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.

No abstract

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.

No abstract

No abstract

A quartz enhanced photoacoustic spectroscopy (QEPAS) sensor, employing an erbium-doped fiber amplified laser source and a custom quartz tuning fork (QTF) with its two prongs spaced $800 lm apart, is reported. The sensor employs an acoustic micro-resonator (AmR) which is assembled in an "on-beam" QEPAS configuration. Both length and vertical position of the AmR are optimized in terms of signal-to-noise ratio, significantly improving the QEPAS detection sensitivity by a factor of $40, compared to the case of a sensor using a bare custom QTF. The fiber-amplifier-enhanced QEPAS sensor is applied to H 2 S trace gas detection, reaching a sensitivity of $890 ppb at 1 s integration time, similar to those obtained with a power-enhanced QEPAS sensor equipped with a standard QTF, but with the advantages of easy optical alignment, simple installation, and long-term stability.

No abstract

A sub-ppb level photoacoustic spectroscopy (PAS)-based sensor for nitrogen dioxide (NO 2) detection was developed by means of a 3.5 W CW multimode diode laser emitting at 447 nm. A differential photoacoustic cell was designed to match the imperfect laser beam and reduce the external acoustic as well as the electromagnetic noise. The diode laser power, gas flow and pressure of the sensor were optimized, which resulted in a NO 2 sensor system with a detection limit of 54 pptv with a 1-s averaging time and an excellent linear dynamic range over > three orders of magnitude. The impact of water vapor as the catalyst on the photoacoustic signal amplitude was also investigated. Continuous measurements covering an eight-day period were performed to demonstrate the stability and robustness of the reported PAS-based NO 2 sensor system.