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Method Article

TiO2-coated Hollow Glass Microspheres with Superhydrophobic and High IR-reflective Properties Synthesized by a Soft-chemistry Method

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DOI:

10.3791/55389

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April 26th, 2017

 ,  ,  , 
1Department of Biomedical Science, City University of Hong Kong, 2Technological and Higher Education Institute of Hong Kong, 3Hong Kong Polytechnic University, 4Pavotech Co. Ltd.

In This Article

Summary

This manuscript proposes a soft-chemistry method to synthesize superhydrophobic, TiO2-coated hollow glass microspheres (HGM) with high IR-reflective properties.

Abstract

This manuscript proposes a soft-chemistry method to develop superhydrophobic and highly IR-reflective hollow glass microspheres (HGM). The anatase TiO2 and a superhydrophobic agent were coated on the HGM surface in one step. TBT and PFOTES were selected as the Ti source and the superhydrophobic agent, respectively. They were both coated on the HGM, and after the hydrothermal process, the TBT turned to anatase TiO2. In this way, a PFOTES/TiO2-coated HGM (MCHGM) was prepared. For comparison, PFOTES single-coated HGM (F-SCHGM) and TiO2 single-coated HGM (Ti-SCHGM) were synthesized as well. The PFOTES and TiO2 coatings on the HGM surface were demonstrated through X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive detector (EDS) characterizations. The MCHGM showed a higher contact angle (153°) but a lower sliding angle (16°) than F-SCHGM, with a contact angle of 141.2° and a sliding angle of 67°. In addition, both Ti-SCHGM and MCHGM displayed similar IR reflectivity values, which were about 5.8% higher than the original HGM and F-SCHGM. Also, the PFOTES coating barely changed the thermal conductivity. Therefore, F-SCHGM, with a thermal conductivity of 0.0479 W/(m·K), was quite like the original HGM, which was 0.0475 W/(m·K). MCHGM and Ti-SCHGM were also similar. Their thermal conductivity values were 0.0543 W/(m·K) and 0.0543 W/(m·K), respectively. The TiO2 coating slightly increased the thermal conductivity, but with the increase in reflectivity, the overall heat-insulation property was enhanced. Finally, since the IR-reflecting property is provided by the HGM coating, if the coating is fouled, the reflectivity decreases. Therefore, with the superhydrophobic coating, the surface is protected from fouling, and its lifetime is also prolonged.

Introduction

Hollow glass microspheres (HGM) are inorganic materials ranging in size from 10 to 100 µm. They demonstrate many useful features, such as excellent dispersion, high flow ability, low density, and superior thermal insulation properties1,2,3,4. Because of their hollow structure, HGM have an extremely low thermal conductivity10,11. For these reasons, they are applied in many areas, including aerospace engineering5, deep-sea exploration6,7, hydrogen storage8,9, etc. However, they still demonstrate some disadvantages, such as low strength. In addition, IR light is able to transmit through HGM and heat the subject behind. Therefore, surface modifications on HGM are essential to reduce the radiative thermal transfer. An effective method is to coat an IR-blocking material onto the HGM surface. As a semiconductor, TiO2 has been used in many areas, such as photo-catalysis12,13, solar cell development, sensor fabrication14, environmental applications15, and energy storage16. In addition, it also shows low emissivity in the visible light and infrared band17,18,19. Therefore, for our purposes, TiO2 was a prudent selection due to its relatively low price and high performance.

However, the coating is quite easy for pollutants to foul, which seriously affects the reflectivity of TiO2. The reflectivity must reduce gradually. Therefore, a self-cleaning coating is essential to prevent the coating from fouling and to prolong the working time of such a coating.

In this manuscript, a soft-chemistry method was used to develop superhydrophobic TiO2-coated HGM. Tetrabutyl titanate (TBT) and 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES) were selected as the Ti source and superhydrophobic agent, respectively. They were hydrolyzed and deposited on the HGM surface. Then, after the hydrothermal process, the anatase TiO2 formed on the HGM surface, and the superhydrophobic properties remained. For comparison, PFOTES single-coated HGM (F-SCHGM) and TiO2 single-coated HGM (Ti-SCHGM) were synthesized as well. The synthesis scheme is shown in Figure 1.

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Protocol

1. Pre-treatment of HGM

  1. Place the HGM into a 500-mL beaker with 200 mL of absolute alcohol; the low density of unbroken HGM causes it to suspend in the alcohol, but because the density of broken HGM is larger than that of alcohol, it precipitates in the solution. After 30 min, collect the suspended HGM using a clean spoon and dry at 80 °C in an oven for further application.

2. Synthesis of MCHGM

  1. Place 5 g of unbroken HGM, 47.5 mL of ethanol, and 2.5 mL of DI water in a three-necked flask. Stir using a mixing motor at 400 r/min for 20 min (pre-mixing).
  2. Mix 15 g of TBT, 1 g of PFOTES, and 30 mL of absolute alcohol in a 200-mL beaker. Pour the mixture into a constant-pressure funnel.
  3. Insert the constant-pressure funnel into one of the holes of the three-necked flask. Drop the mixture contained in the constant-pressure funnel into the three-necked flask at a speed of 1 drop per 7 s, which is achieved by adjusting the valve of the constant-pressure funnel. Continue the reaction for 3 h.
  4. Pour the mixture from the three-necked flask into a hydrothermal reactor. Put the sealed reactor in a suitable steel sleeve in a 180 °C oven for 6 h.
    NOTE: Make sure that the reactor has a suitable cover. After it is covered, put the reactor into the steel sleeve. The sleeve should also be sealed with a cover.
  5. After the reaction is over, collect the samples suspended in the hydrothermal reactor using a big spoon. Dry the samples at 80 °C for 4 h to obtain MCHGM.

3. Synthesis of F-SCHGM

  1. Add 5 g of unbroken HGM, 47.5 mL of absolute ethanol, and 2.5 mL of DI water to a three-necked flask. Stir using a mixing motor at 400 r/min for 20 min (pre-mixing). Mix 1 g of PFOTES and 30 mL of absolute ethanol in a 200-mL beaker. Transfer the PFOTES and absolute ethanol mixture to a constant-pressure funnel.
  2. Insert the constant-pressure funnel into the three-necked flask. Drop the mixture contained in the constant-pressure funnel into the three-necked flask at a speed of 1 drop per 7 s. Let the reaction run for 3 h.
  3. Transfer the mixture from the three-necked flask to a hydrothermal reactor. Put the sealed reactor in a 180 °C oven for 6 h. After the reaction is over, collect the samples suspended in the hydrothermal reactor using a big spoon. Dry the samples at 80 °C for 4 h to obtain F-SCHGM.

4. Synthesis of Ti-SCHGM

  1. Place 5 g of unbroken HGM, 47.5 mL of absolute ethanol, and 2.5 mL of DI water in a three-necked flask. Stir at 400 r/min for 20 min (pre-mixing). Mix 15 g of TBT and 30 mL of absolute ethanol in a 200-mL beaker. Transfer the TBT and absolute ethanol mixture to a constant-pressure funnel.
  2. Insert the constant-pressure funnel into the three-necked flask. Drop the mixture in the constant-pressure funnel into the three-necked flask at a speed of 1 drop per 7 s. Let the reaction run for 3 h.
  3. Transfer the mixture from the three-necked flask to a hydrothermal reactor. Put the sealed reactor in a 180 °C oven for 6 h. Collect the samples in the hydrothermal reactor after the reaction is over. Dry the samples at 80 °C for 4 h to obtain Ti-SCHGM.

5. Characterizations

  1. Conduct XRD characterizations on all samples. Collect the data using a highly versatile, multipurpose X-ray diffraction system with Cu Kα radiation (λ = 0.15406 nm) and a 2θ ranging from 10° to 80°.
  2. Acquire scanning electron microscope20,21 images after spraying the samples with gold. During the SEM tests, conduct the EDS is on a specific area.
  3. Measure the contact angle by using a contact-angle goniometer22; the water drop volume should be 10 µL.
  4. Measure the sliding angle23 by changing the tilt angle of the surface. Minimize the angle until the water drop can just slide down.
    1. Stick double-sided tape on a glass sheet (size: 26 mm x 76 mm x 2 mm). Using a spoon, uniformly place the powders (F-SCHGM or MCHGM) on the tape. Using an injector, add a water drop (volume: 0.05 mL) to the powder surface.
    2. Put the glass sheet on the motor platform of the contact-angle goniometer. Tilt the glass sheet by leaning the motor platform at a rate of 1°/s. Stop the motor when the water drop starts to slide; the tilt angle is the sliding angle.
  5. Measure the reflectivity spectra using a spectrophotometer24. NOTE: The wavelength is from 450 nm to 2,550 nm.
  6. Measure the thermal conductivity of all samples using a thermal conductivity meter25.

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Results

The tests in step 4.4 reveal many features and properties of the samples. The XRD (Figure 2) reflects the formation of anatase TiO2. The SEM (Figure 3) and EDS (Figure 4) display the TiO2 and PFOTES that are coated on the HGM surface. The contact angle (Figure 5) and sliding angle (Figure 6) tests represent the superhydrophobicity. The Vis-NIR transmittance test (Figure 8

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Discussion

In this manuscript, the critical step in the protocol is the hydrothermal process. It influences the formation of TiO2, the final reflectivity, and the superhydrophobicity. The temperature control and reaction time are also quite significant. If the reaction conditions change, the final products can be flawed.

This method provides a simple way to synthesize superhydrophobic and highly IR-reflective HGM in one step. In previous research, the superhydrophobic and reflectivity properti...

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Disclosures

The authors have nothing to disclose.

Acknowledgements

The work described in this paper was supported by a grant from the CII-HK/PolyU Innovation Fund. Further support was provided by the Shenzhen Peacock Plan (KQTD2015071616442225) and the Chinese Government "Thousand Talent" Program (Y62HB31601). Also, the help from the Department of Applied Biology & Chemical Technology of the Hong Kong Polytechnic University and the Hong Kong Polytechnic University Research Institute for Sustainable Urban Development (RISUD) is appreciated.

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Materials

List of materials used in this article
NameCompanyCatalog NumberComments
HGMTechnical Institute of Physics and Chemistry, Chinese Academy of ScienceN/AN/A
TBTSigma-AldrichCAS#: 5593-70-4Analytical grade
Ethyl AlcoholSigma-AldrichCAS#: 64-17-5Analytical grade
PFOTESSigma-AldrichCAS#: 51851-37-798%

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Superhydrophobic CoatingAnatase TiO2 CoatingPFOTES Superhydrophobic AgentX-ray DiffractionScanning Electron MicroscopyContact Angle MeasurementThermal Conductivity Analysis
TiO2-coated Hollow Glass Microspheres with Superhydrophobic and High IR-reflective Properties Synthesized by a Soft-chemistry Method
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