An Overview of Electrochemical Sensors Based on Transition Metal Carbides and Oxides: Synthesis and Applications

Sensors play vital roles in industry and healthcare due to the significance of controlling the presence of different substances in industrial processes, human organs, and the environment. Electrochemical sensors have gained more attention recently than conventional sensors, including optical fibers, chromatography devices, and chemiresistors, due to their better versatility, higher sensitivity and selectivity, and lower complexity. Herein, we review transition metal carbides (TMCs) and transition metal oxides (TMOs) as outstanding materials for electrochemical sensors. We navigate through the fabrication processes of TMCs and TMOs and reveal the relationships among their synthesis processes, morphological structures, and sensing performance. The state-of-the-art biological, gas, and hydrogen peroxide electrochemical sensors based on TMCs and TMOs are reviewed, and potential challenges in the field are suggested. This review can help others to understand recent advancements in electrochemical sensors based on transition metal oxides and carbides.

ZnO@CuO hollow nanosphere-based composites used for the sensitive detection of hydrogen sulfide with long-term stability

In this study, zinc oxide@cupric oxide hollow nanospheres (ZnO@CuO HNS, 330 nm in diameter) were successfully prepared by a hard-template method using amino-phenolformaldehyde resin spheres (APF) as the templates. A new type of thin-film gas sensor toward hydrogen sulfide (H₂S) was fabricated by means of drop-coating on the gold electrode of an alumina ceramic tube. The microstructure and morphology of the nanosphere composites were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the gas-sensing performance of the composites toward the detection of H₂S were investigated. The ZnO@CuO nanocomposite with a hollow structure exhibited good gas-sensing properties. Under the optimum operating temperature of 260 °C, ambient temperature of 30 °C, and ambient humidity of 70%, the linear response of the sensor to H₂S was in the concentration range of 0.1-100 ppm, and its detection limit reached 0.0611 ppm, with a quick response time of 78 s. Also, the sensor possessed good repeatability, selectivity, and stability. The long-term stability and run duration of such sensors were pronounced, with only a 1.9% reduction in the signal after the continuous monitoring of H₂S gas in a pig farm for 18 months using Alibaba's cloud remote transmission system, which presents an important practical application prospect in atmosphere environment monitoring on livestock-raising fields.

Confined interfacial micelle aggregating assembly of ordered macro-mesoporous tungsten oxides for H2S sensing

Porous tungsten oxides (WO3) have been implemented in various application fields including catalysis, energy storage and conversion, and gas sensing. However, the construction of hierarchically ordered porous WO3 nanostructures with highly crystalline frameworks remains a great challenge. Herein, a confined interfacial micelle aggregating assembly approach has been developed for the synthesis of ordered macro-mesoporous WO3 (OMMW) nanostructures using three-dimensional SiO2 photonic crystals (PCs) as nanoreactors for the confined assembly of tungsten precursor and poly(ethylene oxide)-block-polystyrene (PEO-b-PS) template. After the heat treatment and etching processes, the obtained OMMW could achieve hierarchically ordered porous nanostructures with close-packed spherical mesopores (∼34.1 nm), interconnected macro-cavities (∼420 nm), high accessible surface areas (∼78 m2 g-1), and highly crystalline frameworks owing to the protection of dual templates. When OMMW nanostructures were assembled into gas sensors for the detection of H2S, the resulting sensors exhibited excellent comprehensive sensing performance, including a rapid response-recovery kinetics, in addition to high selectivity and long-term stability, which are significantly better than the previously reported WO3-based sensors. This study paves a promising way toward the development of hierarchically ordered porous semiconductors with large and interconnected porous channels for sensing applications.

Nickel/iron-based bimetallic MOF-derived nickel ferrite materials for triethylamine sensing

The sensors based on the different sized MOF derived NiFe₂O₄ polyhedrons exhibit fast TEA response speed and distinguishing sensitivity.

Ultrafast Response/Recovery and High Selectivity of the H2S Gas Sensor Based on α-Fe2O3 Nano-Ellipsoids from One-Step Hydrothermal Synthesis

Ultrafast response/recovery and high selectivity of gas sensors are critical for real-time and online monitoring of hazardous gases. In this work, α-Fe₂O₃ nano-ellipsoids were synthesized using a facile one-step hydrothermal method and investigated as highly sensitive H₂S-sensing materials. The nano-ellipsoids have an average long-axis diameter of 275 nm and an average short-axis diameter of 125 nm. H₂S gas sensors fabricated using the α-Fe₂O₃ nano-ellipsoids showed excellent H₂S-sensing performance at an optimum working temperature of 260 °C. The response and recovery times were 0.8 s/2.2 s for H₂S gas with a concentration of 50 ppm, which are much faster than those of H₂S gas sensors reported in the literature. The α-Fe₂O₃ nano-ellipsoid-based sensors also showed high selectivity to H₂S compared to other commonly investigated gases including NH₃, CO, NO₂, H₂, CH₂Cl₂, and ethanol. In addition, the sensors exhibited high-response values to different concentrations of H₂S with a detection limit as low as 100 ppb, as well as excellent repeatability and long-term stability.

Enhanced H2S gas-sensing performance of Zn2SnO4 hierarchical quasi-microspheres constructed from nanosheets and octahedra

Most of the reported ternary oxides based sensors have not been realized to detect ppb-level H₂S till now. In this work, Zn₂SnO₄ hierarchical quasi-microspheres were prepared through a facile surfactant-free hydrothermal method followed by calcination in air atmosphere. The quasi-microspheres are composed of nanosheets with the thickness of 100 nm and octahedra with the average size of 0.63 μm, respectively. The sensor fabricated from such Zn₂SnO₄ hierarchical quasi-microspheres shows excellent selective response to H₂S at 133 °C with the lowest detection limit of 1 ppb. The gas response exhibits good linear relationship in the concentration range of 1-1000 ppb. Such outstanding H₂S sensing property might be attributed to its porous structure, the synergistic effect of the two typical building blocks and the surface adsorbed oxygen, and the possible sensing mechanism is also discussed.

A silver nanoparticle-anchored UiO-66(Zr) metal–organic framework (MOF)-based capacitive H2S gas sensor

Metal–organic frameworks anchored with metal oxide nanoparticles for the detection of H₂S gas with enhanced sensitivity.

Hierarchical Flowerlike Highly Synergistic Three-Dimensional Iron Tungsten Oxide Nanostructure-Anchored Nitrogen-Doped Graphene as an Efficient and Durable Electrocatalyst for Oxygen Reduction Reaction

A unique and novel structural morphology with high specific surface area, highly synergistic, remarkable porous conductive networks with outstanding catalytic performance, and durability of oxygen reduction electrocatalyst are typical promising properties in fuel cell application; however, exploring and interpreting this fundamental topic is still a challenging task in the whole world. Herein, we have demonstrated a simple and inexpensive synthesis strategy to design three-dimensional (3D) iron tungsten oxide nanoflower-anchored nitrogen-doped graphene (3D Fe-WO₃ NF/NG) hybrid for a highly efficient synergistic catalyst for oxygen reduction reaction (ORR). The construction of flowerlike Fe-WO₃ nanostructures, based on synthesis parameters, and their ORR performances are systematically investigated. Although pristine 3D Fe-WO₃ NF or reduced graphene oxides show poor catalytic performance and even their hybrid shows unsatisfactory results, impressively, the excellent ORR activity and its outstanding durability are further improved by N doping, especially due to pyridinic and graphitic nitrogen moieties into a graphene sheet. Remarkably, 3D Fe-WO₃ NF/NG hybrid nanoarchitecture reveals an outstanding electrocatalytic performance with a remarkable onset potential value (∼0.98 V), a half-wave potential (∼0.85 V) versus relative hydrogen electrode, significant methanol tolerance, and extraordinary durability of ∼95% current retention, even after 15 000 potential cycles, which is superior to a commercial Pt/C. The exclusive porous architecture, excellent electrical conductivity, and the high synergistic interaction between 3D Fe-WO₃ NF and NG sheets are the beneficial phenomena for such admirable catalytic performance. Therefore, this finding endows design of a highly efficient and durable nonprecious metal-based electrocatalyst for high-performance ORR in alkaline media.

Synthesis and trimethylamine sensing properties of spherical V2O5 hierarchical structures

Spherical V₂O₅ hierarchical structures assembled from nanosheets exhibit rapid response/recovery speeds to TMA gas.

Hierarchical NiO Cube/Nitrogen-Doped Reduced Graphene Oxide Composite with Enhanced H2S Sensing Properties at Low Temperature

A novel hierarchical NiO cube (hc-NiO)/nitrogen-doped reduced graphene oxide (N-rGO) composite is synthesized via a facile hydrothermal method and a postcalcination treatment without any templates and surfactants added. The NiO cubes assembled by abundant nanoparticles in situ grow on the surface of N-rGO layers. The combination of hc-NiO and N-rGO results in enhanced sensing properties with the contributions of the N-rGO providing high specific surface area and more efficient active sites for the adsorption of H₂S molecules and the hierarchically structured NiO cubes providing high sensitivity and distinctive selectivity to H₂S gas. At the optimal operating temperature of 92 °C, the hc-NiO/N-rGO composite based sensor shows not only high response to H₂S in a range of 0.1-100 ppm but also excellent selectivity for H₂S against the other seven gases. The gaseous product, produced from the contact of H₂S with the hc-NiO/N-rGO composite at 92 °C, is measured by GC-MS technique. The change of the surface composition and the chemical state of the hc-NiO/N-rGO composite before and after exposure to H₂S are investigated by XPS. The possible sensing mechanism of the hc-NiO/N-rGO composite is similar to that of semiconductor oxides. The H₂S molecules that absorbed on the sensor surface transform to SO₂ by reacting with the adsorbed oxygen anions. Meanwhile, the electrons restricted by the surface-adsorbed oxygen return to the bulk and neutralize the holes, producing a change in resistance.

Shape Evolution of Hierarchical W18O49 Nanostructures: A Systematic Investigation of the Growth Mechanism, Properties and Morphology-Dependent Photocatalytic Activities

Hierarchical tungsten oxide assemblies such as spindle-like structures, flowers with sharp petals, nanowires and regular hexagonal structures are successfully synthesized via a solvothermal reduction method by simply adjusting the reaction conditions. On the basis of the experimental results, it is determined that the reaction time significantly influences the phase transition, microstructure and photocatalytic activity of the prepared samples. The possible mechanisms for the morphology evolution process have been systematically proposed. Moreover, the as-prepared products exhibit significant morphology-dependent photocatalytic activity. The flower-like W₁₈O₄₉ prepared at 6 h possesses a large specific surface area (150.1 m²∙g⁻¹), improved separation efficiency of electron-hole pairs and decreased electron-transfer resistance according to the photoelectrochemical measurements. As a result, the flower-like W₁₈O₄₉ prepared at 6 h exhibits the highest photocatalytic activity for the degradation of Methyl orange aqueous solution. The radical trap experiments showed that the degradation of MO was driven mainly by the participation of h⁺ and •O₂⁻ radicals.

Low-Temperature H2S Detection with Hierarchical Cr-Doped WO3 Microspheres

Hierarchical Cr-doped WO3 microspheres have been successfully synthesized for efficient sensing of H2S gas at low temperatures. The hierarchical structures provide an effective gas diffusion path via well-aligned micro-, meso-, and macroporous architectures, resulting in significant enhancement in sensing response to H2S. The temperature and gas concentration dependence on the sensing properties elucidate that Cr dopants remarkably improve the response and lower the sensor' operating temperature down to 80 °C. Under 0.1 vol % H2S, the response of Cr-doped WO3 sensor is 6 times larger than pristine WO3 sensor at 80 °C. We suggest the increasing number of oxygen vacancies created by Cr dopants to be the underlying reason for enhancement of charge carrier density and accelerated reactions with H2S.

Room temperature triethylamine sensing properties of polyaniline–WO3 nanocomposites with p–n heterojunctions

A smart sensor based on PANI–WO₃ nanocomposite loaded on PET thin film not only exhibits high sensitivity and selectivity to triethylamine at room temperature, but also has flexibility, simple fabrication and portable characters.

One-step synthesis of flower-shaped WO3 nanostructures for a high-sensitivity room-temperature NOx gas sensor

Flower-shaped WO₃ nanoparticles were successfully synthesized by using a facile hydrothermal method. These particles exhibited excellent room-temperature NO_x gas-sensing performance with high sensitivity, short response time and low detection limit.