In the present work we synthesized
In the present work, we synthesized CuO–Ag2O bimetallic oxide nanoparticles by using microwave assisted and solid state diffusion routes. Different characterization techniques were used to characterize these samples. The LPG sensing characteristics were investigated at room temperature as well as different temperature.
Results and discussion In Fig. 1, the XRD pattern of CuO–Ag2O bimetallic oxide nanoparticles shows broad peaks indicating nanometric dimensions of the synthesized materials. The symbolic representation of diffraction peaks for CuO and Ag2O exactly indexed to JCPDS file no. 89-5897 and 75-1532, respectively. It is also well recognized that CuO and Ag2O alloy possesses easily phase separation, due to the radius inequality between Cu and Ag within the alloy-nanoparticles . The overlapped diffraction peak appears around 41°, which might be shows some indication of alloy formation. The crystal grain size estimated using Debye–Scherrer equation , where D is average crystallite size (nm), K is a shapes factor that is K = 0.89, λ is the wavelength of used X-ray source equals 1.540 Å, β is the full width at half maxima, and θ is the diffraction peak angle. The crystal grain size was found to be 17.3 and 20.4 nm for microwave assisted and solid state diffusion route, respectively. Figs. 2(a) and (b) shows the SEM images of CuO/Ag2O bimetallic nanoparticles synthesized by microwave assisted and solid state diffusion route, respectively. SEM micrograph reflects the agglomeration of nanoparticles, which may due to reaction temperature for the both route is high. The crystal grain size estimated from SEM micrographs was found to ranges between 18 and 21 nm. This variation in particle size strengthens with XRD analysis. Fig. 2(a) shows the three-dimensional aggregate morphology of as-synthesized material, which formed by a group of primary particles. Whereas, Fig. 2(b) depicts that primary aggregation particles form sheets-like shape. The FTIR spectra of the as-synthesized products are shown in Figs. 3(a) and (b). The FTIR spectrum of both samples match finally with each other. The peak at 535 and 601 cm−1 is likely to be from Cu–O stretching . The insignificant band at 513 cm−1 is assigned to Ag–O stretching vibration . The broadband in the lower energy region (2200–4000 cm−1) is attributed to the presence of free electron tail in inorganic materials. In Fig. 4, we present the UV–vis spectra of CuO/Ag2O bimetallic nanoparticles synthesized by microwave assisted and solid state diffusion route. The mitotic inhibitor between 300 and 325 nm for both route may assigned to strong surface plasmon resonance in Ag nanoparticles . From plot, it is also scrutinized that CuO/Ag2O bimetallic nanoparticles synthesized by solid state diffusion route shows red-shift over the microwave assisted route. This is strong evidence of increment of particle size . This statement is also supported by XRD and SEM analysis. The results revels that reaction route can affect the absorption properties of as-synthesized CuO/Ag2O bimetallic nanoparticles. Fig. 5 shows the TGA curves for the CuO/Ag2O bimetallic nanoparticles by microwave assisted and solid state diffusion route to show the thermal behavior during heat treatment. There are two major mass change steps with increasing temperature. The weight loss up to 375 K may assign to removal of constituent water molecules. The TGA curves for both samples shows small weight loss in the region 373–450 K. The thermal stability in this region is very important for gas sensing materials, which discussed later. This thermal stability again witnessed around the 600–650 K. Fig. 6 shows the good dependence of the sensing response of the CuO/Ag2O bimetallic nanoparticles based sensors on the concentration of LPG at room temperature (303 K). The baseline resistance for sensors developed using microwave assisted and solid state diffusion route, were found to be 2.736 × 106 and 2.869 × 106 Ω, respectively. Upon exposure to LPG (reducing gas) the resistance of the both sensors increased . An increase resistance of sensing surface in the presence of reducing species clearly reflects p-type behavior of sensing surface . The sensing response of the sensors was found to vary nearly linearly as a function of LPG concentration. Saturation was not observed for LPG up to 200 ppm for both sensors. This may indicate the optimum detection limit for sensors toward the LPG is beyond 200 ppm. In comparative analysis, the nanoparticles synthesized by microwave assisted route show high sensing response over the particles synthesized by solid state diffusion route. This may be due to lower particle size. The critical analysis of gas sensing properties shows that gas sensing performance is significantly affected by particle size. In our case, XRD and SEM analysis shows nanoparticles synthesized by microwave assisted route exhibits lower particle size than solid state diffusion route. Therefore, Conduction model magnitude of sensing response is inversely proportional to particle size. As particle size decreases, surface to volume ratio increase drastically, this provides much area to gas molecules for interaction .