Self-assembly of Pt nanocrystals into three-dimensional superlattices results in enhanced electrocatalytic performance for methanol oxidation

Guangran Xu a, Jiayin Liu a, Baocang Liu *ab and Jun Zhang *ab
aCollege of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, P.R. China. E-mail: cebcliu@imu.edu.cn; cejzhang@imu.edu.cn; Fax: +86 471 4995400; Tel: +86 471 4995400
bInner Mongolia Key Lab of Nanoscience and Nanotechnology, Inner Mongolia University, Hohhot 010021, P.R. China

Received 17th August 2018 , Accepted 4th October 2018

First published on 8th October 2018


Because of the collective effects emerging from integrated nanostructures, more and more research endeavors have been dedicated to realizing the self-organization of nanostructures into complex integrated architectures with multiple functionalities. Thus, the self-organization of nanostructures into higher-order superstructures is becoming an attractive theme in materials research of nanoscience. Herein, we develop a simple and low-temperature solution approach without the need of any preformed Pt seeds to directly realize a series of three-dimensional (3D) Pt nanocrystal superlattices (NSLs) composed of well-defined interior Pt nanocrystals assembled into 3D face-centered cubic (fcc) superlattice structures. The effects of experimental parameters including solvents, surfactants and structure-directing agents on the assembly behavior, structural configuration, and morphological arrangement of 3D Pt NSLs are systematically studied. Through modulating the experimental parameters, the self-organization of 3D Pt NSLs can be elegantly controlled, and an in-depth mechanism depicting the pathway for Pt nanocrystals self-organized into various 3D Pt NSLs is proposed. The optimal 3D Pt NSLs used as an efficient electrocatalyst exhibit excellent catalytic performance with CO-tolerant catalytic ability and long-term stability for the methanol oxidation reaction (MOR) with a mass activity of 403 mA mgPt−1, which is 2 times higher than that of a commercial 20% Pt/C electrocatalyst (200 mA mgPt−1). This work provides a pathway to realize robust higher-order 3D Pt NSLs with enhanced electrocatalytic performance for the MOR. The long-term stability and the CO-tolerant catalytic ability of 3D Pt NSLs are anticipated to lead to an ideal system with relevance to applications in the renewable energy field.


Introduction

Platinum (Pt) has received increasing research interest in recent decades because of its wide applications in catalysis, sensing, electrochemical energy storage, biomedical diagnosis, therapy, etc.1,2 However, the commercial development of Pt electrocatalysts is hindered by their sluggish kinetics, instability, and easy poisoning by intermediates during operation. To overcome these limitations, rationally adjusting the structure, composition, shape, and self-organized geometry of Pt nanocrystals may provide a new strategy for designing innovative Pt-based electrocatalysts with exotic properties.3–6

In recent years, the development of structurally controllable nanomaterials has made great progress; for instance, nanoclusters,7–9 nanowires,10,11 nanorods,12–14 nanopolyhedra15–17 and three-dimensional nanomaterials18 have been widely reported. Impressively, 3D superlattices are promising in the field of nanomaterials,19 as a result of their excellent electronic properties,20 along with controllable structural properties by controlling their shape, size, and composition.19,21 The interparticle interactions contribute to collective properties in physics, chemistry, and catalysis, which are greatly distinguished from isolated nanoparticles.22 The external operational factors during synthesis have been found to have a large influence on the arrangement of nanostructures23 and post-assembly strategies synthesizing nanocrystal superlattices have been employed, such as interface-assisted assembly,24–27 selective-adsorption,28 and the template and external field-driven method,29,30 which considerably improve the ability to fabricate and use these superstructures. However, realizing highly ordered Pt superstructures on a large scale with a simple method is still a big challenge.

In fact, the direct synthesis and assembly of nanocrystals into superlattices starting from interactions of nanocrystals and organic ligands in solution is still very difficult because of the weaker molecular recognition during the self-assembly process. Actually, the operational factors, especially the kind of organic solvent and surfactant, are very important for the formation of high-quality 3D superlattices. It is well known that nanocrystals are easy to disperse in solvents, so as long as the appropriate conditions are available, the nanocrystals can self-assemble into a regular arrangement. Commonly, if the nanocrystals are “dissolved” in appropriate organic solvents, the nanocrystals tend to self-assemble into a regular arrangement, that is superlattices, along with solvent evaporation.31 Meanwhile, the surfactants on nanocrystal surfaces can act as structure-directing agents and induce the organization of nanocrystals via the interaction of nanocrystals. However, profound understanding of the influences of these operational factors on the self-assembly behaviors of Pt nanocrystals into structurally controllable superstructures is quite demanded to deeply explore the relationship between their physicochemical characteristics and electrocatalytic performance for possible applications in green energy sustainable technologies. In this aspect, the studies of well-organized/self-assembled Pt electrocatalysts with varied electrocatalytic performance derived from their structural or morphological variations are very important.

In this paper, we report a simple and low-temperature solution approach without the need of any preformed Pt seeds for direct synthesis of three-dimensional (3D) Pt nanocrystal superlattices (NSLs). The synthesis of 3D Pt NSLs is conducted in a special device named a Wattecs parallel autoclave system (WPAS) that allows the reaction to take place in a vacuum or an inert N2 atmosphere under steady stirring following a solvothermal operation. The effects of various experimental parameters including solvents, surfactants and structure-directing agents on the self-assembly process of Pt NSLs are systematically explored. By modulating the above mentioned experimental parameters, a series of Pt 3D NSLs with various morphologies can be successfully obtained and their catalytic performances for the methanol oxidation reaction (MOR) are quite different depending on their variable interparticle interactions derived from the morphological variation of the 3D superstructures. The optimal 3D Pt NSL electrocatalyst exhibits excellent catalytic performance with CO-tolerant catalytic ability and long-term stability for the methanol oxidation reaction (MOR) with a mass activity of 403 mA mgPt−1, which is 2 times higher than that of a commercial 20% Pt/C electrocatalyst (200 mA mgPt−1). The synthetic method offers the advantages one-step solution-based synthesis, low temperature, easy operation, and feasible morphological modulation to facilitate the availability of 3D Pt NSLs for their property optimization. The results advance the understanding of the formation mechanism of 3D Pt NSLs and afford the opportunity for improving their functionalities for possible electrocatalytic applications.

Experimental section

Chemicals

Platinum acetylacetonate (Pt(acac)2, 98%), N,N-dimethylformamide (DMF), formyldiethylamine (FDEA), dimethylacetamide (DMAA), acetylacetone (AcAc), polyvinylpyrrolidone (PVP), hexadecyl trimethyl ammonium bromide (CTAB), polyethylene oxide–polypropylene oxide–polyethylene oxide (P123), hydroxypropyl cellulose (HPC), ascorbic acid (AA), NaCl, NaBr, and NaI were all obtained from Acros. All chemical reagents were of analytical (AR) grade. All aqueous solutions were prepared using ultrapure water.

Synthesis of Pt superstructures with a WPAS

The synthesis of Pt superstructures was performed with a Wattecs parallel autoclave system (WPAS), which was similar to a normal solvothermal synthesis but allowed the reaction to take place in a N2 atmosphere under constant stirring (Scheme S1). The magnetite microspheres were prepared according to the following method.32 Typically, 76.18 mg of Pt(acac)2 and 100 mg of PVP (CTAB, P123, or HPC in other cases) were dissolved in 6 mL of DMF (FDEA, DMAA, or AcAc in other cases) in a small vial with stirring for 30 min to form a clear yellow solution. Then, 60 mg of AA and 500 mg of NaBr (NaI or NaCl) were dissolved in 0.5 mL of ultrapure water. Finally, the above solutions were transferred quickly to a 10 mL small vial under continuous stirring. After the small vial was sealed, the obtained solution was further stirred for 30 min at room temperature. The sealed small vial was degassed by vacuum pumping and purging N2 through a Schlenk line of the WPAS three times. After the sealed small vial was fixed on the inner-reactor module, the inner-reactor module was sealed in the stainless steel autoclave of the WPAS. The space between the vial and stainless steel autoclave was degassed by vacuum pumping and filled with 200 psi N2 gas. The autoclave was heated at 120 °C in the WPAS for 4 h under continuous stirring. After cooling to room temperature, the precipitate was collected by centrifugation (12[thin space (1/6-em)]000 rpm for 15 min). The obtained product was then washed once with acetone and several times with ethanol and ultrapure water (1[thin space (1/6-em)]:[thin space (1/6-em)]1) to remove excess PVP and inorganic ions and dried in a vacuum to obtain dry powder.

To investigate the effects of the amount of H2O, PVP surfactant, and NaBr structure-directing agent on the morphological variation of Pt nanocrystals, the amount of H2O, PVP, and NaBr was adjusted from 0, 0.5, 1, 2 to 4 mL, 0, 50, 100 to 200 mg, and 0, 100, 250 to 500 mg, respectively, during the synthetic process.

Characterization

The morphologies of the synthesized nanomaterials were characterized using a field emission transmission electron microscope (FE-TEM, FEI Tecnai F20). Chemical-state analysis of the fabricated nanomaterials was carried out using an X-ray photoelectron spectroscope (XPS, VG Scientific ESCALAB Mark II) equipped with two ultrahigh vacuum (UHV) chambers.

Electrochemical measurements

Prior to the surface coating, a glassy carbon electrode (GCE) (3.0 mm in diameter) was polished with 1.0, 0.3, and 0.05 mm α-Al2O3 powder, respectively, then ultrasonically rinsed with ethanol, acetone and ultrapure water and then dried in N2 at room temperature. To prepare a catalyst-coated working electrode, 5 mg of catalyst was dispersed in a mixture of solvents containing 25 μL of 0.05% Nafion solution, 250 μL water, and 250 μL ethanol. 2 μL catalyst ink was deposited onto the polished electrode which was dried under vacuum conditions to finish the surface coating. As the contrast experiment, a commercial Pt/C catalyst was prepared in the same way.

Cyclic voltammetric (CV) measurements were carried out with a CHI 750D electrochemical workstation, and the electrochemical properties of the prepared electrocatalysts were studied in a standard three-electrode electrolytic cell, in which a GCE, Ag/AgCl and a Pt wire were used as the working electrode, reference electrode and counter electrode, respectively. The electrodes were immersed in nitrogen-saturated 0.5 M H2SO4 solution at 25 °C. Before each experiment, the electrode potential was scanned from −0.2 to 1.0 V vs. Ag/AgCl at a scan rate of 100 mV s−1 until a stable voltammogram was obtained. The scan was repeated several times to ensure that a stable cyclic voltammogram (CV) was obtained. After potential cycling, CVs for the MOR were obtained from −0.2 V to 1.0 V vs. Ag/AgCl at a scan rate of 50 mV s−1 in nitrogen-saturated 0.5 M H2SO4 + 1.0 M CH3OH solution. The amperometric current density–time (it) curves were obtained at a fixed potential for 3600 s under the same test conditions as above. For each catalyst, the test results were normalized to the loading of Pt to obtain the mass activity. All experiments were conducted at room temperature.

Results and discussion

The synthesis process of Pt nanocrystals with various assembled superstructures is shown in Scheme 1. It is found that regulation of the experimental parameters of solvents, surfactants and structure-directing agents during the synthetic process permits the formation of Pt nanocrystals with various self-assembled superstructures including 3D Pt NSLs (DMF or FDEA as solvents, PVP as a surfactant, and NaBr as a structure-directing agent), spherical Pt superstructures, (DMAA or AcAc as solvents, PVP as a surfactant, and NaBr and NaI as structure-directing agents), Pt nanorings (DMF as a solvent, HPC as a surfactant, and NaBr as a structure-directing agent), branched Pt nanocrystals (DMF as a solvent, PVP as a surfactant, and NaCl as a structure-directing agent), monodispersed Pt nanocrystals (DMF as a solvent, CTAB or P123 as surfactants, and NaBr as a structure-directing agent), and aggregated Pt nanocrystals in an irregular state (DMF as a solvent, PVP as a surfactant, and NaI as a structure-directing agent).
image file: c8ce01382a-s1.tif
Scheme 1 Schematic illustration of the formation of 3D Pt nanocrystal superstructures obtained by solvent-mediated, surfactant-induced, and structure-directing-agent-modulated self-assembly processes using DMF, FDEA, DMAA, and AcAc as solvents, PVP, CTAB, P123, and HPC as surfactants, and NaCl, NaBr, and NaI as structure-directing agents.

As shown in Fig. 1a, 3D Pt NSLs with average sizes in the range of 80–100 nm in a cubic self-assembled superstructure are obtained when employing DMF as a solvent, PVP as a surfactant, and NaBr as a structure-directing agent for the synthesis. The magnified TEM images shown in Fig. 1b and c illustrate that the 3D Pt NSLs possess higher-order 3D superlattices, which are assembled from well-defined interior Pt nanocrystals with an average size of around 5 nm. The HRTEM image displayed in Fig. 1d gives lattice distances of 0.197, 0.226, and 0.139 nm that correspond to the (200), (111) and (220) crystal planes, further confirming that the 3D Pt NSLs are indeed assembled from single Pt nanocrystals. The appearance of Pt nanocrystals formed in 3D superlattices suggests that a self-assembly behaviour occurs during the synthetic process.


image file: c8ce01382a-f1.tif
Fig. 1 TEM and HRTEM images of 3D Pt NSLs synthesized in various solvents. (a–d) DMF, (f–i) FDEA, (k–n) DMAA and (p–s) AcAc. (e, j, o and t) Molecular structures of DMF, FDEA, DMAA, and AcAc, respectively.

To explore the formation mechanism of 3D Pt NSLs, the solvents DMF, FDEA, DMAA, and AcAc are utilized for the synthesis for comparison. The DMF has a molecular configuration with a central nitrogen atom connected by two methyl groups to form a formamido group as displayed in Fig. 1e. The formed formamido group can be hydrolyzed into a carboxyl group under the action of H2O, and the oxygen atoms on the carboxyl group have a certain adsorption effect on Pt particles,33 which may play an important role in determining the nucleation and growth of Pt nanocrystals. In comparison, the FDEA has a similar molecular structure to DMF and can form the same formamido group but connected by two ethyl groups with longer chains, which may cause the discrepancies on the morphological appearance of 3D Pt NSLs with a larger assembled superstructure during the self-assembly process. As demonstrated in the TEM image of Fig. 1f, similar 3D Pt NSLs composed of well-defined interior Pt nanocrystals can be also obtained when keeping other experimental parameters identical but using FDEA as a solvent. The magnified TEM image shown in Fig. 1g verifies a trend of self-assembled regular stacking structures, but differently, the accumulation structure assembled from Pt nanocrystals shows an obviously increased gap. The regularity of the stacking structure is partially destroyed (Fig. 1h). The lattice distances of 0.197 and 0.226 nm that correspond to the (200) and (111) crystal planes of Pt are clearly discerned, suggesting the formation of well-defined Pt nanocrystals.

By comparing the molecular structures of the two amino solvents DMF and FDEA (Fig. 1e and j), we found that the two solvents contain the same formamido group, which can be hydrolyzed into a carboxyl to adsorb metallic Pt ions during the synthetic process and thus to influence the nucleation and growth of Pt nanocrystals. It is well known that because the atom or group near the reaction center in molecules occupies a certain space position, the molecular reaction activity is influenced by the so-called space resistance effect.34 Due to the difference in the molecular chain length of methyl and ethyl attached to the central N atom in solvents DMF and FDEA, when the formamido group reacts with metallic Pt ions, the space resistance effect caused by the longer molecular chain of the ethyl group attached to the central N atom also affects the nucleation and growth of Pt nanocrystals, thus resulting in the discrepancies in assembly behaviors for 3D Pt NSLs.

Similarly, we also conducted the synthesis in another amino-containing solvent DMAA with two methyl groups connected to a nitrogen atom linked to the acetamido group with two carbon atoms (Fig. 1o). Fig. 1k–n display the TEM images of 3D Pt superstructures obtained in the DMAA solvent. A great change is observed in the assembly behaviour and morphological evolution of 3D Pt superstructures when performing the reaction in the DMAA solvent. As seen in Fig. 1k–m, the particle size of Pt nanocrystals is around 3–5 nm and most of them are assembled into a spherical aggregated structure. The lattice distances of 0.197 and 0.226 nm are well indexed to the (200) and (111) crystal planes of Pt, indicating the formation of well-defined Pt nanocrystals.

By comparing the structural formula of the two solvents DMF and DMAA, we notice that the two solvent molecules have the same –N–(CH3)2 functional groups but the other ends are varied and connected with formamido and acetamido groups, respectively. As mentioned above, with the help of H2O existing in the solvents, the C[double bond, length as m-dash]O bond of the amide group existing in the form of an aldehyde group in DMF can be further hydrolyzed into a carboxyl group, which has strong electronegativity and can be adsorbed on the surface of Pt seeds, thus posing a great impact on the nucleation and growth of Pt nanocrystals. Conversely, the C[double bond, length as m-dash]O bond of the amide group existing in the form of a keto-group in DMAA can be hardly hydrolyzed, thus showing little impact on the self-assembly of Pt nanocrystals. Therefore, the results show that the amide-group-containing molecules in solvents play a decisive role in forming the Pt superstructures in our synthetic system. This can also be proven by the formation of spherical Pt superstructures in the AcAc solvent. As displayed in Fig. 1p–s, the spherical Pt superstructures are obtained in the AcAc solvent, which are assembled from Pt nanocrystals with particle sizes of 8–10 nm.

As it is proposed above that the presence of H2O is very important for hydrolysis of the aldehyde group into a carboxyl group, we thus explored the effect of the amounts of H2O existing in the synthetic system on the morphological evolution of 3D Pt superstructures. The formation of Pt superstructures with different morphologies is indeed dependent on the hydrolysis of the aldehyde group with the help of H2O. The hydrolysis chemisorption of DMF at the surface of Pt nanocrystals can supply the electron and hydronium ion. The aldehydes can be transformed into carboxylates with the help of the adsorbed oxygen supplied by the activated H2O at nearby sites. With the presence of the hydronium ion, the carboxylate can be further transformed into carbamic acid, and then desorbed from the surface of nanocrystals to promote the reaction. Therefore, the assistance of H2O can facilitate the assembly of Pt nanocrystals.35

Due to the importance of H2O, the influence of H2O consumption on the formation of 3D Pt superstructures was studied. The 3D Pt superstructures obtained in the DMF solvent in the presence of different volumes of H2O are illustrated in Fig. 2. It is found from Fig. 2a–d that when H2O is absent in the synthetic system, monodispersed Pt nanocrystals with diameters of 5–8 nm are obtained and no obvious self-assembly behaviours are observed. However, when the amount of H2O is added at 0.5 mL in the synthetic system, the 3D Pt NSLs assembled from well-defined Pt nanocrystals are obtained (Fig. 1a–d). As the amount of water increased to 1 mL, the magnified TEM image shown in Fig. 1g verifies a trend of self-assembled regular stacking structures, but differently, the accumulation structure assembled from Pt nanocrystals shows an obvious disorder. Further increasing the amount of H2O, the self-assembly behaviours may be limited and higher-ordered 3D Pt NSLs are partially reduced (Fig. 2i–p). This corroborates the proposition that the presence of H2O in the synthetic system plays a crucial role in the formation of 3D Pt NSLs.


image file: c8ce01382a-f2.tif
Fig. 2 TEM and HRTEM images of Pt superstructures prepared in the presence of different amounts of H2O: (a–d) 0 mL, (e–h) 1 mL, (i–l) 2 mL, and (m–p) 4 mL.

It is well known that surfactants as capping agents are often introduced to control the nucleation and growth of Pt nanocrystals and further to inhibit the aggregation of the nanocrystals,35–38 so we investigated the influences of surfactants with various structural conformations on the nucleation and growth of Pt nanocrystals. Scheme 2 shows three typical conformations of surfactants that may affect the morphological variation of Pt nanocrystals due to their adsorption ability on various surfaces of Pt nanocrystals. Linear long-chain hydrocarbons (CTAB) stand in an array on nanocrystal surfaces with a tilt angle (Scheme 2A). Unbranched polymers (PVP and P123) entangle nanocrystals with partial functional groups attached onto their surface (Scheme 2B). Branched polymers and dendrimers (HPC) encapsulate nanocrystals in their inner void space (Scheme 2C). Thus, it is predicted that the arrangement of Pt nanocrystals may be largely varied when using these surfactants with different spatial conformations as capping agents.40


image file: c8ce01382a-s2.tif
Scheme 2 Spatial conformation of surfactants on Pt nanocrystal (blue ball) surfaces: (A) linear long-chain hydrocarbons, (B) unbranched polymers, and (C) branched dendrimeric polymers. Red triangles represent the anchoring sites of surfactants on nanocrystal surfaces.

To explore the effects of surfactants on the morphological variations of Pt nanocrystals, we employed PVP, CTAB, P123, and HPC for the synthesis. The TEM images of Pt superstructures obtained using these surfactants are presented in Fig. 1a–d and 3. When using PVP as a surfactant, the well-defined 3D Pt NLSs are obtained as described above (Fig. 1a–d). However, when altering the surfactant to CTAB, large chunks of Pt nanocrystals with disordered and agglomerated features are obtained (Fig. 3a–d). Interestingly, when using P123 as a surfactant, monodispersed Pt nanocrystals with a diameter of around 10 nm are achieved (Fig. 3e–h). As a contrast, when using HPC as a capping agent, Pt nanocrystals with a diameter of 5 nm assembled into a nanoring structure are achieved (Fig. 3i–l). Thus, we find that the surfactants play an important role in the formation and self-assembly of Pt nanocrystals in our synthetic system. The molecular structures of surfactants PVP, CTAB, P123, and HPC have large effects on the morphological variations of Pt nanocrystals.


image file: c8ce01382a-f3.tif
Fig. 3 TEM and HRTEM images of Pt nanocrystals prepared with various surfactants: (a–d) CTAB, (e–h) P123, and (i–l) HPC.

Comparing the molecular structures of the surfactants used in the synthetic system, it is found that the CTAB is a long-chain molecule (Scheme 3A), and the sites interacting with the surface of Pt nanocrystals are at the end of molecular chains. So the combination behaviours of CTAB with Pt nanocrystals are linear long-chain hydrocarbons (Scheme 2A). However, the molecular chains of CTAB are relatively short, which makes the interaction of CTAB with Pt nanocrystals relatively weak. So Pt nanocrystals can continue to grow into a disordered stacking structure of large size nanocrystals (Fig. 3a–d). In contrast, P123 is a non-ionic surfactant, and its sites adsorbed on the surface of Pt nanocrystals are mainly polar functional groups located on the molecular chain (Scheme 3B). P123 can be effectively adsorbed onto the surface of Pt seeds and has a strong inhibitory effect on the growth of Pt nanocrystals. Thus, monodispersed Pt nanocrystals can be obtained (Fig. 3e–h). HPC belongs to a branched macromolecular polymer (Scheme 3C). Its molecular chain has a large number of polar functional groups that can be adsorbed onto the surface of Pt nanocrystals, so that Pt seeds can adsorb onto the HPC at the initial stage of nucleation. However, due to the complex molecular chain structure of HPC, Pt nanocrystals adsorbed onto the molecular chains can hardly arrange closely, resulting in the special nanoring as shown in Fig. 3i–l. In contrast, a large number of ordered C[double bond, length as m-dash]O bonds exist in PVP (Scheme 3D), and it has been reported that the interaction between PVP and Pt nanocrystals is caused by the coordination between the C[double bond, length as m-dash]O bond on PVP and Pt nanocrystals. Pt nanocrystals can be sequentially adsorbed onto PVP chains during the nucleation process, which makes Pt nanocrystals efficiently and regularly adsorbed onto PVP molecular chains after the nucleation, providing the possibility and basis for ordered self-assembled arrangement of Pt nanocrystals.39,40


image file: c8ce01382a-s3.tif
Scheme 3 Molecular structures of (A) CTAB, (B) P123, (C) HPC, and (D) PVP.

From the above analysis, it is found that 3D Pt NSLs with a uniform size distribution are obtained in the presence of PVP surfactant with a molecular weight of 1[thin space (1/6-em)]300[thin space (1/6-em)]000. To study the influence of the amounts of PVP on Pt superstructures, we performed the experiments by adjusting the amounts of PVP. Fig. 4 displays the TEM images of Pt superstructures obtained with different amounts of PVP. It is found that due to the lack of protection of the PVP surfactant, only Pt nanocrystals with a particle size of ∼5 nm are obtained when no PVP is used in the synthesis and the Pt nanocrystals show obviously random arrangement and aggregation (Fig. 4a–d). When a small amount of PVP (50 mg) is added, Pt nanocrystals are obviously different from those obtained without PVP (Fig. 4e–h). Pt nanocrystals are found to be preliminarily arranged in an ordered and self-assembled manner, showing a superlattice structure. Interestingly, 3D Pt NSLs with a uniform size distribution and highly ordered arrangement are obtained when the addition of PVP is controlled at 100 mg (Fig. 1a–d). However, when the amount of PVP is further increased (200 mg), Pt nanocrystals rather than 3D Pt NSLs are formed in a disordered aggregation manner (Fig. 4i–l). This is because that too much PVP adsorbed on the surface of Pt nanocrystals may result in a more prominent physical barrier function, which has a negative impact on the ordered self-assembly of Pt nanocrystals.


image file: c8ce01382a-f4.tif
Fig. 4 TEM and HRTEM images of Pt nanocrystals synthesized with different amounts of PVP with a molecular weight of 1[thin space (1/6-em)]300[thin space (1/6-em)]000. (a–d) 0 mg, (e–h) 50 mg, and (i–l) 200 mg.

From the above discussion, the solvents and surfactants have important influences on the morphological variation and self-assembly of Pt nanocrystals in our synthetic system. As a large number of reports indicated that structure-directing agents such as halogen ions also have significant influences on the morphological evolution.41,42 In addition, the morphological appearance of nanocrystals mainly depends on the relative growth velocity of each crystal surface during the growth. At present, most synthetic methods mainly control the growth rate of the crystal surface by adding halogen anions (such as Cl, Br, and I), which have strong adsorption on each crystal surface.43 Halogen ions are often used to inhibit the excessive growth and aggregation of nanocrystals and to control the structural characteristics of nanocrystals in precise ways.44Fig. 5 shows Pt nanocrystals obtained using NaCl, NaBr, and NaI as structure-guiding agents. It is clear that under the same conditions, different halogen anions have obvious effects on the morphological variations of Pt nanocrystals.


image file: c8ce01382a-f5.tif
Fig. 5 TEM and HRTEM images of Pt nanocrystals synthesized using different halogen ions as structure-directing agents: (a–d) NaCl and (e–h) NaI.

It has been reported that the ability of halogen anions to form an adsorption layer on precious metal surfaces follows the order Cl < Br ≪ I.45,46 The adsorption capacity of Cl on various crystal surfaces of Pt is weak and unable to form a relatively dense barrier adsorbed layer, which cannot effectively limit the growth of certain crystal planes. Additionally, according to the Wulff theory, the growth rate of each crystal surface is different.47 Therefore, various crystal surfaces of Pt nanocrystals grow freely according to the crystal lattice energy and, finally, the Pt nanocrystals are aggregated to branched Pt nanocrystals (Fig. 5a–d). Meanwhile, due to the strong interaction of I on the crystal surface of Pt nanocrystals, the presence of a certain amount of I in the solution can form a dense adsorption layer on a particular surface.48 Therefore, there is almost no difference in the growth speed of various crystal surfaces of Pt nanocrystals, and the Pt nanocrystals are eventually aggregated to disordered stacking spherical nanostructures (Fig. 5e–h). What's more, the studies of the adsorption of I ions on the Pt surface indicate that the interaction between I and Pt causes a large amount of negative charge to exist on the surface of Pt. This greatly hinders the spread and adsorption of DMF and PVP, leading to a negative effect on the self-assembly of Pt nanocrystals. This proves that the surface adsorption of DMF and PVP on the Pt surface plays a significant role in the self-assembly of Pt nanocrystals into higher-ordered superlattices, so the existence of I has a negative effect on the self-assembly of superlattices. When the Br ions exist in the solution, because the adsorption ability of Br on nanocrystals is different from that of Cl and I, the Br forms a suitable adsorption layer to control the reduction of Pt2+ on the specific crystal surface of Pt and regulate the relative growth rate of different crystal surfaces. It has no negative effect on the adsorption of DMF and PVP on the Pt nanocrystals, providing the preconditions of forming 3D Pt NSLs (Fig. 1a–d).

Fig. 6 displays the representative TEM images of the gradient experimental results on the amount of NaBr. It can be seen from Fig. 6a–d that no Pt nanocrystals can self-assemble into 3D superlattices without the addition of a structure-guiding agent.


image file: c8ce01382a-f6.tif
Fig. 6 TEM and HRTEM images of Pt nanocrystals synthesized with different various amounts of NaBr: (a–d) 0 mg, (e–h) 100 mg, and (i–l) 250 mg.

Instead, monodispersed Pt nanocrystals with a diameter of about 5 nm are obtained. When slight addition of NaBr (100 mg) is introduced into the synthetic system, monodispersed Pt nanocrystals can be also observed (Fig. 6e); however, these monodispersed Pt nanocrystals tend to assemble into a superstructure in a single short-range ordered arrangement (Fig. 6f–h). It is noteworthy that along with the increase of the amount of NaBr (250 and 500 mg), monodispersed Pt nanocrystals eventually self-assemble into 3D superlattices, indicating that Br adsorbed on the surface of Pt nanocrystals interacts with other particles in the system and induces their self-assembly into a superlattice structure (Fig. 6i–l and 1a–d).

Owing to their attractive structural features, the electrocatalytic performance of Pt superstructures with various assembled morphologies as electrocatalysts for the methanol oxidation reaction (MOR) was evaluated by electrochemical cyclic voltammetry (CV) and chronoamperometry measurements. As a contrast, the commercial 20% Pt/C electrocatalyst was also examined. Fig. 7A shows the CV curves of all types of Pt electrocatalysts recorded in 0.5 M H2SO4 from −0.2 V to 1.0 V at a sweep rate of 50 mV s−1. The signals appearing in the potential region of −0.2 to 0.05 V can be well-defined as the peaks for hydrogen adsorption/desorption, and the signals in the ranges of 0.05 to 0.35 V and 0.35 V to 0.8 V correspond to the double-layer region and metal oxidation/reduction peaks, respectively. Fig. 7B presents the typical CV curves of different Pt electrocatalysts for the MOR measured in 0.5 M H2SO4 + 1 M CH3OH solution. Due to the reactivation of Pt associated with the removal of the residual carbon species, the MOR activity is assessed by the numerical value of the anodic peak current in the forward scan. As the two major parameters evaluate the catalytic performance, the onset potential of methanol oxidation and the forward anodic peak potential are assessed.49,50 The mass catalytic activity of the Pt electrocatalyst with a 3D superlattice structure as reflected by the peak current densities after being normalized to the Pt loading is 403 mA mgPt−1, which is about 2 times higher than that of the commercial 20% Pt/C electrocatalyst (200 mA mgPt−1). Fig. 7C is the graphical comparison of the mass activities of all Pt electrocatalysts. Furthermore, the long-term stability and tolerant ability to CO intermediates are evaluated by the chronoamperometry measurements performed in 0.5 M H2SO4 + 1 M CH3OH solution as shown in Fig. 7D. Because of the hydrogen adsorption and the double-layer discharge, the current densities of all Pt electrocatalysts attenuate rapidly at the initial stage. And the adsorption of the CO on the electrocatalyst surface during methanol electro-oxidation is believed to cause the subsequent attenuation.51 Among the different Pt electrocatalysts, the Pt electrocatalyst with a 3D superlattice structure exhibits the slowest current decay over time, suggesting the excellent electrocatalytic performance.


image file: c8ce01382a-f7.tif
Fig. 7 Electrocatalytic performance of (a) 3D Pt NSLs, (b) 3D spherical Pt superstructures, (c) Pt monodispersed nanocrystals, (d) bulk monodispersed Pt nanocrystals and the commercial 20% Pt/C catalysts. (A) CV curves recorded at room temperature in a N2-purged 0.5 M H2SO4 solution with a sweep rate of 100 mV s−1, (B) mass activity, (C) graphical comparison of mass activities of all catalysts, and (D) stability of all catalysts. The methanol oxidation was performed in 0.5 M H2SO4 + 1.0 M CH3OH solution at a scan rate of 50 mV s−1. The inset is a schematic of Pt nanostructures with representative morphologies and superstructures.

The discrepancies in the electrocatalytic performance of the Pt superstructures with different representative morphologies may be attributed to their surface composition and valence state; we thus investigated their surface characteristics with the XPS technique. The XPS scans of Pt 4f for 3D Pt NSLs and 3D spherical Pt superstructures show identical peaks with binding energies at 70.25 and 73.58 eV assigned to Pt0 4f7/2 and 4f5/2 (Fig. 8A(a and b)), while the XPS spectra of Pt 4f for monodispersed Pt nanocrystals display doublet peaks at 70.48 and 73.85 eV ascribed to Pt0 4f7/2 and 4f5/2 (Fig. 8A(c and d)). The binding energies of Pt 4f have a smaller negative shift about 0.2 eV for monodispersed Pt nanocrystals compared with the 3D Pt NSLs and 3D spherical Pt superstructures, indicating the changes in the electronic structure and density around Pt atoms. The shifts of XPS spectra are usually relevant to the changes in the d-band center, Fermi energy and work function.52 Moreover, the metallic Pt0 is the predominant species and the high ratio of metallic Pt0 is crucial for high catalytic performance, which is in consistency with our electrocatalytic measurements.53 The surface atomic ratios of Pt0/Pt2+ at 0.83[thin space (1/6-em)]:[thin space (1/6-em)]0.17 and 0.8[thin space (1/6-em)]:[thin space (1/6-em)]0.2 for 3D Pt NSLs and 3D spherical Pt superstructures are higher than those at 0.79[thin space (1/6-em)]:[thin space (1/6-em)]0.21 and 0.76[thin space (1/6-em)]:[thin space (1/6-em)]0.24 for the two kinds of monodispersed Pt nanocrystals (Fig. 8B). The above results may imply the enhancement of catalytic activity for Pt superstructures with well-defined self-organized configurations.


image file: c8ce01382a-f8.tif
Fig. 8 (A) XPS spectra of Pt 4f of (a) 3D Pt NSLs, (b) 3D spherical Pt superstructures, (c) monodispersed Pt nanocrystals and (d) bulk monodispersed Pt nanocrystals; (B) the surface elemental compositions of Pt0 and Pt2+.

Conclusions

In summary, unique bifunctional Pt electrocatalysts with superior electrocatalytic activities for the MOR were developed via a facile one-pot method with a Wattecs parallel autoclave system under hydrothermal conditions in an inert atmosphere, which allowed the regulation of the surface elemental composition and valence state of Pt electrocatalysts. The synergistic electronic effects among metal elements would modulate the surface electronic structures of Pt and thus enhance the electrocatalytic activity. Due to their particular structure and valence composition, the Pt electrocatalysts show a remarkable enhancement for the MOR with superior electrocatalytic activity and robust durability, even better than those of a commercial 20% Pt/C catalyst. This study opens up a new avenue for the design of novel highly-efficient bifunctional electrocatalysts for use in water-splitting, fuel cells and other renewable energy technology fields.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the NSFC (21661023 and 21601096), the Application Program from Inner Mongolia Science and Technology Department (2016), the Program of Higher-level Talents of IMU (21300-5155105), and the Cooperation Project of State Key Laboratory of Baiyun Obo Rare Earth Resource Researches and Comprehensive Utilization (2017Z1950).

Notes and references

  1. S. Zhang, S. Guo, H. Zhu, D. Su and S. Sun, J. Am. Chem. Soc., 2012, 134, 5060–5063 CrossRef CAS.
  2. N. Kakati, J. Maiti, S. H. Lee, S. H. Jee, B. Viswanathan and Y. S. Yoon, Chem. Rev., 2014, 114, 12397–12429 CrossRef CAS.
  3. Z. Quan, Y. Wang and J. Fang, Acc. Chem. Res., 2013, 46, 191–202 CrossRef CAS PubMed.
  4. S. Guo and E. Wang, Nano Today, 2011, 6, 240–264 CrossRef CAS.
  5. L. Zhang, W. Niu and G. Xu, Nano Today, 2012, 7, 586–605 CrossRef CAS.
  6. D. Wang and Y. Li, Adv. Mater., 2011, 23, 1044–1060 CrossRef CAS.
  7. A. D. Allian, K. Takanabe, K. L. Fujdala, X. Hao, T. J. Truex, J. Cai, C. Buda, M. Neurock and E. Iglesia, J. Am. Chem. Soc., 2011, 133, 4498–4517 CrossRef CAS.
  8. S. I. Sanchez, M. W. Small, J.-m. Zuo and R. G. Nuzzo, J. Am. Chem. Soc., 2009, 131, 8683–8689 CrossRef CAS.
  9. A. D. Allian, K. Takanabe, K. L. Fujdala, X. Hao, T. J. Truex, J. Cai, C. Buda, M. Neurock and E. Iglesia, J. Am. Chem. Soc., 2012, 134, 743–743 CrossRef CAS.
  10. C. Zhu, S. Guo and S. Dong, Adv. Mater., 2012, 24, 2326–2331 CrossRef CAS.
  11. J. Lai, L. Zhang, W. Qi, J. Zhao, M. Xu, W. Gao and G. Xu, ChemCatChem, 2014, 6, 2253–2257 CrossRef CAS.
  12. S. Maksimuk, S. Yang, Z. Peng and H. Yang, J. Am. Chem. Soc., 2007, 129, 8684–8685 CrossRef CAS.
  13. Y. Wang and H. Yang, J. Am. Chem. Soc., 2005, 127, 5316–5317 CrossRef CAS.
  14. N. N. Kariuki, W. J. Khudhayer, T. Karabacak and D. J. Myers, ACS Catal., 2013, 3, 3123–3132 CrossRef CAS.
  15. X. Huang, H. Zhang, C. Guo, Z. Zhou and N. Zheng, Angew. Chem., Int. Ed., 2009, 48, 4808–4812 CrossRef CAS.
  16. B. Lim, J. Wang, P. H. C. Camargo, C. M. Cobley, M. J. Kim and Y. Xia, Angew. Chem., 2009, 121, 6422–6426 CrossRef.
  17. A.-X. Yin, X.-Q. Min, W. Zhu, H.-S. Wu, Y.-W. Zhang and C.-H. Yan, Chem. Commun., 2012, 48, 543–545 RSC.
  18. Y. W. Li, J. L. Hart, M. L. Taheri and J. D. Snyder, ACS Catal., 2017, 7, 7995–8005 CrossRef CAS.
  19. D. V. Talapin, J.-S. Lee, M. V. Kovalenko and E. V. Shevchenko, Chem. Rev., 2010, 110, 389–458 CrossRef CAS.
  20. Y. Nagaoka, O. Chen, Z. Wang and Y. C. Cao, J. Am. Chem. Soc., 2012, 134, 2868–2871 CrossRef CAS.
  21. S. Eyheramendy, F. I. Martinez, F. Manevy, C. Vial and G. M. Repetto, Nat. Commun., 2015, 6, 6472 CrossRef CAS.
  22. D. L. Keeling, N. S. Oxtoby, C. Wilson, M. J. Humphry, N. R. Champness and P. H. Beton, Nano Lett., 2003, 3, 9–12 CrossRef CAS.
  23. G. Singh, H. Chan, A. Baskin, E. Gelman, N. Repnin, P. Kral and R. Klajn, Science, 2014, 345, 1149–1153 CrossRef CAS.
  24. T. Ming, X. Kou, H. Chen, T. Wang, H.-L. Tam, K.-W. Cheah, J.-Y. Chen and J. Wang, Angew. Chem., Int. Ed., 2008, 47, 9685–9690 CrossRef CAS.
  25. A. R. Tao, J. Huang and P. Yang, Acc. Chem. Res., 2008, 41, 1662–1673 CrossRef CAS.
  26. C.-Y. Chiu, C.-K. Chen, C.-W. Chang, U. S. Jeng, C.-S. Tan, C.-W. Yang, L.-J. Chen, T.-J. Yen and M. H. Huang, J. Am. Chem. Soc., 2015, 137, 2265–2275 CrossRef CAS.
  27. M. H. Huang and S. Thoka, Nano Today, 2015, 10, 81–92 CrossRef CAS.
  28. G. A. DeVries, M. Brunnbauer, Y. Hu, A. M. Jackson, B. Long, B. T. Neltner, O. Uzun, B. H. Wunsch and F. Stellacci, Science, 2007, 315, 358–361 CrossRef CAS.
  29. Z. Yang, J. Wei, P. Bonville and M.-P. Pileni, J. Am. Chem. Soc., 2015, 137, 4487–4493 CrossRef CAS.
  30. C. Zhang, R. J. Macfarlane, K. L. Young, C. H. J. Choi, L. Hao, E. Auyeung, G. Liu, X. Zhou and C. A. Mirkin, Nat. Mater., 2013, 12, 741–746 CrossRef CAS.
  31. J. R. Heath, Science, 1995, 270, 1315 CrossRef CAS.
  32. G. Xu, J. Liu, B. Liu, X. Gong, S. Wang, Q. Wang and J. Zhang, CrystEngComm, 2017, 19, 7322–7331 RSC.
  33. C. Gumeci, A. Marathe, R. L. Behrens, J. Chaudhuri and C. Korzeniewski, J. Phys. Chem. C, 2014, 118, 14433–14440 CrossRef CAS.
  34. Z.-Q. Lin, J. Liang, P.-J. Sun, F. Liu, Y.-Y. Tay, M.-D. Yi, K. Peng, X.-H. Xia, L.-H. Xie, X.-H. Zhou, J.-F. Zhao and W. Huang, Adv. Mater., 2013, 25, 3664–3669 CrossRef CAS.
  35. E. Roduner, Chem. Soc. Rev., 2006, 35, 583–592 RSC.
  36. I. Lee, R. Morales, M. A. Albiter and F. Zaera, Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 15241–15246 CrossRef CAS PubMed.
  37. M. Sankar, N. Dimitratos, P. J. Miedziak, P. P. Wells, C. J. Kiely and G. J. Hutchings, Chem. Soc. Rev., 2012, 41, 8099–8139 RSC.
  38. J. W. Hong, D. Kim, Y. W. Lee, M. Kim, S. W. Kang and S. W. Han, Angew. Chem., Int. Ed., 2011, 50, 8876–8880 CrossRef CAS.
  39. Z. Niu and Y. Li, Chem. Mater., 2014, 26, 72–83 CrossRef CAS.
  40. Y. Borodko, S. E. Habas, M. Koebel, P. Yang, H. Frei and G. A. Somorjai, J. Phys. Chem. B, 2006, 110, 23052–23059 CrossRef CAS.
  41. B. Wu and N. Zheng, Nano Today, 2013, 8, 168–197 CrossRef CAS.
  42. M. Rycenga, C. M. Cobley, J. Zeng, W. Li, C. H. Moran, Q. Zhang, D. Qin and Y. Xia, Chem. Rev., 2011, 111, 3669–3712 CrossRef CAS.
  43. Y. Xia, Y. Xiong, B. Lim and S. E. Skrabalak, Angew. Chem., Int. Ed., 2009, 48, 60–103 CrossRef CAS.
  44. A. Carrasquillo, J.-J. Jeng, R. J. Barriga, W. F. Temesghen and M. P. Soriaga, Inorg. Chim. Acta, 1997, 255, 249–254 CrossRef CAS.
  45. P. Diao, J. Wang, D. Zhang, M. Xiang and Q. Zhang, J. Electroanal. Chem., 2009, 630, 81–90 CrossRef CAS.
  46. T. H. Ha, H.-J. Koo and B. H. Chung, J. Phys. Chem. C, 2007, 111, 1123–1130 CrossRef CAS.
  47. I. Pastoriza-Santos and L. M. Liz-Marzán, Adv. Funct. Mater., 2009, 19, 679–688 CrossRef CAS.
  48. M. Yamada, S. Kon and M. Miyake, Chem. Lett., 2005, 34, 1050–1051 CrossRef CAS.
  49. Z. Zhang, Y. Wang and X. Wang, Nanoscale, 2011, 3, 1663–1674 RSC.
  50. J. Xu, X. Liu, Y. Chen, Y. Zhou, T. Lu and Y. Tang, J. Mater. Chem., 2012, 22, 23659–23667 RSC.
  51. L.-X. Ding, A.-L. Wang, G.-R. Li, Z.-Q. Liu, W.-X. Zhao, C.-Y. Su and Y.-X. Tong, J. Am. Chem. Soc., 2012, 134, 5730–5733 CrossRef CAS PubMed.
  52. Y. Hu, A. Zhu, Q. Zhang and Q. Liu, J. Power Sources, 2015, 299, 443–450 CrossRef CAS.
  53. J. Zheng, D. A. Cullen, R. V. Forest, J. A. Wittkopf, Z. Zhuang, W. Sheng, J. G. Chen and Y. Yan, ACS Catal., 2015, 5, 1468–1474 CrossRef CAS.

Footnote

Electronic supplementary information (ESI) available: Diagram of the WPAS. See DOI: 10.1039/c8ce01382a

This journal is © The Royal Society of Chemistry 2019