Hafnium-Oxide 3‑D Nanofilms via the Anodizing of Al/Hf Metal Layers

Hafnium-oxide films with self-organized nanostructured 3-D architectures and variable dimension (10 to 400 nm) are synthesized via the high-current anodizing of thin aluminum-on-hafnium layers in phosphoric, malonic, and oxalic acid electrolytes. In the approach, the self-organized growth of a porous anodic alumina (PAA) film is immediately followed by the fast PAA-assisted reanodizing of the hafnium underlayer. The PAA-dissolved films consist of arrays of upright-standing hafnium-oxide nanorods held on the substrate by the tiny needle-like “nanoroots” widespread over a continuous hafnium-oxide bottom layer. The roots are amorphous Hf₂O₃, while the rods are amorphous HfO₂–Hf₂O₃–Al₂O₃ mixed oxides, the bottom layer being, however, highly textured nanocrystalline HfO₂. The calculated transport numbers for O^2– and Hf⁴⁺⁽³⁾⁺ ions are, respectively, ∼0.55 and ∼0.45, which is a unique situation for anodic hafnium oxide, which normally grows by O^2– transport only. Annealing the films in air at 600 °C oxidizes the remaining Hf metal to polycrystalline HfO₂, still leaving the roots and rods amorphous. The annealing in vacuum results in partial oxide reduction and crystallization of the roots and rods to stable orthorhombic and monoclinic HfO₂ phases. A model of the anodic film growth and solid-state ionic transport is proposed and experimentally justified. Potential applications of the 3-D hafnium-oxide nanofilms are in advanced electronic, photonic, or magnetic micro- and nanodevices

Unraveling the Gas-Sensing Mechanisms of Lead-Free Perovskites Supported on Graphene

Lead halide perovskites have been attracting great attention due to their outstanding properties and have been utilized for a wide variety of applications. However, the high toxicity of lead promotes an urgent and necessary search for alternative nanomaterials. In this perspective, the emerging lead-free perovskites are an environmentally friendly and harmless option. The present work reports for the first time gas sensors based on lead-free perovskite nanocrystals supported on graphene, which acts as a transducing element owing to its high and efficient carrier transport properties. The use of nanocrystals enables achieving excellent sensitivity toward gas compounds and presents better properties than those of bulky perovskite thin films, owing to their quantum confinement effect and exciton binding energy. Specifically, an industrially scalable, facile, and inexpensive synthesis is proposed to support two different perovskites (Cs3CuBr5 and Cs2AgBiBr6) on graphene for effectively detecting a variety of harmful pollutants below the threshold limit values. H2 and H2S gases were detected for the first time by utilizing lead-free perovskites, and ultrasensitive detection of NO2 was also achieved at room temperature. In addition, the band-gap type, defect tolerance, and electronic surface traps at the nanocrystals were studied in detail for understanding the differences in the sensing performance observed. Finally, a comprehensive sensing mechanism is proposed

Temperature-Dependent NO₂ Sensing Mechanisms over Indium Oxide

The surface species responsible for NO₂ gas sensing over indium oxide was studied by <i>operando</i> DRIFTS coupled to a multivariate spectral analysis. It revealed the important roles of surface nitrites on the temperature-dependent gas sensing mechanism and the interaction of such nitrites with surface hydroxyls. A highly hydroxylated surface with high concentration of surface adsorbed H₂O is beneficial to enhance the concentration of adsorbed NO₂, present as nitrites, thus explaining superior sensing response at lower operating temperatures

Three-Dimensional Assemblies of Edge-Enriched WSe₂ Nanoflowers for Selectively Detecting Ammonia or Nitrogen Dioxide

Herein, we present, for the first time, a chemoresistive-type gas sensor composed of two-dimensional WSe2, fabricated by a simple selenization of tungsten trioxide (WO3) nanowires at atmospheric pressure. The morphological, structural, and chemical composition investigation shows the growth of vertically oriented three-dimensional (3D) assemblies of edge-enriched WSe2 nanoplatelets arrayed in a nanoflower shape. The gas sensing properties of flowered nanoplatelets (2H-WSe2) are investigated thoroughly toward specific gases (NH3 and NO2) at different operating temperatures. The integration of 3D WSe2 with unique structural arrangements resulted in exceptional gas sensing characteristics with dual selectivity toward NH3 and NO2 gases. Selectivity can be tuned by selecting its operating temperature (150 °C for NH3 and 100 °C for NO2). For instance, the sensor has shown stable and reproducible responses (24.5%) toward 40 ppm NH3 vapor detection with an experimental LoD < 2 ppm at moderate temperatures. The gas detecting capabilities for CO, H2, C6H6, and NO2 were also investigated to better comprehend the selectivity of the nanoflower sensor. Sensors showed repeatable responses with high sensitivity to NO2 molecules at a substantially lower operating temperature (100 °C) (even at room temperature) and LoD < 0.1 ppm. However, the gas sensing properties reveal high selectivity toward NH3 gas at moderate operating temperatures. Moreover, the sensor demonstrated high resilience against ambient humidity (Rh = 50%), demonstrating its remarkable stability toward NH3 gas detection. Considering the detection of NO2 in a humid ambient atmosphere, there was a modest increase in the sensor response (5.5%). Furthermore, four-month long-term stability assessments were also taken toward NH3 gas detection, and sensors showed excellent response stability. Therefore, this study highlights the practical application of the 2H variant of WSe2 nanoflower gas sensors for NH3 vapor detection

Aerosol-Assisted CVD-Grown WO₃ Nanoneedles Decorated with Copper Oxide Nanoparticles for the Selective and Humidity-Resilient Detection of H₂S

A gas-sensitive hybrid material consisting of Cu₂O nanoparticle-decorated WO₃ nanoneedles is successfully grown for the first time in a single step via aerosol-assisted chemical vapor deposition. Morphological, structural, and composition analyses show that our method is effective for growing single-crystalline, n-type WO₃ nanoneedles decorated with p-type Cu₂O nanoparticles at moderate temperatures (i.e., 380 °C), with cost effectiveness and short fabrication times, directly onto microhot plate transducer arrays with the view of obtaining gas sensors. The gas-sensing studies performed show that this hybrid nanomaterial has excellent sensitivity and selectivity to hydrogen sulfide (7-fold increase in response compared with that of pristine WO₃ nanoneedles) and a low detection limit (below 300 ppb of H₂S), together with unprecedented fast response times (2 s) and high immunity to changes in the background humidity. These superior properties arise because of the multiple p–n heterojunctions created at the nanoscale in our hybrid nanomaterial

Vertical heterostructure of graphite-MoS2 for gas sensing

2D materials, given their form-factor, high surface-to-volume ratio, and chemical functionality have immense use in sensor design. Engineering 2D heterostructures can result in robust combinations of desirable properties but sensor design methodologies require careful considerations about material properties and orientation to maximize sensor response. This study introduces a sensor approach that combines the excellent electrical transport and transduction properties of graphite film with chemical reactivity derived from the edge sites of semiconducting molybdenum disulfide (MoS2) through a two-step chemical vapour deposition method. The resulting vertical heterostructure shows potential for high-performance hybrid chemiresistors for gas sensing. This architecture offers active sensing edge sites across the MoS2 flakes We detail the growth of vertically oriented MoS2 over a nanoscale graphite film (NGF) cross-section, enhancing the adsorption of analytes such as NO2, NH3, and water vapor. Raman spectroscopy, density functional theory calculations and scanning probe methods elucidate the influence of chemical doping by distinguishing the role of MoS2 edge sites relative to the basal plane. High-resolution imaging techniques confirm the controlled growth of highly crystalline hybrid structures. The MoS2/NGF hybrid structure exhibits exceptional chemiresistive responses at both room and elevated temperatures compared to bare graphitic layers. Quantitative analysis reveals that the sensitivity of this hybrid sensor surpasses other 2D material hybrids, particularly in parts per billion concentrations.</p