Porous Pt Nanotubes with High Methanol Oxidation Electrocatalytic

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Porous Pt Nanotubes with High Methanol Oxidation Electrocatalytic Activity Based on Original Bamboo-Shaped Te Nanotubes Yue Lou,† Chunguang Li,*,† Xuedong Gao,‡ Tianyu Bai,§ Cailing Chen,† He Huang,† Chen Liang,† Zhan Shi,*,† and Shouhua Feng† †

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, and ‡Department of Materials Science, Key Laboratory of Automobile Materials of MOE and State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic of China § College of Medical Laboratory, Dalian Medical University, Dalian 116044, People’s Republic of China S Supporting Information *

ABSTRACT: In this report, a facile and general strategy was developed to synthesize original bamboo-shaped Te nanotubes (NTs) with well-controlled size and morphology. On the basis of the as-prepared Te NTs, porous Pt nanotubes (NTs) with excellent property and structural stability have been designed and manufactured. Importantly, we avoided the use of surface stabilizing agents, which may affect the catalytic properties during the templated synthesis process. Furthermore, Pt NTs with different morphology were successfully prepared by tuning the experimental parameters. As a result, transmission electron microscopy (TEM) study shows that both Te NTs and Pt NTs have uniform size and morphology. Following cyclic voltammogram (CV) testing, the asprepared porous Pt NTs and macroporous Pt NTs exhibited excellent catalytic activities toward electrochemical methanol oxidation reactions due to their tubiform structure with nanoporous framework. Thus, the as-prepared Pt NTs with specific porous structure hold potential usage as alternative anode catalysts for direct methanol fuel cells (DMFCs). KEYWORDS: bamboo-shaped, porous, platinum, nanotubes, methanol oxidation, electrocatalysis

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INTRODUCTION Precious metals, chiefly Pt-based nanomaterials, are a remarkable class of anode catalyst for direct methanol fuel cells (DMFCs) because of their highly efficient methanol utilization and remarkable potential for further improvement.1−8 It is well-known that the catalytic activity of catalyst is largely determined by the physical structure of nanomaterials.1 Therefore, the shape control of nanomaterials is the emphasis in enhancing the catalytic properties.9,10 Among many kinds of nanostructures, one-dimensional nanomaterials with porous structure have attracted great interest due to their superior physical properties, which can considerably improve their catalytic activities.11−14 Inspired by this, we intend to design and synthesize porous Pt-based nanomaterials with wellcontrolled geometrical configuration to achieve better electrochemical performance. Because it is very difficult to manipulate the morphology of nanomaterials by controlling the nucleation and growth of nanomaterials in a one-step synthesis process, template synthesis becomes a powerful strategy. It is prone to employing mild synthetic conditions and simple synthetic processes to obtain well-controlled products.11,15 Generally, there are three template methods. A contrast of hard template method and soft template method with sacrificial © 2016 American Chemical Society

template method shows that the core superiority of sacrificial template method is that the sacrificial template acts as a reactant involved in replacement reaction, so that the template elimination process template is omitted. Recently, it has been widely used in the fabrication of porous and hollow nanomaterials, in which the template sacrificed itself on the basis of the principles of galvanic replacement reaction.1 It has also been found that various types of nanomaterials can be used as sacrificial templates, such as Cu NWs,16 carbon NTs,17 Zn2O NRs,11,18 and Te NWs.19−27 Obviously, semimetal Te nanowires (NWs) were frequently used as sacrificial templates for the synthesis of one-dimensional nanomaterials because the preparation method was simple, and they could be synthesized in large quantities at low cost. According to our investigation, Te NWs that have been reported as templates were all prepared by using the method reported previously, and Te NWs always possess small diameter and ultralong length.26−32 Moreover, surface stabilizing agents (PVP, CTAB, etc.) always play an important role in the formation of extraordinary structure at Received: May 5, 2016 Accepted: June 10, 2016 Published: June 16, 2016 16147

DOI: 10.1021/acsami.6b05350 ACS Appl. Mater. Interfaces 2016, 8, 16147−16153

ACS Applied Materials & Interfaces

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template-based methods.26,28−30 Unfortunately, surfactant adsorbed on the catalyst surface could decrease the active sites and further reduce the catalytic activity of the catalyst.33 Besides, the morphology uncontrollability of Te NWs restricts the shape control of target products. Therefore, these templates can be further optimized. Herein, we created a facile strategy for the synthesis of bamboo-shaped Te NTs. By carefully adjusting the reaction kinetics, the morphology and size of as-prepared Te NTs can be controlled. On the basis of the fine-tuned bamboo-shaped Te NTs, we synthesized high-quality porous Pt NTs with narrow diameter and length distribution. Fine-tuning of the size and morphology of Te NTs affords Pt NTs with controlled shape. Moreover, there is no use of extra surfactants during the templated synthesis process. We have also discussed the formation mechanism of the template-oriented synthesis. As a result, the as-prepared Pt NTs exhibit excellent catalyst activity for M/EOR in alkaline conditio, confirming that as-prepared Pt NTs have a potential application as anode catalysts for the DMFCs.

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Research Article

RESULTS AND DISCUSSION Morphology Regulation and Structure Characterizations of Te NTs. In this study, we have shown a synthesis of original bamboo-shaped Te NTs with controllable size and morphology. We adopted a facile metalloid oxyanion tellurite reduction method to synthesize Te NTs. As illustrated in the chemical eq 1, TeO32− was reduced by N2H4 when it was added in EG solution. Ethylene glycol (EG) was used as both solution and reducing agents. TeO32 − + N2H4 → Te + N2 + 2OH− + H 2O

(1)

In fact, because the hydrazine hydrate solution is alkaline, the remaining hydrazine hydrate can make Te react with N2H4 in the presence of OH− and further convert to Te2−: 2Te + N2H4 + 8OH− → 2Te 2 − + N2 + 4OH− + 4H 2O (2)

So in this reaction, the morphology of Te NTs greatly depends on the amount of added hydrazine hydrate. Figure 1

EXPERIMENTAL SECTION

Chemicals. Polyvinylpyrrolidone (PVP, Mw ca. 55 000), polyacrylic acid (PAA, Mw ca. 1800), and Nafion ethanol solution (5 wt %) were bought from Sigma-Aldrich. Chloroplatinic acid hexahydrate (H2PtCl6· 6H2O, AR), ethylene glycol (EG), Na2TeO3, hydrazine hydrate (85%, AR), methanol, ethanol, and NaOH were purchased from Sinopharm Chemical Reagent Co., Ltd. 20 wt % Pt/C was produced by Johnson Matthey Corp. All reagents were directly used without removing impurities unless special instructions dictated. Synthesis of Bamboo-Shaped Te NTs. In a typical synthesis, PVP (2 mmol), PAA (2 mmol), and 9 mL of EG were added into a 50 mL three-neck flask with nitrogen protection under magnetic stirring. The temperature then was increased to 150 °C, and 150 μL of Na2TeO3 aqueous solution (0.02 M) was injected under magnetic stirring for 2 min, followed by adding 80 μL of hydrazine hydrate solution (diluted to 4.4 times with water). The mixture was held at 150 °C for 10 min, and then it was quickly cooled to room temperature. The Te NTs were washed with ethanol and double distilled water three times. The final product was dispersed into ethanol for further characterization. Synthesis of Porous Pt NTs. Te NTs (about 0.006 mmol) dispersed in 8 mL of EG were added into a three-neck flask. The temperature then was raised to 80 °C with nitrogen protection under magnetic stirring. The temperature was kept for 2.5 h after adding 120 μL of H2PtCl66H2O aqueous solutions (0.05 g/mL). The as-prepared Pt NTs were washed with ethanol several times and finally dispersed into ethanol for further characterizations. Material Characterization. X-ray diffraction (XRD) was used to determine crystal structure by a Rigaku D/max-2500 diffractometer with Cu Kα radiation operating at 200 mA and 40 kV. Morphology information on Te NTs and Pt NTs was obtained by transmission electron microscopy (TEM) and high-resolution transmission electron microscopic (HRTEM). The instrument model is FEI Tecnai G2STwin. EDS elemental mapping images were obtained with the same instrument. Scanning electron microscopy (SEM) images were obtained with a JEOL JSM-6700F microscope operating at 10 kV. X-ray photoelectron spectroscopy (XPS) was tested by ESCALAB 250 (Mg Kα X-ray). Element composition of as-prepared nanomaterials was characterized by ICP-AES technique collected on Thermo iCAP Qc. The cyclic voltammograms (CV) were used for electrochemical characterization on a CHI 760 electrochemical workstation (CH Instruments Corp., Shanghai, China). UV−visible spectra were made on a UV-2450 UV−visible spectrophotometer.

Figure 1. (a−e) TEM image of the bamboo-like Te nanotubes using different amounts of hydrazine hydrate in the presence of Na2TeO3/ EG stock solution: (a) 60 μL, (b) 80 μL, (c) 100 μL, (d) 150 μL, (e) 300 μL. The insets in (a−e) are SEM images of the tip of Te nanotubes. (f) XRD pattern of Te nanotubes.

demonstrates the typical TEM images of the bamboo-shaped Te nanotubes with hydrazine hydrate dosage for 60, 80, 100, 150, and 300 μL while maintaining the other experimental conditions. These nanotubes possess special bamboo-shaped morphology with narrow diameter and length distribution. We can observe that when different amounts of hydrazine hydrate were added, the lengths and diameters of the Te NTs are 16148

DOI: 10.1021/acsami.6b05350 ACS Appl. Mater. Interfaces 2016, 8, 16147−16153

Research Article

ACS Applied Materials & Interfaces different. Interestingly, when the amount of hydrazine hydrate was increased, the tip of Te NTs became obviously longer, because extra hydrazine hydrate promotes reaction 2, which could further change the morphology of Te NTs. Moreover, to confirm that the products have the hollow structure, we picked out one Te NT corresponding to each of the reaction conditions for detailed observation. As shown in the inset in Figure 1, it is clearer that as-prepared Te NTs possess the hollow structure. These as-prepared Te NTs with different size or morphology have the same crystal structure. The crystal structures are characterized by the X-ray diffraction (XRD) pattern (Figure 1f). The result agrees with the standard literature values (JCPDS, 36-1452) reported by Luo’s group, which indicates that the trigonal phase of Te NTs was successfully synthesized.34,35 The morphology of Te NTs was observed by TEM under different magnifications. The as-prepared Te NTs are one-dimensional configuration measured up to 800− 900 nm long with the outer diameter of about 35−40 nm (Figure 2a). HRTEM images (Figure 2b) exhibit high

Figure 3. (a−c) TEM image of the bamboo-shaped Te nanotubes with experimental temperature for 120, 150, and 180 °C, respectively.

Pt NTs with excellent catalytic activity through surfactant-free galvanic replacement reactions. Here, Te NTs were simultaneously used as sacrificial template and reducing agent, and PtCl62− was reduced to Pt by Te as illustrated in chemical eq 3. PtCl 6 2 − + Te + 3H 2O → Pt + TeO32 − + 6Cl− + 6H+ (3)

In this reaction, the morphology of Pt NTs depends on not only the morphology of Te templates but also on the amount of added PtCl62−. Figure 4a,b shows the TEM images of the

Figure 2. (a) TEM image of single Te nanotubes, (b) HRTEM image of single Te nanotubes, and (c) selected-area FFT of (b).

crystallinity of Te NTs, and the distance of atomic lattice fringes is 0.59 nm, which corresponds to the (001) planes. As a parallel result, its FFT (Figure 2c) shows two planes ascribed to (001), (1, −2, 0), which indicates the Te NTs grew along the direction [001]. As for the formation of bamboo-shaped Te nanotubes, two polymeric surfactants (PVP and PAA) played an important role in the formation of special tubular structure. We found that Te nanorods or Te nanowires were generated without the existence of PAA,27,24 because tubular morphology is the result of the interaction of two surfactants. As an acid, PAA benefits tellurite reduction and promotes the generation of hollow structures. PVP has effect on controlling longitudinal growth dynamics and avoiding the particle aggregation. So the special tubular structure is produced merely in the case of a suitable concentration ratio of PVP/PAA. Furthermore, we also explored the effects of reaction temperature on morphology. Bamboo-shaped Te NTs with different lengths and outer diameters were synthesized by adjusting the temperature to 120, 150, and 180 °C while maintaining the other experimental conditions (Figure 3). We found that the higher was temperature we set, the shorter and thicker are the nanotubes we can get. This shape evolution is also caused by changing the reaction kinetics. The lateral growth rate was significantly increased when the higher reaction temperature was set, resulting in final products with different size and morphology.36 Thus, the reaction temperature has an impact on reaction kinetics for the growth process of Te NTs. Morphology Regulation and Structure Characterizations of Pt NTs. On the basis of these fine-tuned bamboo-shaped Te NTs, we designed and synthesized porous

Figure 4. (a−c) TEM image of porous Pt NTs under different magnifications. The inset in (c) is the HRTEM image. (d) EDS elemental mapping image of porous Pt NTs. The inset is the HAADFSTEM image of a single Pt nanotube.

samples prepared with 120 μL of H2PtCl6·6H2O aqueous solutions (0.05 g/mL). Those as-prepared Pt NTs maintain tubular shape with narrow diameter and length distribution. They have a shorter length of 550−650 nm with a higher diameter between 45 and 50 nm. The sharp tip disappeared and porous structure formed after galvanic replacement reactions. When the dosage of H2PtCl6·6H2O aqueous solutions was increased to 180 μL, porous Pt NTs with some holes formed (Figure S1). Increasing the amount of the H2PtCl6·6H2O aqueous solution accelerates the reaction rate, and partial loss occurs in the Te NTs template. As a result, the special nanostructure is produced, which we called macroporous Pt NTs. These as-prepared Te NTs with different morphology have the same crystal structure. The X-ray diffraction (XRD) pattern (Figure 5a) shows that Pt NTs exhibited five distinct peaks, which can be indexed to (111), (200), (220), (311), and (222) 16149

DOI: 10.1021/acsami.6b05350 ACS Appl. Mater. Interfaces 2016, 8, 16147−16153

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Figure 5. (a) XRD pattern of Pt NTs. (b) UV−visible spectra of Te NTs and Pt NTs. Figure 6. XPS spectra of (a) Te NTs and (b) Pt NTs.

crystal planes of face-centered cubic (fcc) platinum. It agreed with the standard literature values (JCPDS no. 01-1194), suggesting that Te template has been successfully changed to Pt NTs. Figure 4c shows the HRTEM image. The inset reveals a 0.23 nm d-spacing of adjacent fringes, which corresponds to the (111) plane of the fcc Pt cubic crystal. The fact indicates that highly crystalline Pt NTs grow along the (111) direction, which agree with previous research.26,28 Platinum element distribution was studied by EDS elemental mapping image (Figure 4d), which shows that a strong platinum signal evenly displays among the Pt NTs. Obviously, platinum element is equally distributed among the nanotubes. The UV−visible spectra of Pt NTs were tested and compared to those of template Te NWs (Figure 5). Te NTs show two characteristic peaks, and they disappeared after the replacement reaction was accomplished, which confirms the formation of Pt NTs and the complete decomposition of Te NTs at the end of the reaction.20 The composition of Te NTs and Pt NTs was determined by the EDS curves attached to SEM (Figure S3). There is no impurity peak, which demonstrates Te NTs are completely transformed to Pt NTs. Typical SEM images under low magnification (Figure S3) also confirm that the as-synthesized products possess narrow diameter and length distribution. In addition, XPS spectra (Figure 6) show the Te 3d and Pt 4f core levels. Te 3d core splits into two intense peaks located at 573.0 and 583.4 eV, which correspond to Te (0) 3d5/2 and Te 3d (0)3/2, respectively. For Pt NTs, the Pt 4f peaks split into 71.0 and

74.3 eV with a peak separation of 3.3 eV, which can be assigned to the Pt0 species.22,29 The result is consistent with other test results, which clearly demonstrates that the formation of alloy does not exist. Schematic Formation of Porous Pt NTs. Schematic illustration of the galvanic replacement reaction is shown in Scheme 1, which illustrates the major process involved in the Scheme 1. Schematic Illustration of the Galvanic Replacement

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Figure 7. (a) CV curves for the porous Pt NTs, macroporous Pt NTs, and Pt/C (20% Pt) in 1 M KOH solution at 50 mV s−1. (b,c) Mass activity and specific activity of the three catalysts recorded in 1 M KOH + 1 M CH3OH solution at 50 mV s−1. (d) Chronoamperograms of the three catalysts in 1 M KOH + 1 M CH3OH solution at −0.3 V.

formation of Pt NTs. First, PtCl62− aggregated to the surface of Te NTs when an aqueous H2PtCl6 solution was added into the EG solution of the bamboo-like Te NTs. Te and EG both acted as the reducing agent. The Te@Pt transition state then was gradually formed. XRD pattern of Te@Pt transition state (Figure S2) tested as the reaction progressed shows the first two peaks correspond to Te NTs, due to the incomplete substitution of Te NTs. As the reaction was carried out in aqueous, a porous structure can be achieved while the galvanic exchange reactions are occurring. The reason is illustrated in eq 4,26,28 with Vm represent molar volume, M representing molar mass, and ρ representing density. The ratio of the molar volume of Te and Pt is constant at 2.24. The volume reduces while the Te atom was substituted by Pt atom, so the nanoporous framework was constructed. V m(Te)/V m(Pt) = M Te*ρPt /M Pt*ρTe = 2.24

electrodes and counter electrode, respectively. Glassy carbon (GC) (3 mm diameter) acting as the working electrode was polished with 0.05 mm alumina slurries on microcloth about 8 min, followed by washing with deionized water before use. Test solution was prepared by mixing water, alcohol, and Nafion (5.0 wt %) with a ratio of 20:20:0.075 (v/v/v) under sonication. A certain amount of Pt NTs catalysts (approximately 0.2 mg) was then dispersed in 1 mL of test solution under ultrasound. This suspension (6 μL) was dropped on the surface of GC, and the modified electrodes were dried thoroughly in air. As a comparison, 20 wt % Pt/C (Johnson Matthey Corp.) was prepared with the procedure described above. Figure 7a shows cyclic voltammogram curves of porous Pt NTs, macroporous Pt NTs, and commercial Pt/C at the potential from −0.9 to 0.6 V versus Ag/AgCl in N2-saturated 1 M KOH solution at 50 mV s−1. The mass of Pt was determined by inductively coupled plasma mass spectrometry (ICP-MS). We can observe peaks for the Hupd adsorption (−0.9 to −0.6 V) in detail in Figure S4a, in which we can integrate hydrogen adsorption charge (excluded electric double layer capacity) to calculate the electro chemical surface area (ECSA). On the basis of the normalization of the mass of Pt, the specific ECSA of porous Pt NTs and macroporous Pt NTs was 47.17 and 25.99 m2 g−1, respectively, which is higher than that of Pt/C (12.60 m2 g−1). The result can be attributed to their hollow and porous structure with large specific area. The catalytic activities for methanol oxidation were studied in KOH (1 M) and methanol (1 M) solutions at 50 mV s−1. The results were normalized by the ECSA and mass, which

(4)

During the replacement process between Te NTs and the platinum precursor, the reaction rate can be adjusted by changing the temperature or the amount of platinum, thereby achieving morphology regulation. Scheme 1 gives an example of two Pt NTs models with different morphologies, which are caused by changing the amount of platinum precursor. Also, the morphology of Pt nanomaterials can be controlled by directly changing the Te NTs template with controllable size and morphology. Electrochemical Investigation. To study the electrochemical properties of the Pt NTs, the cyclic voltammograms of the Pt NTs were tested. A three-electrode system was used. Ag/ AgCl and platinum sheet electrode were used as reference 16151

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and the S&T Development Program of Jilin Province of China (no. 20160101325JC). This project was also supported by State Key Laboratory of Luminescence and Applications (SKLA2015-08).

were used to assess specific activity and mass activity. Figure 7b shows that the forward anodic peak current density of porous Pt NTs is 4.69 times that of Pt/C (20% Pt, 0.497 A mg−1), and macroporous Pt NTs is 2.28 times that of Pt/C. What is more, the specific activities of the as-prepared Pt NTs (Figure 7c) are higher than those of commercial Pt/C catalyst. The results demonstrate that the porous structure can effectively improve the catalytic activity. It is of great significance to control the size and morphology because catalyst structure has a great impact on catalytic activity, which means that the optimization of catalytic activity can be accomplished by altering the morphology of nanomaterials. Furthermore, chronoamperometry curves (Figure 7d) recorded at −0.3 V vs Ag/AgCl were used to determine the stability of the electroctatalysts, and the results demonstrate that current densities decay rapidly during the initial period (Figure S4b), which is attributed to insufficient methanol and poison effect by the reaction intermediates (CO). This situation is consistent with the reported literature.26,29 We compared the long-term poisoning rate, and we can observe that the current density of as-prepared Pt NTs decays slowly during the entire process, which indicates an enhanced stability effect of hollow porous structure during the long time range.

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CONCLUSION In summary, we have designed a comprehensive method to produce uniform Te NTs with controlled size and morphology. The as-prepared Te NTs can be applied for the templateoriented synthesis of nanoporous Pt NTs without surfactant PVP. We accomplished morphology regulation, and the asprepared porous Pt nanotubes exhibit excellent catalytic activities toward electrochemical methanol oxidation reactions. Thus, these special porous Pt NTs hold potential usage as alternative anode catalysts for DMFCs. The original Te NTs with controllable morphology provide more choices of templates, and they offer more possibilities of controlling size and morphology of nanomaterials on template-oriented synthesis. The template optimization method we provided is a promising alternative method for the design and the synthesis of desired morphology or structure, and we believe it can be extended to the controlled synthesis of other noble metals and bimetallic and multimetallic alloys for enhanced application properties in other fields.

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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/acsami.6b05350. TEM image of macroporous Pt NTs, XRD pattern of Pt nanotubes, Te nanotubes and Te@Pt transition state, details of CV curves, and chronoamperograms (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *Tel./fax: +86-431-85168662. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Foundation of the Natural Science Foundation of China (nos. 21371069 and 21301068) 16152

DOI: 10.1021/acsami.6b05350 ACS Appl. Mater. Interfaces 2016, 8, 16147−16153

Research Article

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DOI: 10.1021/acsami.6b05350 ACS Appl. Mater. Interfaces 2016, 8, 16147−16153