[Background introduction]

Measuring humidity is a common practice in health care, such as breathing monitoring and mood detection. To date, researchers have studied a variety of nanomaterials such as graphene oxide, MoS2, metal oxides, and nanohybrids as humidity sensing materials. These types of humidity sensors can be miniaturized for wearable devices and have low energy consumption. However, such devices are often less sensitive at low relative humidity (RH) levels (<40% RH).

[Achievement Profile]
Recently, Professor Peng Zhengchun‘s group at Shenzhen University reported a method for growing TiO2 nanowires in situ on two-dimensional Ti3C2 MXene. Compared with pure Ti3C2 or TiO2 materials, the specific surface area of Ti3C2 / TiO2 composites prepared by this method is an order of magnitude larger. This sea-urchin-like composite structure shows unprecedented high sensitivity in low relative humidity (RH) environments. The author uses complex impedance spectroscopy and Schottky junction theory to explain the potential sensing mechanism of Ti3C2 / TiO2 composites under different humidity conditions, and demonstrates that a humidity sensor made of Ti3C2 / TiO2 composites can be used for various liquids and human fingers Application in non-contact detection.
The results were published online in ACS Applied Materials & Interfaces: High-Performance Humidity Sensor Based on Urchin-Like Composite of Ti3C2 MXene-Derived TiO2 Nanowires. Dr. Peng Zhengchun‘s research group Dr. Li Ning and Jiang Yue co-authored the article, and Assistant Professor Xing Chenyang co-authored.

[Picture and text guide]

Figure 1: (a) Material preparation process: HF etching, LPE, alkali oxidation and other methods. (B) Schematic diagram of experimental device

Figure 2 (a) Humidity response of Ti3C2 / TiO2 composites prepared under different conditions (samples A1-A9). (B) Moisture sensitivity comparison of bulk TiO2, Ti3C2 nanosheets, Ti3C2 / TiO2 composite prepared by hydrothermal method and Ti3C2 / TiO2 composite prepared by alkaline oxidation method (sample A5). (C) The relationship between the capacitance of the sensor A5 and the operating frequency under different relative humidity. (D) The relationship between the capacitance output of sensor A5 and RH. (E) Response and recovery characteristics of sensor A5 exposed to a range of RH conditions. (F) Repeatability of sensor A5 exposed to 33% RH, 54% RH, and 84% RH, respectively. (G) Hysteresis curve of the adsorption-desorption response measured in the range of 7-97% RH. (H) The stability of sensor A5 under different humidity conditions for 40 days.

Figure 3 Humidity fluctuations are reflected by the capacitance output of a humidity sensor integrated in the fingers of a robotic hand close to a wet diaper. (A) Water cups at different temperatures (b) (c) High-resolution capacitance maps on the near surface of the fingertip measured by an established humidity sensor (D) Photograph of three fingertip positions above the 5 × 5 matrix. (E) A 3D map of the approaching three fingertips.

Figure 4 (a) Schematic diagram of the adsorption process of water molecules on the Ti3C2 / TiO2 composite membrane. Complex impedance diagram of Ti3C2 / TiO2 The complex impedance diagram of Ti3C2 / TiO2 composite film at 7-23% RH (b) and 33-97% RH (c). Inset in (b, c): EC corresponds to the complex impedance spectrum (CIS) spectrum. (D) Bode diagram of Ti3C2 / TiO2 composite under different RH conditions. (E) Complex impedance diagram of pure Ti3C2 film.

Figure 5 (a) I-V curves of Ti3C2 / TiO2 composite films measured at different RH levels. (B) Band diagram of Ti3C2 / TiO2 composite film (E0 vacuum level; W work function; Ef Fermi level; EC conduction band; EV valence band)

[Summary of this article]
A Ti3C2 / TiO2 composite humidity sensor was prepared by oxidizing Ti3C2 in KOH solution. The sensor has a very high specific surface area, and has excellent sensitivity compared to sensors made of pure Ti3C2 or pure TiO2, especially at low relative humidity. CIS and Bode diagrams were used to explain the sensing mechanism of Ti3C2 / TiO2 composites. The barrier of the Ti3C2 / TiO2 Schottky junction also helps improve the sensitivity of the composite in low humidity environments. The sensor can detect diaper humidity and water temperature levels in a non-contact manner, and monitor the proximity of fingertips, indicating that it has great potential in technologies such as health monitoring, environmental protection, and human-computer interaction.


Literature link:
DOI: 10.1021 / acsami.9b12168