Heretofore, in order to optimize the efficiency of the semiconductor’s photocatalytic performance, many solutions have been suggested; among them, the combination of plasmonic noble metals and semiconductors as the plasmonic photocatalysts can be a very effective solution. This kind of composites is the proper candidates for investigating the effects of localized surface plasmon resonances on the semiconductor photocatalytic performance. Since the localized surface plasmon resonance of the noble metals depends on the various factors such as the size, morphology, substances and environments of the nanostructures, so by changing these factors, one can control and finally optimize the photocatalytic efficiencies of the composites containing the noble metals.
In the present research, in order to investigate the plasmonic photocatalysts and their performance, firstly, various nanostructures such as nanowires, nanocubes and nanospheres in different sizes and also zinc oxide nanoparticles were synthesized. Then, the dependency of the plasmonic properties of Ag nanostructures on size, morphology and nanostructure environment was analyzed and characterized. Next, in order to induct the context of surface plasmonic resonance and the effective parameters on it, the simulations were performed by Finite Difference Time Domain (FDTD) method. Afterward, composites consist of zinc oxide nanoparticles and various silver nanostructures with different concentrations of 1, 4.5 and 8 wt % were synthesized. In order to investigate the photocatalytic properties of these composites, the photocatalysts were coated on glass substrates. The results of the photocatalytic degradation of methylene blue using such plasmonic composites showed that the photocatalytic performance was improved compared to the pure ZnO. The performance was better when the silver nanostructure was nanocube. Also in the presence of nanospheres the performance was better compared to silver nanowire. The investigations showed that the morphology, size and concentration of silver nanostructures can affect the photocatalytic activities. It was found that the composite containing 4.5 wt % silver nanocubes with average size of 65 nm has the most photocatalytic efficiency among the other composites. Such composite could degrade more than 68% of the methylene blue dye after 120 minutes with the constant degradation rate of 0.00921 min-1. Finally, the physical mechanisms governing the present plasmonic photocatalysts were studied, and was confirmed that in present work, the energy transfer mechanism can be the dominant mechanism. The energy transfer mechanism is due to the near-field effect and the far-field scattering of the silver nanostructures’ LSPR phenomena in the plasmonic photocatalyst composites.

 transparent and conductive thin layers of random networks of functionalized multiwall carbon nanotubes were fabricated by spin coating techniques without using surfactant. In order to study the electro optical properties of carbon nanotube thin films and their dependence on carbon nanotubes length, carbon nanotubes were refluxed at 15, 30, 60 and 120 minutes in a mixture of sulfuric and nitric acids by a ratio of 3:1 at 80 °C. Such functionalization was necessary to disperse CNTs in solutions homogenously and also cut them to different lengths. 3- Aminopropyltriethoxysilane was used for improving the adhesion between glass substrates and functionalized CNTs. Electro optical properties of thin films prepared using different refluxing times CNTs were studied in the visible region and observations showed the electrical properties of the films with 30 minutes refluxed CNTs, were extremely improved in comparison with the 60 and 120 minutes refluxing times CNTs. Post annealing treatments showed that the electrical properties were improved one degree of magnitude for all samples at ambient pressures and at 285 °C. The best results were observed in samples with 30 minutes refluxed CNTs. The figure of merit (FoM) of different refluxed times CNTs films were calculated. It was 37 for the films which were prepared with 30 minutes refluxed CNTs which was higher compared with the films prepared with 60 and 120 minutes refluxed CNTs. For improving the electrical properties of fabricated layers, Ag nanowires were added to carbon nanotubes with different weight percentages. The best results were observed for 5.1 weight percentage of Ag nanowires. The films figure of merit was improved significantly and the maximum of 94.2 was obtained for samples with 5.1 % wt. of Ag nanowires. Photocatalytic, hydrophobic and hydrophilic properties of different functionalized CNTs films were investigated. The photocatalytic results showed some CNTs length dependence as well as thickness dependence. For films which were fabricated from 30 minutes refluxed CNTs the photocatalytic properties was better in comparison to 60 and 120 minutes refluxed CNTs. Hydrophobic properties of different refluxed times samples were studied and the observation showed the transition of hydrophobic to hydrophilic state with increasing the films thickness.

Abstract :
In the present thesis, thick film conductometric gas sensors based on ZnO hollow-sphere (ZHSs) nanostructures and corresponding hybrid sensors, with the multi-wall carbon nanotubes (MWCNTs) were prepared and their sensing properties in response to the some volatile organic compounds: Ethanol, Acetone, Methanol, Ether, Formaldehyde, Xylene and Toluene were investigated. In order to prepare initial ZnO hollow spheres, a template assisted chemical method using a solution of zinc-acetate dihydrate in dimethylformamide (DMF) as starting solution and dispersed carbon microspheres (CSs) as templates, was employed. For this purpose, at first, spherical carbon templates were synthesized by a hydrothermal method using a solution of Sucrose and de-ionized water in a closed autoclave. Results obtained from a set of experiments showed that by reducing initial solution concentration from 0.5 to 0.1 mollit-1 final CSs sizes were reduced from about 2500 nm to 300 nm. The formation of carbon spheres and ZnO hollow spheres (ZHSs) and estimation of their sizes and purities were investigated using XRD, SEM and TEM measurements. In order to prepare sensing hybrid material powders, ZHSs powders and MWCNTs with the various weight percent of 0.02, 0.05, 0.1, 0.3, 0.5 were homogenously mixed by ultrasonic assisted method in ethanol. SEM images investigation indicated that MWCNTs were dispersed fairly homogenous among ZHSs matrix and proper contacts between MWCNTs and ZHSs were constructed. The results of BET measurements showed that addition of only 0.05 wt% MWCNTs to the matrix of ZHSs increased specific surface area of the hybrid powder to about 1.3 times. Thick film gas sensors were fabricated in 6 batches by screen printing method; one of them was corresponded to the pure ZHSs (S-Blank) and the 5 others were corresponded to the hybrid powders with different CNTs contents and denoted by S-0.02, S-0.05, S-0.1, S-0.3, S-0.5. In the sensing measurement step; from response-temperature curves it was found that not only all hybrid sensors showed higher responses to all target VOCs than blank sensor, but maximum response enhancement to a specified VOC occurred at a given CNT content. For Ethanol and Methanol 0.05 wt% of MWCNTs was found as the optimum content; whereas for Acetone and Ether, S-0.1 hybrid sensor showed maximum responses. Also, it was found that addition of small amounts of carbon nanotubes to matrix of ZHSs reduced the optimum operating temperatures considerably. For the Methanol, adding 0.02wt% of CNTs reduced the optimum operating temperature as much as 20 °C; whereas for the Ethanol adding 0.05wt% of CNTs lowered the optimum operating temperature as much as 50 °C. For the blank and hybrid sensors, response-concentration curves showed that the saturation concentration has been affected by the carbon nanotube content; so that for Ethanol and Methanol, S-0.05 and for the Acetone and Ether S-0.1 hybrid sensors showed maximum linear correlation and saturation limit. Plotting the response bar diagrams for the S-0.05 and S-0.1 hybrid sensors in response to various VOCs showed a considerable selectivity that was related to CNTs content and temperature. Also, measurements showed that hybrid sensors represented lower response and recovery times compared to the blank sensors to some VOCs. For instance, response and recovery times for the S-0.05 hybrid sensor to 320 ppm ethanol at 300 °C, were recorded 44, 72 s that in comparison with those measured for blank sensor at the same temperature, namely: 82, 103 s revealed a significant reduction. Measuring the responses of blank and S-0.05 hybrid sensor to 320 ppm ethanol at 300 °C for 11 consecutive weeks and recording values of response intensities and characteristics times with differences less than 7%, indicated that both blank and hybrid sensors had good durability and stability.
In order to justify the results and observations, a mechanism based on synergistic action between ZHSs and MWCNTs was proposed.
 

 In order to attain smaller and more effective heat transfer systems, various techniques have been proposed to enhance the heat transfer of fluids. Nanofluids are new engineering materials which contain dispersed nanoparticles in the base fluids (water, ethylene glycol and oil). Nanofluids will be more stable, have higher thermal conductivity, less corrosion effects and need less pumping power compared to ordinary suspensions. The dispersed nanoparticles in the base fluids might be metal nanoparticles (Ag, Au, Fe, Al, Cu), nonmetal nanoparticles (SiO2, SiC, TiO2, Fe3O4, CuO, Al2O3), nanofibers, nanorods or nanotubes. Carbon nanotubes (CNTs) are a good choice as nanoadditives to increase the thermal conductivity of the base fluids due to having very high thermal conductivity (about 3000 W/m.K) and high aspect ratio.
In this work, nanofluids containing CNTs with different lengths and 0.1, 0.25 and 0.5 Vol% concentrations at different temperatures in deionized water, ethylene glycol, 50% deionized water + 50% ethylene glycol and engine oil base fluids were prepared and their thermal conductivities were measured. In order to change the CNTs lengths and enhance the dispersibility of them at different fluids, the CNTs were functionalized. The mixture of sulfuric and nitric acids with the ratios of 3:1 was used for their functionalization and the refluxing time was investigated as the variable parameter. By immersing the CNTs in the acids mixture, the CNTs will be broken at the defects sites and the functional groups like –COOH, -OH and so on are attached to the broken bonds. Therefore, it was expected to have carbon nanotubes with different lengths by changing the refluxing time. According to our investigation using SEM images and measurement software, the length of CNTs after 1, 2 and 4 hours refluxing times were reduced to 203, 171 and 135 nm respectively. In order to estimate the stability of nanofluids, UV-vis spectroscopy and zeta potential analysis were used to measure the stability of the nanofluids quantitatively. The results showed that the deionized water based nanofluids containing 4 hours refluxed CNTs had higher stability than the other samples, for which, no significant decrease of UV-vis absorbance peak intensity was observed after about 3 months from nanofluids preparation.
The results of measuring thermal conductivity showed that the dispersion of CNTs on the base fluids led to increase thermal conductivity for all base fluids. In addition, the thermal conductivity of nanofluids containing functionalized CNTs was higher than that of for nanofluids containing pristine CNTs. Also by increasing the refluxing time, the thermal conductivity was increased for ethylene glycol and engine oil base fluids while the higher thermal conductivity of deionized water based nanofluids was observed for 1 hour refluxed CNTs. On the other hand, the increasing of thermal conductivity was observed by increasing the concentration of CNTs for all base fluids but the higher increase was observed for engine oil base fluid. The effect of increasing temperature on the thermal conductivity of nanofluids was investigated at 20, 30, 40 and 50 ̊C. The results showed that the thermal conductivity of water-CNTs nanofluids were increased with increasing temperature while the increasing of thermal conductivity of EG-CNTs and engine oil-CNTs nanofluids were lower than that of water-CNTs nanofluids.
Also, the effect of CNTs decoration with Ag nanoparticles by the mass ratios of 1%, 2% and 4% on the thermal conductivity of nanofluids was investigated. The results showed that the thermal conductivity of nanofluids containing decorated CNTs was increased with respect to the undecorated ones and the maximum increase of 20% was observed for nanofluids containing CNTs decorated with 4 wt.% of Ag nanoparticles with respect to the deionized water.
The artificial neural networks method was used to model the experimental data of thermal conductivities by many researchers. In this work the artificial neural networks method was which is the method based on experimental data was used. In fact, by using this method one can design a virtual laboratory to predict the results on different conditions. The artificial neural network modelling was applied on the deionized water and engine oil based nanofluids results. The investigation showed that the neural network was trained well for two base fluids in order to predict results on different conditions but the network was trained fairly better for the engine oil based nanofluids with an error lower than 4% due to the ordered experimental data of engine oil based nanofluids.