Nanoporous tree-like SiO2 films fabricated by sol–gel assisted electrostatic spray deposition

https://doi.org/10.1016/j.micromeso.2011.09.003Get rights and content

Abstract

A novel nanoporous tree-like SiO2 film was synthesized by a sol–gel assisted electrostatic spray deposition (SG-ESD) approach. The sol–gel process employed was better to create SiO2 with linear cross-link chains for the electrostatic spray deposition. From electron microscopic study, it was found that the as-deposited SiO2 films possess nanoporous tree-like morphology in macroscale; and monodispersed hollow nanoporous spherical structure in microscale. The formation of the nanoporous “tree” structure is related to the preferential landing of droplets on the substrate and the electrostatic repulsion force among landing droplets during the ESD process. The catalytic effect of hydrochloric acid in the sol–gel system contributes to the formation of the hollow and porous structured SiO2 spheres. The charge-discharge characteristics of the porous SiO2 as an anode for Li-ion batteries were briefly evaluated and good long-term cycle performance was reached.

Highlights

► Nanostructured SiO2 was synthesized. ► The structure possesses porous and nano-tree-like morphology in macroscale. ► The structure possesses monodispersed nanoporous spherical structure in microscale. ► Obtained SiO2 shows good cycle performance as anode for lithium ion batteries application.

Introduction

Porous films show anomalous optical, electronic, thermal, magnetic, and other properties superior to those of their bulk counterparts [1], [2], [3]. Of them, porous SiO2 films have been drawing tremendous research interest in the last two decades because of their wide-range applications, such as microelectronics [4], [5], [6], [7], optoelectronic [8], [9], [10], multilayer pyroelectric thin-film infrared detectors [11], [12], catalysis [13], adsorption [14], sensors [15], membranes [16], template [17], selective separation [18], etc. For instance, porous SiO2 films have been employed as a super-low dielectric constant material to effectively decrease the interconnection RC signal delay and cross talk in ultra large-scale integrated circuits [6]. Porous SiO2 films were also shown the refractive index as low as 1.23 and its reflective effect can be controlled by adjusting its porosity [8], [12]. Furthermore, it was demonstrated that SiO2 synthesized by hydrothermal reaction [19], reactive radio frequency sputtering [20], and laser treatment [21] can be employed as anode material for lithium ion batteries [19], [20], [21]. Fu et al. reported that the capacity of SiO2 thin films faded with a slow rate, accounting for about 13.7% after 100 charge/discharge cycles [20]. It is expected that a porous SiO2 film as a stress-resistant structure with high surface area as an anode can further improve the cycle performance of a lithium ion battery.

Several dry and wet methods have been employed to prepare porous SiO2 films, including chemical vapor deposition (CVD) [22], electrochemical anodized oxidation [23] and thermal oxidation [24] of a silicon substrate, self-assembled soft templates [25], sol–gel method [26], etc. Of them, CVD method and thermal oxidation are widely used in semiconductor processing because their vapor processes are more suitable for depositing dense SiO2 thin films of low porosity. Electrochemical anodized oxidation method is the most reported wet electrochemical etching process suitable for the formation of thin porous Si with native SiO2 film. This method requires a careful selection of several main experimental parameters (such as current, density of illumination, etc.) and special type of Si substrates. Consequently, it remains challenging in order to achieve a precise and repeatable control of the porous structures and dimensions of SnO2 films. Moreover, the solution-based nature of the electrochemical anodization process limits its application resulting from the intolerable compatibility issues in some device fabrication processes. In comparison, the sol–gel method exhibits many manufacturing advantages such as low cost, high purity, low processing temperature, and good controllability of the chemical composition [27], [28]. Hydrolysis of a silicon alkoxide precursor such as tetraethylorthosilicate (TEOS) was previously adopted to prepare the porous SiO2 films [29]. However, decomposition of the Si based precursor gel alone is usually difficult to precisely control the morphology and porosity of SiO2 films; besides, mechanical problem such as cracking and peeling are likely to occur during the post annealing process. As Lou et al. reported, the synthetic approaches of hollow structures are divided into four categories: (1) conventional hard templating synthesis, (2) sacrificial templating synthesis, (3) soft templating synthesis, and (4) template-free methods [30]. The template approach is based on the silica formed as the shell on the surfaces of the template particle via the catalyzed hydrolysis and condensation of TEOS. After the silica coating, the template particles are dissolved subsequently to form the hollow structure. However, the disadvantages related to high cost and tedious synthetic procedures have impeded scale-up of the template approach for large scale applications [30].

Regular porous SiO2 can be synthesized in the aforementioned various approaches, but no research was reported on formation of multi-level hollow and nanoporous SiO2 with tree-like nanostructure. It is expected that the hollow as well as tree-like nanostructure can be beneficial to increase the effective surface area of SiO2. Recently we attempted an approach to deposit porous and hollow SiO2 films with tree-like nanostructure by combining sol–gel method and electrostatic spray deposition (ESD), so-called sol–gel assisted ESD (SG-ESD). The SG-ESD technique provides a simple and versatile method for generating a rich variety of morphologies, such as thin-films, porous and fibrous matrices, and single or multi-component films [31], [32]. In this work, we demonstrated that the SG-ESD has many advantages in precise and repeatable fabrication of nanoporous tree-like SiO2 nanofilms. The sol–gel step produced monodispersed SiO2 nanoparticles. Moreover unipolar ions induced by the ESD technique charged the sol-solution of low viscosity and thereby helped atomize it into liquid droplets. In addition, the electrical field existing between the nozzle and substrate dominantly determined the trajectories of the droplets and thereby their locations on the substrate.

Section snippets

Synthesis of SiO2 films

A sol–gel method was utilized to prepare the precursor solution: 4.46 mL Tetraethyl orthosilicate (TEOS) and 11.68 mL ethanol (EtOH) were mixed and vigorously stirred using a magnetic stirrer for 10 min. Then a mixture of home-made deionized water (H2O) and hydrochloric acid (HCl) were added dropwise while stirring to induce the TEOS hydrolysis. The final molar ratio was TEOS:EtOH:H2O:HCl = 1:10:4:0.05. The solution were then stirred at 40 °C for 5 h and further aged for 24 h at room temperature after

Results and discussion

Thermal behavior of SiO2 gel is presented in Fig. 2a. From the weight loss vs. temperature curve, the apparent weight loss below 200 °C is assigned to the evaporation behavior of ethanol solvent, HCl catalyst, and deionized (DI) water, which also resulted from the desorption of the adsorbed water molecules. In the temperature range from 200 to 600 °C, the residual organic groups, such as hydroxyl and methyl, decomposed and further resulted in mass loss [33], [34]. The differential scanning

Conclusion

In summary, we have successfully synthesized novel nanoporous tree-like SiO2 films by a sol–gel assisted electrostatic spray deposition approach. The process began with the hydrolysis and condensation of tetraethylorthosilicate via a sol–gel method to form SiO2 based sol solution. The sol solution was atomized by a high electrostatic field applied in the ESD and the spray was deposited onto a heated substrate. It was confirmed that the SiO2 films possess porous and nanoporous tree-like

Acknowledgments

We acknowledge financial support from the US Defense Advanced Research Projects Agency (DARPA) Young Faculty Award Program (Project No. HR0011-08-1-0036), the Air Force Office of Scientific Research (AFOSR FA9550-08-1-0287), American Chemical Society (Petroleum Research Fund, 49301-0N110). We also appreciate the experimental assistant from Mr. Chiwon Kang and support from FIU AMERI facility.

References (51)

  • F. Chen et al.

    Thin Solid Films

    (2008)
  • M. Herrmann et al.

    Microelectron. Eng.

    (2008)
  • S.Y. Chang et al.

    Microelectron. Eng.

    (2006)
  • L. Li et al.

    Ceram. Int.

    (2004)
  • S.H. Lee et al.

    Diamond Relat. Mater.

    (2007)
  • B.K. Guo et al.

    Electrochem. Commun.

    (2008)
  • Q. Sun et al.

    Appl. Surf. Sci.

    (2008)
  • A.Y. Pidluzhna et al.

    J. Non-Cryst. Solids

    (2008)
  • J.Y. Park et al.

    Mater. Chem. Phys.

    (2003)
  • C.H. Chen et al.

    Thin Solid Films

    (1999)
  • Z.J. Wu et al.

    J. Non-Cryst. Solids

    (2003)
  • C.J. Brinker

    J. Non-Cryst. Solids

    (1988)
  • K. Bredereck et al.

    J. Colloid Interf. Sci.

    (2011)
  • I. Strawbridge et al.

    J. Non-Cryst. Solids

    (1985)
  • Y. Liu et al.

    Thin Solid Films

    (1999)
  • M.T. Mark et al.

    Science

    (2002)
  • E.D. Mark

    Nature

    (2002)
  • A. Link et al.

    J. Appl. Phys.

    (2006)
  • N. Kawakami et al.

    Jpn. J. Appl. Phys.

    (2000)
  • K. Makita et al.

    J. Sol–Gel Sci. Technol.

    (1999)
  • J.Q. Xi et al.

    Nat. Photonics

    (2007)
  • J.Q. Xi et al.

    Nano Lett.

    (2005)
  • X.Q. Wu et al.

    Integr. Ferroelectr.

    (2003)
  • X. Bu et al.

    Science

    (1997)
  • H.K. Jeong et al.

    Nat. Mater.

    (2002)
  • Cited by (0)

    1

    Tel.: +1 519 661 2111x87759; fax: +1 519 661 3020.

    View full text