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Method Article

Synthesis of Substrate-Bound Au Nanowires Via an Active Surface Growth Mechanism

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DOI:

10.3791/57808

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July 18th, 2018

 ,  ,  ,  ,  ,  ,  , 
1Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, 2Chemistry and Biological Chemistry, Nanyang Technological University

* These authors contributed equally

In This Article

Summary

We report a solution-based method to synthesize substrate-bound Au nanowires. By tuning the molecular ligands used during the synthesis, the Au nanowires can be grown from various substrates with different surface properties. Au nanowire-based nanostructures can also be synthesized by adjusting the reaction parameters.

Abstract

Advancing synthetic capabilities is important for the development of nanoscience and nanotechnology. The synthesis of nanowires has always been a challenge, as it requires asymmetric growth of symmetric crystals. Here, we report a distinctive synthesis of substrate-bound Au nanowires. This template-free synthesis employs thiolated ligands and substrate adsorption to achieve the continuous asymmetric deposition of Au in solution at ambient conditions. The thiolated ligand prevented the Au deposition on the exposed surface of the seeds, so the Au deposition only occurs at the interface between the Au seeds and the substrate. The side of the newly deposited Au nanowires is immediately covered with the thiolated ligand, while the bottom facing the substrate remains ligand-free and active for the next round of Au deposition. We further demonstrate that this Au nanowire growth can be induced on various substrates, and different thiolated ligands can be used to regulate the surface chemistry of the nanowires. The diameter of the nanowires can also be controlled with mixed ligands, in which another "bad" ligand could turn on the lateral growth. With the understanding of the mechanism, Au nanowire-based nanostructures can be designed and synthesized.

Introduction

Typical of one dimensional nanomaterials, nanowires possess both the bulk-related properties and the unique properties originated from the quantum effects of the nanoscale structure. As a bridge between the nanoscale and the bulk scale materials, they have been widely applied in various fields of catalysis, sensing, and nanoelectronic devices, etc.1,2,3.

However, the synthesis of nanowires has long been a great challenge, as it usually requires breaking the intrinsic symmetry in the crystals. Traditionally, a template is employed to regulate the deposition of materials. For instance, template-electrodeposition has been used for the formation of various types of nanowires like Ag nanowires and CdS nanowires4,5,6,7,8,9,10. Another common approach is vapor-liquid-solid (VLS) growth, which employs a molten catalyst to induce the anisotropic growth on the substrate at an elevated temperature11. Common strategies for the synthesis of metal nanowires are the polyol methods for Ag nanowires and the oleylamine-assisted ultrathin Au nanowires12,13,14,15. Both approaches are material-specific, and the nanowire parameters are not readily tuned during the synthesis. In addition, metal nanowires can also be formed by the pressure-driven method, where the assembled metal nanoparticles are mechanically compressed and fused into nanowires16,17,18.

Recently, we reported a distinctive method to synthesis Au nanowires19. With the assist of a thiolated small molecule ligand, the nanowires could grow and form a vertically aligned array on the bulk Si wafer substrate at ambient conditions. It was found that the ligands play an important role in the symmetry-breaking growth. It binds to the surface of the substrate-adsorbed Au seeds strongly, forcing the Au to deposit selectively at the ligand-deficient interface between seeds and substrate. The interface between the newly deposited Au and the substrate remains ligand deficient, therefore, the active surface exists throughout the whole growth. By tuning the ligand concentration, the seed type and concentration as well as several other parameters, a series of Au nanowire-based nanostructures could be synthesized.

In this work, we will provide a detailed protocol for this convenient Au nanowires synthesis. The derived synthesis is also presented, including the synthesis of Au nanowires with hydrophobic surface property, Au nanowires on other substrates, tapered Au nanowires by mixing two ligands and the nanowire-based Au nanostructures formed by tuning the growth conditions.

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Protocol

Caution: Please check the material safety data sheets (MSDS) of the chemicals for detailed handling and storage instruction. Please be careful while handling the nanomaterials, as there may be unidentified risk. Please perform the experiments in a fume hood and wear appropriate personal protective equipment.

1. Synthesis of Seed Nanoparticles

Note: To avoid failure caused by the premature nucleation during the nanoparticle synthesis, wash the glassware and stir bar used in the synthesis with aqua regia and rinse thoroughly with water.

  1. Synthesis of 3-5 nm Au nanoparticles
    1. Prepare a hydrogen tetrachloroaurate (III) (HAuCl4) solution by dissolving 10 mg of HAuCl4∙3H2O in 1 mL of deionization water (DI water) in a 4 mL vial. Pipette 0.197 mL of the HAuCl4 solution into a 50 mL round-bottom flask.
    2. Add 19.7 mL of DI water to the flask to dilute the HAuCl4 solution.
    3. Dissolve 10 mg of sodium citrate in 1 mL of DI water in another 4 mL vial to prepare a 1% sodium citrate stock solution. Add 0.147 mL of the 1% sodium citrate solution to the diluted HAuCl4 solution prepared in step 1.1.2.
    4. Prepare a 0.1 M sodium borohydride (NaBH4) solution by dissolving 2.3 mg of NaBH4 in 0.6 mL of DI water.
    5. Quickly inject 0.6 mL of 0.1 M NaBH4 solution into the mixture from step 1.1.3 under vigorous stirring. Check for an immediate color change of the solution from pale yellow to bright orange.
    6. Stir the mixture for another 10 min. Wait for a gradual color change of the solution to reddish orange.
    7. Confirm the size of the obtained Au nanoparticles with UV-Vis spectroscopy and scanning electron microscopy (SEM).
  2. Synthesis of 15 and 40 nm Au nanoparticles
    1. Add 100 mL of DI water in a 250 mL round-bottom flask. Weigh 10 mg of HAuCl4∙3H2O solid and dissolve it in the round-bottom flask.
    2. Add a magnetic stir bar into the flask and equip the flask with a condenser. Stir and heat the solution prepared in 1.2.1 to 100 °C in an oil bath. Reflux the solution for 10 min.
    3. Weigh 40 mg of sodium citrate and dissolve it in 4 mL of DI water to prepare a 1% sodium citrate stock solution.
    4. To synthesize 15 nm Au nanoparticles, add 3 mL of the 1% sodium citrate solution from step 1.2.3 to the boiled mixture with a syringe.
      Note: The color of the solution turns gray in 1 min, and then gradually to red.
      1. To synthesize 40 nm Au nanoparticles, inject 1.5 mL of the 1% sodium citrate solution to the boiling solution from step 1.2.2 with a syringe. Keep the solution boiling until it changes to red in about 10 min.
        Note: The color of the solution changes from transparent to dark gray, then to black, and finally to purple in about 1 min.
    5. Continue to reflux the reaction solution for 30 min. Cool down the solution to room temperature at ambient conditions.
    6. Characterize the size and uniformity of the resulting Au nanoparticles with UV-Vis spectroscopy and SEM.

2. Synthesis of Au Nanowires (Length = ~500 nm ) on Silicon (Si) Wafers and Various Substrates

  1. Prepare the substrate for seed adsorption.
    1. Cut the Si wafer into 5 mm ´ 5 mm pieces. Clean the Si wafer pieces with DI water and ethanol sequentially in an ultrasonic bath, each for 15 min.
    2. Treat the Si wafer with 29.6 W of O2 plasma (operated at 220 V) for 20 min.
      Note: The surface of the wafer becomes hydrophilic.
    3. Prepare a 5 mM 3-aminopropyltriethoxysilane (APTES) solution by dissolving 11.1 mg of APTES to a mixture of DI water (5 mL) and ethanol (5 mL) in a 20 mL vial.
    4. Soak a piece of Si wafer in the APTES solution prepared in step 2.1.3 in a 20 mL vial for 30 min.
    5. Take out the Si wafer and wash it thoroughly with ethanol and DI water.
  2. Adsorb seed nanoparticles onto the substrate.
    1. Soak the Si wafer in the 3-5 nm Au seeds solution prepared in step 1.1 for 2 h.
      1. To grow Au nanowires from 15 nm Au seeds, soak the Si wafer in the 15 nm Au nanoparticle solution for 2 h.
      2. To grow Au nanowires from 40 nm Au seeds, soak the Si wafer in the 40 nm Au nanoparticle solution for 2 h.
    2. Take out the Si wafer and wash it with 50 mL of DI water to remove unabsorbed Au nanoparticles.
  3. Grow the substrate-bound Au nanowires.
    1. Prepare a 1.65 mM 4-mercaptobenzoic acid (4-MBA) solution by dissolving 2.5 mg of 4-MBA in 10 mL of ethanol.
    2. Prepare a 5.10 mM HAuCl4 solution by dissolving 20.1 mg of HAuCl4∙3H2O in a mixture of 5 mL of ethanol and 5 mL of DI water.
    3. Prepare a 12.3 mM L-ascorbic acid solution by dissolving 21.7 mg of L-ascorbic acid in 10 mL of DI water.
    4. Mix 0.5 mL of 5.10 mM HAuCl4 solution with 0.5 mL of 1.65 mM 4-MBA solution in a 10 mL vial.
    5. Soak the seed-adsorbed Si wafer from step 2.2.2 in the mixed solution prepared in step 2.3.4.
    6. Add 0.5 mL of 12.3 mM L-ascorbic acid solution into the wafer-soaked mixed solution. Shake the vial gently to evenly mix the solution.
    7. Leave the wafer and the solution undisturbed for 15 min. Check the bubble formation during the growth process, and the color change of the surface of the Si wafer from shiny gray to reddish brown.
    8. Take out the Si wafer and rinse it with ethanol and DI water. Dry the Si wafer at ambient conditions and wait for the surface of the wafer to turn to gold.
    9. Characterize the morphology of the Au nanowires with the SEM.
  4. For growth of Au nanowires on a glass slide, Al2O3, SrTiO3, LaAlO3, indium tin oxide (ITO) and F-doped tin oxide (FTO) substrates, follow the same procedures, including the cleaning processes.

3. Synthesis of Au Nanowires with Different Ligands

  1. Synthesis of Au nanowire forests with various thiolated ligands: 2-naphthalenethiol (2-NpSH), 4-mercaptophenylacetic acid (4-MPAA) and 3-mercaptobenzoic acid (3-MBA).
    1. Treat the Si wafer following the same procedures in steps 2.1-2.2.
    2. Prepare a 1.65 mM 2-NpSH solution by dissolving 2.6 mg of 2-NpSH in 10 mL of ethanol.
      1. Prepare a 1.65 mM 4-MPAA solution by dissolving 2.8 mg of 4-MPAA in 10 mL of ethanol.
      2. Prepare a 1.65 mM 3-MBA solution by dissolving 2.5 mg of 3-MBA in 10 mL of ethanol.
    3. Prepare the 5.10 mM HAuCl4 solution and 12.3 mM L-ascorbic acid solution following the same procedure in steps 2.3.1-2.3.3.
    4. In a 10 mL vial, mix 0.5 mL of the HAuCl4 solution with 0.5 mL of 2-NpSH solution, and shake the mixture to get a homogeneous solution.
      1. In a 10 mL vial, mix 0.5 mL of the HAuCl4 solution with 0.5 mL of 4-MPAA solution, and shake the mixture to get a homogeneous solution.
      2. In a 10 mL vial, mix 0.5 mL of the HAuCl4 solution with 0.5 mL of 3-MBA solution, and shake the mixture to get a homogeneous solution.
    5. Add 0.5 mL of 12.3 mM L-ascorbic acid solution to the wafer-soaked mixed solution. Shake the vial gently to get an evenly mixed solution.
    6. Leave the wafer and the solution undisturbed for 15 min. Observe the surface of the Si wafer turning slowly from shiny gray to reddish brown.
    7. Take out the Si wafer and rinse it with ethanol and DI water, and dry the Si wafer at ambient conditions until the surface of the wafer turns to gold.
    8. Confirm the Au nanowires forest structure with SEM.
  2. Synthesis of tapered Au nanowires with mixed ligands.
    1. Synthesis of thickened Au nanowires with mixed ligands of 4-MBA and 3-mercaptopropanoic acid (3-MPA) (c4-MBA/c3-MPA = 3:1).
      1. Treat the Si wafer following the same procedures in steps 2.1-2.2, except dilute the seed solution 100 times.
      2. Prepare a 3 mM 4-MBA solution by dissolving 4.6 mg of 4-MBA in 10 mL of ethanol.
      3. Prepare a 3 mM 3-MPA solution by dissolving 3.2 mg of 3-MPA in 10 mL of ethanol.
      4. Mix 0.75 mL of 3 mM 4-MBA solution with 0.25 mL of 3 mM 3-MPA solution (in a 10 mL vial. Shake gently to get a homogeneous solution.
      5. Prepare the 5.10 mM HAuCl4 solution and 12.3 mM L-ascorbic acid solution following the same procedure in steps 2.3.2-2.3.3.
      6. Add 0.5 mL of 5.10 mM HAuCl4 solution to the mixed solution in step 3.2.1.4 and shake gently to homogenize the solution.
      7. Soak the Si wafer from step 3.2.1.1 in the mixed solution in a 10 mL vial. Add 0.5 mL of 12.3 mM L-ascorbic acid solution to the mixed solution.
      8. After 10 min, take the Si wafer out and rinse with ethanol and DI water.
      9. Dry the wafer at ambient conditions and confirm the structure by SEM.
    2. Synthesize tapered Au nanowires with mixed ligand of 4-MBA and 3-MPA (c4-MBA/c3-MPA = 6:4). Follow the same procedures in step 3.2.1 with 0.6 mL of 3 mM 4-MBA solution and 0.4 mL of 3 mM 3-MPA solution instead.
    3. Synthesize tapered Au nanowires with mixed ligands of 4-MBA and 3-MPA (c4-MBA/c3-MPA = 1:1). Follow the same procedures in step 3.2.1 with 0.5 mL of 3 mM 4-MBA solution and 0.5 mL of 3 mM 3-MPA solution instead.

4. Synthesis of Au Nanowire-Based Complex Nanostructures

  1. Synthesis of segmental Au nanowires with thick-thin-thick-thin segments.
    1. Treat the Si wafer following the same procedures in steps 2.1-2.2.
    2. Prepare a 1.65 mM 4-MBA solution by dissolving 2.5 mg of 4-MBA in 10 mL of ethanol.
    3. Prepare a 0.0830 mM 4-MBA solution by diluting the 1.65 mM 4-MBA solution 20 times.
    4. Prepare the HAuCl4 and L-ascorbic acid solution following the same procedures in steps 2.3.2-2.3.3.
    5. Prepare 1.5 mL of growth solution A by mixing 0.5 mL of 1.65 mM 4-MBA solution, 0.5 mL of 5.10 mM HAuCl4 solution, and 0.5 mL of 12.3 mM L-ascorbic acid solution (the final concentration: c4-MBA = 0.550 mM, cHAuCl4 = 1.70 mM, cL-ascorbic acid = 4.10 mM).
    6. Prepare 1.5 mL of growth solution B by mixing 0.5 mL of 0.0830 mM 4-MBA solution, 0.5 mL of 5.10 mM HAuCl4 solution, 0.5 mL of 12.3 mM L-ascorbic acid solution (the final concentration: c4-MBA = 0.0280 mM, cHAuCl4 = 1.70 mM, cL-ascorbic acid = 4.10 mM).
    7. Immerse the Si wafer in a 10 mL vial containing growth solution B for about 1 min.
    8. Rapidly transfer the Si wafer without drying to another 10 mL vial containing growth solution A and leave it to grow for 2 min.
    9. Repeat steps 4.1.7-4.1.8 one more time.
    10. Take the Si wafer out and rinse with 50 mL of ethanol and 50 mL of DI water.
    11. Confirm the structure of the resulting segmented Au nanowires by SEM.
  2. Synthesis of the nanoflowers
    1. Treat the Si wafer following the same procedures in steps 2.1-2.2, except with a 5 min O2 plasma treatment.
    2. Soak the Si wafer in a 10 mL vial containing a 10000x diluted 3-5 nm Au seeds solution for 15 min.
    3. Wash the Si wafer from step 4.2.2 thoroughly with DI water to remove unabsorbed Au nanoparticles.
    4. Prepare a growth solution containing 0.550 mM 4-MBA, 1.70 mM HAuCl4, and 4.10 mM L-ascorbic-acid, following the same procedures in step 4.1.5.
    5. Soak the wafer in a 10 mL vial containing the growth solution for 30 s.
    6. Remove the wafer from the growth solution and leave a thin layer of the solution (~ 13- 15 μL) on the wafer.
    7. Quickly blow-dry the wafer at room temperature.
    8. Confirm the nanoflower structure by SEM.

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Results

The Au nanoparticle seeds, substrate-bound Au nanowires and Au nanowire-based derivative nanostructures are characterized with SEM. Figure 1 shows the representative SEM images of the 3-5 nm Au nanoparticles, 15 nm Au Nanoparticles and 40 nm Au nanoparticles adsorbed on the Si wafer, confirming their sizes, adsorption and distribution. The Au nanowires grown from the respective seeds on the Si wafer substrate are also presented. The representative SEM images ...

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Discussion

The mechanism of this active surface growth governed nanowire synthesis has been discussed comprehensively in previous work19. In addition, the effects of seed sizes and types as well as the effect of ligand types and sizes have also been investigated20,21. Generally. the nanowire growth is very different from previous reported routes. No template is required, and the asymmetric growth is induced by the differences between the ligand-cappe...

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Disclosures

The authors have nothing to disclose.

Acknowledgements

We gratefully acknowledge the financial support from National Natural Science Foundation of China (21703104), Jiangsu Science and Technology Plan (SBK2017041514) Nanjing Tech University (39837131), and SICAM Fellowship from Jiangsu National Synergetic Innovation Center for Advanced Materials.

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
Trisodium citrate dihydrateAlfa AesarLoT: 5008F14U
Sodium borohydrideFlukaLoT: STBG0330VNaBH4
Hydrogen tetrachloroaurate(III) trihydrateAlfa AesarLoT: T19C006HAuCl4
3-aminopropyltriethoxysilaneJ&K ScientificLoT: LT20Q102APTES
L-ascorbic acid Sigma-AldrichLoT: SLBL9227V
4-mercaptobenzoic acidSigma-AldrichLoT: MKBV5048V4-MBA
2-NaphthalenethiolSigma-AldrichLoT: BCBP4238V2-NpSH
4-Mercaptophenylacetic acidAlfa AesarLoT: 101991604-MPAA
3-mercaptobenzoic acidAladdinLoT: G12130273-MBA
3-Mercaptopropionic acidAladdinLoT: E16180953-MPA
absolute ethanolSinopharm chemical Reagent20170802
Silicon waferZhe Jiang lijingPSi
Scanning Electron MicroscopeQuanta FEG 250SEM
Centrifuge Eppendorf5424
Ultrasonic cleaner Kun Shan hechuang
Ultra-pure water systemNanJing qianyanUP6682-10-11for deionized water
Plasma cleanerHarrick PlasmaPDC-002for oxygen plasma

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Gold Nanowire SynthesisSubstrate Bound NanowiresThiolated LigandsChloroauric AcidSodium BorohydrideAPTES TreatmentGold NanoparticlesLigand ConcentrationAlternating Segments
Synthesis of Substrate-Bound Au Nanowires Via an Active Surface Growth Mechanism
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