Vardan Galstyan

TiO2 Nanotubes: Recent Advances in Synthesis and Gas Sensing Properties

Synthesis—particularly by electrochemical anodization-, growth mechanism and chemical sensing properties of pure, doped and mixed titania tubular arrays are reviewed. The first part deals on how anodization parameters affect the size, shape and morphology of titania nanotubes. In the second part fabrication of sensing devices based on titania nanotubes is presented, together with their most notable gas sensing performances. Doping largely improves conductivity and enhances gas sensing performances of TiO2 nanotubes.

Flexible dye sensitized solar cells using TiO2 nanotubes

The growth of TiO2 nanotube arrays on plastic flexible substrates is researched. The approach uses anodization of a titanium thick film for obtaining nanotubes directly on poly(ethylene terephthalate) (PET) and Kapton HN substrate. The morphological features of the tubes can be finely tuned by varying the preparation conditions, and tube morphology affects the functional properties of the nanotube array. Crystallization of the anatase phase in nanotubes on Kapton HN substrate is obtained via post growth annealing. The nanotube arrays have been dye-sensitized using the commercial Ru-based N719 dye. The system was tested as photoanode in a flexible dye sensitized solar cell. Photoconversion efficiency of 3.5% was obtained.

Vertically Aligned TiO2 Nanotubes on Plastic Substrates for Flexible Solar Cells

Electrochemical anodization of a titanium film on a Kapton HN substrate leads to the formation of closely packed aligned nanotubes, whose shape can be finely tuned by tailoring the anodization parameters. An amorphous-to-anatase phase transition is induced on nanotubes by annealing at 350 °C. The nanotubes are applied as photoanodes in flexible dye-sensitized solar cells (N719 dye; I3-/I- redox couple), resulting in a photoconversion efficiency of up to 3.5% under simulated sunlight irradiation air mass 1.5 global (AM 1.5G).

Fabrication and investigation of gas sensing properties of Nb-doped TiO(2) nanotubular arrays.

Synthesis of Nb-containing titania nanotubular arrays at room temperature by electrochemical anodization is reported. Crystallization of pure and Nb-doped TiO(2) nanotubes was carried out by post-growth annealing at 400°C. The morphology of the tubes obtained was characterized by scanning electron microscopy (SEM). Crystal structure and composition of tubes were investigated by glancing incidence x-ray diffraction (GIXRD) and total reflection x-ray fluorescence (TXRF). For the first time gas sensing characteristics of Nb-doped TiO(2) nanotubes were investigated and compared to those of undoped nanotubes. The functional properties of nanotubular arrays towards CO, H(2), NO(2), ethanol and acetone were tested in a wide range of operating temperature. The introduction of Nb largely improves conductivity and enhances gas sensing performances of TiO(2) nanotubes.

Reduced graphene oxide/ZnO nanocomposite for application in chemical gas sensors

Coupling of graphene-based materials with metal oxide nanostructures is an effective way to obtain composites with improved gas sensing properties. In this work, we prepared a hybrid structure based on graphene oxide (GO) and ZnO nanostructures. The morphological, compositional and structural analyses of the composite material have been investigated using scanning electron microscopy, X-ray diffraction spectroscopy, energy dispersive X-ray analysis and Raman spectroscopy. The gas sensing properties of the obtained structure have been studied towards nitrogen dioxide, hydrogen and methane at relatively low (about 200 °C) operating temperatures. It has been demonstrated that the reduced graphene oxide (RGO)/ZnO composites exhibit 40–50% better response to NO2 and H2 compared to pure ZnO sensors. The obtained results show that the functionalization of the nanostructured ZnO with the RGO sheets is a promising strategy to develop chemical gas sensors with improved gas sensing properties.

Metal Oxide Gas Sensors, a Survey of Selectivity Issues Addressed at the SENSOR Lab, Brescia (Italy)

This work reports the recent results achieved at the SENSOR Lab, Brescia (Italy) to address the selectivity of metal oxide based gas sensors. In particular, two main strategies are being developed for this purpose: (i) investigating different sensing mechanisms featuring different response spectra that may be potentially integrated in a single device; (ii) exploiting the electronic nose (EN) approach. The former has been addressed only recently and activities are mainly focused on determining the most suitable configuration and measurements to exploit the novel mechanism. Devices suitable to exploit optical (photoluminescence), magnetic (magneto-optical Kerr effect) and surface ionization in addition to the traditional chemiresistor device are here discussed together with the sensing performance measured so far. The electronic nose is a much more consolidated technology, and results are shown concerning its suitability to respond to industrial and societal needs in the fields of food quality control and detection of microbial activity in human sweat.

Synthesis of self-ordered and well-aligned Nb2O5 nanotubes

In the present work we demonstrate the fabrication of self-assembled and highly aligned Nb2O5 nanotubes by means of electrochemical anodization of metallic Nb at room temperature. We have investigated the effect of anodization parameters (the type and the concentration of the electrolyte, the anodization voltage and the current) on the formation of Nb2O5 nanotube arrays. The structural properties of the nanotubes as a function of annealing treatment conditions (temperature and duration) have been studied. The current–voltage characterization of the samples in the dark and under UV light illumination has been carried out. The photocurrent of the Nb2O5 nanotubes has been improved by improving their crystalline performance.

Synthesis of self-assembled chain-like ZnO nanostructures on stiff and flexible substrates

We have developed a new chemical route for ZnO nanostructures preparation that combines anodization and thermal annealing in a very convenient technique to obtain crystalline nanostructures on stiff and flexible (Kapton HN®) substrates. The preparation parameters like the type and the concentration of electrolyte, the anodization voltage and the current were tailored to obtain the nanosized ZnO structure. The relation between size and morphology of the nanostructures and the preparation parameters were investigated by means of scanning electron microscopy (SEM), atomic force microscopy (AFM), Glancing Incidence X-Ray Diffraction (GIXRD) and photoluminescence (PL). The structures are composed of nanoparticles forming elongated aggregates with chain-like morphology. The diameter of the particles is ranging from 10 to 75 nm and the length of then chains is more than 1 μm. ZnO crystalline phase was confirmed by structural analysis with GIXRD for samples annealed at 350 and 400 °C.

Porous TiO2-Based Gas Sensors for Cyber Chemical Systems to Provide Security and Medical Diagnosis

Gas sensors play an important role in our life, providing control and security of technical processes, environment, transportation and healthcare. Consequently, the development of high performance gas sensor devices is the subject of intense research. TiO2, with its excellent physical and chemical properties, is a very attractive material for the fabrication of chemical sensors. Meanwhile, the emerging technologies are focused on the fabrication of more flexible and smart systems for precise monitoring and diagnosis in real-time. The proposed cyber chemical systems in this paper are based on the integration of cyber elements with the chemical sensor devices. These systems may have a crucial effect on the environmental and industrial safety, control of carriage of dangerous goods and medicine. This review highlights the recent developments on fabrication of porous TiO2-based chemical gas sensors for their application in cyber chemical system showing the convenience and feasibility of such a model to provide the security and to perform the diagnostics. The most of reports have demonstrated that the fabrication of doped, mixed and composite structures based on porous TiO2 may drastically improve its sensing performance. In addition, each component has its unique effect on the sensing properties of material.

A composite structure based on reduced graphene oxide and metal oxide nanomaterials for chemical sensors

Summary A hybrid nanostructure based on reduced graphene oxide and ZnO has been obtained for the detection of volatile organic compounds. The sensing properties of the hybrid structure have been studied for different concentrations of ethanol and acetone. The response of the hybrid material is significantly higher compared to pristine ZnO nanostructures. The obtained results have shown that the nanohybrid is a promising structure for the monitoring of environmental pollutants and for the application of breath tests in assessment of exposure to volatile organic compounds.