Atomic Layer Deposition of SnO2-Coated Anodic One-Dimensional TiO2 Nanotube Layers for Low Concentration NO2 Sensing

Wearable Nano-Based Gas Sensors for Environmental Monitoring and Encountered Challenges in Optimization

With a rising emphasis on public safety and quality of life, there is an urgent need to ensure optimal air quality, both indoors and outdoors. Detecting toxic gaseous compounds plays a pivotal role in shaping our sustainable future. This review aims to elucidate the advancements in smart wearable (nano)sensors for monitoring harmful gaseous pollutants, such as ammonia (NH3), nitric oxide (NO), nitrous oxide (N2O), nitrogen dioxide (NO2), carbon monoxide (CO), carbon dioxide (CO2), hydrogen sulfide (H2S), sulfur dioxide (SO2), ozone (O3), hydrocarbons (CxHy), and hydrogen fluoride (HF). Differentiating this review from its predecessors, we shed light on the challenges faced in enhancing sensor performance and offer a deep dive into the evolution of sensing materials, wearable substrates, electrodes, and types of sensors. Noteworthy materials for robust detection systems encompass 2D nanostructures, carbon nanomaterials, conducting polymers, nanohybrids, and metal oxide semiconductors. A dedicated section dissects the significance of circuit integration, miniaturization, real-time sensing, repeatability, reusability, power efficiency, gas-sensitive material deposition, selectivity, sensitivity, stability, and response/recovery time, pinpointing gaps in the current knowledge and offering avenues for further research. To conclude, we provide insights and suggestions for the prospective trajectory of smart wearable nanosensors in addressing the extant challenges.

Additives in Nanocrystalline Tin Dioxide: Recent Progress in the Characterization of Materials for Gas Sensor Applications

Tin dioxide has huge potential and is widely studied and used in different fields, including as a sensitive material in semiconductor gas sensors. The specificity of the chemical activity of tin dioxide in its interaction with the gas phase is achieved via the immobilization of various modifiers on the SnO2 surface. The type of additive, its concentration, and the distribution between the surface and the volume of SnO2 crystallites have a significant effect on semiconductor gas sensor characteristics, namely sensitivity and selectivity. This review discusses the recent approaches to analyzing the composition of SnO2-based nanocomposites (the gross quantitative elemental composition, phase composition, surface composition, electronic state of additives, and mutual distribution of the components) and systematizes experimental data obtained using a set of analytical methods for studying the concentration of additives on the surface and in the volume of SnO2 nanocrystals. The benefits and drawbacks of new approaches to the high-accuracy analysis of SnO2-based nanocomposites by ICP MS and TXRF methods are discussed.

3D self-assembled indium sulfide nanoreactor for in-situ surface covalent functionalization: Towards high-performance room-temperature NO2 sensing

Thiol functionalization of two-dimensional (2D) metal sulfides has been demonstrated as an effective approach to enhance the sensing performances. However, most thiol functionalization is realized by multiple-step approaches in liquid medium and depends on the dispersity of 2D materials. Here, we utilize a three-dimensional (3D) In₂S₃ nano-porous structure that self-assembled from 2D components as the nanoreactor, in which the surface-absorbed thiol molecules from the chemical residues of the nanoreactor are used for the in-situ covalent functionalization. Such functionalization is realized by facile heat the nanoreactor at 100 °C, leading to the recombing sulfur vacancies with thiol-terminated groups. The NO₂ sensing performances of such functionalized nanoreactor are investigated at room temperature, in which In₂S₃-100 exhibits a response magnitude of 21.5 towards 10 ppm NO₂ with full reversibility, high selectivity, and excellent repeatability. Such high-performance gas sensors can be attributed to the additional electrons that transferring from the functional group into the host, thus significantly modifying the electronic band structure. This work provides a guideline for the facile in-situ functionalization of metal sulfides and an efficient strategy for the high performances gas sensors without external stimulus.

Gigahertz-Based Visible Light Detection Enabled via CdS-Coated TiO2 Nanotube Layers

Detection of visible light is a key component in material characterization techniques and often a key component of quality or purity control analyses for health and safety applications. Here in this work, to enable visible light detection at gigahertz frequencies, a planar microwave resonator is integrated with high aspect ratio TiO₂ nanotube (TNT) layer-sensitized CdS coating using the atomic layer deposition (ALD) technique. This unique method of visible light detection with microwave-based sensing improves integration of the light detection devices with digital technology. The designed planar microwave resonator sensor was implemented and tested with resonant frequency between 8.2 and 8.4 GHz and a resonant amplitude between -15 and -25 dB, depending on the wavelength of the illuminated light illumination on the nanotubes. The ALD CdS coating sensitized the nanotubes in visible light up to ∼650 nm wavelengths, as characterized by visible spectroscopy. Furthermore, CdS-coated TNT layer integration with the planar resonator sensor allowed for development of a robust microwave sensing platform with improved sensitivity to green and red light (60 and 1300%, respectively) compared to the blank TNT layers. Moreover, the CdS coating of the TNT layer enhanced the sensor's response to light exposure and resulted in shorter recovery times once the light source was removed. Despite having a CdS coating, the sensor was capable of detecting blue and UV light; however, refining the sensitizing layer could potentially enhance its sensitivity to specific wavelengths of light in certain applications.

Cross-Interference of VOCs in SnO2-Based NO Sensors

In this work, we studied the influence of cross-interference effects between VOCs and NO on the performance of SnO₂ and Pt-SnO₂-based gas sensors. Sensing films were fabricated by screen printing. The results show that the response of the SnO₂ sensors to NO under air is higher than that of Pt-SnO₂, but the response to VOCs is lower than that of Pt-SnO₂. The Pt-SnO₂ sensor was significantly more responsive to VOCs in the NO background than in air. In the traditional single-component gas test, the pure SnO₂ sensor showed good selectivity to VOCs and NO at 300 °C and 150 °C, respectively. Loading noble metal Pt improved the sensitivity to VOCs at high temperature, but also significantly increased the interference to NO sensitivity at low temperature. The explanation for this phenomenon is that the noble metal Pt can catalyze the reaction between NO and VOCs to generate more O^-, which further promotes the adsorption of VOCs. Therefore, selectivity cannot be simply determined by single-component gas testing alone. Mutual interference between mixed gases needs to be taken into account.

Gas Sensors Based on Titanium Oxides (Review)

Nanostructured titanium compounds have recently been applied in the design of gas sensors. Among titanium compounds, titanium oxides (TiO2) are the most frequently used in gas sensing devices. Therefore, in this review, we are paying significant attention to the variety of allotropic modifications of titanium oxides, which include anatase, rutile, brukite. Very recently, the applicability of non-stoichiometric titanium oxide (TiO2−x)-based layers for the design of gas sensors was demonstrated. For this reason, in this review, we are addressing some research related to the formation of non-stoichiometric titanium oxide (TiO2−x) and Magnéli phase (TinO2n−1)-based layers suitable for sensor design. The most promising titanium compounds and hetero- and nano-structures based on these compounds are discussed. It is also outlined that during the past decade, many new strategies for the synthesis of TiO2 and conducting polymer-based composite materials were developed, which have found some specific application areas. Therefore, in this review, we are highlighting how specific formation methods, which can be used for the formation of TiO2 and conducting polymer composites, can be applied to tune composite characteristics that are leading towards advanced applications in these specific technological fields. The possibility to tune the sensitivity and selectivity of titanium compound-based sensing layers is addressed. In this review, some other recent reviews related to the development of sensors based on titanium oxides are overviewed. Some designs of titanium-based nanomaterials used for the development of sensors are outlined.

High-Frequency TiO2 Nanotube-Adapted Microwave Coplanar Waveguide Resonator for High-Sensitivity Ultraviolet Detection

Ultraviolet (UV) sensors are a key component in growing applications such as water quality treatment and environmental monitoring, with considerable interest in their miniaturization and enhanced operation. This work presents a passive gold coplanar waveguide split ring resonator integrated with anodic self-organized TiO₂ nanotube (TNT) membranes with a thickness of 20 μm to provide real-time UV detection. The resonator operated as a one-port device to capture the reflection coefficient (S₁₁) signal, with a center frequency of 16 GHz and a notch amplitude of -88 dB. It was experimentally analyzed for its UV sensing capability in the range of 36.5-463 μW/cm². The high-frequency resonator was improved through design choices including the addition of a tapered input transmission line, wire bonding for practical device design, and an interdigitated capacitive ring gap. The high frequency also helped mitigate noise due to water vapor or environmental contaminants. S₁₁ amplitude variation was found through both experiments and modeling to follow a linear trend with UV illumination intensity. The resonator exhibited over 45 ± 2 dB shift in the resonant amplitude under the highest UV illumination conditions, with a sensitivity of 0.084 dB/μW cm^-2 and the potential to sense UV intensity as low as 2.7 μW/cm². The presented device enabled a repeatable and accurate microwave response under UV illumination with very high sensitivity, entirely through the use of passive circuit elements.

Anatase porous titania nanosheets for resonant-gravimetric detection of ppb-level NO2 at room-temperature

The trace-level detection of harmful NO2 gas at room-temperature is very important for environmental protection and public health. This paper reports the resonant-gravimetric detection of ppb-level NO2 at room-temperature using two-dimensional porous TiO2 nanosheets (PTNSs) as highly active sensing materials. They are synthesized by a facile high-temperature calcination approach based on a graphene oxide self-sacrificial template. The PTNS sample prepared at 500 °C (TiO2-500 °C) show an anatase structure, while the sample prepared at 800 °C (TiO2-800 °C) contains an impurity rutile phase. By loading pure anatase PTNSs onto resonant microcantilevers, the sensors exhibit high sensitivity to NO2 gas with a limit of detection as low as 15 ppb. Compared with the TiO2-800 °C sample, the much higher sensitivity of the TiO2-500 °C sample can be attributed to the bigger adsorption enthalpy (-ΔH°) of pure anatase TiO2 to NO2 gas molecules (21.7 and 57.8 kJ mol-1, respectively). Density functional theory calculations further demonstrate that the existence of the rutile impurity phase in the TiO2-800 °C sample results in its significantly decreased adsorption activity to NO2. This work approves the great application potential of anatase PTNSs for the highly sensitive resonant-gravimetric detection of NO2 gas at room-temperature.

Effect of Plasma-Enhanced Atomic Layer Deposition on Oxygen Overabundance and Its Influence on the Morphological, Optical, Structural, and Mechanical Properties of Al-Doped TiO2 Coating

The chemical, structural, morphological, and optical properties of Al-doped TiO₂ thin films, called TiO₂/Al₂O₃ nanolaminates, grown by plasma-enhanced atomic layer deposition (PEALD) on p-type Si <100> and commercial SLG glass were discussed. High-quality PEALD TiO₂/Al₂O₃ nanolaminates were produced in the amorphous and crystalline phases. All crystalline nanolaminates have an overabundance of oxygen, while amorphous ones lack oxygen. The superabundance of oxygen on the crystalline film surface was illustrated by a schematic representation that described this phenomenon observed for PEALD TiO₂/Al₂O₃ nanolaminates. The transition from crystalline to amorphous phase increased the surface hardness and the optical gap and decreased the refractive index. Therefore, the doping effect of TiO₂ by the insertion of Al₂O₃ monolayers showed that it is possible to adjust different parameters of the thin-film material and to control, for example, the mobility of the hole-electron pair in the metal-insulator-devices semiconductors, corrosion protection, and optical properties, which are crucial for application in a wide range of technological areas, such as those used to manufacture fluorescence biosensors, photodetectors, and solar cells, among other devices.

Insights in the Application of Stoichiometric and Non-Stoichiometric Titanium Oxides for the Design of Sensors for the Determination of Gases and VOCs (TiO2-x and TinO2n-1 vs. TiO2)

In this review article, attention is paid towards the formation of various nanostructured stoichiometric titanium dioxide (TiO2), non-stoichiometric titanium oxide (TiO2-x) and Magnéli phase (TinO2n-1)-based layers, which are suitable for the application in gas and volatile organic compound (VOC) sensors. Some aspects related to variation of sensitivity and selectivity of titanium oxide-based sensors are critically overviewed and discussed. The most promising titanium oxide-based hetero- and nano-structures are outlined. Recent research and many recently available reviews on TiO2-based sensors and some TiO2 synthesis methods are discussed. Some promising directions for the development of TiO2-based sensors, especially those that are capable to operate at relatively low temperatures, are outlined. The applicability of non-stoichiometric titanium oxides in the development of gas and VOC sensors is foreseen and transitions between various titanium oxide states are discussed. The presence of non-stoichiometric titanium oxide and Magnéli phase (TinO2n-1)-based layers in 'self-heating' sensors is predicted, and the advantages and limitations of 'self-heating' gas and VOC sensors, based on TiO2 and TiO2-x/TiO2 heterostructures, are discussed.