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Synthesis of Metallic Nanostructures Using Chemical Fluid Deposition Candy S. Lin,† Frank Leung-Yuk Lam,† Xijun Hu,† Wing Yim Tam,*,‡ and Ka M. Ng† Department of Chemical Engineering, and Department of Physics and Institute of Nano Science and Technology, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China ReceiVed: March 2, 2008; ReVised Manuscript ReceiVed: April 8, 2008
We report the synthesis of metallic nanostructures by a chemical fluid deposition method using carbon dioxide supercritical fluid as the reaction medium. Ordered platinum nanowires were molded out from a mesoporous silica template, SBA-15, with a pore channel diameter of 7 nm. A solubility study of the platinum precursor, platinum(II) dimethylcyclooctadiene, in the supercritical carbon dioxide was carried out to obtain a phase diagram in order to optimize the operating parameters required for the metal deposition process. High-purity and uniform nanowires with large aspect ratios were obtained at moderate temperatures and pressures. Introduction Metallic nanostructures are gaining interest for their extraordinary properties encountered at such small scales as potential building blocks for developing nanodevices in a variety of applications.1–3 In particular, the unique electrical, magnetic, and optical properties4,5 hold promise for applications in modern electronics and photonics.6 Current methods for nanostructure synthesis include electrodeposition,7,8 laser ablation,9 vaporliquid-solid growth methods,10,11 pressure injection method,12,13 evaporation mask method,14,15 and chemical vapor deposition (CVD).16,17 However, these methods usually require extreme reaction conditions, poisonous chemicals, or precise catalysis control. For example, CVD requires that the metals or metal oxides be in the vapor phase, which can only be achieved with suitable precursors. Recently, inorganic material synthesis by chemical fluid deposition (CFD) has received much attention because, taking advantage of the special properties of supercritical fluids, it enables the fabrication of nanostructures at much lower risks and costs than conventional methods. The supercritical fluid is a good solvent for the metal precursors and exhibits high diffusivity and low viscosity which enable an effective infiltration of the metal precursors into mesoscaled templates. Also, the density of the fluid can be adjusted by controlling the pressure and temperature, which in turn will change the solubility and diffusivity of the metal precursors several orders of magnitude greater than that of the normal phases. Watkins and Blackburn and co-workers had explored extensively the use of the CFD method for synthesizing metallic nanofilms (Pt, Pd, Au, Ni) on silica wafer and polymer membranes.18–21 The purity of these films was high with thickness controlled by changing the concentration of reactants in the system. Korgel and Hanrath and co-workers had also published work on synthesizing nanowires of semiconducting materials, such as silicon, germanium, and gallium arsenide22–25 using gold seeding particles. Formation of metallic nanowires were first explored by Crowley et al. by using copper and other semiconductor precursors to infiltrate into silica matrixes.26,27 * Corresponding author. E-mail:
[email protected]. Phone: 852-2358-7490. Fax: 852-2358-1652. † Department of Chemical Engineering. ‡ Department of Physics and Institute of Nano Science and Technology.
They found that metallic nanotubes could be produced by controlling the precursor and reducing agent concentrations. Wai and co-workers had published work on the different forms of metallic and semimetallic nanostructures on different substrates such as carbon nanotubes, silicon, and germanium substrates, and other metallic nanocomposite applications.28–30 However, thorough research is required for obtaining a high success rate of producing good quality nanowires at high yield to satisfy the increasing demand for exploring nanowire applications or in fabricating nanodevices. Among the various metals used in CFD, platinum is of particular interest due to recent investigations in its optical properties in the nanoscale. In this work, we have developed a CFD technique that would synthesize platinum nanostructures using mesoporous templates as molds. A solubility study of the metal precursor, platinum(II) dimethylcyclooctadiene (PtCODMe2), in supercritical CO2 was first conducted to maximize the amount of metal precursor in the system. Then the operating condition was optimized by tuning the pressure and temperature to enhance the infiltration into the porous templates and to produce uniform metallic nanostructures. After successful metal deposition in the porous substrate, the template was removed to give the final product of pure metallic nanostructures. High-purity and uniform nanowires with large aspect ratios were obtained at moderate temperatures and pressures. Experimental Section The method used in this work utilizes the supercritical CO2 (SC-CO2) fluid as a solvent for an organometallic precursor to undergo reduction and deposition inside a mesoporous template. The carbon dioxide used in this study was supplied by Chun Wang Industrial Gases with a purity of 99.99%. The metallic precursor, PtCODMe2, used directly as received was supplied by Strem Chemicals with 99% purity. A phase equilibrium analyzer (PEA) unit, manufactured by Thar Design Technologies (PEA-30 ML), was used to determine the solubility of PtCODMe2 in SC-CO2. It consists of a syringe pump (240 mL), a volume-variable view cell (maximum volume 25 mL), a PEA vessel housing with a magnetic stirrer to agitate the contents, and an external temperature-controlled circulating bath using water/ethylene glycol mixture as the heat transfer fluid. A schematic diagram of the PEA apparatus is
10.1021/jp801845q CCC: $40.75 2008 American Chemical Society Published on Web 06/18/2008
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Figure 2. Phase diagram of PtCODMe2 in supercritical CO2 (SC-CO2) at 60 °C. Figure 1. Schematic diagram of the PEA vessel (enclosed in blue) and CFD equipment setup.
shown in the blue enclosure of Figure 1. Located at the bottom of the vessel is a sapphire window, which provides visual access via a camera connected to a monitor. The view window of the vessel is used to visually determine the point of solubility and solute precipitation (cloud point) from the appearance and disappearance of solid inside the vessel.31,32 The phase diagram can thus be obtained for the corresponding parameters such as pressure, temperature, volume, and density.33 The amount by weight of the platinum precursor PtCODMe2 was varied from 1.7 to 4.9 mg in this experiment to determine its solubility in SC-CO2. The syringe pump was first filled with CO2 to be compressed, and the pressure was maintained using a computerized autocontrol movable piston that allowed the system volume (6-25 mL) to be adjusted during operation at a temperature preset to 60 ( 0.2 °C. Temperature was monitored by a type-K thermocouple, and the pressure was measured by a Honeywell pressure transducer accurate to (0.1%. First, the volume in the vessel was adjusted until the appearance of a clear cloud point (the point where solid precursors dropped out of the solution). Once this happened, the pressure was increased again to the point where all solid precursors disappeared. The final pressure at this condition was thus recorded. With a known amount of CO2 transferred from the syringe pump to the PEA vessel, the solubility at this particular temperature and pressure could be calculated. Each solubility point was repeated twice to ensure reproducibility. After each run, the system was vented, cleaned with acetone, and dried prior to the next experiment. The solubility results are plotted in a phase diagram shown in Figure 2. Our results agree well with that obtained by Ashenbrenner et al.34 We chose operating points near the solubility surface for the synthesis of nanostructures in mesoporous templates using the CFD method as will be discussed further in the Results and Discussion section below. To fabricate the metallic nanostructures a CFD setup was built for this work with a reactor designed to withstand high temperature and pressure. A schematic diagram of this setup is shown in Figure 1 together with the PEA system for the solubility study which shares the same CO2 syringe pump. The mechanism of the CFD process is illustrated in Figure 3. First, known quantities of platinum precursor and substrate were placed inside the reaction chamber and brought to solubility temperature at 60 °C (inset 1 of Figure 3). Then CO2 was pumped into the chamber to the desired solubility pressure (ranging from 80 to 150 bar). The platinum precursor was then given a fixed time, typically around 2-4 h, to be dissolved in
Figure 3. Mechanism of metal deposition using the CFD technique.
the supercritical medium and to diffuse into the porous channels of the mesoporous silica used for this experiment, SBA-15 (inset 2 of Figure 3). The temperature was then heated to 80 °C and kept for a set time, typically around 2-4 h, for the reduction to take place. We used hydrogen as the reducing agent for the metal precursor. The production of solid platinum can be expressed in the equation below:
Excess amount of hydrogen was charged into the reactor in our experiment. Solid platinum would be deposited on the surface of the mesoporous channels of the silica template until the metal precursor was used up (insets 3 and 4 in Figure 3). A fixed period of time was also given for this reduction reaction to take place. The operating time for each of the steps above was varied from 1 to 4 h in order to optimize the conditions for a complete infiltration of the metal in the porous template for good quality of nanowires. At the end of the experiment, the reactor was allowed to cool to room temperature before the pressure of the reactor was slowly released. The metal-filled templates were collected for analysis and template removal by submerging in 5% hydrofluoric acid (HF) for 20 min to remove the silica template (insets 5 and 6 of Figure 3) to reveal the metallic nanostructures fabricated. Characterization techniques such as transmission electron microscopy (TEM) and energy-dispersive X-ray analysis (EDX) were used to confirm the presence of metal in the templates
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TABLE 1: Operating Conditions for the Precursor-Template Ratio in CFD Experiments mole fraction precursor to of precursor weight of template template ratio pressure label (mol/mol × 105) used (g) (weight ratio) (bar) L1 L2 L3
19.17 39.20 65.33
0.0015 0.0015 0.0015
5:1 10:1 20:1
100 130 150
and to study the morphology of the metal deposited in the substrates. The resulting nanostructures were then further analyzed using the scanning electron microscope (SEM). Results and Discussion Concentration and Loading Effect. From the solubility data shown in Figure 2, operating conditions can be chosen to investigate the effect of different parameters involved in the process. Among these conditions, the initial amount of metal precursor placed in the reactor is the most relevant parameter. This would control of the amount of platinum in the reaction and thus was first considered before varying other parameters. We used the operating points, labeled as L1-L3 in Figure 2, taken along the solubility surface and varied the concentration of the metal precursor as shown in Table 1 which shows the concentration in molar fraction of metal precursor in the supercritical fluid medium and the ratio by weight of the precursor to the template. It has been previously investigated by Xu et al.35 and Sun et al.36 that, as the concentration of precursor is increased, more metal deposition can be observed in the pore channels of the template. As shown in Figure 4, the precursor-template ratio had a large effect when it was doubled from L1 (Figure 4a) to L2 (Figure 4b). The infiltration of metal can clearly be seen to increase in the pore channels of the SBA substrate. For the ratio of 10:1, infiltration was even and the template was well-filled. However, little change was observed when the ratio was doubled again from L2 to L3 (Figure 4c). Instead, abundant platinum was observed to have deposited on the outside surface of the SBA template for the sample with precursor-template ratio at 20:1. This result showed that increasing the precursor to template ratio beyond 10:1 was not beneficial. Thus, the ratio of 10:1 was chosen for our CFD experiment.
Figure 4. Platinum infiltration for precursor-template ratio of (a) 5:1, (b) 10:1, and (c) 20:1 at time frames tD ) 4 h, tH ) 2 h, tR ) 4 h, and P ) 100 bar.
Figure 5. (a) TEM images of an unfilled SBA-15 silica template. (b) EDX pattern of a blank SBA-15 sample at “A” indicating the absence of platinum. (c) Infiltration carried out at different pressures, all with a precursor-template ratio of 10:1, for T ) 60 °C, tD ) 1 h, tH ) 0.5 h, tR ) 1 h.
Pressure Effect. We have investigated the deposition pressure dependence and found it to have a significant effect on the amount of platinum infiltrated into the SBA-15 substrate. The pressure was varied from 80 to 150 bar, labeled as P1-P4 in Figure 2, but keeping the precursor to template ratio of 10:1 and other parameters fixed. At lower pressure, 80 bar, many small platinum particles were found to have deposited in the pore channels of the SBA substrates. This was likely due to the higher diffusivity at low pressures leading to the diffusion of more precursor molecules into the porous template within the set time period.37 Thus, more particles could be formed in the pore channels of the substrate when hydrogen was introduced into the reactor. However, the particles formed with this condition were found to be quite small (∼2-3 nm) and did not fill the pores. On the other hand as the pressure was increased to 100 bar, Figure 5d, longer particles were observed inside the pore channels. On the contrary, upon further increase of the pressure (130 and 150 bar in Figure 5, parts e and f), only small crystals were found to have deposited. There seemed to be fewer particles filling the channels at these higher pressures, which might have been due to the decrease in the diffusivity of the precursor in the SC-CO2.35 The condition at 100 bar was therefore chosen for better metal infiltration and crystalline structure. Time Duration Effect. The durations of the procedures discussed in the Experimental Section above were very critical for crystal growth. Three time frames were monitored to optimize the CFD nanowire synthesis process: the dissolution time (tD, time for the metal precursors to dissolve in the SCCO2 medium), the heating time (tH, time given for the thermal
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Figure 8. Platinum infiltrated into SBA-15 with conditions carried out at 100 bar by varying tR (a) 2 h and (b) 4 h with tD and tH both fixed at 2 h.
Figure 6. TEM images of platinum infiltrated into SBA-15 with conditions carried out at 100 bar by varying tD (a) 1 h, (b) 2 h, (c) 4 h, and (d) 6 h with tH and tR both fixed at 2 h and P ) 100 bar.
Figure 7. Platinum infiltrated into SBA-15 with conditions carried out at 100 bar by varying tH (a) 2 h and (b) 4 h with tD and tR both fixed at 2 h.
Figure 9. Platinum infiltrated into SBA-15 with conditions carried out at 100 bar by setting time frames to (a) tD ) 2 h, tH ) 4 h, tR ) 4 h, (b) tD ) 4 h, tH ) 2 h, tR ) 4 h, and (c) tD ) 4 h, tH ) 4 h, tR ) 2 h.
equilibration at the reaction temperature), and the reaction time (tR, time allowed for the reduction reaction). Figure 6 shows the results by varying the dissolution time tD while keeping the other times fixed. It can be seen that the dissolution time has an obvious effect on the amount of metal infiltrated into the SBA substrate. Note also that tD is also the time allowed for the dissolved metal precursor to diffuse into the porous channels of the SBA substrate. When a longer time was allowed for this stage, more metal precursor would be dissolved in the SC-CO2 (Figure 6a-c). However, the amount of metal deposition at 4 h (Figure 6c) and 6 h (Figure 6d) appeared to be similar, suggesting that 4 h would be long enough to reach diffusion equilibrium. In contrast to the dissolution time, the heating time tH did not show an obvious effect when it was varied (Figure 7, parts a and b). Thus, a heating time of 2 h would be sufficient for the temperature to stabilize when increasing the temperature from dissolution temperature (60 °C) to reaction temperature (80 °C). Similar to the dissolution time, the reaction time tR showed a significant effect in obtaining continuous nanowires. As shown in Figure 8, parts a and b, a longer reaction time (4 h) would allow a more complete reaction to take place and allow crystals to grow inside the porous channels. Hence, it could be concluded that dissolution time and reaction time must be carefully controlled in order to obtain high-quality nanowires with high aspect ratio. The optimal time operations were the following: 4 h dissolution time, 2 h heating time, and 4 h of reaction time.
Figure 9 shows the comparison of the results using the optimum operation times Figure 9b to other operating times, Figure 9, parts a and c. It seems that Figure 9b has achieved the best infiltration as compared to Figure 9, parts a and c. With the use of the optimum conditions, nanowires up to 1.4 µm in length, corresponding to aspect ratios ∼200:1, can be found inside the pores of the SBA templates. Template Removal. The silica templates were removed by soaking the samples in 5% concentrated HF to obtain bare platinum nanowires. A sample fabricated using the conditions tD ) 1 h, tH ) 2 h, tR ) 2 h, P ) 100 bar, and ratio ) 10:1 shown in Figure 6a was processed further to remove the template to obtain the bare platinum nanoparticle clusters as shown in Figure 10a. Since the metal infiltration was incomplete in this sample, the template was only partially filled with small nanocrystals of platinum particles throughout the pore channels of the substrate, and after the removal of the SBA-15 template, the platinum particles formed clusters. Another sample was taken from that of Figure 9b (tD ) 4 h, tH ) 2 h, tR ) 4 h, P ) 100 bar, and ratio ) 10:1) for template removal, and its TEM image is shown in Figure 10c. The image shows bundles of platinum nanowires after the template was removed. The insets of Figure 10, parts a and c, are diffraction patterns showing the polycrystalline nature of the particles. EDX patterns of the nanowires in Figure 10, parts a and c, were used to confirm the purity of the platinum as well as the absence of silica substrate shown, respectively, in Figure and 10, parts b and d. It is clear that
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Figure 10. Comparison of platinum nanostructures obtained after template removal from conditions operated at (a) shorter time frames and (c) longer time frames with (b) and (d) as their EDX patterns, respectively.
Figure 10d has a higher platinum content than Figure 10b, in agreement with the TEM images in Figure 10, parts a and c, respectively. Conclusion We have shown CFD to be a reliable method for the synthesis of metallic nanostructures by using porous template SBA-15 and SC-CO2 as the reaction medium. A solubility study of the platinum precursor PtCODMe2 was first carried out to obtain a phase diagram in order to optimize the operating parameters required for this process. Ordered platinum nanostructures have successfully been molded out by the SBA templates forming platinum nanobundles with 7 nm in diameter and large aspect ratios higher than 200:1. Even though we have achieved good metal infiltration into the templates using the optimized conditions stated above, it was still difficult to produce individual single-crystalline metallic nanowires, after the template was removed. Thermal annealing may be applied as a post treatment to realign the metal atoms to form single crystal structures. This heat treatment can be performed inherently to the experimental apparatus by applying heat to the bottom of the reactor with a heating mantle. Efforts in this direction are underway. Acknowledgment. Support from Hong Kong RGC Grants HKUST603405, HKUST602606, and a Central Allocation Grant HKUST3/06C is gratefully acknowledged. References and Notes (1) Huang, Y.; Duan, X.; Cui, Y.; Lauhon, L. J.; Kim, K. H.; Lieber, C. M. Science 2001, 294, 1313.
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