The surface chemistry and the morphology of SnO₂ nanowires of average length and diameter of several µm and around 100 nm, respectively, deposited by vapor phase deposition (VPD) method on Au-covered Si substrate, were studied before and after subsequent air exposure. For this purpose, surface-sensitive methods, including X-ray photoelectron spectroscopy (XPS), thermal desorption spectroscopy (TDS) and the scanning electron microscopy (SEM), were applied. The studies presented within this paper allowed to determine their surface non-stoichiometry combined with the presence of carbon contaminations, in a good correlation with their surface morphology. The relative concentrations of the main components [O]/[Sn]; [C]/[Sn]; [Au]/[Sn], together with the O–Sn; O–Si bonds, were analyzed. The results of TDS remained in a good agreement with the observations from XPS. Moreover, conclusions obtained for SnO₂ nanowires deposited with the use of Au catalyst were compared to the previous obtained for Ag-assisted tin dioxide nanowires. The information obtained within these studies is of a great importance for the potential application of SnO₂ nanowires in the field of novel chemical nanosensor devices, since the results can provide an interpretation of how aging effects influence gas sensor dynamic characteristics.

In this paper, the results of detailed X-ray photoelectron spectroscopy (XPS) studies combined with atomic force microscopy (AFM) investigation concerning the local surface chemistry and morphology of nanostructured ZnO thin films are presented. They have been deposited by direct current (DC) reactive magnetron sputtering under variable absolute Ar/O₂ flows (in sccm): 3:0.3; 8:0.8; 10:1; 15:1.5; 20:2, and 30:3, respectively. The XPS studies allowed us to obtain the information on: (1) the relative concentrations of main elements related to their surface nonstoichiometry; (2) the existence of undesired C surface contaminations; and (3) the various forms of surface bondings. It was found that only for the nanostructured ZnO thin films, deposited under extremely different conditions, i.e., for Ar/O₂ flow ratio equal to 3:0.3 and 30:3 (in sccm), respectively, an evident and the most pronounced difference had been observed. The same was for the case of AFM experiments. What is crucial, our experiments allowed us to find the correlation mainly between the lowest level of C contaminations and the local surface morphology of nanostructured ZnO thin films obtained at the highest Ar/O₂ ratio (30:3), for which the densely packaged (agglomerated) nanograins were observed, yielding a smaller surface area for undesired C adsorption. The obtained information can help in understanding the reason of still rather poor gas sensor characteristics of ZnO based nanostructures including the undesired ageing effect, being of a serious barrier for their potential application in the development of novel gas sensor devices.

The aim of this research is to study the role of nanocrystalline TiO₂/SnO₂ n–n heterojunctions for hydrogen sensing. Nanopowders of pure SnO₂, 90 mol % SnO₂/10 mol % TiO₂, 10 mol % SnO₂/90 mol % TiO₂ and pure TiO₂ have been obtained using flame spray synthesis (FSS). The samples have been characterized by BET, XRD, SEM, HR-TEM, Mössbauer effect and impedance spectroscopy. Gas-sensing experiments have been performed for H₂ concentrations of 1–3000 ppm at 200–400 °C. The nanomaterials are well-crystallized, anatase TiO₂, rutile TiO₂ and cassiterite SnO₂ polymorphic forms are present depending on the chemical composition of the powders. The crystallite sizes from XRD peak analysis are within the range of 3–27 nm. Tin exhibits only the oxidation state 4+. The H₂ detection threshold for the studied TiO₂/SnO₂ heterostructures is lower than 1 ppm especially in the case of SnO₂-rich samples. The recovery time of SnO₂-based heterostructures, despite their large responses over the whole measuring range, is much longer than that of TiO₂-rich samples at higher H₂ flows. TiO₂/SnO₂ heterostructures can be intentionally modified for the improved H₂ detection within both the small (1–50 ppm) and the large (50–3000 ppm) concentration range. The temperature T_max at which the semiconducting behavior begins to prevail upon water desorption/oxygen adsorption depends on the TiO₂/SnO₂ composition. The electrical resistance of sensing materials exhibits a power-law dependence on the H₂ partial pressure. This allows us to draw a conclusion about the first step in the gas sensing mechanism related to the adsorption of oxygen ions at the surface of nanomaterials.

In this work, comparative studies of the surface morphology and surface chemistry of SnO₂ nanolayers prepared by spin coating with subsequent thermal oxidation (SCTO) in the temperature range of 400–700 °C using scanning electron microscopy (SEM), atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS) methods, are presented. The SEM images show that SCTO SnO₂ nanolayers contain partly connected irregular structures strongly dependent on the final oxidation temperature, with interconnected single grains of longitudinal shape and size, resulting in a flatter surface morphology with respect to the commonly used three-dimensional (3D) SnO₂ thin films. In turn, AFM studies additionally confirm that SCTO SnO₂ nanolayers after post-oxidation annealing at higher temperatures contain isolated grains of average lateral dimensions in the range of 20–50 nm having a rather flat surface morphology of average surface roughness defined by the root mean square factor at the level of ∼2 nm. From the XPS experimental research it can be concluded that, for our SCTO SnO₂ samples, a slight surface nonstoichiometry defined by the relative [O]/[Sn] concentration at the level of 1.8–1.9 is observed, also depending on the final post-oxidation temperature, being an evident contradiction to recently published literature using x-ray diffraction data. Moreover, XPS experiments show that there is also a permanent small amount of carbon contamination present at the surface of internal grains of our SCTO SnO₂ nanolayers, creating an undesired potential barrier for interactions with gaseous species when they are used as the active materials for gas sensing devices.

The aim of this research is to study size – dependent effects and gas sensing properties of TiO2 nanomaterials prepared on the base of nanopowders obtained by Flame Spray Synthesis, FSS. The specific surface area (SSA) and the resulting particle sizes of the prepared powders are controlled by the flow rate of the precursor. The results concerning morphological properties of nanomaterials investigated by BET, adsorption isotherms, X – ray Diffraction, XRD, Scanning Electron Microscopy, SEM are discussed. Electrical resistivity of gas sensing tablets is measured as a function of hydrogen concentration within the range of 50 – 3000 ppm at constant temperature of 200 oC – 400 oC. Dynamic changes in the electrical resistivity upon interaction with hydrogen are large and reproducible. The sensor response increases systematically with the decrease in the temperature of operation down to 250oC. Smaller grain size results in a higher electrical conductivity suggesting the predominant influence of intergrain contacts

This work presents, for the very first time, very promising nitrogen dioxide (NO₂) sensing characteristics of SnO_x nanolayers obtained by the innovative and unique rheotaxial growth and vacuum oxidation (RGVO) processing technique. The NO₂ gas sensing experiments were performed using the novel surface photovoltage gas sensing device. The measured detection limit at room temperature (RT) is as low as 10 ppb NO₂ in synthetic air, whereas the detection limit calculated on the basis of signal to noise ratio is around 6 ppb NO₂. For the complementary study of surface chemistry of RGVO SnO_x nanolayers, including nonstoichiometry, presence of carbon contamination and surface bondings, the X-ray photoelectron spectroscopy (XPS) method was applied. The SnO_x RGVO samples reveal nonstoichiometry because the relative concentration [O]/[Sn] equals 0.94 for the as deposited sample and increases upon subsequent air exposure and NO₂ sensing. Moreover, carbon contamination has been recognized after exposing the RGVO SnO_x nanolayers to the air and during the NO₂ detection.