Müezzinoğlu, Esra Demet; Tuna Yıldırım, Sümeyra; Çağırtekin, Ali Orkun; Ateş, Aytunç; Acar, Selim; Yıldırım, M. Ali

Cd(OH)2/CdO thin films were synthesized on glass substrates by the SILAR method at 30, 40, and 50 SILAR cycles. The effects of the SILAR cycle and sensor operating temperature on the structural, morphological, and CO gas-sensing properties of the films were investigated. XRD studies revealed that synthesized films had a polycrystalline hexagonal phase of Cd(OH)2, and after static gas sensing measurements, the hexagonal phase was transformed to cubic phase of CdO with the effect of operating temperature. SEM analyses confirmed the phase conversion and showed that the morphological properties of films improved with operating temperature. Cd(OH)2/CdO thin film-based sensors were fabricated and static-dynamic gas sensing measurements were made towards CO gas. The detection limit and operating temperature values of sensors were determined. The optimal operating temperature was found to be 157 °C for all sensors. The CO sensing results demonstrated that the sensor with 40 SILAR cycles has the highest sensitivity for 50–500 ppm CO gas concentration values at 157 °C compared to the others. The sensing responses of sensors increased from 24% to 40% for 50 ppm CO gas and from 44% to 69% for 500 ppm CO gas at 157 °C, depending on SILAR cycle. The activation energy (Ea) values of sensors were found to change between 0.396–0.684 eV in the Cd(OH)2 phase regions and 0.495–0.912 eV in the CdO phase regions, with the SILAR cycle. The response and recovery times of sensor with 40 SILAR cycles were found to be 84.2 s and 40.8 s for 50 ppm CO gas at 157 °C, respectively. Furthermore, the sensor demonstrated excellent selectivity to CO gas compared to CO2, CH4, and NH3 gases. Finally, our findings highlight the potential of Cd(OH)2/CdO nanomaterials synthesized using a simple process and in different SILAR cycles as a promising material to enhance the sensing properties of CO gas sensors. Graphical Abstract: (Figure presented.)

Horoz, Banu; Tuna Yıldırım, Sümeyra; Soltabayev, Baktiyar; Ateş, Aytunç; Acar, Selim; Yıldırım, M. Ali

In2O3 thin films were deposited via Successive Ionic Layer Adsorption and Reaction (SILAR) method on glass substrates at 20, 30, 40, and 50 SILAR cycles. The effect of SILAR cycle on the general and CO gas sensing properties of the films was investigated. The GIXRD and FE-SEM results indicated that the films had cubic phase and porous morphology. As a function of temperature and gas concentration, CO gas sensing measurements of In2O3 thin film-based sensors were made, and the detection limit and operating temperature values were determined. The optimal operating temperature was found to be 222 °C for all sensors. The CO sensing results demonstrated that the sensor with 30 SILAR cycle had higher sensitivity for 1–100-ppm gas concentration values at 222 °C operating temperature than the others. The sensing responses of the sensors increased from 12 to 29% for 1-ppm CO gas and from 52 to 91% for 100-ppm CO gas at 222 °C, depending on the SILAR cycle. The detection limit of the sensors toward CO gas at 222 °C reached 1 ppm, and the response and recovery times of the sensor with 30 SILAR cycle were found to be 54.2 s and 49 s for 1-ppm CO, and 47.4 and 62.5 s for 100-ppm CO gas at 222 °C, respectively. The activation energy (E a) values of the sensors were found to change between 0.08 and 0.15 eV in the temperature range of 300–340 K and between 0.700 and 0.749 eV in the temperature range of 350–520 K, with SILAR cycle number. Finally, in this study, it was revealed that SILAR cycle number changed the structural, morphological, and CO gas sensing properties of the In2O3 thin films, and SILAR cycle optimization was performed for the highly sensitive In2O3 thin film-based CO gas sensor.

Yıldırım, M. Ali; Tuna Yıldırım, Sümeyra; Karademir, Mert; Çağırtekin, Ali Orkun; Ateş, Aytunç; Acar, Selim

In this study, a nanowire network-based V2O5 nanostructure, designed as the active surface in H2S gas sensors, was produced using an environmentally friendly method known as hydrothermal synthesis. The morphological, structural, and electrical properties of the produced nanostructure were characterized using various measurement techniques. XRD, SEM, and EDX analyses confirmed that the nanostructure was in a pure monoclinic phase, had a two-dimensional (2-D) nanowire network morphology, and was deposited stoichiometrically. From UV–Vis analysis, the energy band gap value of the nanostructure was calculated to be 2.65 eV. In gas sensing measurements conducted between 27 °C and 102 °C, the optimal operating temperature of the fabricated sensor was determined to be 42 °C. At this temperature, which is close to room temperature, the sensor exhibited a high sensitivity of 21.16 % for 50 ppm H2S gas, along with good selectivity, stable repeatability, and baseline stability. Moreover, for 2 ppm H2S gas, the response and recovery times were 39 s and 58 s, respectively. A comprehensive electrical analysis of the sensor was carried out through impedance spectroscopy measurements over a wide temperature range. Our findings indicate that the 2-D V2O5 nanowire network structure (NNS) results in several notable effects, including low operating temperature, high selectivity, and stability for H2S gas, demonstrating considerable potential for industrial applications.

Barin, Özlem; Ajjaq, Ahmad; Çağırtekin, Ali Orkun; Karaduman Er, Irmak; Yıldırım, Memet Ali; Ateş, Aytunç; Acar, Selim

Although hydrogen is a non-poisonous gas that can dissipate quickly once is released, it causes suffocation at high concentrations and is very risky on some occasions if not suitably handled. In this study, an effective approach was suggested for developing and tuning hydrogen gas sensors, where the crucial effect of nucleation layers and chemical stabilizing agents on the morphology of hydrothermally-assisted growth and its gas sensing performance was demonstrated. For this purpose, ZnO seed layers were separately synthesized by different solution-based chemical processes based on the successive ionic layer adsorption and reaction, spin, and dip coating techniques followed by a hydrothermal treatment under the same conditions. X-ray diffraction measurements revealed that all the synthesized samples have a similar pure hexagonal wurtzite structure with a big difference in the degree of textural orientation along the c-axis of ZnO. Optical transmittances, forbidden bandgap energies, and Urbach energies of the produced samples were influenced to a great extent by the coating techniques and the chemical solutions used. Aside from that, the morphology of the synthesized samples and their overall thicknesses together with the available oxygen vacancies detected by X-ray photoelectron spectroscopy changed considerably. The change in structure and morphology have influenced the wettability of the synthesized samples. They have also greatly influenced the gas sensing performance of the sensors in terms of optimal working temperature, sensitivity, and response/recovery periods. The hydrogen gas detection mechanism of the fabricated sensors and the logic behind the change that occurred in gas sensing performance based on morphology are discussed.

Karaduman Er, Irmak; Yıldırım, Memet Ali; Örkçü, H. Hasan; Ateş, Aytunç; Acar, Selim

This paper presents the preparation of Zn1−xSnxO thin films by Successive Ionic Layer Adsorption and Reaction (SILAR) method and a systematic investigation of their low-level NO gas detection properties. By changing Sn dopant ratio “x” from 0 to 1 with 0.25 increment steps, a wide range analysis from pure ZnO to pure SnO2 was carried out. X-ray diffraction (XRD), scanning electron microscope (SEM), energy-dispersive X-ray analysis (EDAX) and optical absorption measurements were used in the characterization of the synthesized films. Characterization data showed that the films have polycrystalline nature with hexagonal wurtzite (pure ZnO) and tetragonal (pure SnO2) structure and that the increase in the concentration of Sn dopant leads to an increase in the optical bandgap and a decrease in the average crystallite size. The 25% Sn doping value gave the smallest dislocation density, and the film with this doping value showed the best crystalline structure among all others. From SEM images, different doping ratios appeared to affect the distribution of surface particles. And by gas sensing analysis, the individual responses of the synthesized sensors to 100 ppb NO gas were examined, among which Zn0.75Sn0.25O recorded the highest responsivity of 37% at 55 °C as well as the highest selectivity towards NO gas as compared with other control group gases, and the latter was also proved through principal component analysis (PCA). Doping- and temperature-dependent gas sensing analysis can provide quite efficient information on the sensing mechanism of metal oxide semiconductor (MOS) materials. It was concluded from the measurements that Sn doping has a significant effect on gas sensing properties with 25% doping value regarded as the optimal doping value within the investigated series.

Soltabayev, B.; Er, I Karaduman; Surel, H.; Coşkun, A.; Yildirim, M.A.; Ateş, A.; Acar, S.

Zn1-xNixO thin films (x = 0, 0.05, 0.10, 0.15 and 0.20) were systematically synthesized by Successive Ionic Layer Adsorption And Reaction (SILAR) method with 40 SILAR cycle and then were annealed at 300 °C. Influence of nickel doping and annealing treatment on the structural, morphological, optical and gas sensing properties of Zn1-xNixO thin films were investigated. X-ray diffraction (XRD), scanning electron microscope (SEM) and optical absorption measurements were used for characterization of the films. The gas sensing properties of the Zn1-xNixO sensors for low nitric oxide (NO) gas concentrations were investigated as a function of operating temperature and doping concentration. The gas sensing results showed that the doping process strongly influenced the response of ZnO sensors. The optimal operating temperature of the sensors was found of 95 °C, the response of the annealed Zn00.90Ni0.10O sensor for 25 ppm NO reached a maximum of 63%. The Zn0.90Ni0.10O sensor exhibited higher response and better response/recovery times than the others. It was concluded that Ni doping and annealing treatment enhanced the gas sensing properties of sensors by changing microstructure, morphology, bandgap of ZnO films.

Karaduman Er, Irmak; Çağirtekin, Ali Orkun; Çorlu, Tuğba; Yildirim, Memet Ali; Ateş, Aytunç; Acar, Selim

Zn1-xSnxO (x = 0, 0.05, 0.10, 0.15, 0.20) nanostructures have been grown through the successive ionic layer adsorption and reaction method. The structural, morphological and compositional properties of the nanostructures have been characterized through X-ray diffraction, scanning electron microscope and energy dispersive X-ray analysis, respectively. The NO gas sensing properties of sensors to 20 ppb have been systematically investigated in the dark and under UV light irradiation. A Zn0.90Sn0.10O sensor has exhibited the highest response for 20 ppb NO gas compared with other sensors. The sensor response has increased from 1.9 to 43% depending on the UV light irradiation for the Zn0.90Sn0.10O sensor. Zn0.90Sn0.10O nanostructure can be used as a suitable gas sensor material for detection of low concentration levels of NO gas.

Pepe, Yasemin; Yildirim, Mehmet Ali; Karatay, Ahmet; Ates, Aytunc; Unver, Huseyin; Elmali, Ayhan

The effects of annealing and Al dopant on the nonlinear absorption characteristics of amorphous ZnO thin films were studied experimentally and theoretically. The structural patterns of AZO thin films grown at various growth temperatures (80ᵒC, 90ᵒC, 100ᵒC) and Al doping ratios (%5, %10, %20) were determined by using x-ray diffraction. The results revealed that the crystallinity increased with increasing growth temperature and Al doping ratio. The optical band gap decreased from 3.91 to 3.78 eV and the transmittance of AZO thin films decreased from 90% to 55% with increasing Al molar ratio. The open aperture Gaussian beam Z-scan measurements indicated that all studied films exhibited nonlinear absorption behavior due to the defect states inside energy bandgap. The nonlinear absorption coefficients increased from 1.34 × 10−4 to 6.12 × 10−4 cm/W for 4 ns pulse duration with increasing of doping concentration. The experimental curves were fitted to the theory of open aperture Gaussian beam Z-scan based on the Adomian decomposition method to obtain nonlinear absorption coefficients along with saturation intensity thresholds. In order to investigate the effect of free carrier absorption, the OA Z-scan experiments were also performed for 65 ps pulse duration. The nonlinear absorption coefficients were found to be 0.58 × 10−4, 1.81 × 10−4 and 3.29 × 10−4 cm/W for 65 ps pulse duration. For nanosecond pulse durations, the obtained nonlinear absorption coefficient values of thin films are bigger than the values of picosecond pulse durations due to the greater contribution of free carrier absorption. Increasing annealing temperature from 100ᵒC to 400ᵒC leads to decreasing nonlinear absorption coefficients owing to decreasing localized defect states.

Soltabayev, Baktiyar; Yıldırım, Memet Ali; Ateş, Aytunç; Acar, Selim

In-doped ZnO thin films with 0%,1%, 3%, 5% and 7% In were deposited by Successive Ionic Layer Adsorption and Reaction (SILAR)method on glass substrates for nitric oxide (NO)gas sensing application. X-ray diffraction (XRD), scanning electron microscope (SEM)and energy dispersive X-ray analysis (EDAX)were used to evaluate the effect of In doping concentrations on properties of the films. The XRD study clearly shows the existence of the polycrystalline structure of the films. The morphological study indicates the formation of a uniform particle such as granular and nano-flower structures on the surface of the films. The most important morphological change was observed at 3% indium doped thin film, which has nano-flower surface morphology. The gas sensing measurements were performed for different temperature and different NO gas concentrations. The maximum gas sensitivity was revealed at an optimal indium doping concentration of 3 at.% that achieved the highest sensitivity with a value of 22.4 for 50 ppm NO gas at 167 °C.

Yıldırım, M. Ali; Tuna Yıldırım, Sümeyra; Çağirtekin, Ali Orkun; Karademir, Mert; Karaduman Er, Irmak; Coşkun, Aysun; Ateş, Aytunç; Acar, Selim

V2O5 nanostructures were successfully deposited on glass substrates by hydrothermal method at 180 °C for the deposition times of 4, 6, 8, 10 h. The effect of deposition time on the structural, morphological, compositional and H2S gas sensing properties of the nanostructures were investigated by XRD, SEM, EDAX and gas measurement system, respectively. The XRD and SEM studies indicated that the nanostructures had polycrystalline nature with monoclinic phase of V2O5 and the structural and morphological properties of the nanostructures depended on the deposition time. The sensing measurements of the sensors were made based on temperature and gas concentration. The sensors exhibited high responses towards 50 ppm H2S gas concentration at operating temperature of 145 °C. In addition, the sensors showed acceptable responses at temperatures below the operating temperature of 50 °C. It was seen that the gas sensing properties of the nanostructure deposited for 6 h deposition time were better than others.