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Discipline
Chemical
Keywords
Nanohorns
Carbon
Oxidation
Observation Type
Standalone
Nature
Standard Data
Submitted
Jun 3rd, 2016
Published
Jul 15th, 2016
  • Abstract
    The morphological change of nanohorns aggregates is presented. Scanning electron microscope (SEM) analysis was conducted on a pristine sample and dispersions. Polar solvents (water and ethanol) were used and stored for one day and one week, observing different packing aggregates.
  • Figure
  • Introduction
    A single-walled carbon nanohorn (SWCNH or NH) is an allotrope form of carbon. It is made of a cone shape (20-40nm length, 5nm widths) with a definite apex composed by pentagons. The pristine nanohorns powder shows a tendency to assembly into spherical-like aggregates, often named Dahlia flower or bud-like. These structures appear as large as 70-100nm diameter from a sonicated dispersion. However, when the morphology of the pristine dry powder (without sonication) is observed, larger spherical aggregates are found.
  • Objective
    To observe whether the morphology of assembled nanohorns is affected by ageing and solvent.
  • Results & Discussion
    Figure (a) shows the commercial nanohorn, carried out using SEM analysis. This material is made into spherical or ellipsoidal clusters exhibiting a size distribution of 5-20 μm. The dimension of the NH aggregates appear to be larger by about two orders of magnitude compared to the values reported from other authors (∼100 nm) by means of TEM analysis. To date, we have found only one publication that reports a SEM image for pristine NH, Cioffi and coworkers. In the consulted literature, sample preparation comprises a sonication step to allow a homogenous dispersion. To avoid a possible alteration of the NH aggregates, in our experiments we limit the high-energy treatment by mixing all samples manually. In order to observe the effect of a polar solvent on the assembling character, we dispersed NH in distilled water and ethanol and stored up to one week. 
    In Figure (b), an aqueous dispersion, stored for 1 day, shows NH assembly into spherical aggregates with average sizes 5-15 μm. By keeping the dispersion for 1 week, the average size of the spherical aggregates is increased up to 10-30 μm (as seen in Figure (c)). On the other hand, the solvent substitution with ethanol produces parallelepiped-like aggregates, as seen in Figure (d1) and (e1). It is worth noting that the top side of the parallelepiped aggregate appears to be composed of a cluster of spherical aggregates possessing an average size in the range 5-10 μm (Figures (d3), (e3)). By contrast, the bulk of this parallelepiped-like aggregate turns into a layered structure, as observed in Figures (d2) and (e2). We posit that the solvent influences the size of the spherical aggregates, which are reduced when the polarity of the solvent is changed. The morphology of nanohorns can be explained by a parallelism with surfactants. These classes of materials exhibit the formation of spherical aggregates, micelles, that spontaneously assemble, minimising the van der Waals interactions. Eventually, surfactants will form compact structures (liquid crystals). Similarly, nanohorns assemble into a complex and hierarchical structure that determines its packing assembly character.

  • Conclusions
    In conclusion, we observe a larger spherical aggregates for pristine NH. The use of distinctive polar solvents changes the interaction between NH units, leading to the formation of a distinct structure. So far, spherical morphology turns to generate more compact crystalline-like structure.
  • Methods
    Materials. Nanohorns powder was purchased from Carbonium S.r.l. 
    Preparation and Characterisation. The NH powder was dried at 150ºC in an oven to prevent gas or water adsorption, and then was stored in a desiccator. All the dispersions were prepared using  a single  powder batch and freshly prepared with a nominal concentration of 2mg/mL. In practice, 8mg of NH were dispersed in 4mL of solvent and homogenised for three minutes by means of a vortex. Before being used, all dispersions are set for  2-3 hours up to equilibrium. In the next step, each dispersion was dried in an oven at 150ºC until a dry powder is obtained (3-4 hours). Morphology of nanohorns were recored by means of a scanning electron microscope (SEM, Philips XL30 TMP, 20KV) on the dry powders. 
  • Funding statement
    This work was financially supported by the University of Padova in the framework of the progetto strategico MAESTRA.
  • Acknowledgements
    The author thanks CE.A.S.C. for providing  SEM service.               
      
  • Ethics statement
    Not applicable.
  • References
  • 1
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    Matters14/30

    Solvent effect on morphology of pristine nanohorns

    Abstractlink
    The morphological change of nanohorns aggregates is presented. Scanning electron microscope (SEM) analysis was conducted on a pristine sample and dispersions. Polar solvents (water and ethanol) were used and stored for one day and one week, observing different packing aggregates.
    Figurelink
    SEM Images of Pristine Nanohorn. (a) Dry NH. (b) NH stored for 1 day in water. (c) NH stored for 1 week in water. (d) NH stored for 1 day in ethanol. (e) NH stored for 1 week in ethanol.
    Introductionlink
    A single-walled carbon nanohorn (SWCNH or NH) is an allotrope form of carbon. It is made of a cone shape (20-40nm length, 5nm widths) with a definite apex composed by pentagons[1][2][3]. The pristine nanohorns powder shows a tendency to assembly into spherical-like aggregates, often named Dahlia flower or bud-like. These structures appear as large as 70-100nm diameter from a sonicated dispersion. However, when the morphology of the pristine dry powder (without sonication) is observed, larger spherical aggregates are found.
    Objectivelink
    To observe whether the morphology of assembled nanohorns is affected by ageing and solvent.
    Results & Discussionlink
    Figure (a) shows the commercial nanohorn, carried out using SEM analysis. This material is made into spherical or ellipsoidal clusters exhibiting a size distribution of 5-20 μm. The dimension of the NH aggregates appear to be larger by about two orders of magnitude compared to the values reported from other authors (∼100 nm) by means of TEM analysis[4][5]. To date, we have found only one publication that reports a SEM image for pristine NH, Cioffi and coworkers[6]. In the consulted literature, sample preparation comprises a sonication step to allow a homogenous dispersion. To avoid a possible alteration of the NH aggregates, in our experiments we limit the high-energy treatment by mixing all samples manually. In order to observe the effect of a polar solvent on the assembling character, we dispersed NH in distilled water and ethanol and stored up to one week. 
    In Figure (b), an aqueous dispersion, stored for 1 day, shows NH assembly into spherical aggregates with average sizes 5-15 μm. By keeping the dispersion for 1 week, the average size of the spherical aggregates is increased up to 10-30 μm (as seen in Figure (c)). On the other hand, the solvent substitution with ethanol produces parallelepiped-like aggregates, as seen in Figure (d1) and (e1). It is worth noting that the top side of the parallelepiped aggregate appears to be composed of a cluster of spherical aggregates possessing an average size in the range 5-10 μm (Figures (d3), (e3)). By contrast, the bulk of this parallelepiped-like aggregate turns into a layered structure, as observed in Figures (d2) and (e2). We posit that the solvent influences the size of the spherical aggregates, which are reduced when the polarity of the solvent is changed. The morphology of nanohorns can be explained by a parallelism with surfactants. These classes of materials exhibit the formation of spherical aggregates, micelles, that spontaneously assemble, minimising the van der Waals interactions. Eventually, surfactants will form compact structures (liquid crystals). Similarly, nanohorns assemble into a complex and hierarchical structure that determines its packing assembly character.

    Conclusionslink
    In conclusion, we observe a larger spherical aggregates for pristine NH. The use of distinctive polar solvents changes the interaction between NH units, leading to the formation of a distinct structure. So far, spherical morphology turns to generate more compact crystalline-like structure.
    Methodslink
    Materials. Nanohorns powder was purchased from Carbonium S.r.l. 
    Preparation and Characterisation. The NH powder was dried at 150ºC in an oven to prevent gas or water adsorption, and then was stored in a desiccator. All the dispersions were prepared using  a single  powder batch and freshly prepared with a nominal concentration of 2mg/mL. In practice, 8mg of NH were dispersed in 4mL of solvent and homogenised for three minutes by means of a vortex. Before being used, all dispersions are set for  2-3 hours up to equilibrium. In the next step, each dispersion was dried in an oven at 150ºC until a dry powder is obtained (3-4 hours). Morphology of nanohorns were recored by means of a scanning electron microscope (SEM, Philips XL30 TMP, 20KV) on the dry powders. 
    Funding Statementlink
    This work was financially supported by the University of Padova in the framework of the progetto strategico MAESTRA.
    Acknowledgementslink
    The author thanks CE.A.S.C. for providing  SEM service.               
      
    Ethics Statementlink
    Not applicable.

    No fraudulence is committed in performing these experiments or during processing of the data. We understand that in the case of fraudulence, the study can be retracted by Matters.

    Referenceslink
    1. Iijima, S, Ichihashi,more_horiz, Y
      Pentagons, heptagons and negative curvature in graphite microtubule growth
      Nature, 356/1992, pages 776-778 chrome_reader_mode
    2. Hashem Rafii-Tabar,
      Computational Physics of Carbon Nanotubes
    3. Kvashnin, Alexander G, Sorokin,more_horiz, Boris I
      Flexoelectricity in Carbon Nanostructures: Nanotubes, Fullerenes, and Nanocones
      J. Phys. Chem. Lett., 6/2015, pages 2740-2744 chrome_reader_mode
    4. Costa, Rubén D, Feihl,more_horiz, Dirk M
      Carbon nanohorns as integrative materials for efficient dye-sensitized solar cells.
      Adv. Mater., 25/2013, pages 6513-6518 chrome_reader_mode
    5. Bandow, S, Kokai,more_horiz, S
      Interlayer spacing anomaly of single-wall carbon nanohorn aggregate
      Chem. Phys. Lett., 321/2000, pages 514-519 chrome_reader_mode
    6. Cioffi, Carla, Campidelli,more_horiz, Maurizio
      Functionalisation of carbon nanohorns
      Chem. Commun., 2006, pages 2129-2131 chrome_reader_mode
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