Quartz-enhanced photoacoustic spectroscopy (QEPAS) is a sensitive gas detection technique which requires frequent calibration and has a long response time. Here we report beat frequency (BF) QEPAS that can be used for ultra-sensitive calibration-free trace-gas detection and fast spectral scan applications. The resonance frequency and Q-factor of the quartz tuning fork (QTF) as well as the trace-gas concentration can be obtained simultaneously by detecting the beat frequency signal generated when the transient response signal of the QTF is demodulated at its non-resonance frequency. Hence, BF-QEPAS avoids a calibration process and permits continuous monitoring of a targeted trace gas. Three semiconductor lasers were selected as the excitation source to verify the performance of the BF-QEPAS technique. The BF-QEPAS method is capable of measuring lower trace-gas concentration levels with shorter averaging times as compared to conventional PAS and QEPAS techniques and determines the electrical QTF parameters precisely.

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A new visible light-driven photocatalyst, Bi5O7I, prepared by a hydrothermal method was studied. The as-prepared Bi5O7I exhibited efficient photocatalytic activity in the decomposition of a widely used dye, tetraethylated rhodamine (RhB), in water and acetaldehyde (CH3CHO) in air under visible light irradiation. Besides decoloring, the reduction of chemical oxygen demand concentration was also observed in the degradation of RhB, further demonstrating the photocatalytic performance of Bi5O7I. The results of density functional theory calculations indicated that the conduction band bottom of Bi5O7I is mainly composed of Bi 6p orbits, and the valence band top primarily consists of I 5p and O 2p orbits. The as-prepared Bi5O7I exhibited much higher photocatalytic activity than Bi2O3, which may be ascribed to the hybrid states of the valence bands as well as the internal electric fields between Bi5O7 and I slabs. According to experimental results, a possible photocatalytic mechanism of Bi5O7I was proposed.