Enhanced Refractive Index Sensitivity through Combining a Sol-Gel Adsorbate with a TiO2 Nanoimprinted Metasurface for Gas Sensing

Designing a highly near infrared-reflective black nanoparticles for autonomous driving based on the refractive index and principle

HYPOTHESIS

The development of highly NIR reflective black single-shell hollow nanoparticles (BSS-HNPs) can overcome the Light Detection and Ranging (LiDAR) sensor limitations of dark-tone materials. The crystalline phase of TiO2 and the refractive index can be controlled by calcination temperature. The formation of hollow structure and the refractive index is expected to simultaneously increase the light reflection and LiDAR detectability.

EXPERIMENTS

The BSS-HNPs are synthesized using the sol-gel method, calcination, NaBH4 reduction, and etching to form a hollow structure with true blackness. The computational bandgap calculation is conducted to determine the bandgap energy (Eg) of the white and black TiO2 with different crystalline structures. The blackness of the as-synthesized materials is determined by the Commission on Illumination (CIE) L*a*b* color system.

FINDINGS

The hydrophilic nature of BSS-HNPs enables the formulation of hydrophilic paints, allowing the mono-layer coating. With the synergistic effects of hollow structure and the refractive index, BSS-HNPs manifested superb NIR reflectance at LiDAR detection wavelengths. The high detectability, blackness, and hollow structure of BSS-HNPs can expand the variety of LiDAR-detectable dark-tone materials.

Designing Novel LiDAR-Detectable Plate-Type Materials: Synthesis, Chemistry, and Practical Application for Autonomous Working Environment

Plate-type hollow black TiO2 (HL/BT) with a high NIR reflectance was fabricated for the first time as a LiDAR-detectable black material. A TiO2 layer was formed on commercial-grade glass by using the sol-gel method to obtain a plate-type structure. The glass template was then etched with hydrofluoric acid to form a hollow structure, and blackness was further achieved through NaBH4 reduction, which altered the oxidation state of TiO2 to black TixO2x-1 or Ti4+ to Ti3+ and Ti2+. The blackness of the HL/BT material was maintained by a novel approach that involved etching prior to reduction. The thickness of the TiO2 layer was controlled to maximize the NIR reflectance when applied as paint. The HL/BT material with a thickness of 140 nm (HL/BT140) showed a blackness (L*) of 13.3 and high NIR reflectance of 23.6% at a wavelength of 905 nm. This is attributed to the effective light reflection at the interface created by the TiO2 layer and the hollow structure. Plate-type HL/BT140 provides excellent spreadability, durability, and thermal stability in practical paint applications compared with sphere-type materials due to the higher contacting area to the applied surface, making it suitable for use as a LiDAR-detectable inorganic black pigment in autonomous environments.

Application of Polyoxometalates in Chemiresistive Gas Sensors: A Review

Polyoxometalates (POMs) are a series of molecular metal compounds based on W and Mo elements, exhibiting excellent physical and chemical properties. POMs have been widely used in the fields of photoelectric materials, catalytic materials, and coordination chemistry. In recent years, POMs have emerged in the field of chemiresistive gas sensors. They can work as electron acceptors and improve the gas-sensing performance of traditional sensing materials by means of capturing electrons from semiconductors, separating electrons produced by light excitation or thermal excitation and delaying the recombination of electrons and holes. So far, the highest sensing sensitivity response of POMs-based chemiresistive gas sensor is 231 to 1 ppm NO₂ gas. In this review, an overview is investigated about how POMs have evolved as sensing materials in gas sensors. First, some POMs and POMs-based sensing materials in recent years are introduced and classified. After that, brief analyses for each kind of sensing materials are provided. Then we compare the reported POMs-based sensors in different sensing parameters. Finally, the future outlooks are discussed on the basis of the current developments. This work is the first comprehensive overview of POMs-based chemiresistive gas sensors. This work can provide valuable information for developing high-performance POMs-based gas sensors.