Enhancement of Gas Sensing Properties of Metal Oxides Thin Films by Embedding of Noble Metals Nanoparticles

Abstract

Abstract In this study, Au and Ag NPs were synthesized in distilled water by Pulsed Laser Ablation in Liquid (PLAL) technique using Q-switched (Nd:YAG) pulsed laser with laser parameters (520 mJ laser energy, 1064 nm wavelength, and 1Hz frequency). The effect of six different laser pulses (150, 250, 350, 450, 550, and 650) on the properties of these NPs were investigated systematically. ZnO and SnO2 thin films were deposited on glass substrates by sol-gel spin-coating method. Volumetric ratios of ZnO and SnO2 solutions with Au and Ag colloidal solution (3:2 and 4:1) were used to prepare ZnO and SnO2 thin films embedded with Au and Ag NPs, and the morphological, structural, optical and Hall Effect properties of all the films were investigated. TEM micrographs of Au and Ag NPs shows that all samples were spherical in shape. The XRD patterns of the Au and Ag NPs exhibited the pure cubic crystalline structure at all the pulses used. The XRD patterns of Ag NPs also showed the formation of another cubic crystal structure attributed to AgO. FESEM images of the films exhibited spherical particles and randomly scattered spherical particles having irregular sizes for ZnO+Au (3:2) and ZnO+Au (4:1),respectively and showing cauliflower like shapes with irregular size-distribution for ZnO+Ag (3:2) and randomly scattered cauliflower particles having irregular shapes and sizes for ZnO+Ag (4:1). The images of SnO2+Au (3:2) and SnO2+Au (4:1) display that most of the particles obtained have spherical shapes and are agglomerated due to their small sizes. The XRD patterns of all ZnO thin films showed that the diffraction peaks belong to the hexagonal phase with a wurtzite structure, while the XRD patterns of all SnO2 thin films exhibited diffraction peaks which belong to the tetragonal rutile crystal. Raman spectroscopy studies show that the peaks of ZnO and SnO2 thin films embedded with Au and Ag NPs at volume ratio (3:2) are Raman-active peaks consistent vibration modes. It can be notice four typical peaks at Raman shift of 312, 453, 562 and 790 cm-1 for (ZnO +Au) thin film and observed three peaks at 420, 560 and 825 cm-1 are the transverse optical (TO), longitudinal optical (LO) polar branches, respectively for (ZnO +Ag) thin film. Also, show two of the four fundamental Raman-active peaks at 751and 807 cm−1 for (SnO2+Au) thin film and five peaks at 237, 421, 473, 561and 749 cm−1 for (SnO2+Ag) thin film. The absorbance values of the films decrease with the increase of wavelength, it is increase as the molarity of ZnO and SnO2 increases. It can be notice that the absorbance increase with the volumetric ratios of ZnO and SnO2 thin films embedded with Au and Ag NPs increasing. The absorption coefficient of ZnO and SnO2 thin films embedded with Au and Ag NPs at different volume ratios (3:2 and 4:1) increase gradually with the increase in the energy of the incident photons until it reaches the value of 10 4 cm-1, the values of the absorption coefficient greater than 104 cm-1 indicate the possibility of allowed direct electronic transitions. The value of optical energy gap of ZnO and SnO2 thin films embedded with Au and Ag NPs at different volume ratios (3:2 and 4:1) obtained are (3.92, 3.90, 3.99 and 3.97 eV) and (3.56, 3.58, 3.80 and 3.85 eV), respectively. Hall Effect measurements of ZnO thin films embedded with Au and Ag NPs show that the conductivity increases as carrier concentration increases. It can be notice that the resistivity decreases as carrier concentration increases for AuNPs embedded with ZnO, while it is increases for AgNPs embedded with ZnO. The measurements of SnO2 thin films embedded with Au and Ag NPs show the conductivity increases as carrier concentration decreases, and the resistivity is decreases at a high amount of Au and Ag. Gas sensor results show that the maximum sensitivity of (SnO2+Au) sensor to 60 ppm of NO2 gas is greater than (ZnO+Ag) sensor. The results show the sensitivity increase as the operating temperature and as the concentration of the NO2 and NH3 gas increases. Also can be notice that the response and recovery times decrease as the operating temperature increases.