Photoluminescence based H2 and O2 gas sensing by ZnO nanowires

Abstract

Gas sensing properties of ZnO nanowires prepared via thermal chemical vapor deposition method were investigated by analyzing change in their photoluminescence (PL) spectra. The as-synthesized nanowires show two different PL peaks positioned at 380 nm and 520 nm. The 380 nm emission is ascribed to near band edge emission, and the green peak (520 nm) appears due to the oxygen vacancy defects. The intensity of the green PL signal enhances upon hydrogen gas exposure, whereas it gets quenched upon oxygen gas loading. The ZnO nanowires' sensing response values were observed as about 54% for H2 gas and 9% for O2 gas at room temperature for 50 sccm H2/O2 gas flow rate. The sensor response was also analyzed as a function of sample temperature ranging from 300 K to 400 K. A conclusion was derived from the observations that the H2/O2 gases affect the adsorbed oxygen species on the surface of ZnO nanowires. The adsorbed species result in the band bending and hence changes the depletion region which causes variation in PL signal. A photoluminescence based gas sensing technique has advantage over a conductometric technique due to its higher sensitivity and faster response time.

Abnormal gas pressure sensitivity of the visible emission in ZnO quantum dots prepared by improved sol–gel method: the role of surface polarity

“…Owing to the wide band gap (i.e., $3.37 eV at 300 K) and large exciton binding energy (60 meV), as well as many other outstanding properties, bulk ZnO and nanostructures of ZnO have attracted a great deal of interest for potential applications in the elds of optoelectronic devices, photocatalysts, gas sensor, and biological labels. [1][2][3][4][5][6][7][8][9][10][11] As one of the typical nanostructures, ZnO quantum dots (QDs) have unique physical properties due to their strong quantum size effect and surface effect. [12][13][14][15][16][17] The photoluminescence (PL) spectrum of ZnO QDs usually features two emission bands, a relatively narrow ultraviolet (UV) emission from the band-edge excitonic recombination, and a broad visible emission induced by defects.…”

Low Energy Pulsed Laser Excitation in UV Enhances the Gas Sensing Capacity of Photoluminescent ZnO Nanohybrids

“…The onset of this low plateau ( Figure 5) occurs at about 0.2 mJ/cm 2 for the two wavelengths investigated and might be indicative either for a saturation behavior or for a change in the mechanism of interaction between the semiconductor and the analyte. The widely manifested increase of the ZnO PL emission upon exposure of the semiconductor to ethanol vapors has been interpreted on the basis of charge transfer occurring from the alcohol to ZnO that leads to an increase in the density of charge carriers, which contribute to the enhancement of radiative recombination following excitation of the semiconductor [25]. Even though the detailed chemistry, taking place on the semiconductor surface during the interaction with the analyte has not been clearly identified, a series of reactions of the photogenerated electrons and holes ( + ℎ → + ℎ ) with the adsorbed molecules have been proposed to explain the PL sensing mechanism in metal oxides [23].…”

“…The extraction of electrons from the conduction band [12] leads to the formation of an electron depleted layer, the thickness of which has been suggested to have an impact on the UV photoluminescence leading to a reduction of its intensity [21]. When ethanol is present, it interacts with the photo-generated oxygen ions (O − 2 ) [9] causing the release of trapped electrons back to ZnO [25], thereby reducing the depletion layer thickness and enhancing the ZnO PL emission over that observed in air. An alternative mechanism, suggesting a competing adsorption between the oxygen species and ethanol on the surface of ZnO nanostructures, has also been proposed, on the basis of a limited number of theoretical [30] and experimental [19] studies, to be responsible for ethanol sensing, owing to the ability of ethanol to remove oxygen from the semiconductor surface, while releasing back the electrons trapped by oxygen.…”

“…The density of carriers plays a decisive role in the sensing process by affecting the kinetics of redox reactions occurring between the analyte gas and the semiconductor surface. [25]. In the present work a comparative study is reported, for the first time, investigating the response of ZnO-PDMS composites exposed to ethanol and oxygen, as a function of photoexcitation parameters with emphasis on the excitation wavelength, λ exc (photon energy), and the energy fluence, F exc (photon number density).…”

Role of oxygen vacancies and In‐doping on the sensing performance of ZnO particles

“…Selectivity is the different sensitivity of the sensing device upon exposure to different gases at a fixed concentration and device temperature. ZnO has been used as the sensing media of a wide variety of gases at ppm concentrations: ethanol [6], H 2 [7], CO [8], propane [9], etc. In particular CO and propane gases are of special interest due to their toxicity at low concentrations and their presence in domestic and industrial environments.…”