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    Advanced NH3 Detection by 1D Nanostructured La:ZnO Sensors with Novel Intrinsic p-n Shifting and Ultrahigh Baseline Stability
    (Amer Chemical Soc, 2024) Ajjaq, Ahmad; Bulut, Fatih; Ozturk, Ozgur; Acar, Selim
    Due to its stability, transportability, and ability to be produced using renewable energy sources, NH3 has become an attractive option for hydrogen production and storage. Detecting NH3 is then essential, being a toxic and flammable gas that can pose dangers if not properly monitored. ZnO chemiresistive sensors have shown great potential in real NH3 monitoring applications; yet, research and development in this area are ongoing due to reported limitations, like baseline instabilities and sensitivity to environmental factors, including temperature, humidity, and interferent gases. Herein, we suggest an approach to obtain sensors with competitive performance based on ZnO semiconducting metal oxides. For this purpose, one-dimensional nanostructured pure and La-doped ZnO films were synthesized hydrothermally. Incorporating large rare earth ions, like La, into the bulk lattice of ZnO is challenging and can lead to surface defects that are influential in gas-sensing reactions. The sensors experienced a temperature-induced p-n shifting at about 100 degrees C, verified by the Hall effect and AC impedance measurements. The doped sensor showed exceptional stepwise baseline stability and outstanding performance at a relatively low operating temperature (150 degrees C) with a sensing response of 91 at best (@ 50 ppm NH3) and recorded a tolerance to water vapor up to 70% RH. Alongside p-n shifting, the enhanced performance was discussed in correlation with La doping-triggered changes in the nanostructural and surfacial properties of the films. We validated the proposed technique by producing similar sensors and performing multiple replicates to ensure consistency and reproducibility. We also introduced the fill factor concept into the gas sensor field as a new trustworthy parameter that could improve sensor performance assessment and help rate sensors based on deviation from ideality.
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    Chemical precursor-dependent dual effect of doping on the gas-sensing performance of metal oxide semiconducting materials
    (Elsevier Science Sa, 2024) Ajjaq, Ahmad; Bulut, Fatih; Ozturk, Ozgur; Acar, Selim
    In this study, we report a chemical precursor-dependent dual effect of doping on the gas-sensing performance of metal oxide semiconducting materials. Our findings challenge the conventional notion that optimal doping consistently enhances gas-sensing properties. Acetate and nitrate salts were used as chemical precursors, lanthanum (La) was used as a dopant, and ZnO was used as a semiconducting material. All materials were synthesized under identical conditions by a two-step process involving dip coating and hydrothermal methods. Gas-sensing results demonstrated an improvement in the performance of the acetate-based doped film and a decline in that of the nitrate-based doped film compared to their respective pure counterparts. Among the produced sensors, 1 wt% La-doped ZnO sensor produced by the acetate precursor proved to be convenient for usages in real markets. It showed superior performance with a high response (62) at a relatively low operating temperature (150degree celsius) towards 50 ppm of NH3 gas. The sensor also demonstrated exceptional baseline stability, high short-term and long-term consistency, good selectivity, and strong tolerance to humidity (up to 70 RH%) with slightly slow adsorption-desorption rates. The dual effect was discussed with respect to dopant- and precursor-induced variations in structural and surficial characteristics, revealed by XRD, Raman, FESEM, AFM, and XPS. The discussion delved deeper into the role of chemical precursors on nanostructure growth and, for the first time, illuminated a temperature-dependent complex gas-sensing principle governed by the detected p-n shift of the semiconductor type of the sensing elements, confirmed by Hall effect.
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    Effect of Ni and Al doping on structural, optical, and CO2 gas sensing properties of 1D ZnO nanorods produced by hydrothermal method
    (Wiley, 2022) Bulut, Fatih; Ozturk, Ozgur; Acar, Selim; Yildirim, Gurcan
    In the present study, the one-dimensional ZnO nanorod structures are produced within the different nickel and aluminum molecular weight ratios of 0-7% using the hydrothermal method. It is found that the aluminum (Al) and nickel (Ni) impurities with different ionic radius, chemical valence, and electron configurations of outer shell cause to vary the fundamental characteristic features including the crystallinity quality, crystallite size, surface morphology, nanorod diameter, optical absorbance, energy band gap, resistance, gas response, and gas sensing properties. The structural analyses performed by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) indicate that the samples are found to crystallize in the hexagonal wurtzite structure. The presence of optimum nickel and aluminum in the crystal system improves considerably the crystallinity quality and surface morphology. Additionally, the combination of electron dispersive X-ray (EDX) and XRD results declare that the Ni and Al impurities incorporate successfully into the ZnO crystal structure. Moreover, the diameters of nanorod structures in 1D orientation are determined to be 80 nm or below. The hexagonal wurtzite-type ZnO nanorod structure prepared by 5% Ni has more space between the nanorods and thus presents higher response to the CO2 detection. Further, the optical absorbance spectra display that the band gap value is observed to decrease regularly with the increment in the doping level as a result of band shrinkage effect depending on the enhancement of mobile hole carrier concentrations in the crystal structure. In other words, the doping mechanism leads to vary the homogeneities in the interfacial charges, nanorod diameters, ZnO oxide layer composition and thickness. The last test conducted in this study is responsible for the determination of CO2 gas sensing levels. The obtained gas sensing results are further compared with each other and literature findings. It is observed that 5% Ni-doped sample provides more successful results than other samples in the sensing CO2 gas at the different concentrations. All in all, the paper establishing a strong methodology between doping mechanism and change in the fundamental characteristic features of hexagonal wurtzite-type ZnO with the aid of advanced microscopy techniques will become pioneering research to answer key questions in materials sciences and electronic research.
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    The Investigation of CO2 Gas Sensing Performance of ZnO Nanorods Growth on RF Sputtered Seed Layer
    (2023) Bulut, Fatih; Ozturk, Ozgur; Acar, Selim
    In this study, one-dimensional ZnO nanorod structures with different ratios of nickel doping were produced using the hydrothermal method. The presence of nickel doping in different ratios caused variations in the fundamental characteristics of the nanorods that grew on the RF sputtered seed layer, such as crystallinity quality, morphology, diameter of the nanorods, band gap energy, resistance of the sample, and CO2 gas sensing. Produced samples were found to form like hexagonal rods and crystallize in a wurtzite structure, and the ratio of nickel doping improved the crystallin quality and the morphology of sample surface. This study showed that the 5% nickel doped sample provided the most effective results in sensing CO2 gas at different concentrations. Overall, the study provided valuable insights into the relationship between doping system and the basic characteristics of wurtzite-type hexagonal ZnO.