ZnO-Based nanostructures for gas sensing applications.

Metal oxide chemical sensors based on nanomaterials are gaining popularity and finding extensive use in automotive industries, process control and environmental monitoring. ZnO, a semiconducting metal oxide has attracted great interest over the years for its sensitivity to a variety of gases. Nanostructured sensing materials, such as thin films, nanowires, tetrapods, nanoflackes offer an inherently high surface area, reducing operating temperatures and increasing sensitivity to low concentrations of analytes. In this thesis, ZnO nanostructures have been tested as chemical sensors and a detailed study on the effect of different process parameters such as grain size, roughness, surface-to-volume ratio, depletion layer, temperature, gas concentration and material properties on gas sensitivity is presented. Initially, ZnO nanodevices were prepared with a variety of techniques, such as RF sputtering, electrodeposition, hydrothermal growth, chemical vapour deposition, thermal evaporation and controlled oxidation. The structural characterization of the nanodevices has been done by a FEI QUANTA 3D dual beam SEM/FIB machine and by a Dimension 3100 Atomic Force Microscope (AFM) (Digital Instruments) in tapping mode. X-ray diffraction (XRD) spectra were recorded on an AXS D8 diffractometer (Bruker) with a Cu Kα X-ray tube. The gas sensor substrate based on alumina consisted of Pt grid of 50nm thickness and golden contacts of 200nm thickness creating an alumina patterned substrate. The sensor deposition area was coated with ZnO nanostructures to form the sensing material. Sensing measurements are performed in a closed steel chamber where air and tested gases have been inserted. ZnO based nanostructures’ response was measured in different concentrations of Ethanol, CO and NO2. Initially the role of grain size and roughness has been investigated in several thin film based nanodevices. Grain size is decreasing with increasing RF sputtering power and increasing by post-annealing treatment. Roughness instead is increasing with both the increasing of RF sputtering power and post-annealing treatment. High response was observed for those films with smaller grain size, while the roughness seems to influence very little the response of the sensor. For all thin films, the response is increasing with ii temperature and gas concentration. Recovery time and response time seem to follow a non-linear behavior with the above parameters. Extended studies have investigated the role of surface-to-volume ratio and depletion layer in the sensing performance. It has been observed that the increase of surface-to volume ratio has an important effect on the sensitivity, increasing, more than twice the response of such a device in respect to another that is based on a ZnO thin film. On the other hand, the dimensions of a nanostructure play the most crucial role in the depletion layer width in respect to the sensing properties. The diameter of a nanowire should be comparable with its depletion layer width. In this case the depletion layer has strong effect, which makes the sensor’s response depend also on it. The sensing properties of all fabricated structures have been compared to find the optimum sensor that could face the demands of automotive industries. All fabricated structures have been compared in different configurations to find out which one presents the best sensing performance. To that direction sensors based on thin film, tetrapods, nanowires, nanoflackes have been tested in same environmental conditions. Advanced nanostructures present better sensing properties. Sensing response of every advanced nanostructure presents more than double sensing response than every thin film-based nanostructure. Comparing the advanced nanostructures with each other, tetrapods based sensor has higher response and recovery time, while the sensitivity is slightly higher for the nanowires-based sensor. Theoretical studies have been performed by ab-initio simulations in NO2 environment. They have revealed that the sensing mechanism is driven almost exclusively by competitive adsorption between NO2 and atmospheric oxygen mediated by temperature change. The influence of the NO2 on the electronic properties of ZnO has been assessed and it is in accordance with the experiments. Our future work is the investigation of other materials for the development of sensing nanodevices targeting to develop more sensitive nanosensors in the same or lower cost. Additionally, the investigation of other growth techniques that could develop more complicated structures in low cost is another point of interest for the future

Nano-materials employment in energy harvesting and storage devices.

The present thesis focuses on the development of a new generation of miniature electronic devices by employing nano-scale materials. Specifically, ZnO nanowire arrays were investigated to increase the conversion efficiency of energy harvesting devices and graphene nano-platelets employed to enhance supercapacitors’ energy storage capability. The results obtained in this work pave the way to the possibility of conceiving novel autonomous devices integrating both energy units. The present thesis has been structured in five chapters. A first introduction chapter reviews the pros and cons of renewable energies against the conventional ones produced from fossil fuels as well as their impact on the modern societies. The theoretical background on vibration energy harvesting and electrochemical energy storage is provided. Vibration energy harvesting mechanism relies on piezoelectric phenomena, where a pressure applied on a piezoelectric material turns ultimately into energy. Instead, supercapacitors store large quantity of energy for time unit by high surface material dielectric polarization. In this chapter, the reasons why ZnO nanowire arrays and graphene nano-platelets were considered are introduced. The second chapter presents promising methods to synthesize piezoelectric ZnO nano-materials prior their integration into energy harvesting devices. Since the highest piezoelectric properties of the ZnO-crystal are along its c-axis, the most suitable growing methods were selected to tailor the crystal’s unit-cell best orientation. In this chapter physical and chemical growing methods are reported. Physical vapor deposition (PVD) was used to grow ZnO thin film, then employed as a seed layer for the growth of 1D-ZnO nanowires by chemical methods in a second step. ZnO nanowires were synthesized either with or without a nanoporous template by: i) electrochemical deposition (ECD), and ii) hydrothermal technique. The fundamental process parameters to tailor the chemical growth are reported as well as the morphological and microstructural characterization of the structures fabricated. In the third chapter, the characteristics of the energy harvesting device fabricated from the piezoelectric ZnO nanostructures are reported. Piezoresponse force microscopy was initially used to measure the d33 piezoelectric coefficient of the ZnO nanostructures fabricated fairly matching the theoretical expectations. Finally, this chapter reports the energy harvested by the devices fabricated, measured by connecting an external resistive load to it: a maximum energy harvested equal to 2 μJ/cm2 was found. The fourth chapter focuses on nano-scale graphene based materials for supercapacitors’ electrodes. Specifically, the synthesis and the characterization of the graphene nano-platelets used in this work is described. XRD and Raman spectroscopy were used to distinguish pure graphite from graphene, BET and SEM to measure its specific surface area and morphology. To determine the graphene’s properties functional to the application thermogravimetric analysis (TGA) was carried out. To identify the types of oxygen groups present in the graphene materials, the corresponding Fourier Transform Infrared spectra (FTIR) were recorded and their contribution in rGO was examined by X-Ray photoelectron spectroscopy (XPS) analysis. Overall this chapter reviews the relevant analysis to be performed in candidate materials for fabrication of supercapacitor electrodes. The fifth chapter discusses the fabrication of supercapacitor electrodes made with the graphene nano-platelets previously described as well as the methods for their electrochemical characterization. As being the standard of the energy storage industry, cyclic voltammetry (CV) and constant current charge and discharge experiments were carried out for capacitance estimation. The electrochemical characteristics of the device were then linked to the properties of the graphene nano-materials employed. All measurements were done in a full-scale electrochemical cell mimicking a real supercapacitor device. The results suggest that mechanically exfoliated graphene nano-platelets (GNP) best perform among the variety of materials investigated

Voluntary self-poisoning as a cause of admission to a tertiary hospital internal medicine clinic in Piraeus, Greece within a year

BACKGROUND: Out of 1705 patients hospitalised for various reasons in the 3(rd) Internal Medicine Department of the Regional General Hospital of Nikaea, in Piraeus, 146(8,5%) persons were admitted for drug intoxication between November 1999 and November 2000. METHODS: On average, these persons [male 50(34,2%) – female 96(65,8%)] were admitted to the hospital within 3.7 hours after taking the drug. RESULTS: The drugs that were more frequently taken, alone or in combination with other drugs, were sedatives (67.1%), aspirins and analgesics (mainly paracetamol) (43.5%). 38.3% of patients had a mental illness history, 31.5% were in need of psychiatric help and 45.2% had made a previous suicide attempt. No death occurred during the above period and the outcome of the patients' health was normal. After mental state examination, the mental illnesses diagnosed were depression (20.96%), psychosis (15.32%), dysthymic disorder (16,2%), anxiety disorder (22.58%) and personality disorder (8.87%). CONCLUSIONS: Self-poisoning remains a crucial problem. The use of paracetamol and sedatives are particularly important in the population studied. Interpersonal psychiatric therapy may be a valuable treatment after people tried to poison themselves

NO2 Gas Sensing Mechanism of ZnO Thin-Film Transducers: Physical Experiment and Theoretical Correlation Study

In this work, ZnO thin films were investigated to sense NO2, a gas exhausted by the most common combustion systems polluting the environment. To this end, ZnO thin films were grown by RF sputtering on properly designed and patterned substrates to allow the measurement of the electrical response of the material when exposed to different concentrations of the gas. X-ray diffraction was carried out to correlate the material's electrical response to the morphological and microstructural features of the sensing materials. Electrical conductivity measurements showed that the transducer fabricated in this work exhibits the optimal performance when heated at 200 °C, and the detection of 0.1 ppm concentration of NO2 was possible. Ab initio modeling allowed the understanding of the sensing mechanism driven by the competitive adsorption of NO2 and atmospheric oxygen mediated by heat. The combined theoretical and experimental study here reported provides insights into the sensing mechanism which will aid the optimization of ZnO transducer design for the quantitative measurement of NO2 exhausted by combustion systems which will be used, ultimately, for the optimized adjustment of combustion resulting into a reduced pollutants and greenhouse gases emission

Piezoelectric properties of template-free electrochemically grown ZnO nanorod arrays

In this work, we evaluated the piezoelectric properties of the ZnO nanorods (NRs), subsequently grownby a template-free electrochemical deposition (ECD) approach. The one-step preparation method yieldedin a relatively short-time synthesis large arrays of high aspect-ratio ZnO NRs, vertically aligned to the substrate and exhibiting a well-defined hexagonal symmetry coupled with an excellent crystallinity. Piezoresponse force microscopy (PFM) was employed to extract the effective piezoelectric coefficient (d33) of the ZnO [0 0 0 1]-oriented NRs. A mean effective d33 coefficient of 11.8 pm/V was measured overseveral individual NRs. The results have shown an increased piezoelectric response of the ZnO NRs, with∼18% in respect to the ZnO bulk material, and with a factor ranging from 28% to 167% compared to similar ZnO nanostructures prepared by other counterpart techniques. Both morphology and crystalline quality of the obtained ZnO NRs, as well as the piezoelectric investigations, ranked the ECD as a promising low-cost pathway for possibly producing high quality nanostructured ZnO-based piezoelectric devices with enhanced performance

In-Hospital Mortality in a Tertiary Referral Hospital: Causes Of Death and Comparison between Patients with and without Diabetes

Introduction: Subjects with type 2 diabetes mellitus (T2DM) have increased morbidity and mortality mainly due to macrovascular complications. In addition, diabetic patients show increased in-hospital admissions in comparison with nondiabetic patients. However, in-hospital mortality data for patients with T2DM are not available in our country. The aim of the present study was to examine mortality rates in diabetic compared to nondiabetic patients admitted to a tertiary hospital during a 10-year period (1998-2007). Material and Methods: We performed a retrospective analysis of mortality rates in patients with and without T2DM hospitalised in a tertiary care hospital during the years 1998-2007. Demographic characteristics, medical history and outcome were collected from the patients’ medical records. Patients with type 1 diabetes were excluded from the analysis. Results: A total of 16 125 patients’ records were studied (14 005 without diabetes and 2 120 with T2DM). In the total sample, 1 467 (9.1%) deaths were recorded. Mortality rates were higher in the diabetic than in the nondiabetic patients (11.2% versus 8.7%, respectively, p < 0.001). Age of death did not differ between diabetic and nondiabetic patients (age +/- SD: 77.1 +/- 9.5 vs. 77.6 +/- 16.3 years, p = 0.73). Median length of hospital stay was higher in the diabetic than in the nondiabetic patients (p = 0.03). Mortality was higher in the diabetic in comparison with the nondiabetic females (59.9% vs. 52.7%, respectively, p = 0.04), while no gender difference was found in males. Cerebrovascular disease was the commonest cause of death in both diabetic and nondiabetic patients (41.6% vs. 30.3%, p = 0.001), followed by infections (23.1% vs. 21.7%, respectively, p = 0.62). Death rates from malignancies were more common in the nondiabetic in comparison with the diabetic patients (18.8% vs. 4.2%, p < 0.001). No significant differences were observed between the two study groups regarding mortality caused by cardiovascular events and chronic renal failure. Conclusion: The present study showed that diabetic patients and especially females had increased in-hospital mortality compared with nondiabetic patients. Cerebrovascular disease and infections were the more common cause of death in both groups