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

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 properties^1,2,3,4. Because of their hollow structure, HGM have an extremely low thermal conductivity^10,11. For these reasons, they are applied in many areas, including aerospace engineering⁵, deep-sea exploration⁶

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

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

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.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
HGM	Technical Institute of Physics and Chemistry, Chinese Academy of Science	N/A	N/A
TBT	Sigma-Aldrich	CAS#: 5593-70-4	Analytical grade
Ethyl Alcohol	Sigma-Aldrich	CAS#: 64-17-5	Analytical grade
PFOTES	Sigma-Aldrich	CAS#: 51851-37-7	98%