAg and PtAgCo nanostructures prepared with different molar ratios of Pt(acac)2/AgNO3/Co(acac)2: (PtAg) 0.01: 0.01, (PtAgCo-I) 0.01:0.01:0.005, (PtAgCo-II) 0.01:0.01:0.01, (PtAgCo-III) 0.01:0.01:0.015, and (PtAgCo-IV) 0.01:0.01:0.02.

ascorbic acid and Co, NSs get aggregated at 150 °C (Figure S6). This is due to the fact that, in the presence of reducing agents at high temperature, the generating rate of nuclei is higher and all the seeds are used before anisotropic growth of trimetallic NSs is formed.18 Therefore, we conducted the synthesis of trimetallic NSs without reducing agent if compared to bimetallic NSs. HAADF-STEM analysis (Figure 1c) showed that as-prepared PtAgCo NSs were largely composed of Pt, especially at the edges if compared to PtAg NSs (Figure 2d). The incorporation of the Co also took place at the edges of the PtAgCo NSs, while the contents of the Ag were less due to oxidative etching. This Pt enrichment at the edges of PtAgCo NSs compared to PtAg NSs can be observed by line scanning profiles (Figure 1c and Figure 2d). These results along with ICP-MS analysis have led us to hypothesize an element-specific anisotropic growth of the trimetallic nanosheets, where the Pt 6331

DOI: 10.1021/acs.chemmater.7b01598 Chem. Mater. 2017, 29, 6329−6335

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Chemistry of Materials and Ag reduce first, forming twinned nanoparticles, has already been reported by our group13 (Figure S7). Alloy nanoparticles were enriched in Pt, which is in line with Yang and coworkers.16 These particles were first transferred into multiple twinned multipods within the next few minutes (Figure S8), then to intermediate nanostructures (Figure S9), and finally into NSs (Figure 1a). The concentrations of Co in twinned particles and multipods were negligible, while, in the intermediate nanostructures, they were significantly lower and analogous to the under deposition potential (UDP) process. This slow reduction of Co, followed by a further reduction of Pt onto the (111) facets, is identical to what Strasser and co-workers have observed.17 The initial deposition of trace amounts of Co on the NSs can offer active sites at the edges. Such a process will lead to further Pt growth9 as well as assist in maintaining the shape of NSs. Due to this, when we add lower or higher amounts of Co, NSs start to aggregate (Figure 3b,d). Furthermore, we have analyzed the structure of trimetallic NSs with an appropriate concentration (0.01 mmol) of Co and by setting different reaction times at 150 °C (Figure S10). These results have led us to propose that UDP change in the reduction kinetics due to the coexistence of the precursors (Pt, Ag, and Co)14 and an element-specific anisotropic growth16,17 are the preferential deposition of Co on the edges of trimetallic NSs. Another reason for the preferential deposition of Co at the edges of the NSs is due to edge side faces. There is a correlation between edge side faces of the as-prepared NSs, which have higher surface energy than the basal planes.14,19 The third reason for the incorporation of the Co is carbon monoxide (CO). The utilized gel-like material was composed of formaldehyde. The decomposition of formaldehyde after longer reaction time at higher temperature may produce CO, which acts as a reducing agent for the precursors.11,13,17 On the basis of our previous work, CO binds strongly on the PtAg nanosheets (111) surfaces,13 and therefore, due to further oxidative etching of stacking faults, it confines the growth of nanosheets maximizing (111) surface exposure. CO that is generated from the decomposition of formaldehyde is thus considered essential to the anisotropic growth of PtAgCo NSs. It also becomes evident that crystallinity alteration relies on the type of foreign cations.27 In order to confirm the mechanism, we have applied this synthetic scheme to another metal (Cu). When Cu2+ is introduced, it is observed that crystallinity alteration takes place and it forms tetrapods (Figure S11) instead of NSs.13 While the coexistence of Co with Ag and Pt precursors in the reaction medium changes the reduction kinetics, mutual interactions have a modifying effect on the growth of NSs.14 To make sure we conducted additional experiments, in the absence of Ag precursor, PtCo spherical nanoparticles were obtained, while, in the absence of Pt, we were able to obtain an undefined morphology instead of NSs (Figure S12). Thus, it is reasonable to propose that the presence of stacking faults is the main cause of forming PtAgCo nanosheets, while CO facilitates the growth by strongly binding on (111) faces of sheets. To support such a proposition, a reaction was carried out with an oxidative etchant (iron nitrate) which removes the twins and the formation of stacking faults does not take place. The role of Fe(NO3)2 is pivotal, as the iron species reduces the net rate of precursors.28 It also removes the twins at the early stage of the reaction, by forming the single crystalline nanostructures (Figure S13), and in turn, the formation of NSs

does not takes place. Thus, the formation of PtAgCo NSs reported here undergoes an oxidative etching of stacking faults by element-specific anisotropic growth. At the same time, CO acts as a reducing agent which strongly binds on (111) faces and facilitates the growth. The proposed growth mechanism is schematized in Figure 4.

Figure 4. Schematic illustration of PtAg and PtAgCo nanosheets.

To evaluate the catalysis, this controllable synthesis of PtAgCo NSs offers an ideal platform for the HER studies. For the HER process, tailoring the Pt surface charge demonstrates that incorporation of Co induces more charge polarization,10,29 which well explains the increasing HER activity of PtAgCo NSs. The electrochemical performance of carbon supported NSs, AgCo nanostructures, and commercial Pt/C catalyst (JM, 20 wt % Pt) was investigated in a N2-saturated 0.5 M H2SO4 solution at 50 mV s−1. All the samples and commercial Pt/C catalyst were deposited on a glassy carbon (GC) electrode with the same loading. All potentials (overpotentials) were reported versus a reversible hydrogen electrode (RHE). Figure 5a illustrates the polarization curves of all the NSs samples and AgCo nanostructures with the commercial 20% Pt/C catalyst,

Figure 5. (a) Polarization curves for PtAg, AgCo, and PtAgCo nanostructures prepared with different molar ratios of Pt(acac)2/ AgNO3/Co(acac)2: (PtAg) 0.01:0.01, (AgCo) 0.01:0.01, (PtAgCo-I) 0.01:0.01:0.005, (PtAgCo-II) 0.01:0.01:0.01, and (PtAgCo-III) 0.01: 0.01:0.015 in reference to 20% Pt/C at the same total metal loading weights in a 0.5 M H2SO4 (aq) electrolyte. (b) The corresponding Tafel plots. Durability tests: (c) 20% Pt/C and (d) PtAgCo-II samples. 6332

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S19) to gain insights into the origin of the superior catalytic activity of PtAgCo NSs. It has been revealed that the catalyst approaches the optimal performance at an appropriate content of Co (0.01 mmol). To further study the relationship between catalytic activity and Co content in NSs catalysts, the NSs samples with different Co percents were studied (Figure 5a). It becomes evident that an appropriate concentration of Co (0.01 mmol) not only helps to maintain the structure of NSs (Figure 1a) but also induces significant charge polarization on Pt. All of these results are in line with already reported research outcomes by Xiang and co-workers in which they have explained theoretically and experimentally the interfacial polarization between Pt and other metals.9,10 Second, the surface structures (edge thickness and lateral size) of the PtAgCo NSs also contribute to the HER performance enhancement. Compared with PtAg NSs (Figures 3a and S20a), when due to the incorporation of the Co, the edge thickness of NSs increases (Figures 3b and S20b) and the current density of PtAgCo NSs also increases. In fact, the sample with 0.01 mmol concentration of Co generates PtAgCoII NSs with an appropriate edge thickness, and lateral size (Figures 3c and S20c) that displays an extremely large current density of 705 mA cm−2. Such density is even greater than that of commercial 20% and 40% Pt/C (Figure S21). Furthermore, at higher concentration of Co, the edge thickness of NSs increases (Figures 3d and S20d) with the decrease of lateral size and current density. This may be due to the static polarization effect, in which dynamic charge distribution is being disturbed by higher concentration of Co for HER.9 This polarization effect is being due to the incorporation of Co, which has been verified by performing VSM analysis. The magnetic field dependence of magnetization (M−H) curves at 300 K of PtAg and PtAgCo ultrathin NSs is used to investigate the heterogeneous spin states. As displayed in Figure 3e, the M− H curves confirmed the expected ferromagnetic nature of PtAgCo NSs. In contrast, weak ferromagnetism (Figure S3) in the virgin PtAg ultrathin NSs is showing that, due to the strong d-electron interaction of cobalt, surface polarization occurs in PtAgCo NSs.32 All of the Co-doped NSs possess better HER performances compared to PtAg NSs. Among them, PtAgCo-II exhibits the exceptionally high HER performance, confirming the successful introduction of the Co and its polarization effect.10,32 According to the existing literature, the edges of 2D NSs are considered to be active.14,19 It is found that a very large amount of element Pt is distributed at the edges/outer surface; meanwhile, the incorporation of the Co also takes place at the exterior of the NSs (Figure S22). It significantly modifies the electronic structure of Pt by the so-called electronic and ligand effects33 by alloying of Pt with Co atoms. This in turn leads to downshifts of the d-band centers of Pt with respect to the Fermi levels. According to d-band theory, it will facilitate the HER process by decreasing the adsorption of Hads on Pt.34,35 The reduced bonding strength of Pt-Hads on PtAgCo NSs is according to the Sabatier principle.36,37 Experimentally, we have verified it by XPS (Figure S23); the Pt 4f binding energy of the PtAgCo NSs has shifted, if compared with that of PtAg NSs. The positions of Pt 4f7/2 are 70.73 and 70.21 eV for PtAg and PtAgCo NSs, respectively, which could be attributed to the electronic interaction of Pt with Co and Ag atoms in biand trimetallic NSs. The down-shifting of the peak of trimetallic NSs compared with bimetallic NSs is implying a charge transfer from Co to Pt.38,39 As mentioned earlier, the formation of

acting as a reference sample. The current densities of all catalysts at the same potential are found to be in the ascending order of AgCo/C < PtAg/C < 20% Pt/C < PtAgCo-I/C < PtAgCo-III/C < PtAgCo-II/C with the increase of Co contents. The activity of all the Co-doped NSs samples significantly exceeds that of 20% Pt/C, while samples without Pt and Co have shown less activity than 20% Pt/C. Among the samples, the PtAgCo-II with the medium edge thickness and ratio of Co has displayed the highest activity with a large current density of 705 mA cm−2 at a potential of −400 mV. To explain the current density−composition relationship of NSs, we have collected cyclic voltammetry (CV) curves (Figure S14) and electrochemical impedance spectroscopy (EIS) plots (Figure S15). The Nyquist plots and CV represent the conductivities and surface areas of the NSs, respectively. The data suggest that the variation in the composition of Co has an impact on the conductivities and electrochemical surface areas of the NSs samples, which are in the following descending order: PtAgCo-II/C > PtAgCo-III/C > PtAgCo-I/C > 20% Pt/ C > PtAg/C. Thus, the altering rates of current densities by composition control should be related to the reaction process (i.e., charge transportation) occurring on the catalyst’s surface. Meanwhile, the excellent electrocatalytic performance of NSs is further confirmed by the Tafel slope, which is a significant parameter that is revealing hydrogen-evolution dynamics. As shown in Figure 5b, the Tafel plots vary from 27 to 35 mV dec−1 and are in inverse order to the current densities. Such kind of Tafel slopes trend of Pt-based alloys has been previously reported.10,30,31 This trend, as a reference in addition to the current density assessment, indicates a significant change of electronic effect by the incorporation of Co and demonstrates that the HER performance of NSs can be tuned by tailoring the contents of Co in the alloy lattices.10,30 Apart from the HER activity, the electrochemical stability of the samples is another important criterion for evaluating practical applications. For this assessment, we cycled the PtAgCo-II/C catalyst continuously for 5000 cycles in 0.5 M H2SO4 by repeating CV between 0.4 and 0.1 V versus RHE at 50 mV s−1. As displayed in Figure 5d, the negligible decline in the current density under cycling indicates that the ultrathin PtAgCo-II nanosheets have exceptional stability. This may give the catalyst a role within real applications. In contrast, the commercial 20% Pt/C shows relatively weak stability under the given conditions (Figure 5c). Furthermore, a chronoamperometric test of PtAgCo-II/C and 20% Pt/C catalysts was conducted under a static overpotential of 0.27 V over 7200 s (Figure S16), and we have found that asprepared PtAgCo-II NSs exhibited excellent stability. The aforementioned results indicates that our sample has an excellent durability compared with commercial 20% Pt/C. This better performance of PtAgCo-II/C catalyst could be due to the stable morphology and surface structure of the nanosheets during the reaction (Figure S17). Upon identifying the improved HER performance of cobalt-doped Pt-based alloys, we have pointed out the several reasons behind this process. The incorporation of Co into PtAg not only generates the trimetallic NSs but also tunes the contents of Pt and efficient activity; Pt atoms near the Co atom can arise more electrons compared with Ag, and have the potential to generate highly active sites.10,29 It has been further demonstrated that, in the trimetallic system, Co can induce more charge polarization on Pt than the case of the bimetallic PtAg and PtCo system. To ensure, additional studies were conducted (Figures S18 and 6333

DOI: 10.1021/acs.chemmater.7b01598 Chem. Mater. 2017, 29, 6329−6335

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Chemistry of Materials

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PtAgCo NSs takes place by the oxidative etching of surface confinement defects and formed (111) faces. In this regard, we have therefore proposed that Pt atoms adjacent to the defects can contribute to the enhanced HER activities, due to the lower coordination numbers.40 Another reason for better HER performance of PtAgCo-II NSs compared to PtAg NSs is attributed to the porous surface of ternary alloy NSs (Figure S24). The nanoporous structure of the catalyst offers more reaction sites and enhances the activity via the so-called nanoconfinement effect.41 Finally, as compared to the PtAg/C, the PtAgCoII/C catalyst possesses smaller charge transfer resistance (Figure S15), suggesting that a faster reaction rate and highly efficient charge transportation occur on trimetallic NSs.10,29,35 We have also applied PtAgCo-II/C catalyst to the methanol oxidation reaction (MOR). Due to the unique structure, in the case of MOR, the activity was also enhanced (Figure S25) relative to 20% Pt/C.

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CONCLUSION In summary, trimetallic PtAgCo alloy nanosheets with controllable compositions have been successfully synthesized by a new robust and efficient synthetic approach. Thus, it is possible to maximize the exposure of certain facets that exhibit better HER catalytic properties. In particular, the chemical composition of nanosheets is potentially the key for altering the hydrogen-evolution reaction. As a result, the PtAgCo-II ultrathin nanosheets possess useful HER properties, which achieve a current density up to 705 mA cm−2 at a potential of −400 mV. We envision that our present work would provide some fresh insights into the synthesis of ultrathin 2D nanosheets with enhanced electrocatalytic activities.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01598. Experimental details and characterization data (TEM, EDX mapping images, XRD, XPS, electrochemical measurement tests, and so forth) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xun Wang: 0000-0002-8066-4450 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (21431003, 21521091) and the China Ministry of Science and Technology under Contract of 2016YFA0202801.

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REFERENCES

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DOI: 10.1021/acs.chemmater.7b01598 Chem. Mater. 2017, 29, 6329−6335