Formation and Catalytic Application of Electrically Conductive Pt

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Formation and Catalytic Application of Electrically Conductive Pt Nanowires Subrata Kundu,* David Huitink, and Hong Liang* Materials Science & Mechanical Engineering, Texas A&M UniVersity, College Station, Texas 77843-3123 ReceiVed: December 21, 2009; ReVised Manuscript ReceiVed: March 30, 2010

Pt nanowires with variable lengths have been synthesized by the reduction of Pt(IV) ions using alkaline 2,7-DHN in CTAB micellar media under 3 h of UV photoirradiation. The synthesized Pt nanowires are stable for more than 3 months in ambient conditions. The particles’ morphology can be tuned by simply changing the surfactant-to-metal ion molar ratios and other reaction parameters. The mechanisms of the nanowire formation and effects of different reaction parameters are studied in detail. The resulting nanowires are found to be electrically conductive and maintain Ohm’s law with the electrodes. Moreover, they serve as a good catalyst for the reduction of organic dye molecules in the presence of NaBH4. The catalysis rate was compared by considering the electron-transfer process during reduction. The present method would lead to a quick process for the synthesis of other monometallic and composite nanowires with variable lengths. The Pt nanowires offer promising applications in different types of organic and inorganic catalysis reactions, nanoelectronics, and biomedical applications. Introduction Colloidal metal nanoparticles (NPs) have been of great interest in recent years because of the realization of their unique properties that may not be obtained from other ordinary sources. The properties of NPs differ dramatically from those of their bulk counterparts due to the nature of nanoscale interactions. Among different metals, coinage metals, such as Au, Ag, Pd, Pt, etc., receive more attention because of their size-dependent optical properties,1 magnetic properties,2 and catalytic properties.3 These particles are widely used as catalysts,3 adsorbents and sensors,4 and as elements in optical,1 electrical,5 and magnetic2 devices. The catalytic properties of the nanostructured materials depend on the size and shape of the particles.6,7 As the size and shape of the particles change, their catalytic activity also changes. Among the different metals studied so far, Pt NPs have been a major interest because of their excellent reactivity as catalysts for many purposes.8-10 These include catalysis in proton-exchange membranes and methanol fuel cells,8 removal of NO in combustion processes,9 CO oxidation,10 etc. These applications demand a large amount of Pt NPs, resulting in high cost. Thus, their commercialization remains a challenging task. The unique control of sizes and shapes of Pt at the nano dimension can help to lower Pt usage and facilitate the necessary cost reduction. Over the past few years, the noble metal NPs, such as Au,3,6 Ag,11 Pd,12 Pt,13 and oxide NPs,14 have been studied as effective catalysts in different types of homogeneous and heterogeneous catalysis reactions, even though the synthesis of size- and shapecontrolled NPs still remained a challenging task. There are not many reports for the shape control synthesis of Pt NPs in comparison to Au and Ag. Among the different shapes, the rods,15 cubes,16 quasispherical,17 tetrahedral,18 and the 1-D wires19 are most important due to their variety of applications. Most of the syntheses of nanowires are done by either controlled growth on porous membranes or in solid support or template assisted chemical processes. The first control synthesis of Pt * To whom correspondence should be addressed. E-mail: [email protected] (S.K.), [email protected] (H.L.). Phone: 979-862-2578. Fax: 979-845-3081.

NPs was reported by Ahmadi et al. using a polymer as a protective agent in 1996.20 They synthesized cubic and tetrahedral Pt NPs by changing the molar concentrations of polymers with the Pt salt. El-Sayed’s group also studied the shapedependent change in activation energy during the electrontransfer process catalyzed by Pt NPs.21 Shelnutt’s group reported the synthesis of Pt nanowire networks using a soft template.22 They reduced the Pt complex using sodium borohydride in a two-phase water-chloloform system. Zhong et al. synthesized mesoporous Pt nanowire arrays using direct current (dc) electrodeposition into the pores of aluminum oxide templates.23 Fukuoka et al. synthesized Pt NPs and nanowires on mesoporous silica templates using the UV irradiation method for 48 h.24 Yang et al. grew Pt nanowire arrays by electrodeposition in a polycarbonate membrane and studied their application in biosensors.25 Bore et al. synthesized Pt nanowires inside aerosolderived mesoporous silica particles.26 We also synthesized nanowires of Au and CdS using a deoxyribonucleic acid (DNA) template by different irradiation techniques.5,27 In both cases, DNA acted as a template and NPs grew on the same to produce nanowires. Recently, Teng et al. synthesized Pt nanowires into a mixture of octadecylamine (ODA) and toluene in the presence of n-dodecyl trimethyl ammonium bromide (DTAB) as a phasetransfer catalyst.28 They reduced the Pt ions using sodium borohydride. We recently synthesized shape-selective Pt NPs using UV photoirradiation techniques.29 Most of the former synthesis methods of nanowires require the addition of Pt seed particles as a separate step, long reaction times, and multiple steps, or they produce a mixture of variable shapes with low yields. In the present work, we report the synthesis of electrically conductive Pt nanowires using a simple photochemical approach. The synthesis was done by the reduction of Pt ions in CTAB (cetyl trimethyl ammonium bromide) micellar media in the presence of a new reducing agent, alkaline 2,7-dihydroxy naphthalene (2,7-DHN), under 3 h of UV photoirradiation. The method generates nanowires of different sizes just by altering the Pt (IV) ion-to-CTAB molar ratios and by changing other reaction parameters. The synthesized long Pt nanowires were

10.1021/jp9120589  2010 American Chemical Society Published on Web 04/13/2010

Electrically Conductive Pt Nanowires

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TABLE 1: Final Concentrations of All the Reaction Parameters, Time of UV Photoirradiation, and Particle Size and Shape Distribution for the Formation of Pt Nanowires set no. 1 2 3

final concn of final concn of final concn of final concn of time of UV CTAB (M) Pt(IV) solution (M) 2,7-DHN (M) NaOH (M) irradiation (h) 7.59 × 10-2 7.40 × 10-2 7.22 × 10-2

7.59 × 10-4 9.87 × 10-4 1.20 × 10-3

1.51 × 10-3 1.48 × 10-3 1.44 × 10-3

1.26 × 10-2 1.23 × 10-2 1.20 × 10-2

found to be electrically conductive and follow Ohm’s law. Moreover, the Pt nanowires act as an effective catalyst for the reduction of organic dye molecules in the presence of NaBH4 in a short time scale. To the best of our knowledge, the synthesis of electrically conductive Pt nanowires using a simple photochemical method and their catalytic activity have not been explored before. The present synthesis process and the catalysis study are simple, straightforward, reproducible, and costeffective. Experimental Section Reagents. The 2,7-dihydroxynapthalene (2,7-DHN) was obtained from Sigma and recrystallized in hot water. Cetyltrimethylammonium bromide (CTAB, 99%), platinum chloride (PtCl4), and sodium hydroxide (NaOH) were also purchased from the same source. Three different dye molecules, Rose Bengal (C20H2Cl4I4Na2O5, RB), Methylene Blue (C16H18N3SCl, MB), and Rhodamine B (C28H31N2O3Cl, RhB) were obtained from Sigma and used as received. Sodium borohydride (NaBH4) was also purchased from Sigma. Deionized (DI) water was used for the entire synthesis. Instruments. The UV-visible (UV-vis) absorption spectra were recorded in a Hitachi (model U-4100) UV-vis-NIR spectrophotometer equipped with a 1 cm quartz cuvette holder for liquid samples. A high-resolution transmission electron microscope (HR-TEM) (JEOL JEM 2010) was used at an accelerating voltage of 200 kV. The energy-dispersive X-ray spectrum (EDS) was recorded with an Oxford Instruments INCA energy system connected with the TEM. The XRD analysis was done with a scanning rate of 0.020 s-1 in the 2θ range of 25-65° using a Bruker-AXS D8 Advanced Bragg-Brentano X-ray powder diffractometer with Cu KR radiation (λ ) 0.154178 nm). The X-ray photoelectron spectroscopy (XPS) analysis was carried out using a Kratos Axis Ultra Imaging X-ray photoelectron spectrometer with monochromatic Al KR line (1486.7 eV). The instrument integrates a magnetic immersion lens and charge neutralization system with a spherical mirror analyzer, which provides real-time chemical state and elemental imaging using a full range of pass energies. The emitted photoelectrons were detected by the analyzer at a passing energy of 20 eV with an energy resolution of 0.1 eV. The incident X-ray beam is normal to the sample surface, and the detector is 45° away from the incident direction. The analysis spot on the sample is 0.4 mm × 0.7 mm. An electrically conducting atomic force microscope (AFM) was used for scanning the surfaces for the conductivity (I-V) study. A Nano-R AFM system (Pacific Nanotechnology Inc./Agilent Tech.) adapted for measuring currents through conducting samples was utilized in measuring the conductance of the nanowires. The highly doped n-type silicon tip was coated with a CVD deposited diamond-like carbon (DLC) approximately 100 nm thick. The doped diamond coating enhances the conductivity of the tip and allows electrical continuity. A voltage divider (Shark Box, PNI) was used to separate a supply voltage from a picoameter that was set up to measure currents flowing from the tip to the sample. Because even small currents can damage the tip’s thin conducting layer,

3 3 3

shape of the particles longer wires medium wires short wires

particle shape distribution length (L) and diameter (D) of the wires 100% wires 100% wires 100% wires

L ) >2 µm and D ) ∼45 ( 5 nm L ) 1.3 ( 0.2 µm and D ) ∼40 ( 5 nm L ) 300-500 nm and D ) ∼40 ( 5 nm

a 10 MΩ impedance was inserted to reduce currents. Furthermore, small contact forces were maintained in order to preserve the fidelity of the probe during measurement. A xenon lamp from Newport Corporation at a wavelength of 260 nm on the sample was used for UV photoirradiation. The approximate intensity was 20 µW, and the distance of the sample from the light source was 16 cm. The sample was placed over a wooden box with a stand to make the light shine on it directly. Photochemical Synthesis of Size-Selective Pt Nanowires. Pt nanowires with different lengths were synthesized by varying the concentration of Pt(IV) salt in a solution mixture containing CTAB, 2,7-DHN, and NaOH. For a typical synthesis process, 30 mL of 0.1 M CTAB was mixed with 3 mL of 10-2 M Pt(IV) solution. A 6 mL portion of 10-2 M 2,7-DHN and 500 µL of 1 M NaOH were then mixed together. The solution mixture was stirred for 10-15 s using a magnetic stirrer. Finally, the solution mixture was placed in front of the UV irradiation source continuously for 3 h, and stirring was continued during the whole period of the irradiation process. The process exclusively generates longer Pt nanowires. For the synthesis of other sizes of nanowires, we varied the concentration of CTAB to the Pt(IV) ions, keeping other reaction parameters fixed. The overall final concentrations of all reaction parameters and UV irradiation time are summarized in Table 1. Initially, the mixed solution became slightly reddish-orange in color after 10-15 min of UV irradiation. The mixed solution became yellowish-green after 30 min of UV irradiation, dark green after 1-2 h, and finally blackish-green after completion of the reaction (3 h of UV irradiation). The solution mixture was centrifuged at 6000 rpm for 20 min and 4000 rpm for 10 min to remove the excess surfactant and other chemicals from the Pt NPs solution. The precipitated Pt NPs were blackish-brown in color and redispersed in DI water. The Pt NPs solution was found to be stable for more than 3 months in the dark and stored at 4 °C in a refrigerator. Catalytic Reduction of Organic Dye Molecules Using Pt Nanowires As Catalyst. The reduction of organic dye molecules, such as RB, MB, and RhB, with NaBH4 were chosen as a model reaction for the efficiency testing of Pt nanowires as a catalyst. A stock NaBH4 0.1 M solution was freshly made and stored in the refrigerator in the dark. The reduction of the dyes was carried out in a quartz cuvette having a path length of 1 cm. For a typical reduction, the dye solution was diluted to a concentration of 10-5 M. Now, 6 mL of 10-5 M dye solution was mixed with 500 µL of 0.1 M) NaBH4, and the solution was mixed well by shaking. Finally, 200 µL of Pt nanowire solution was added, and the progress of the reduction was monitored spectrophotometrically using an in situ UV-vis spectrophotometer. The reduction of all the dyes started within a minute and was completed in less than an hour, as observed from the UV-vis spectrum. After the completion of the reduction, the pinkish-color RB and RhB and bluish-color MB became colorless, which was further proved by the UV-vis spectrum. Finally, the catalysis rate was calculated, and a comparison study was done.

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Figure 1. UV-visible absorption spectra for the synthesis of Pt nanowires at different reaction conditions of the synthesis process: (A) the absorption spectrum of aqueous CTAB solution, (B) the absorption spectrum of aqueous 2,7-DHN solution in water showing two distinct bands peaking at 280 and 323 nm, (C) the absorption spectrum of aqueous Pt(IV) chloride solution showing a hump ranging from ∼230 to 270 nm, (D) the absorption spectrum of the mixture of alkaline 2,7DHN with CTAB and Pt(IV) solution before UV irradiation, (E) the optical absorption spectrum of the smaller Pt nanowires showing a broad absorption band in the range of 230-600 nm, (F, G) the optical absorption spectrum of medium-sized Pt nanowires and longer-sized Pt nanowires. The inset shows three different Pt nanowire solutions indicated with P1 (short in length), P2 (medium length), and P3 (longer length).

Preparation of Samples for TEM, EDS, XRD, XPS, and I-V Studies. The Pt nanowires were characterized using TEM, EDS, XPS, XRD, and conductivity (I-V) measurements. The samples for TEM and EDS were prepared by placing a drop of the corresponding Pt nanowire solution onto a carbon-coated Cu grid, followed by slow evaporation of solvent at ambient conditions. For XPS and XRD analyses, glass slides were used as substrates for thin-film preparation. The slides were cleaned thoroughly in acetone and sonicated for about 20 min. The cleaned substrates were covered with the Pt nanowire solution and dried in air. After the first layer was deposited, subsequent layers were deposited by repeatedly adding more Pt nanowire solution and drying. Final samples were obtained after 6-8 depositions and then analyzed using XPS and XRD techniques. The I-V measurements were performed using a conductive AFM probe scanning the substrate in contact mode. The aqueous nanowire solution was deposited on a silicon wafer coated with thermally evaporated gold. The surface of this sample was connected to the reference ground of the measurement circuit. The AFM’s standard scanning mode was implemented to locate a suitable area of deposited nanowires. The AFM probe was placed in contact with the nanowires, and an electrical potential was applied across the nanowire via the AFM probe. The applied electrical potentials through the nanowire and AFM probe were increased from -3500 to 3500 mV in 1000 mV in each step in both the directions. For each potential, the current flow through the nanowire was measured with a Keithley picoammeter and recorded with a LabView program. Results and Discussion UV-vis Spectroscopic Study. Pt nanowires with variable lengths were synthesized by the chemical reduction of Pt(IV) ions in the presence of alkaline 2,7-DHN in CTAB micellar media under 3 h of UV photoirradiation. Figure 1 shows the UV-vis spectra of the reaction mixture at different stages of

Kundu et al. the synthesis process. An aqueous CTAB solution has no specific absorption band, as shown in curve A in Figure 1. An aqueous, colorless solution of 2,7-DHN shows two distinct absorption bands in the UV-vis region at 280 and 323 nm (curve B, Figure 1). These two bands appear due to the presence of aromatic rings in 2,7-DHN. Yellowish-color aqueous solutions of Pt(IV) show a small hump in the region of 230-270 nm due to ligand-to-metal charge transfer (LMCT) spectra (curve C, Figure 1).30 After the addition of NaOH with the reaction mixture containing CTAB, Pt(IV), and 2,7-DHN, two new peaks appeared at wavelengths of 249 and 340 nm, which may be due to the formation of the CTAB-Pt(IV) complex31 or due to interaction of 2,7-DHN with the complex or with Pt(IV) ions before UV photoirradiation (curve D, Figure 1). After the UV irradiation starts, the solution mixture is initially slightly reddish-orange in color and it turns to greenish-yellow after 30 min of UV irradiation. With increasing irradiation time, the color changes to dark greenish and finally blackish-brown after 3 h of UV photoirradiation. This solution exclusively contains Pt nanowires. The UV-vis spectrum of the smaller Pt nanowires shows a broad absorption band in the range of 230-600 nm (curve E, Figure 1) having similarities with the literature.22,32-34 It was also reported that the Pt NPs do not exhibit an intense band in the visible region due to a very small imaginary part of its dielectric constant. Pt nanowires with two other different sizes show similar types of adsorption bands shown in curves F and G in Figure 1. Curve F is the absorption band of medium-sized Pt nanowires, whereas curve G is the absorption band of longer Pt nanowires. The synthesized Pt nanowire solutions are stable and were stored in a sealed bottle in a refrigerator at 4 °C for long-term use. The inset of Figure 1 shows three different Pt nanowire solutions indicated with P1 (short in length), P2 (medium length), and P3 (longer length) after successive centrifugation and redispersion in DI water. Transmission Electron Microscopy (TEM) Analysis. Figure 2 displays the transmission electron microscopy (TEM) images of the different sizes of Pt nanowires at different magnifications after 3 h of UV photoirradiation. Figure 2A,B shows the low- and high-magnified TEM images of short Pt nanowires. The insets show their corresponding higher-magnified images. From Figure 2A, the length of the nanowires are in the range of 300-500 nm. There are a few short nanowires also observed. The average diameters of the wires are ∼40 ( 5 nm. The inset of Figure 2B shows a very high-magnified image of the nanowires. The orientation of different crystal planes is clearly visible from that image. Figure 2C,D shows the TEM images of medium length Pt nanowires at different magnifications. Insets show their corresponding higher-magnified images. From Figure 2C, the average length of the nanowires is 1.3 ( 0.2 µm and the average diameter of the nanowires is ∼40 ( 5 nm. Figure 2D shows a single nanowire, and the inset shows the higher-magnified image. Panels E and F in Figure 2 are TEM images of longer Pt nanowires. Figure 2E is the low-magnified image, where the average length of the wires is >2 µm. The inset shows the corresponding highermagnified image. Figure 2F shows the higher-magnified image, and the inset shows the image of a single nanowire. The average diameter of the nanowires is ∼45 ( 5 nm. From the above TEM analysis, it is clear that we are able to synthesize nanowires with a short length, medium length, and longer length by changing the reaction parameters. One thing is interestingly noted that the diameters of the wires are almost the same in all the cases, whereas the length of the wires increases with the decrease in Pt(IV) concentrations in the reaction medium.



Figure 2. Transmission electron microscopy (TEM) images of different sizes of Pt nanowires are shown in A-F. (A, B) The low- and highmagnified TEM images of short Pt nanowires. The lengths and diameters of the nanowires are in the ranges of 300-500 and ∼40 ( 5 nm, respectively. The insets show their corresponding higher-magnified images. (C, D) The TEM images of medium length Pt nanowires. The average length and diameter of the nanowires are 1.3 ( 0.2 µm and ∼40 ( 5 nm, respectively. Insets show their corresponding higher-magnified images. (E, F) The TEM images of longer Pt nanowires. The average lengths of the wires are >2 µm. The average diameters of the wires are ∼45 ( 5 nm.

Energy-Dispersive X-ray Spectroscopy (EDS) Analysis. Figure 3 represents the results obtained from the energydispersive X-ray spectroscopy (EDS) analysis. An EDS analysis is used to determine the chemicals present in the reaction product. The spectrum consists of different peaks for Pt, C, Cu, Cl, Cr, and Br. The C and Cu peaks came from the Cu TEM grid used for the analysis. The Cl peak came from the

chlorinated salt of Pt used for the synthesis. The Cr peak came from the TEM sample holder. The Pt peak came from the Pt nanowires, and the Br peak came from the CTAB used for the stabilization of the Pt nanowires. X-ray Diffraction (XRD) Analysis. The X-ray diffraction (XRD) patterns of the Pt nanowires are shown in Figure 4. The diffraction pattern in the 2θ range of 25-65° corresponds to

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Figure 3. Energy-dispersive X-ray spectroscopic (EDS) analysis of the Pt nanowires shows different peaks for Pt, C, Cu, Cl, Cr, and Br.

Figure 5. X-ray photoelectron spectra (XPS) of the Pt nanowires: (A) the overall survey spectra and (B) the spectra of the Pt 4f region. Figure 4. Powder X-ray diffraction (XRD) patterns of the Pt nanowires.

diffraction from the (111) and (200) lattice planes. As the intensity of the reflection is directly proportional to the X-ray coherence length of the crystal, the relative intensities of these reflections also vary with NP shape. The X-ray diffraction pattern confirmed the formation of face-centered cubic (fcc) Pt having space group Fm3m (225) with lattice constant a ) 0.3923 nm (JCPDS card no. 04-0802). All the peaks are well matched with the previous theoretical and experimental XRD analysis by Fukuoka et al. for their study on Pt nanowires.24 A small intense peak at a 2θ value of 53.7° is observed and matched with the report by Somorjai’s group for their synthesis of Pt NPs on mesoporous silica.35 Another two high intensity peaks at low 2θ values (28° and 31.5°) are also observed and matched with the literature report on Pt NPs.36 We used CTAB as a stabilizing agent, and the selective interaction of CTAB with different crystal planes of Pt nanowires might change the growth rates and intensity of the different crystal planes. X-ray Photoelectron Spectroscopy (XPS) Analysis. Figure 5 represents the overall survey and Pt 4f XPS spectra of the Pt nanowires. The survey spectrum in Figure 5A contains the characteristic peaks of O 1s at 528.3 eV and C 1s at 281.9 eV. The Pt 4d region is characterized by a doublet that arises from spin-orbit coupling (4d5/2 and 4d3/2). The peak positions are

313.8 and 331.3 eV for 4d5/2 and 4d3/2, respectively. The Pt 4f region is characterized by a doublet that arises from spin-orbit coupling (4f7/2 and 4f5/2). This Pt 4f region is enlarged in Figure 5B. The peak positions are 71.10 and 73.99 eV for 4f7/2 and 4f5/2, respectively. These two binding energies of Pt 4f7/2 and 4f5/2 in the XPS spectra of the CTAB-coated Pt nanowires correspond to platinum in the zero-valent state. The intensity of these peaks is somehow lower than the literature reports. Two other high intensity peaks due to Pt were observed at binding energy values of 72.7 eV (Pt 4f7/2) and 75.98 eV (Pt 4f5/2). These two peaks are probably due to the formation of organometallic compounds with Pt. The positions of the above peaks are in good agreement with the literature.32,37 A few other small features at other binding energies are also observed. The peaks at 399.68 and 151.7 eV are attributed to Na 1s and Br 3p, respectively. The Na 1s peak came as we used NaOH during our synthesis, and Br 3p came from the surfactant CTAB used as a capping agent in our synthesis. A small peak at 101.2 eV for Si 2p is also identified and probably came from the glass substrate used for sample preparation. Study of Other Reaction Parameters. In our experiment, we conducted control experiments in order to evaluate the effects of other reaction variables. We tested varied concentrations of Pt(IV) ions, CTAB, 2,7-DHN, and NaOH. The Pt nanowires with different lengths are formed only at the concentrations



Figure 6. Transmission electron microscopy (TEM) image of Pt nanowires when the irradiation time is ∼2 h. From the image, it is clear that the wires are not fully formed.

given in Table 1. From Table 1, it is clearly observed that nanowires are formed at higher concentrations of CTAB (∼10-1 M). At lower concentrations of CTAB (e10-3 M), mostly spherical particles with a mixture of other anisotropic shapes are formed (not shown here). We varied the concentration of Pt(IV) ions and have seen that nanowires formed when the Pt(IV) ion concentration was ∼10-3 M. At lower concentrations of Pt(IV) ions (e10-4 M), longer irradiation time is required for the formation of particles, whereas at very high concentrations of Pt(IV) ions (g10-2 M), particles formed anisotropic shapes with an aggregated structure. In addition, we varied the concentration of 2,7-DHN and observed that, at higher 2,7-DHN concentration, the Pt particles are formed and get precipitated immediately, whereas at lower concentration, longer irradiation time is required for the formation of particles. The effects of NaOH for the formation of wires are studied in detail. The addition of NaOH changes the pH in the reaction medium, which exerts a strong influence on the reducing capability of 2,7-DHN and, in turn, on the nanowire formation. In our reaction, in the absence of NaOH, keeping all other reaction parameters fixed, no Pt nanowires are formed. Under the alkaline condition in the presence of NaOH, Pt nanowires are formed with higher yields. We tested other concentrations of NaOH and observed that, at very high concentration (g10-1 M), the Pt particles formed but were not very stable and precipitated within a few hours after the synthesis. This might be due to the formation of its oxide or hydroxide compound at higher alkaline conditions. Therefore, an appropriate concentration of OH- ions plays the key role for the nanowire formation by increasing the reducing power of 2,7-DHN. We also examined the effect of varying UV irradiation time and saw that a 3 h time is sufficient for the formation of the nanowires. When the irradiation time is longer (g6 h), the particles formed aggregated structures. When the irradiation time is shorter (∼2 h), the nanowires were not fully formed, as represented in Figure 6. From Figure 6, it is clear that the particles are growing wirelike shapes but are not continuous, as indicated with red arrows. The proper reagent concentrations and reaction parameters are extremely important for the formation of definite shaped Pt NPs. Conductivity (I-V) Measurement. The electrical conductivity of the CTAB-terminated long Pt nanowires were measured using an AFM probe. Here, we first deposited the Pt nanowires on a conducting substrate and then measured their conductivity. AFM probes have been used extensively to image and manipu-

Figure 7. Current (I) vs voltage (V) measurements of the Pt nanowires. (A) The interactive drawing of the conductivity (I-V) measurement. (B) The I-V curve of the long Pt nanowires, which maintains the Ohm’s law, having a resistance of 10 GΩ.

late atoms and structures on a variety of surfaces.38 We previously measured the conductivity of Pd and CdS nanowires embedded in a DNA template using the same AFM setup.39,40 Here, we constructed the current (I)-voltage (V) curve using an AFM with an external applied potential and measured the current. The experimental setup was discussed elsewhere.41 A drawing of the conductivity (I-V) measurement is shown in Figure 7A. The AFM probe was positioned on top of a nanowire, and the current was required to travel through the wires before touching the surface and then to the picoammeter. Figure 7B compares the I-V curve of the long Pt nanowire. From the curve, it is clearly shown that the current is increasing linearly with the applied potential and maintains Ohm’s law, indicating very good contact of the Pt nanowires with the substrate. We have not observed any hysteresis in the I-V measurement, which indicates the continuous structure of the Pt nanowires. The resistance of the sample is 10 GΩ, and resistance for the baseline (circuit resistance) is 10 MΩ. The linear fit equation for the sample is y ) 1 × 10-4 × -0.0446. This high value of resistance is an upper bound on the resistance of the nanowire itself as the nanowire/substrate contact resistance being significant. We believed that the contact resistance played a crucial role for the conductivity measurement. Such contact resistance was due to the nanowire/substrate and the excess surfactant that was present with the particles’ surface. It seems that the surfactant may have inhibited the conductivity of the nanowire by making a thin coating on the conducting AFM tip. The excess surfactant was attempted to be removed from the particles’ surface through repeated centrifugation. Although, a small amount of surfactant cannot be removed from the particles’ surface, which might play an important role in the conductivity measurement. In our previous study with Pd and CdS nanowires in DNA, we also observed the linear relationship and the result

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follows the Ohm’s law.39,40 It is noted that our setup is limited in measuring the lateral conductivity of a single nanowire as they immobilized on the substrate without any directionality. The Pt nanowires we deposited on a substrate were stable for more than 3 months, after which time the measured conductivity was similar to that of nanowires immediately after synthesis. This high stability of the Pt nanowires will make them suitable in nanodevice fabrication for different types of interconnections. Mechanism of the Pt Nanowire Formation. After the redispersion process, Pt nanowires have maintained their shape and been stable for a few months in the DI water solution. The formation of different-sized Pt nanowires is caused by the reduction of Pt(IV) ions in an alkaline 2,7-DHN solution under 3 h of continuous UV irradiation. All experiments were done in an ambient environment at room temperature. In our process, the presence of both 2,7-DHN and CTAB is necessary for the formation of Pt nanowires. In the absence of 2,7-DHN, no Pt nanowires were formed due to the lack of a reducing agent in the solution. In the absence of CTAB, micrometer-sized Pt NPs were formed and precipitated immediately due to lack of a stabilizing agent. A proper concentration of NaOH plays a crucial role by maintaining the pH at 8-9 and by enhancing the reducing power of 2,7-DHN. A proper concentration of hydroxide ions can facilitate the growth of Pt particles into various shapes.42 The alkaline 2,7-DHN has hydroxyl groups on its skeleton and acts as a reducing agent. It was reported earlier that compounds with hydroxyl groups, such as polyvinyl alcohol,43 ascorbic acid,44 TX-100,45 and benzophenone,46 acted as reducing agents in nanomaterial synthesis. In the presence of UV light, the hydroxyl compound undergoes photolytic cleavage of the hydroxyl bond to generate radical species. It was previously reported that photogenerated hydroxyl radicals could reduce the metal ions.43-46 Thus, the hydroxyl radicals generated from 2,7-DHN reduced the Pt(IV) ions to Pt(0). Initially, after the reduction of Pt(IV) to Pt(0), the reaction lead to the formation of smaller-sized Pt nuclei. The small Pt(0) nuclei subsequently formed Pt atoms and assembled together to form Pt particles. Once the particles were formed, the surfactant CTAB molecule attached to the surface of the particles and prevented them from random aggregation. At a low CTAB concentration (e10-2 M), the interaction of the cationic CTA+ part with the surface of the Pt particles was low and growth took place in all possible directions and produced mostly the spherical particles. When the CTAB concentration was high (g10-1 M), mostly anisotropic shapes were formed. It was reported earlier that, with high surfactant concentration (g10-1 M), the aqueous CTAB molecule formed rodlike or wormlike micellar templates.22,47 Murphy et al. described earlier that rodlike micellar templates facilitated the formation of Au nanorods.47 In our study, we believe that, with high CTAB concentrations, the CTAB generated rod-/wormlike micellar templates and the particles grew on that templates and generated nanowires. The formation of definite shapes mostly depended on two factors. The first is the faceting tendency of the stabilizing agent and the second, the growth kinetics, that is, the rate of supply of Pt(0) to the different crystallographic planes.48 In our experiment, we irradiated the solution mixture at ∼260 nm, which was close to the absorption band of 2,7DHN. To check the radical species generated at that wavelength, we irradiated the reaction mixture far above (∼365 nm) and far below (∼190 nm) the absorption band of 2,7-DHN. In those cases, no Pt NPs formed in the experimental time scale, which proved that the generation of radical species was responsible for the reduction of Pt(IV) ions to Pt(0). Initially, small Pt nuclei

Kundu et al. SCHEME 1: Schematic Presentation for the Formation of Pt Nanowires

were formed, and with time, they transformed to Pt atoms and, finally, Pt particles. The formation of small Pt particles during the early stage of reaction was also observed from TEM images (not shown here). These Pt particles grew on the CTAB templates to produce the desired nanowires. The overall process for the nanowire generation is shown in Scheme 1. Shelnutt’s group reported earlier that Pt nanowire networks could be formed using a soft template for metal growth.22 Pileni also reported earlier that a soft template could control the size and shape of the NPs by emphasizing the potential of soft templates for shape selectivity of NPs.49 From Table 1, it is clear that short wires are formed comparatively by low CTAB concentrations but high Pt(IV) concentrations. Long wires were formed at high CTAB and low Pt(IV) concentrations. After the synthesis, we studied the catalytic activity of Pt nanowires for the reduction of different organic dye molecules in the presence of sodium borohydride as a reducing agent. As an example, we used only the short Pt nanowires in the catalysis study. Catalytic Reduction of Organic Dye Molecules Using Pt Nanowires As Catalyst. The catalytic reduction of organic dye molecules using NaBH4 in the presence of Pt nanowires as a catalyst was studied in detail. The synthesized Pt nanowires are centrifuged several times to remove excess surfactant. They were then redispersed in DI water before using as a catalyst for dye reduction. During a catalysis reaction, the reducing agent needed fresh nanoparticle surfaces to transfer electrons. If there was surfactant trapped on the particles’ surface, the catalysis rate would be slow. Thus, we removed most of the excess surfactant by repeated centrifugation. Here, we selected three different dye molecules, one cationic dye, MB, and two anionic dyes, RB and RhB, having different organic skeletons. These dye molecules are highly soluble in water and used as commercial colorants in textile dyes to meet the color requirements in the dye industry. Most of the dye molecules have become a major concern in environmental pollution. Therefore, suitable methods are extremely important to degrade these dyes to avoid wastewater contamination. Several methods have been reported to degrade the dyes but do not seem to be environmentally friendly or need harsh reduction conditions.50-52 The dye reduction in surfactant media using UV irradiation was studied before.53 Here, we chose three different water-soluble dye molecules, and the advantage for this reduction with NaBH4 is that the reduction can be monitored easily using a UV-vis spectrophotometer as there was no noticeable byproduct formed.



Figure 8. UV-vis absorption spectra for the reduction of three different dye molecules with NaBH4 using Pt nanowires as a catalyst. (A) The UV-vis spectrum for the successive reduction of RB at 548 nm. (B) The ln(abs) vs time (T) plot having the rate constant value of 6.05 × 10-2 min-1. (C, D) The UV-vis spectrum for the successive reduction of MB at 663 nm and the ln(abs) vs time (T) plot, respectively, having a rate constant value of 2.08 × 10-1 min-1. (E, F) The successive reduction for RhB at 552 nm and the ln(abs) vs time (T) plot, respectively, having a rate constant value of 1.03 × 10-1 min-1. The insets of (A), (C), and (E) show the corresponding dye images, and those of (B), (D), and (F) show their reduced product.

The pH of the different solution before reduction was measured using a pH meter. The pHs of aqueous RB, MB, and RhB solutions are 6.85, 7.58, and 6.81, respectively. The pH of 0.1 M NaBH4 solutions is 10.56, and the pH of the short Pt nanowire solution is 6.76. The dye reduction using metal NPs as a catalyst

has been focused a little.50-52,54 The reduction of these dyes without Pt nanowires as a catalyst, but in the presence of NaBH4, is very slow. Again, the reduction with Pt nanowires in the absence of NaBH4 does not proceed at all, even after a day or two. A proper combination of the dye molecule, NaBH4, and

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TABLE 2: Final Concentrations of All Reactants, Reduction Time, and Rate Constant Values for the Reduction of Dye Molecules with NaBH4 in the Presence of Pt Nanowires as Catalyst name of the dye molecules

final concn of dye (M)

final concn of NaBH4 (M)

volume of Pt nanowire added (mL)

time for full reduction (min)

first-order rate constant (k) (min-1)

correlation coefficient (R)

standard deviation (SD)

Methylene Blue (MB) Rhodamine B (RhB) Rose Bengal (RB)

8.95 × 10-4 8.95 × 10-4 8.95 × 10-4

2.98 × 10-3 2.98 × 10-3 2.98 × 10-3

0.2 0.2 0.2

10 27 40

2.08 × 10-1 1.03 × 10-1 6.05 × 10-2

0.982 0.988 0.9862

0.129 0.144 0.1462

Pt nanowires is necessary for this catalysis reaction. The characteristic absorption peaks for RB, MB, and RhB are located at 548, 663, and 552 nm, respectively, in the UV-vis spectrum. After the addition of NaBH4 and catalytic amounts of Pt nanowires as catalyst, the reduction starts spontaneously; that was clearly observed by the successive decrease of the absorbance value in the UV-vis spectrum with time. The absorption maximum of RB is at 548 nm, which decreases gradually with time and is completed after 40 min. The UV-vis spectrum for the RB reduction is shown in Figure 8A. Figure 8B shows the ln(abs) versus time (T) plot for the reduction of RB using Pt nanowires as a catalyst, and the rate constant value for this reduction was found to be 6.05 × 10-2 min-1. Figure 8C,D shows the successive reduction in the UV-vis spectrum for MB at 663 nm and ln(abs) versus time (T) plot curves, respectively. Similarly, Figure 8E,F shows the successive reduction in the UV-vis spectrum at 552 nm and the corresponding ln(abs) versus time (T) plot, respectively, for RhB. The concentration of the dye molecules, reduction time, rate constant values, etc. are given in detail in Table 2. From Table 2, it is clear that the reduction is the fastest for MB, medium with RhB, and the slowest with RB. All of the reduction processes maintain first-order kinetics. From a mechanistic point of view, NaBH4 transfers the electron to the dye molecule via the Pt nanowires and the dye molecule is reduced to colorless. The insets of panels (A), (C), and (E) in Figure 8show the corresponding dye images before reduction, and those of panels (B), (D), and (F) show the reduced colorless product of the dye after complete reduction. The fastest reduction in the case of MB might be due to the better adsorption of the dye on the Pt nanowire surfaces. This might be due to the presence of nitrogen and sulfur in the MB skeleton and the metal particles having a strong affinity toward the nitrogen or sulfur compound, so the electron transfer becomes fast and reduction takes a shorter time to complete. In the case of RB, the slowest rate was observed. This might be due to the presence of bulky halide ions in the RB skeleton, and it was observed that adsorption of the dye on the Pt nanowires’ surface became slow. For RhB, an intermediate rate was observed. In the above catalysis study, all the reductions were completed in less than an hour. By exploiting such a procedure, it will be possible to degrade other varieties of dye molecules using Pt nanowires as a catalyst and will be discussed in the near future. Conclusion In summary, we have demonstrated a simple, reproducible, and facile method for the synthesis of electrically conductive Pt nanowires. The nanowires are synthesized by the reduction of Pt(IV) ions in the presence of an alkaline 2,7-DHN solution in a CTAB micellar media under 3 h of UV irradiation. The process generates Pt nanowires with different lengths just by tuning the Pt(IV) ion-to-CTAB molar ratios and other reaction parameters. The nanowires are found to be electrically conductive and maintain Ohm’s law with the electrodes. Finally, the nanowires were found to exhibit excellent catalytic activity for the reduction of different organic dye molecules in the presence

of NaBH4. The Pt nanowire helps during the electron-transfer process from the NaBH4 to the dye molecule. In the future, these results offer a powerful platform for the synthesis of other multifunctional nanowires in a short time scale and offer potential applications in nanoelectronics, catalysis, and biomedical purposes. Acknowledgment. This research was, in part, sponsored by the NSF-0506082; the Department of Mechanical Engineering, Texas A&M University; and the Texas Engineering Experiments Station. Assistance by Mr. Ke Wang in XPS and XRD analysis was greatly appreciated. Support from the Microscopy Imaging Center (MIC) by Dr. Zhiping Luo and the Materials Characterization Facility (MCF), at the Texas A&M University, was greatly appreciated. References and Notes (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Mandal, M.; Kundu, S.; Sau, T. K.; Yusuf, S. M.; Pal, T. Chem. Mater. 2003, 15, 3710. (3) Kundu, S.; Wang, K.; Liang, H. J. Phys. Chem. C 2009, 113, 5157. (4) Thomas, J. M. Pure Appl. Chem. 1988, 60, 1517. (5) Kundu, S.; Liang, H. Langmuir 2008, 24, 9668. (6) Kundu, S.; Lau, S.; Liang, H. J. Phys. Chem. C 2009, 113, 5150. (7) Kundu, S.; Wang, K.; Liang, H. J. Phys. Chem. C 2009, 113, 18570. (8) Rolison, D. R. Science 2003, 299, 1698. (9) Ascarelli, P.; Contini, V.; Giorgi, R. J. Appl. Phys. 2002, 91, 4556. (10) Sakurai, H.; Haruta, M. Appl. Catal., A 1995, 127, 93. (11) Kundu, S.; Mandal, M.; Ghosh, S. K.; Pal, T. J. Colloid Interface Sci. 2004, 272, 134. (12) Lu, Z.; Liu, G.; Phillips, H.; Hill, J. M.; Chang, J.; Kydd, R. A. Nano Lett. 2001, 1, 683. (13) Nakano, S.; Akedo, J.; Ogiso, H. Surf. Coat. Technol. 2004, 187, 167. (14) Wada, K.; Yano, K.; Kondo, T.; Mitsudo, T. Catal. Today 2006, 117, 242. (15) Chen, J.; Xiong, Y.; Yin, Y.; Xia, Y. Small 2006, 2, 1340. (16) Petroski, J. M.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. A 2001, 105, 5542. (17) Jana, N. R.; Peng, X. J. Am. Chem. Soc. 2003, 125, 14280. (18) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343. (19) Chen, Y.; Johnson, E.; Peng, X. J. Am. Chem. Soc. 2007, 129, 10937. (20) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (21) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663. (22) Song, Y.; Garcia, R. M.; Dorin, R. M.; Wang, H.; Qiu, Y.; Coker, E. N.; Steen, W. A.; Miller, J. E.; Shelnutt, J. A. Nano Lett. 2007, 7, 3650. (23) Zhong, Y.; Xu, C.; Kong, L.; Li, H. Appl. Surf. Sci. 2008, 255, 3388. (24) Fukuoka, A.; Sakamoto, Y.; Higuchi, T.; Shimomura, N.; Ichikawa, M. J. Porous Mater. 2006, 13, 231. (25) Yang, M.; Qu, F.; Lu, Y.; He, Y.; Shen, G.; Yu, R. Biomaterials 2006, 27, 5944. (26) Bore, M. T.; Ward, T. L.; Fukuoka, A.; Datye, A. K. Catal. Lett. 2004, 98, 167. (27) Kundu, S.; Liang, H. AdV. Mater. 2008, 20, 826. (28) Teng, X.; Han, W.; Ku, W.; Hucker, M. Angew. Chem. 2008, 120, 2085. (29) Kundu, S.; Liang, H. Langmuir [Online early access]. DOI: 10.1021/ la904070n, 2010. (30) Hoggard, P. E.; Bridgeman, A. J.; Kunkely, H.; Vogler, A. Inorg. Chim. Acta 2004, 357, 639. (31) Krishnaswamy, R.; Remita, H.; Impe´ror-Clerc, M.; Even, C.; Davidson, P.; Pansu, B. Chem. Phys. Chem. 2006, 7, 1510. (32) Yang, W.; Yang, C.; Sun, M.; Yang, F.; Ma, Y.; Zhang, Z.; Yang, X. Talanta 2009, 78, 557.

Electrically Conductive Pt Nanowires (33) Chen, S.; Kimura, K. J. Phys. Chem. B 2001, 105, 5397. (34) Teranishi, T.; Kurita, R.; Miyake, M. J. Inorg. Organomet. Polym. 2000, 10, 145. (35) Konya, Z.; Puntes, V. F.; Kiricsi, I.; Zhu, J.; Alivisatos, P.; Somorjai, G. A. Catal. Lett. 2002, 81, 137. (36) Bera, P.; Priolkar, K. R.; Gayen, A.; Sarode, P. R.; Hegde, M. S.; Emura, S.; Kumashiro, R.; Jayaram, V.; Subbanna, G. N. Chem. Mater. 2003, 15, 2049. (37) Dablemont, C.; Lang, P.; Mangeney, C.; Piquemal, C.; Petkov, V.; Herbst, F.; Viau, G. Langmuir 2008, 24, 5832. (38) Porti, M.; Gerardin, S.; Nafrı´a, M.; Aymerich, X.; Cester, A.; Paccagnella, A. IEEE Trans. Nucl. Sci. 2007, 54, 1891. (39) Kundu, S.; Wang, K.; Huitink, D.; Liang, H. Langmuir 2009, 25, 10146. (40) Kundu, S.; Lee, H.; Liang, H. Inorg. Chem. 2009, 48, 121. (41) Bailon, M. F.; Salinas, P. F.; Arboleda, J. S. Proceedings of 12th IPFA, Singapore, 2005; p 282. (42) Filankembo, A.; Pileni, M. P. J. Phys. Chem. B 2000, 104, 5865. (43) Henglein, A. Langmuir 1999, 15, 6738. (44) Pal, A.; Pal, T. J. Raman. Spectrosc. 1999, 30, 199.

J. Phys. Chem. C, Vol. 114, No. 17, 2010 7709 (45) Pal, A. Talanta 1998, 46, 583. (46) Sato, T.; Maeda, N.; Ohkoshi, H.; Yonezawa, Y. Bull. Chem. Soc. Jpn. 1994, 67, 3165. (47) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (48) Petroski, J. M.; Wang, Z. L.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 3316. (49) Pileni, M. P. Nat. Mater. 2003, 2, 145. (50) Xie, Y.; Chen, F.; He, J.; Zhao, J.; Wang, H. J. Photochem. Photobiol., A 2000, 136, 235. (51) Kamat, P. V.; Das, S.; Thomas, K. G.; George, M. V. Chem. Phys. Lett. 1991, 178, 75. (52) Kosanie, M. M.; Trickovic, J. S. J. Photochem. Photobiol., A 2002, 149, 257. (53) Kundu, S.; Panigrahi, S.; Pal, A.; Ghosh, S. K.; Nath, S.; Praharaj, S.; Basu, S.; Pal, T. Dyes Pigm. 2006, 69, 177. (54) Al-Hassan, L. A.; Al-Amro, F. S. J. Photochem. Photobiol., A 1998, 112, 165.

JP9120589

Formation and Catalytic Application of Electrically Conductive Pt

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