Tuning ZnO Sensors Reactivity toward Volatile Organic Compounds via Ag Doping and Nanoparticle Functionalization.

Nanomaterials for highly selective and sensitive sensors toward specific gas molecules of volatile organic compounds (VOCs) are most important in developing new-generation of detector devices, for example, for biomarkers of diseases as well as for continuous air quality monitoring. Here, we present an innovative preparation approach for engineering sensors, which allow for full control of the dopant concentrations and the nanoparticles functionalization of columnar material surfaces. The main outcome of this powerful design concept lies in fine-tuning the reactivity of the sensor surfaces toward the VOCs of interest. First, nanocolumnar and well-distributed Ag-doped zinc oxide (ZnO:Ag) thin films are synthesized from chemical solution, and, at a second stage, noble nanoparticles of the required size are deposited using a gas aggregation source, ensuring that no percolating paths are formed between them. Typical samples that were investigated are Ag-doped and Ag nanoparticle-functionalized ZnO:Ag nanocolumnar films. The highest responses to VOCs, in particular to (CH3)2CHOH, were obtained at a low operating temperature (250 °C) for the samples synergistically enhanced with dopants and nanoparticles simultaneously. In addition, the response times, particularly the recovery times, are greatly reduced for the fully modified nanocolumnar thin films for a wide range of operating temperatures. The adsorption of propanol, acetone, methane, and hydrogen at various surface sites of the Ag-doped Ag8/ZnO(0001) surface has been examined with the density functional theory (DFT) calculations to understand the preference for organic compounds and to confirm experimental results. The response of the synergistically enhanced sensors to gas molecules containing certain functional groups is in excellent agreement with density functional theory calculations performed in this work too. This new fabrication strategy can underpin the next generation of advanced materials for gas sensing applications and prevent VOC levels that are hazardous to human health and can cause environmental damages.

Interaction of H2O with the Platinum Pt (001), (011), and (111) Surfaces: A Density Functional Theory Study with Long-Range Dispersion Corrections.

Platinum is a noble metal that is widely used for the electrocatalytic production of hydrogen, but the surface reactivity of platinum toward water is not yet fully understood, even though the effect of water adsorption on the surface free energy of Pt is important in the interpretation of the morphology and catalytic properties of this metal. In this study, we have carried out density functional theory calculations with long-range dispersion corrections [DFT-D3-(BJ)] to investigate the interaction of H2O with the Pt (001), (011), and (111) surfaces. During the adsorption of a single H2O molecule on various Pt surfaces, it was found that the lowest adsorption energy (E ads) was obtained for the dissociative adsorption of H2O on the (001) surface, followed by the (011) and (111) surfaces. When the surface coverage was increased up to a monolayer, we noted an increase in E ads/H2O with increasing coverage for the (001) surface, while for the (011) and (111) surfaces, E ads/H2O decreased. Considering experimental conditions, we observed that the highest coverage was obtained on the (011) surface, followed by the (111) and (001) surfaces. However, with an increase in temperature, the surface coverage decreased on all the surfaces. Total desorption occurred at temperatures higher than 400 K for the (011) and (111) surfaces, but above 850 K for the (001) surface. From the morphology analysis of the Pt nanoparticle, we noted that, when the temperature increased, only the electrocatalytically active (111) surface remained.

Stability and mobility of supported Nin (n = 1-10) clusters on ZrO2(111) and YSZ(111) surfaces: a density functional theory study.

The performance of supported metal catalysts, such as nickel nanoparticles decorating yttria-stabilized zirconia (YSZ), depends on their microstructure and the metal-support interface. Here, we have used spin polarized density functional theory (DFT) to evaluate different Ni cluster geometries and determined the electronic structure of the most stable configurations. We have described the interaction of Nin (n = 1-10) clusters supported on the cubic ZrO2(111) and YSZ(111) surfaces, which show a preference for pyramidal shapes rather than flat structures wetting the surface. The interfacial interaction is characterized by charge transfer from the cluster to the surface. We also show how yttrium, present in YSZ, affects the Ni-Ni interaction. Through analysing the difference between the cohesive energy and the clustering energy, we show the preference of Ni-Ni bond formation over Ni-surface interaction; this energy difference decreases with the increase of the Nin cluster size. From the evaluation of the Ni atomic hopping rates on YSZ, we have demonstrated that under different temperature conditions, Ni atoms aggregate with other atoms and clusters, which affects the cluster size stability.