Three-Dimensional Nanostructures Formed from Morphology

Research Article pubs.acs.org/journal/ascecg

Three-Dimensional Nanostructures Formed from Morphology Controlled Synthesis of Pt Particles Based on Gas−Liquid Reaction for Electrocatalytic Application Wushuang Bai, Qinglin Sheng,* and Jianbin Zheng* Institute of Analytical Science, Shaanxi Provincial Key Laboratory of Electroanalytical Chemistry, Northwest University, Xi’an, Shaanxi 710069, China S Supporting Information *

ABSTRACT: In this paper, platinum (Pt) nanomaterials with controlled morphologies are grown on the surface of flowerlike manganese dioxide (MnO2) respectively based on gas−liquid reaction. Then flowerlike three-dimensional (3D) nanostructures are formed, with successful synthesis of corresponding Pt/MnO2 nanocomposites. The obtained nanocomposites are characterized by scanning electron microscopy, energy-dispersive X-ray spectrum, transmission electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. In addition, an interesting colorchange phenomenon appeared with the Pt nucleation and growth progress which may be due to variation of the Mn valence state triggered by the reduction of Pt. This phenomenon can be used for naked-eye observation of materials’ growth states which is beneficial for investigation of synthetic mechanisms. At last, the Pt/MnO2 3D nanostructure exhibits perfect electrocatalytic properties toward oxidation of methanol. The four kinds of Pt/MnO2 composites are all used for electrochemical catalytic sensing of methanol respectively which indicates that the morphology of nanomaterials determines the catalytic properties. This research provides a new platform for controllable synthesis of nanomaterials and investigation of electrocatalysis based on morphology controlled nanomaterials. KEYWORDS: Platinum nanoparticle, Manganese dioxide, 3D nanostructure, Gas−liquid reaction, Electrocatalysis

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Pt. Somorjai et al.15 have synthesized Pt nanoparticles with various sizes (2, 4, 6, 8 nm) based on a kind of colloidal method where polyvinylpyrrolidone (PVP) was used as the capping agent and polyol as the solvent and reducing agent. Huang et al.16 have synthesized Pt nanotubes by a simultaneous alloyingetching strategy with open ends by selective etching Au core from coaxial Au/Pt nanorods. The diameter and wall thickness of nanotubes can be readily controlled in the range of 14−37 and 2−32 nm, respectively. Sun et al.17 have developed a facile strategy for the preparation of Pt nanocubes with a controllable assembling manner. The combined use of formaldehyde and Fe3+ is critical to the successful formation of the Pt nanocube supercrystals in one-pot synthesis. However, most of synthetic methods are performed in one phase, the speed of reaction is too fast to control so that researchers have to find the softer reductive agents and some more effective protective agents to realize the morphology controlled synthesis. Compared with other methods, gas−liquid reaction, based on the chemical reactions which are performed between two different phases, is

INTRODUCTION As the development of nanotechnology, nanomaterials have received the greatest attention in various scientific fields such as chemistry, physics, and biology due to their large surface areas and high surface concentrations of edges, corners, defect sites, and other unusual structural features.1−6 Synthesis of nanomaterials with perfect properties is becoming one of the most challenging issues. Among all the factors which can influence the property of nanomaterials, morphology is a decisive one because the morphology will determines the specific surface area and the ratio of surface to bulk atoms as well as the fractions of atoms at corners and edges.7 Therefore, controllable synthesis of nanomaterials and investigation of synthetic mechanisms is an interesting topic worthy of research. Among all materials, platinum (Pt) nanostructures serve as the primary catalysts for various chemical conversions and also have remarkable performance in petroleum reforming, chemical sensing, and fuel cells.8−12 Catalytic performance of Pt nanoparticles (Pt NPs) is also highly dependent on the exact arrangement between atoms on the exposed facets and production of monodisperse, uniformly faceted particles is desired for high specificity in a catalytic process.13,14 Therefore, much attention has been focused on controllable synthesis of © XXXX American Chemical Society

Received: June 1, 2016 Revised: June 29, 2016

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DOI: 10.1021/acssuschemeng.6b01210 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. TEM patterns of Pt/MnO2 nanocomposites.

different morphologies were grown uniformly on surface of MnO2 respectively through adjusting experimental temperature under the protection of chitosan and the synthetic mechanism was also investigated. According to our previous research, MnO2 would exhibit positive electricity in the presence of a certain amount of chitosan which could increase adsorption of Pt ions.29 Then four kinds of 3D flowerlike Pt/MnO2 nanostructures were used for electro-catalysis of methanol. Methanol, as the simplest alcohol, has attracted considerable attention in the past decade due to its use in the fields of chemical science, industry, environmental science, and energy.32 Therefore, electrochemical research of methanol will be full of significance. Because of the unique 3D structure and homogeneous morphology of Pt particles, the Pt/MnO2 structure exhibits perfect catalytic properties. In addition, electrochemical application of four kinds of Pt/MnO2 structures reveals the relationship between the catalytic properties of the material and its morphology. These impressive results identified the 3D nanostructure as a promising candidate for use in electrocatalytic devices. In addition, synthesis of Pt particles with different morphologies based on color-changed interfacial reaction provides a new platform for controllable synthesis of nanomaterials and investigation of synthetic mechanism.

always a special one which can control the synthesis of nanomaterials effectively.18−21 The reaction can be controlled by adjusting the environmental factors such as reaction temperature, gas pressures, velocity of gas flow, and throughput.22 Therefore, controllable synthesis of Pt nanoparticles through gas−liquid reaction will be of great expectation. In recent years, three-dimensional (3D) structures have attracted more and more attention. Nanomaterials with 3D morphologies have various perfect properties due to their porous structure and large surface area which can accelerate the speed of electronic conduction and amplify the signal.23 Therefore, 3D materials have been used in many fields instead of the general 1D and 2D materials, especially in electrochemical sensing. Li et al.24 have constructed a kind of 3D membrane-modified electrode based on highly dispersed silver nanoparticles doped graphene. The modified electrode exhibits super performance for electrochemical biosensing. Liu et al.25 have synthesized 3D hierarchical porous cobalt oxide nanostructure as direct electrochemical biosensing interface which exhibits low detection limit and high sensitivity toward hydrogen peroxide and glucose. Ma et al.26 have fabricated a multifunctional colorimetric and electrochemical biosensing platform based on 3D graphene network@WO3 nanowire composites. The platform exhibits extra sensitivity and selectivity in detection of hydrogen peroxide, ascorbic acid, and dopamine. In our previous research, a kind of 3D flowerlike manganese dioxide (MnO2) nanomaterial was reported.27 The flowerlike structure exhibits large surface area and perfect 3D conformation. In addition, as one kind of metal oxide semiconductor materials, manganese oxides are always used in the field of electrochemistry,28 and it has been confirmed that unique synergistic effect appeared while manganese oxides are used together with metal. Therefore, they can be used as good templates to fabricate 3D nanostructures, especially in the synthesis process of 3D metal nanomaterials. In this work, 3D flowerlike Pt coated MnO2 (Pt/MnO2) nanostructures were formed through a kind of simple gas− liquid reaction. It was found that chloroplatinic acid (H2PtCl6) can be reduced by HCHO gas under proper experimental conditions and well-dispersed Pt nanoparticles with four

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RESULTS AND DISCUSSION Structural and Morphological Study of Pt/MnO2 Nanocomposite. In this research, flowerlike MnO2 structures were obtained successfully as in our previous work.27 The SEM and TEM results have been shown in Figure S1. The obtained MnO2 nanomaterials exhibit a flowerlike structure which is similar as our previous work.27 In brief, the flowerlike structure is composed by large numbers of MnO2 layers. As seen in Figure S1, the single layer is similar to graphene which exhibits an extra thin and wrinklelike structure. Figure 1 shows the TEM patterns of Pt/MnO2 nanocomposites. Compared with flowerlike MnO2, the Pt/MnO2 composite exhibits similar structure. As shown in Figure 2, large numbers of Pt nanoparticles are dispersed uniformly on surface of MnO2 without aggregation and the flowerlike Pt/MnO2 structures are obtained successfully. Before the experiment, it is worried that B


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results, the as-prepared MnO2 has hydrous nature. It is reported that a considerable amount of water can enhance the diffusion of electrolyte cations in the electrode material, which is helpful in improving the electrochemical performance of the material.30 Compared with Figure 2Aa, Figure 2Ab shows a low absorption-peak intensity of the functional groups. This may be due to the reduction effect of CH2O and coverage of Pt particles. Furthermore, the XRD patterns of MnO2 and Pt/ MnO2 are shown in Figure 2B. As seen from Figure 2Ba, all reflections of XRD pattern of the flowerlike MnO2 are indexed well to a pure layered birnessite-type MnO2 (JCPDS No. 801098, monoclinic, C2/m, a = 5.15 Å, b = 2.84 Å, c = 7.17 Å). Among all peaks, the (001) peak is relatively strong, which indicates the enrichment of the (001) crystalline planes. XRD pattern of the Pt/MnO2 composite (curve b, Figure 2B) exhibits the characteristic fcc platinum lattice: diffraction peaks at 40.0° for Pt(111), 46.2° for Pt(200), 67.5° for Pt(220) confirming that the platinum precursor has been chemically reduced to Pt NPs by HCHO. The diffraction peak for Pt(220) is used to estimate the Pt crystallite size since there is no interference from other diffraction peaks. Calculation using the Scherrer equation yields an average crystallite size of Pt (normal to Pt(220)) on MnO2 of 5.0 nm, which is consistent with the TEM results.11 Effect of Chitosan on Formation of Pt/MnO2 Nanocomposite. As mentioned in the Introduction, chitosan is used as protective agent or dispersant in this method which is benefit for synthesis and adsorption of Pt particles on surface of MnO2. Therefore, it is important to investigate the effect of chitosan on formation of Pt/MnO2 nanocomposite. First, the effect of chitosan on dispersions of Pt NPs was investigated. Figure 3 shows the TEM patterns of Pt/MnO2 which were synthesized under the protection of different amount of chitosan under 60 °C. When 1 mL 5% chitosan were used in experimental process, the obtained Pt NPs were aggregated seriously on surface of MnO2 (Figure 3A). When 3

Figure 2. FTIR spectra (A) and XRD patterns (B) of MnO2 (a) and Pt/MnO2 nanocomposites (b).

the flowerlike structure may be broken. Actually, according to the patterns, the original structure of MnO2 is still exists and the structure provides a good support for Pt loading. As seen in Figure 1C and D, the Pt particle has homogeneous globular morphology which exhibits a size of about 5 nm. The good morphology may be attributed to the slow and soft reduction of volatilized HCHO gas. In addition, in order to further investigate the compounds of nanocomposites, EDX, FTIR, and XRD of obtained Pt/MnO2 are also characterized. Figure S2 shows the EDX pattern of Pt/ MnO2. It is found that the nanocomposites are composed of O, Mn and Pt elements which represent the components of MnO2 and Pt, respectively. This results show that Pt nanoparticles have been loaded on surface of MnO2 successfully. Figure 2 shows the FTIR and XRD patterns of MnO2 (a) and Pt/MnO2 (b), respectively. As seen in Figure 2A, the vibration frequencies at 570 cm−1, which may reveal information about manganese oxide lattice (MnO6 octahedral), can be assigned to the Mn−O stretching vibrations. The absorption at 3450 cm−1 is due to stretching vibrations of hydroxide group and two weak peaks at 1384.41 and 1636.47 cm−1 are associated with bending vibrations of H−O−H, relating to adsorbed and/or crystalline water molecules present in the product. According to the FT-IR

Figure 3. TEM images showing the morphologies of Pt/MnO2 nanocomposites which were protected by different amounts of chitosan: (A) 1, (B) 3, (C) 5, and (D) 8 mL 5% chitosan. C


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ACS Sustainable Chemistry & Engineering and 5 mL 5% chitosan were used in experimental process respectively, the synthesized Pt/MnO2 composites have good morphology (Figure 3B and C, respectively). The Pt NPs are distributed uniformly on surface of MnO2, and a kind of flowerlike structure is obtained. However, it can be seen clearly that the flowerlike structure in Figure 3C is darker than that in Figure 3B which indicates that when 5 mL 5% chitosan were used, more Pt NPs or particles which have larger size have been adsorbed on surface of MnO2. At last, more chitosan (8 mL 5% chitosan) were used for Pt NPs synthesis and the result is shown in Figure 3D. As shown in the TEM pattern, there are amounts of impurities which may be surplus chitosan on the surface of MnO2. Compared with Figure 3B and C, the Pt NPs are aggregated at local levels. Second, the effect of chitosan on size of Pt particles was also investigated. Pt NPs were synthesized at different temperature (40, 50, 60, and 75 °C) under protection of two different amounts of chitosan (3 and 5 mL 5% chitosan), respectively. According to that mentioned in the first investigation, the aggregation of Pt NPs can be avoided under protection of 3 or 5 mL 5% chitosan which is beneficial for observation of particle size. According to experimental results (Figure S4), although different amounts of chitosan were used, the Pt NPs which were synthesized at the same temperature have similar morphologies. However, the particle has larger size and more uniform morphologies when more chitosan was used in the synthesis process. From that mentioned above, it is more appropriate to use 5 mL chitosan in synthetic progress. The effect of chitosan on the formation of Pt/MnO2 has been investigated successfully. First, the amounts of chitosan will influence the adsorption and dispersion of Pt NPs on the surface of MnO2: less chitosan will lead to aggregation of Pt NPs while there will be many impurities that appear on the surface of MnO2 if too much chitosan is used. Second, the size of Pt particle will be influenced in absence of aggregation: in a certain range, more chitosan will lead to the larger size and more uniform morphology. Effect of Temperature on Controllable Synthesis of Pt NPs. Experimental temperature is always an important factor in chemical reaction. In this research, Pt particles with controlled morphology were obtained successfully through regulating experimental temperature. As shown in Figure 4, it can be seen that the obtained Pt particles exhibit different morphology (gravel-like, gatherlike, globular, and cube structures) when the temperature is set from 40 to 75 °C (40, 50, 60, 70, and 75 °C), and the TEM characterizations are shown in Figure 5. Figure 5B and C exhibits the morphology of Pt NPs which were obtained at 40 °C. The obtained Pt particles are distributed uniformly on surface of MnO2 in absence of aggregation. The particles have irregular edge and size and exhibit a kind of gravel-like morphology. When the temperature is 50 °C, the obtained particles exhibit a very different morphology (Figure 5E and F). At first glance, the aggregation is also avoided and the particles have large size (about 20 nm) and the same irregular shape (Figure 5E). However, the big particles are gathered by amounts of small particles which are only few nanometers large (Figure 5F). Then, the experimental temperature is set to 60 °C, and the results are shown in Figure 5H and I. As mentioned in Figure 1, the obtained Pt particles exhibit homogeneous globular morphology. Compared with these, when the temperature is increased to 70 °C, morphology of Pt particle is changed (Figure 5K and L). It can

Figure 4. Scheme of controllable synthesis of Pt NPs under different experimental temperatures.

be seen that there are two kinds of morphology appeared: one is a globular structure, and the other, a cube. In other words, the primary globular structure is transforming into cube. At last, the temperature is set to 75 °C, and the results are shown in Figure 5N and O. At such a high temperature, morphology of Pt particles has been transformed into a cube. In addition, the size of the particle is also increased to about 10 nm. According to that mentioned above, Pt NPs with controlled morphology (gravel-like, gatherlike, globular, and cubelike structures) have been synthesized successfully on surface of flowerlike MnO2 and four kinds corresponding Pt/MnO2 3D nanostructures are obtained. In addition, the corresponding EDX results (Figure S2 and Figure S6−S8), and BET surface areas (Figure S9) are summarized in Table 1. It can be seen that Pt has been loaded successfully on MnO2 and the four kinds of composites have similar Pt loading amount. N2 adsorption−desorption isotherms of four kinds of Pt/MnO2 have been shown in Figure S9. After calculating by BET equation, the results have been shown in Table 1. It can been seen that compared with gatherlike and cubelike structures, gravel-like and globular structures have larger specific surface areas. Growth States and Formation Mechanism. In order to investigate formation mechanism of Pt NPs, growth states of Pt particles under different reaction durations were researched first (experimental temperature was 60 °C) and results are shown in Figure 6. Figure 6A shows the TEM morphology of materials which are obtained at first of reaction (the reaction time was 55 min). It can be seen clearly that there are no particles appeared at this time and the color of reaction system is still black brown (Figure 6Aa). After 5 min, an interesting phenomenon of color change was observed. As shown in Figure 6B, when the reaction is about 1 h, the color of the solution changed quickly, in a few seconds. The black−brown solution changed into light brown (Figure 6Bb). Quickly, it is sampled for TEM characterization (Figure 6B). It can be seen that there are many small particles with the size of 1−2 nm. With the passing of reaction time, the color of the solution becomes darker and darker (Figure 6Cc and Dd) and the size of particles grows (Figure 6C and D). At D


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Figure 5. TEM images showing the morphologies of Pt nuclei and corresponding Pt NPs which were synthesized at different reaction temperatures: (A−C) 40, (D−F) 50, (G−I) 60, (J−L) 65, and (M−O) 70 °C. Pt nuclei (A, D, G, J, M).

experimental conditions are the same as that mentioned above except the presence of H2PtCl6. However, it is found that the color is not changed during the process of the experiment.

last, when the reaction time is about 6 h, the solution became black (Figure 6Ee) and the size of Pt NPs is kept at about 5 nm. Then a contrast test was done as well (Figure S3). The E


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ACS Sustainable Chemistry & Engineering Table 1. Pt Loading and BET Surface Areas of all Pt/MnO2 Composites samples

surface area (m2/g)

Pt (wt %)

Mn (wt %)

gatherlike Pt/MnO2 globular Pt/MnO2 gravel-like Pt/MnO2 cubelike Pt/MnO2

66.8 92.9 113.1 52.1

20.03 22.25 19.79 19.23

40.49 41.73 43.07 44.92

According to the results, first, formation of Pt NPs can be divided into two steps: nucleation and growth. The steps may be presented schematically as (1−2),31 where n is the number of Pt0 atoms. Nucleation step 1 proceeds homogeneously in solution and involves the reduction of PtCl62− to Pt0 atoms, followed by their aggregation to the critical nucleus size, (Pt0)n. As nuclei appear, a fast autocatalytic reduction 2 on the surface of growing particles takes place and limits the rate of their growth.

nPtCl 6 0

2−

0

→ (Pt )n

(Pt )n + PtCl 6

2−

Figure 7. Scheme of nucleation and growth of Pt nanoparticles under different experimental temperatures.

the color of solution begins to change soon after 2.5 h (Figure 7). After sampling quickly, TEM characterization is shown in Figure 5A: Pt nucleus with size of about 1−2 nm exhibit irregular morphology. After 16 h, the color of the solution is transformed to black thoroughly and the obtained Pt NPs with size of about 5 nm also exhibit irregular shape (Figure 5B and C). When the temperature is 50 °C, the Pt nucleus with gatherlike structure appeared when the color of solution changes after about 1.5 h (Figures 5D and 7). Accordingly, Pt NPs with gatherlike morphology are obtained (Figure 5E and F) when the color of the solution is transformed to black after 10 h (Figure 7). Then the investigation continues at the temperatures of 60 and 70 °C respectively. It can be seen that the Pt NPs (Figure 5H, I, K, and L) exhibit the similar morphology as the corresponding nucleus (Figure 5G and J). However, when the temperature is 60 °C, the color is changed first after 1 h while it is 45 min at 70 °C. The color is transformed to black after 6 h at 60 °C which is longer than 3 h at 70 °C (Figure 7). At last, the temperature is increased to 75 °C. It is still the same that the Pt nucleus (Figure 5M) exhibits similar morphology as Pt NPs (5 N and O). However, the first color change appeared after only 30 min (Figure 7).

(1) 0

→ (Pt )n + 1

(2)

Second, the color-changed phenomenon may be due to the following reason: When nucleation happened, PtCl62− is reduced to Pt0 which may induce the transformation of Mn valence state. Then the color of solution is changed from black brown to light brown. With the time passing, the amounts and size of Pt NPs are all increased. Therefore, the color of solution is changed from light brown to black. Due to the remarkable color changes, this phenomenon can be used for naked-eye observation of materials’ growth states which is beneficial for the investigation of synthetic mechanisms. In addition, the phenomenon can be probably applied in the field of photochemistry for investigation of the transformation of Mn valence states which will be researched in our further work. In order to further investigate the morphology controlled formation of Pt NPs, nucleation and growth of particles at different temperatures were researched and results are shown in Figures 5 and 7. When the experimental temperature is 40 °C,

Figure 6. TEM images showing the structural evolution of Pt NPs for different reaction durations: (A) seconds before color change (less than 1 h), (B) color change (1 h), (C) 1.5 h, (D) 6 h, (E) 10 h. (insets a−e) Corresponding colors of reaction system for different reaction durations. F


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Figure 8. (A) EIS of (a) bare GCE, (b) MnO2/GCE, and (c) Pt/MnO2/GCE in 5.0 mM [Fe(CN)6]4−/3− containing 0.1 M KCl from 105 to 10−2 Hz at an amplitude of 5 mV. (B) DPVs of (a, a′) MnO2/GCE and (b, b′) Pt/MnO2/GCE in 0.5 M KOH solution in the absence (a, b) and presence (a′, b′) of 0.1 M CH3OH at a scan rate of 50 mV/s. (C) CVs showing the current responses of Pt/MnO2/GCE in 0.5 M KOH in the presence of CH3OH with different concentrations, (a−g) 10, 50, 60, 70, 80, 90, 100 mM, at a scan rate of 50 mV/s. (inset) Linear fitting program of the oxidation peak currents with CH3OH concentrations. (D) DPV comparison between four kinds of Pt/MnO2 nanocomposites modified GCEs in 0.5 M KOH in the presence of 0.1 M CH3OH. Pt/MnO2 nanocomposites were synthesized at 40 (b), 50 (a), 60 (d), and 75 °C (c), respectively.

Then electrochemical catalytic behaviors of the obtained materials at different stages were investigated by DPV, and the results are shown in Figure S5 and 8B. Figure S5 shows the DPVs of bare GCE in absence and presence of CH3OH. It can be seen that bare GCE exhibits no electrochemical response in the absence of CH3OH. Instead, in the presence of CH3OH, bare GCE exhibits very weak peak at about 0.35 V. Figure 8 B shows the DPVs of MnO2/GCE and Pt/MnO2/GCE. It is obviously seen that in 0.5 M KOH solution, MnO2/GCE and Pt/MnO2/GCE (curves a and b, respectively) exhibit almost no electrochemical response in the absence of CH3OH. However, when 0.1 M CH3OH are added, the electrochemical signals are changed. In contrast, MnO2/GCE also shows no significant current response (curve a′) while Pt/MnO2/GCE (curve b′) shows a remarkable catalytic current peak about 32 μA in intensity at 0.3 V. All the above investigations indicate that the obtained Pt NPs have a notable catalytic performance for CH3OH oxidation. The catalytic responses of Pt/MnO2/GCE by changing the concentration of CH3OH are also investigated (Figure 8C). It can be seen clearly that in 0.5 M KOH, with the increment of CH3OH concentration, the highest oxidation current responses gradually increased due to the excellent electrocatalytic activity of Pt/MnO2 nanocomposites. The inset of Figure 8C shows the calibration curves of catalytic current versus CH3OH concentration. A good linear relationship is found between the catalytic current and CH3OH concentration at a range from 10 to 100 mM (R = 0.9991). In addition, DPV comparison between four kinds of Pt/ MnO2 nanocomposites (synthesized at 40, 50, 60, and 75 °C, respectively) modified electrodes was investigated (Figure 8D). In the presence of 0.1 M CH3OH, the Pt/MnO2/GCEs exhibit different current responses. The gatherlike Pt/MnO2/GCE

According to the investigation at different experimental temperatures, first, it is indicated again that the color change is an obvious signal of nucleation of Pt which can be observed by the naked eye. Second, high temperature will speed up the process of nucleation and growth of Pt particles. Third, with increasing temperature, the morphology of obtained Pt particles is transformed from gravel to cube (gravel-like, gatherlike, globular, and cube structures), while it is the same as the morphological change of Pt nucleus. It can be seen that nucleation is a decisive step for morphology controlled synthesis of Pt NPs. High temperature will provide more energy which is beneficial for the oriented nucleation and growth of particles.27 When the temperature is low, an irregular nucleus appeared and diffusion of smaller particles is lower, gathering occured easily. Therefore, formation of particles is nonoriented. When the temperature is high, the rate of mass transport will be more drastic than that of gathering and the formation of nanostructure proceeds along the low-energy direction. These may be the reasons why the morphology of Pt particles changes from irregular gravel to cube. Electrocatalytic Behavior of Pt/MnO2 Nanocomposites. Before electrocatalytic investigation, electrochemical impedance spectroscopy (EIS) of modified electrodes was characterized. Generally, the semicircle diameter equaled to the electron transfer resistance (Rct). As shown in Figure 8A, the value of Rct is about 500 Ω when bare GCE was used (Figure 8Aa). However, the value is increased to about 2200 Ω when flowerlike MnO2 were modified on surface of GCE (Figure 8Ab). At last, Pt NPs were adsorbed on the surface of MnO2, and the value of Rct is decreased to 800 Ω (Figure 8Ac). In general, Pt NPs were synthesized successfully which could enhance the electron transfer efficiency effectively. G


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Figure 9. Typical amperometric response of four kinds of Pt/MnO2 nanocomposite-modified GCEs on successive injection of CH3OH into stirred 0.5 M KOH (A, C, E, G) and corresponding calibration curve of CH3OH versus its concentration (B, D, F, H): Pt/MnO2 nanocomposites were obtained at 50 (A), 40 (C), 75 (E), and 60 °C (G).

shows the lowest current response of 7 μA with a catalytic potential of 0.4 V (Figure 8Da). However, gravel-like Pt/ MnO2/GCE and cubelike Pt/MnO2/GCE show the similar current responses of 17 μA with catalytic potentials of 0.35 and 0.4 V, respectively (Figure 8Db and c). By construct, globular Pt/MnO2/GCE has the highest current response of 32 μA with the lowest catalytic potential of 0.3 V. In order to further research electrochemical catalytic oxidation sensing of four kinds of Pt/MnO2 nanocomposites modified GCEs, amperometric responses of Pt/MnO2/GCEs were investigated in 0.5 M KOH for different concentration of CH3OH. The results are shown in Figure 9. Figure 9A shows the amperometric response of gatherlike Pt/MnO2/GCE at 0.4 V. As seen in the inset of Figure 9A, the Pt/MnO2/GCE shows the higher background. Figure 9B shows the calibration curve of the modified electrode. The linear detection range is from 3 to 969.25 mM with a correlation coefficient of 0.9934, and the

detection limit is estimated to be 1.25 mM at a signal-to-noise ratio of 3. Figure 9C shows the amperometric response of gravel-like Pt/MnO2/GCE at 0.4 V. As seen in the inset of Figure 9C, Pt/MnO2/GCE shows the lower background. Figure 9D shows the calibration curve of the modified electrode. The linear detection range is from 0.5 to 169.25 mM with a correlation coefficient of 0.9952, and the detection limit is estimated to be 0.1 mM at a signal-to-noise ratio of 3. In addition, amperometric response of cubelike Pt/MnO2/GCE was also investigated at 0.35 V, and the result is shown in Figure 9E. The modified electrode also has lower background. According to the calibration curve (Figure 9F), it can be seen that the Pt/MnO2/GCE exhibits a linear detection range of 6.75−1469.25 mM with a correlation coefficient of 0.9950, and the detection limit is estimated to be 1.25 mM at a signal-tonoise ratio of 3. Figure 9G shows amperometric response of globular Pt/MnO2/GCE at 0.3 V. As seen in inset of Figure 9G, H


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ACS Sustainable Chemistry & Engineering the Pt/MnO2/GCE shows the lowest background. Figure 9H shows the calibration curve of the modified electrode. The linear detection range is from 0.125 to 3219.25 mM with a correlation coefficient of 0.9985, and the detection limit is estimated to be 0.05 mM at a signal-to-noise ratio of 3. According to that mentioned above, it can be seen clearly that morphology of mamomaterial is a decisive factor which can influence its electrochemical catalytic properties. Moreover, the performance comparisons of four kinds of Pt/MnO2/GCEs with others are presented in Table S1. According to these comparisons, it can be seen obviously that performances of the four Pt/MnO2/GCEs are better than that of other modified electrodes, especially the extremely wide linear range and low detection limit. These are due to the 3D flowerlike structure, the large surface area, and unique space structure which are benefit for signal amplification and electron transfer. By construct, the globular Pt/MnO2/GCE has the best properties such as the lowest detection limit and the broadest linear range in four kinds of Pt/MnO2/GCEs. This may be because when the experimental temperature is 60 °C, the obtained Pt NPs have the smallest size and homogeneous morphology. Therefore, the Pt/MnO2 has the largest surface area which can provide more active sites for CH3OH oxidation catalysis. In addition, homogeneous morphology is beneficial for synchronous conduction of electrons and the signal will be amplified more significantly. According to results, the 3D Pt/MnO2 nanostructures based on gas−liquid reaction in this work exhibit perfect electrocatalysis for oxidation of CH3OH which can be applied in fields of electrochemical sensors and fuel cells.

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AUTHOR INFORMATION

Corresponding Authors

*Tel./Fax: +86 29 88303448. E-mail: [email protected] (J.Z.). *E-mail: [email protected] (Q.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of this project by the National Science Fund of China (No. 21275116, No. 21575113), Specialized Research Fund for the Doctoral Program of Higher Education (No. 20126101110013), the Natural Science Fund of Shaanxi Province in China (2013KJXX-25), and the Scientific Research Foundation of Shaanxi Provincial Key Laboratory (13JS097, 13JS098, 14JS094, 15JS100).

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REFERENCES

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CONCLUSIONS In summary, novel flowerlike 3D nanostructures are formed when morphology controlled Pt/MnO2 structures are synthesized based on gas−liquid reaction under the protection of chitosan and Pt nanoparticles with homogeneous morphology are dispersed uniformly on the surface of flowerlike MnO2. By altering the experimental temperature, obtained Pt NPs exhibit four different morphologies (gravel-like, gatherlike, globular, and cube-like) at 40, 50, 60, and 75 °C, respectively. In this method, chitosan has made a significant contribution on synthesis and dispersion of Pt NPs in the process of the gas− liquid reaction under different temperatures which provide a new platform for controllable synthesis of nanomaterials. In addition, due to variation of Mn valence state triggered by reduction of Pt, an interesting color-change phenomenon appeared as Pt nucleation and growth progressed. It can be used for naked-eye observation of materials’ growth states which is beneficial for investigation of synthetic mechanisms. At last, the Pt/MnO2 nanocomposites with four kinds of Pt have been used to electrocatalyze oxidation of CH3OH. The 3D Pt/ MnO2 modified electrode exhibits perfect properties such as low detection limit and broad linear range which can be used in the fields of electrochemical sensors and fuel cells.

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Additional data about experimental sections, TEM characterizations of obtained materials, and tables of comparison (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01210. I


Research Article


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Three-Dimensional Nanostructures Formed from Morphology

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