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Recently, Academician Yu Jihong and Professor Song Xiaowei's research group from the School of Chemistry of Jilin University and the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry of Jilin University published their research work entitled "Zeolites as a Class of Semiconductors for High-Performance Electrically Transduced Sensing" at the Journal of the American Chemical Society. This work reveals the semiconductor characteristics and charge transfer mechanism of zeolite molecular sieve for the first time, and deeply studies the electrical characteristics and energy level structure of molecular sieve, and constructs a resistive ammonia gas sensor based on semiconductor molecular sieve with high sensitivity, high selection, fast response and high stability. This work breaks through the original understanding of molecular sieve materials and further expands the application of molecular sieves to the field of semiconductor devices.

Fig. 1:(a, B) schematic diagram of device structure of conductive molecular sieve-based sensor and comparison diagram of its comprehensive performance;(c,d) single crystal structure characterization of Na-type MFI molecular sieve, with scales of 50 nm and 21/nm in the figure respectively; Schematic diagram of (e) variable temperature impedance test,(f-h)Tauc-plot curve, Mott-Schottky test and energy band structure of Na-ZSM-5 molecular sieve

Zeolite molecular sieves are a class of inorganic microporous crystalline materials with regular pore structure. Due to their large specific surface area, shape-selective adsorption and separation performance, high hydrothermal stability, and adjustable active centers and functional units, they are widely used as catalysis, adsorption and separation and ion exchange materials in the fields of petrochemical, fine chemical and daily chemical industry, and have played an irreplaceable role in the realization of "carbon neutralization. Although molecular sieves have shown great application prospects in the fields of traditional catalysis, energy storage, biomedicine, etc., due to the insulator characteristics of molecular sieves at atmospheric pressure and its extremely low conductivity, the current research on the application of molecular sieves in semiconductor sensor devices is still in the blind area. Therefore, it is of great scientific significance and application value to further study the electrical properties of molecular sieve, and to create molecular sieve materials with excellent charge transfer characteristics and sensing properties, so as to realize the application of molecular sieve in the field of semiconductor sensing.

Figure 2: (a, B) temperature change I-V test of Na-ZSM-5 molecular sieve;(c) temperature change conductivity test;(d,e) electronic structure calculation based on first principles;(f-h) photoelectric properties

In view of this, the team used spectral characterization, electrochemical method, variable temperature electrical characteristic test and photoconductive effect, and DFT theoretical calculation to reveal for the first time that Na-type MFI molecular sieve is a kind of ultra-wide band gap semiconductor (direct band gap: ~ 4.9 eV)(Figure 1), with electronic conductivity and temperature dependence. Through the temperature I-V test (Figure 2), the temperature dependence of the electron transmission characteristics of the molecular sieve is verified, and there is a Schottky barrier between the molecular sieve film and the metal Al electrode, and the I-V curve will show rectification characteristics, which further proves that molecular sieve is a kind of semiconductor material. At the same time, it is proved theoretically and experimentally that the band gap and conductivity of the molecular sieve material can be controlled by optimizing the design of the cation type and concentration of the balance skeleton charge and regulating the silicon-aluminum ratio of the molecular sieve skeleton. On the other hand, this work is the first to use semiconductor conductive molecular sieve to construct a photodetector (Figure 2), which not only obtains a UV detector with fast response/recovery characteristics, but also verifies the banded charge transfer mechanism in the conductive molecular sieve.

Fig. 3: schematic diagram of (a) manufacturing process of molecular sieve-based resistive gas sensor; (B, c) relationship diagram of baseline resistance, sensitivity and temperature;(d) selectivity test;(e,f) ammonia concentration gradient test under different humidity;(g) cycle stability test

In order to further expand the application of semiconductor conductive molecular sieve, the work is the first to construct a resistive gas sensor based on conductive molecular sieve (Figure 3). The Na-ZSM-5-based gas sensor can detect trace ammonia (77 ppb) with high sensitivity, high selectivity and high stability in the humidity range of 2% ~ 90% RH, and has fast response characteristics and excellent long-term stability, its comprehensive performance is better than that of most of the resistive NH3 gas sensors, which confirms that the molecular sieve material is expected to be used in the next generation of sensing materials. At the same time, through solid-phase MAS NMR characterization, NH3-TPD characterization, in-situ Fourier transform infrared spectroscopy (in-situ FTIR) and theoretical calculation, the ammonia gas sensing mechanism of molecular sieve materials was further analyzed (Figure 4). The study pointed out that there is a large amount of charge transfer between Na + ions outside the framework as Lewis acid sites and ammonia molecules, which affects the charge density of molecular sieve framework and leads to significant changes in the conductivity of molecular sieve materials in ammonia atmosphere. Therefore, the sensing properties can be effectively improved by regulating the concentration of Na + ions and the strength of Lewis acid.

Figure 4: Molecular sieve sensing mechanism study:(a, B) 29Si and 27Al solid-phase MAS NMR;(c)NH3-TPD;(d,e) in situ FTIR;(f,g) Adsorption energy of gas molecules on molecular sieve;(h) Calculation of differential charge density of NH3 adsorbed on Lewis acid sites

In summary, this work explores the semiconductor characteristics and charge transfer mechanism of zeolite molecular sieve, and based on this, a resistive ammonia gas sensor with high sensitivity, high selection, fast response and high stability is successfully constructed. The use of molecular sieves with traditional semiconductor materials do not have a regular ordered microporous structure, shape selective screening characteristics, high adsorption capacity, high catalytic activity, high hydrothermal stability and other unique physical and chemical properties, the application of molecular sieves will be further extended to the photoelectric, sensing and other fields.

The relevant research results were recently published in the Journal of the American Chemical Society Journal. The first author of the article is Wang Tianshuang, a postdoctoral fellow in the "Boxin Program" of Jilin University. The corresponding authors are Academician Yu Jihong and Professor Song Xiaowei of Jilin University, and Professor Deng Feng of the Institute of Precision Measurement Science and Technology Innovation of the Chinese Academy of Sciences. This work has been supported by the key project of the National Natural Science Foundation of China, the basic science center project of the National Natural Science Foundation of China and the 111 plan project.

Full text link: https://pubs.acs.org/doi/full/10.1021/jacs.2c1316