Synthesis of Octapod Cu-Au BimetaIIic NanocrystaI with Concave Structure through

Zhen-Ni Wang | Qiang Li | Zhen-Huan Zhao | Xiao-Mei Tong

Abstract—We synthesized octapod Cu-Au bimetallic alloy with a concave structure by employing a replacement reaction between AuPPh3Cl and Cu nanocubes.Using the Cu nanocube as sacrificial templates,we have successfully generated high-active sites on alloy nanocrystals by carefully tuning the replacement reaction and growth.The key is to afford the proper concentration of AuPPh3Cl-TOP to the reaction solution.When the Au precursor with high concentration is injected into the galvanic replacement reaction,the growth dominated the process and hollowed octapod Cu-Au alloy was obtained.In contrast,when the concentration of the Au precursor is low,the replacement reaction can only take place at the nanocrystals,leading to generate Cu-Au nanocages.This work provides an effective strategy for the preparation of hollow bimetallic nanocrystals with high-active sites.

1.Introduction

In recent years,Cu nanostructures have found various applications in photonics,sensing,imaging,and catalysis due to the plasmonic effects and high catalytic activities.They are demonstrated to be active catalysts for many chemical reactions,such as the oxidation of benzyl alcohol,the reduction of CO2,and the synthesis of organic chemicals like drug-like bicyclopentanes[1]-[5].Compared with the noble metals,Cu nanostructures show great superiority because of the low cost and earth abundance.The activity and selectivity of Cu nanostructures are highly dependent on the exposed crystalline facets.For example,Cu (100) preferably converts CO2to CH4,and Cu (111) is more likely to transform CO2to methanol.However,the synthesis of nanostructured Cu with a high content of exposed active facets is a tough task.Present studies using Cu are mainly limited to the Cu nanocube because of its easy acquisition.Even though,the Cu nanocube still suffers from the poor instability to oxidation upon exposure to air or in solutions[6],[7].

Alloying Cu with stable metals is proved to be an effective strategy to improve the stability.Among various metals,Au is one of the most widely used due to the good stability,tunable optical properties,easy functionalization,and facile synthesis[8]-[11].Once alloyed,some unexpected properties are raised,like the improved reaction selectivity,stability,and coupled plasmonic resonance,leading to the wide applications in the fields of catalysis and sensors[12]-[14].

Nanocrystals with surface atomic steps,ledges,and kinks have more low-coordinated surface atoms and therefore possess enhanced catalytic activity than those enclosed by flat surfaces[15]-[18].Therefore,the hollowed structure may have better catalytic performance due to the atomic steps,ledges,and kinks on the surface and the much higher specific surface area.In catalysis applications,hollowed structures show expected remarkable catalytic performance.As a typical example,hollowed Pt3Ni nanoframes exhibit better oxygen reduction activity than Pt nanoparticles[19].Therefore,there is a strong motivation to develop an efficient approach for the preparation of bimetallic hollow nanocrystals.

Recently,Pd nanoframes with well-defined structures have been successfully prepared by dedicatedly tuning the rates of the oxidative etching and the regrowth of the corner,edge,and face of Pd nanocrystals[20].Taking the Pd nanoframes as an inspiration,based on the versatile etching and galvanic replacement mechanism in the synthesis of bimetallic structures,we successfully prepared hollow Cu-Au bimetallic nanocrystals.In this work,AuPPh3Cl was used as the Au precursor to improve the solubility in oleylamine as well as to decrease the reduction potential of Au(I) in order to control the reaction rate.Through the fine control over the replacement and growth process,the Cu-Au bimetallic nanocrystals show a uniform hollow structure.Experimental results indicate that the concentration of Au precursors plays an important role in the formation of the concaves.

2.ExperimentaI Procedures

2.1.Synthesis of Concave Cu-Au AIIoy

The Pd@Cu core-shell nanocubes were synthesized using seed-mediated overgrowth according to our previous report[21].The products were collected by centrifugation,washed three times with water,and redispersed in 6 mL of oleylamine for the further use.In a standard procedure,3 mL of the as-obtained Cu nanocubes was placed in a vial and preheated to 200 °C in an oil bath under magnetic stirring for 10 min.Subsequently,2 mL of AuPPh3Cl-TOP (2 mg/mL) was injected into the solution with a syringe pump (4 mL/h).AuPPh3Cl was synthesized by reacting HAuCl4·4H2O with C18H15P in ethanol[22].After the Au precursor was completely injected,the reaction was allowed to continue at 200 °C for 30 min.The product was collected by centrifugation and washed three times with hexane.

2.2.Synthesis of Cu-Au AIIoy Nanocage

The Pd@Cu core-shell nanocubes were synthesized using seed-mediated overgrowth according to our previous report[21].The products were collected by centrifugation,washed three times with water,and redispersed in 6 mL of oleylamine for further use.In a typical synthesis,3 mL of the as-obtained Cu nanocubes was placed in a vial and preheated to 200 °C in an oil bath under magnetic stirring for 10 min.Then,2 mL of AuPPh3Cl-TOP(0.5 mg/mL) was injected into the solution with a syringe pump (4 mL/h).After the Au precursor was completely injected,the reaction was allowed to continue at 200 °C for 30 min.The products were cooled down to room temperature and washed with hexane three times.

3.ExperimentaI ResuIts and Discussion

In principle,the galvanic replacement can occur between any two metals as long as they have different redox potentials.However,in non-aqueous solvents,the solvent interaction and ligand stabilization may affect the redox properties greatly,like slowing down the rate of the reaction between Au ions and Cu nanoparticles[23].Therefore,for the reaction occurred in the organic medium like oleylamine,the concentration of reactants is critical.For a cubic nanocrystal,the physical and chemical properties of the corners,edges,and faces are different because of the different surface energy and coordination number.The reactivities on these sites are hence different.Generally,the growth rate of these sites tends to follow the decreasing order of corner ＞ edge ＞ face.Fig.1 illustrates the possible evolution routes of the Cu nanocube when preparing the Cu-Au alloy nanostructure.At the high concentration of AuPPh3Cl-TOP,the growth process overwhelms the galvanic replacement process,and the reduced Au atoms from the solution are preferably deposited on the corners rather than the faces of the Cu cube(Step 1).Along with the quick growth process,the slowly galvanic replacement reaction between the Cu atoms from the surface of the cube and Au precursor also exists in the solution,which can release Cu2+ions into the solution.These Cu2+ions are then reduced to Cu atoms by oleylamine who are redeposited on the active corner of the cube.As a result,the alloyed Au-Cu corner on the Cu cube is formed as shown in Step 2.The continuous replacement reaction on the surface of the Cu cube results in the formation of the concave in the starting Cu cube,and finally the Au-Cu hollow octapod structure is obtained as displayed in Step 3.While at the low concentration of AuPPh3Cl-TOP,the reduction reaction is significantly hindered,and the galvanic replacement process between Au ions and Cu atoms is initiated immediately from the surface of the cube to release Cu2+ions into the solution,generating a small hole on the surface of the cube (Step 4).In the meantime,segregated Au and Cu atoms are inclined to alloy to form a thermodynamical stable structure (Step 5).As a result,hollowed Cu-Au nanocages are generated (Step 6).Since the addition method and volume of AuPPh3Cl-TOP is the same for both the synthesis of the concave structure and nanocages,the only parameter affecting the morphology is the difference in the concentration of the Au precursor.At high concentration,there are enough Au(I) ions which are more likely to be reduced to Au atoms making the growth reaction dominating the process; while at low concentration,since the lack of Au(I) ions,the galvanic replacement reaction then overcomes the reduction process.The different reaction pathways lead to the different final morphology.

Fig.1.Schematic illustrating a plausible mechanism for the formation of a Cu-Au alloy nanocrystal through galvanic replacement between a Cu nanocube and AuPPh3Cl.

We conducted the synthesis of Cu-Au alloy structures using the well-defined Cu nanocube as the starting template.Fig.2 shows the scanning electron microscopy (SEM) and transmission electron microscopy (TEM)images of the as-prepared Cu nanocubes according to the method in the literature[21].We monitored the morphology evolution by TEM at different volumes of AuPPh3Cl-TOP with the high concentration(2 mg/mL).Based on the above discussion,the concentration of the Au precursor plays a key role of the morphology evolution.We,therefore,employed TEM to monitor the microstructure of the products by varying the volume of AuPPh3Cl-TOP at the fixed high concentration (2 mg/mL).The TEM and STEM images are shown in Fig.3.When 0.5 mL of the AuPPh3Cl-TOP precursor was injected into the reaction solution(Fig.3 (a)),Au ions were quickly reduced to Au atoms which tended to deposit on the corner of Cu nanocubes.At the same time,at the reaction temperature of 200 °C,the inter-diffusion between Cu and Au occurs to form the alloyed phase.The TEM and scanning TEM (STEM)images in Figs.3 (a) and (b) show the clear concave structures.When the volume of AuPPh3Cl-TOP was increased to 1.0 mL,more Au ions were reduced and deposited onto the alloyed part of the corner of the nanocube,making the corner sharper.Meanwhile,the side faces of the Cu nanocubes are etched due to the galvanic replacement reaction,resulting in the formation of holes on the side faces of the cubes,as shown in Figs.3 (c) and (d).When the volume of AuPPh3Cl-TOP was further increased to 1.5 mL,all the side faces of Cu nanocubes were excavated due to the galvanic replacement.More reduced Au atoms were deposited on the corner of Cu nanocubes,accompanied by the alloying process of Au and Cu.Eventually,the alloyed Cu-Au octapod structure is obtained,as shown in Figs.3 (e) and (f).The corresponding X-ray diffraction(XRD) patterns are shown in Fig.4.We can clearly see that the intensity of the diffraction peaks of Cu nanocubes becomes weak with the increase of the volume of the AuPPh3Cl-TOP solution.The peaks at 2θof (111) and (222) are located between the standard peak positions of face-centred-cubic (fcc) Cu and Au,indicating the formation of the Cu-Au alloyed phase.The intensity of these two peaks increases when the volume increased from 1.0 mL to 1.5 mL.

The octapod Cu-Au alloy was further characterized by TEM and the high-angle annular dark-field scanning TEM (HAADF-STEM).As can be seen in Figs.5 (a) to(c),all nanocrystals surveyed are of high-quality with a uniform structure,demonstrating the excellent control over the morphology.The high-resolution TEM(HRTEM) image in Fig.5 (d) shows the lattice fringes with an interplanar spacing of 0.23 nm,corresponding to the Cu-Au alloy.The different contrast in Fig.5 (c)further demonstrates the proposed evolution process as illustrated in Fig.1.We further analyze the octapod Cu-Au alloy by energy dispersive spectrometer (EDS) mapping,as shown in Fig.5 (e).Both Cu and Au are evenly distributed across the otapod structure.

Fig.2.SEM and TEM images of the Cu nanocube templates:(a) SEM and (b) TEM.

Fig.3.TEM and STEM images of Cu-Au nanocrystals obtained using the standard procedure,except the addition of different amounts of AuPPh3Cl-TOP:(a) and (b) 0.5 mL,(c) and (d) 1.0 mL,and (e) and (f) 1.5 mL.

Fig.4.XRD patterns of Cu-Au nanocrystals obtained using the standard procedure,except the addition of different amounts of AuPPh3Cl-TOP.

Fig.5.Morphology,structure,and composition of the octapod Cu-Au alloy that was prepared using the standard procedure:(a) TEM image,(b) STEM image,(c) and (d) HRTEM images,and (e) EDS mapping.

Fig.6.Cu-Au nanocages were obtained using the standard procedure,except the addition of different concentration of AuPPh3Cl-TOP:(a) TEM image,(b) HAADF-STEM image,(c) and (d) HRTEM images,(e) SAED image,and (f) XRD pattern.

In order to verify the proposed morphology evolution,we decreased the concentration of AuPPh3Cl-TOP solution to 0.5 mg/mL,and carried out the synthesis process at similar conditions.The morphology of the final products is investigated by TEM as shown in Fig.6 (a) which shows the homogeneous morphology.The STEM image in Fig.6 (b) indicates the hollow Cu-Au nanocages.As discussed above,the galvanic replacement reaction between Cu nanocubes and Au ions dominates,resulted in the formation of the Cu-Au alloyed structure without sharpened corners (Fig.6 (c)).The measured lattice spacings of 0.20 nm and 0.23 nm from the HRTEM image(Fig.6 (d)) and the SAED pattern in Fig.6 (e) combined with the result of XRD (Fig.6 (f)) characterization prove the existence of the Cu-Au alloyed phase.

4.ConcIusions

In summary,we demonstrate a unique strategy to create high-active sites in alloy nanocrystals with the hollow structure by changing the concentration of the Au precursor.The success of this synthesis mainly relies on the fine control over the galvanic replacement process and growth process.We believe this work provides a simple and high effective method for the creation of high fractions of the active sites and hollow structure in bimetallic nanocrystals.