The Effect of Film Thickness on the Gas Sensing Properties of Ultra-Thin TiO₂ Films Deposited by Atomic Layer Deposition

High Oxygen Sensitivity of TiO2 Thin Films Deposited by ALD

The gas sensitivity and structural properties of TiO2 thin films deposited by plasma-enhanced atomic layer deposition (ALD) were examined in detail. The TiO2 thin films are deposited using Tetrakis(dimethylamido)titanium(IV) and oxygen plasma at 300 °C on SiO2 substrates followed by annealing at temperatures of 800 °C. Gas sensitivity under exposure to O2 within the temperature range from 30 °C to 700 °C was studied. The ALD-deposited TiO2 thin films demonstrated high responses to O2 in the dynamic range from 0.1 to 100 vol. % and low concentrations of H2, NO2. The ALD deposition allowed the enhancement of sensitivity of TiO2 thin films to gases. The greatest response of TiO2 thin films to O2 was observed at a temperature of 500 °C and was 41.5 arb. un. under exposure to 10 vol. % of O2. The responses of TiO2 thin films to 0.1 vol. % of H2 and 7 × 10-4 vol. % of NO2 at a temperature of 500 °C were 10.49 arb. un. and 10.79 arb. un., correspondingly. The resistance of the films increased due to the chemisorption of oxygen molecules on their surface that decreased the thickness of the conduction channel between the metal contacts. It was suggested that there are two types of adsorption centers on the TiO2 thin films surface: oxygen is chemisorbed in the form of O2- on the first one and O- on the second one.

Large-Area Printed Oxide Film Sensors Enabling Ultrasensitive and Dual Electrical/Colorimetric Detection of Hydrogen at Room Temperature

Commercial hydrogen (H2) sensors operate at high temperatures, which increases power consumption and poses a safety risk owing to the flammable nature of H2. Here, a polymer-noble metal-metal oxide film is fabricated using the spin-coating and printing methods to realize a highly sensitive, low-voltage operation, wide-operating-concentration, and near-monoselective H2 sensor at room temperature. The H2 sensors with an optimized thickness of Pd nanoparticles and SnO2 showed an extremely high response of 16,623 with a response time of 6 s and a recovery time of 5 s at room temperature and 2% H2. At the same time, printed flexible sensors demonstrate excellent sensitivity, with a response of 2300 at 2% H2. The excellent sensing performance at room temperature is due to the optimal SnO2 thickness, corresponding to the Debye length and the oxygen and H2 spillover caused by the optimized coverage of the Pd catalyst. Furthermore, multistructures of WO3 and SnO2 films are used to fabricate a new type of dual-signal sensor, which demonstrated simultaneous conductance and transmittance, i.e., color change. This work provides an effective strategy to develop robust, flexible, transparent, and long-lasting H2 sensors through large-area printing processes based on polymer-metal-metal oxide nanostructures.

Optical Characterization of ALD-Coated Nanoporous Alumina Structures: Effect of Sample Geometry or Coated Layer Material

Optical characterization of nanoporous alumina-based structures (NPA-bSs), obtained by ALD deposition of a thin conformal SiO₂ layer on two alumina nanosupports with different geometrical parameters (pore size and interpore distance), was performed by two noninvasive and nondestructive techniques such as spectroscopic ellipsometry (SE) and photoluminescence (Ph) spectra. SE measurements allow us to estimate the refraction index and extinction coefficient for the studied samples and their dependence with wavelength for the 250-1700 nm interval, showing the effect of sample geometry and cover-layer material (SiO₂, TiO₂, or Fe₂O₃), which significantly affect the oscillatory character of both parameters, as well as changes associated with the light incidence angle, which are attributed to surface impurities and inhomogeneity. Photoluminescence curves exhibit a similar shape independently of sample pore-size/porosity, but they seem to affect intensity values. This analysis shows the potential application of these NPA-bSs platforms to nanophotonics, optical sensing, or biosensing.

Characterization of the Response of Magnetron Sputtered In2O3-x Sensors to NO2

The response of resistive In₂O_3-x sensing devices was investigated as a function of the NO₂ concentration in different operative conditions. Sensing layers are 150 nm thick films manufactured by oxygen-free room temperature magnetron sputtering deposition. This technique allows for a facile and fast manufacturing process, at same time providing advantages in terms of gas sensing performances. The oxygen deficiency during growth provides high densities of oxygen vacancies, both on the surface, where they are favoring NO₂ absorption reactions, and in the bulk, where they act as donors. This n-type doping allows for conveniently lowering the thin film resistivity, thus avoiding the sophisticated electronic readout required in the case of very high resistance sensing layers. The semiconductor layer was characterized in terms of morphology, composition and electronic properties. The sensor baseline resistance is in the order of kilohms and exhibits remarkable performances with respect to gas sensitivity. The sensor response to NO₂ was studied experimentally both in oxygen-rich and oxygen-free atmospheres for different NO₂ concentrations and working temperatures. Experimental tests revealed a response of 32%/ppm at 10 ppm NO₂ and response times of approximately 2 min at an optimal working temperature of 200 °C. The obtained performance is in line with the requirements of a realistic application scenario, such as in plant condition monitoring.

Recent Advances in Sensing Materials Targeting Clinical Volatile Organic Compound (VOC) Biomarkers: A Review

In general, volatile organic compounds (VOCs) have a high vapor pressure at room temperature (RT). It has been reported that all humans generate unique VOC profiles in their exhaled breath which can be utilized as biomarkers to diagnose disease conditions. The VOCs available in exhaled human breath are the products of metabolic activity in the body and, therefore, any changes in its control level can be utilized to diagnose specific diseases. More than 1000 VOCs have been identified in exhaled human breath along with the respiratory droplets which provide rich information on overall health conditions. This provides great potential as a biomarker for a disease that can be sampled non-invasively from exhaled breath with breath biopsy. However, it is still a great challenge to develop a quick responsive, highly selective, and sensitive VOC-sensing system. The VOC sensors are usually coated with various sensing materials to achieve target-specific detection and real-time monitoring of the VOC molecules in the exhaled breath. These VOC-sensing materials have been the subject of huge interest and extensive research has been done in developing various sensing tools based on electrochemical, chemoresistive, and optical methods. The target-sensitive material with excellent sensing performance and capturing of the VOC molecules can be achieved by optimizing the materials, methods, and its thickness. This review paper extensively provides a detailed literature survey on various non-biological VOC-sensing materials including metal oxides, polymers, composites, and other novel materials. Furthermore, this review provides the associated limitations of each material and a summary table comparing the performance of various sensing materials to give a better insight to the readers.

Metal Oxide Heterostructures for Improving Gas Sensing Properties: A Review

Metal oxide semiconductor gas sensors are widely used to detect toxic and inflammable gases in industrial production and daily life. The main research hotspot in this field is the synthesis of gas sensing materials. Previous studies have shown that incorporating two or more metal oxides to form a heterojunction interface can exhibit superior gas sensing performance in response and selectivity compared with single phase. This review focuses on mainly the synthesis methods and gas sensing mechanisms of metal oxide heterostructures. A significant number of heterostructures with different morphologies and shapes have been fabricated, which exhibit specific sensing performance toward a specific target gas. Among these synthesis methods, the hydrothermal method is noteworthy due to the fabrication of diverse structures, such as nanorod-like, nanoflower-like, and hollow sphere structures with enhanced sensing properties. In addition, it should be noted that the combination of different synthesis methods is also an efficient way to obtain metal oxide heterostructures with novel morphologies. Despite advanced methods in the metal oxide semiconductors and nanotechnology field, there are still some new issues which deserve further investigation, such as long-term chemical stability of sensing materials, reproducibility of the fabrication process, and selectivity toward homogeneous gases. Moreover, the gas sensing mechanism of metal oxide heterostructures is controversial. It should be clarified so as to further integrate laboratory theory research with practical exploitation.

Gas Sensor Based on ZnO Nanostructured Film for the Detection of Ethanol Vapor

In this paper, the ZnO<La> target was synthesized by the solid-state reaction method and a nanostructured thin film was deposited by the RF (radio frequency) magnetron sputtering method on a Multi-Sensor-Platform. The obtained ZnO<La> nanostructured film was investigated as the sensing material. Energy-Dispersive X-ray (EDX) analysis indicated the existence of La in the synthesized ZnO<La> material. Scanning Electron Microscope (SEM) images of the film showed the grain sizes in the range of 20–40 nm. Sensor performance characteristics such as a dynamic response, response and recovery times, and ethanol detection range were investigated at 50–300 °C. A sensitivity was observed at extremely low concentrations of ethanol (0.7 ppm). The minimum response and recovery times of the sensor corresponding to 675 ppm ethanol vapor concentration at 250 °C were found to be 14 s and 61 s, respectively. The sensor showed a high response, good selectivity, fast response/recovery behavior, excellent repeatability toward ethanol vapor, and low sensitivity toward humidity. These characteristics enable the use of a ZnO<La> based sensor for ethanol detecting applications.

Superior acetone sensor based on hetero-interface of SnSe2/SnO2 quasi core shell nanoparticles for previewing diabetes

To improve gas sensing performance of SnO₂ sensor, a heterostructure constructed by SnO₂ and SnSe₂ is designed and synthesized via hydrothermal method and post thermal oxidation treatment. The obtained SnSe₂/SnO₂ composite nanoparticles demonstrate a special core-shell structure with SnO₂ nanograins distributed in the shell and mixed SnSe₂ and SnO₂ nanograins in the core. Owning to the promoted charge transfer effect invited by SnSe₂, the sensor based on SnSe₂/SnO₂ composite nanoparticles exhibit expressively enhanced acetone sensing performance compared to the pristine SnO₂ sensor. At the working temperature of 300 °C, the SnSe₂/SnO₂ composite sensor with optimized composition exhibits superior sensing property towards acetone, including high response (10.77-100 ppm), low theoretical limit of detection (0.354 ppm), high selectivity and good reproducibility. Moreover, the sensor shows a satisfactory sensing performance in trace acetone gas detection under high humidity condition (relative humidity: 70-90%), making it a promising candidate to constructing exhaled breath sensors for acetone detection.

Structure, Surface Morphology, Chemical Composition, and Sensing Properties of SnO2 Thin Films in an Oxidizing Atmosphere

We conducted experiments on SnO₂ thin layers to determine the dependencies between the stoichiometry, electrochemical properties, and structure. This study focused on features such as the film structure, working temperature, layer chemistry, and atmosphere composition, which play a crucial role in the oxygen sensor operation. We tested two kinds of resistive SnO₂ layers, which had different grain dimensions, thicknesses, and morphologies. Gas-sensing layers fabricated by two methods, a rheotaxial growth and thermal oxidation (RGTO) process and DC reactive magnetron sputtering, were examined in this work. The crystalline structure of SnO₂ films synthesized by both methods was characterized using XRD, and the crystallite size was determined from XRD and AFM measurements. Chemical characterization was carried out using X-ray photoelectron (XPS) and Auger electron (AES) spectroscopy for the surface and the near-surface film region (in-depth profiles). We investigated the layer resistance for different oxygen concentrations within a range of 1-4%, in a nitrogen atmosphere. Additionally, resistance measurements within a temperature range of 423-623 K were analyzed. We assumed a flat grain geometry in theoretical modeling for comparing the results of measurements with the calculated results.

One-Dimensional Nanomaterials in Resistive Gas Sensor: From Material Design to Application

With a series of widespread applications, resistive gas sensors are considered to be promising candidates for gas detection, benefiting from their small size, ease-of-fabrication, low power consumption and outstanding maintenance properties. One-dimensional (1-D) nanomaterials, which have large specific surface areas, abundant exposed active sites and high length-to-diameter ratios, enable fast charge transfers and gas-sensitive reactions. They can also significantly enhance the sensitivity and response speed of resistive gas sensors. The features and sensing mechanism of current resistive gas sensors and the potential advantages of 1-D nanomaterials in resistive gas sensors are firstly reviewed. This review systematically summarizes the design and optimization strategies of 1-D nanomaterials for high-performance resistive gas sensors, including doping, heterostructures and composites. Based on the monitoring requirements of various characteristic gases, the available applications of this type of gas sensors are also classified and reviewed in the three categories of environment, safety and health. The direction and priorities for the future development of resistive gas sensors are laid out.

Ultraviolet light-emitting diode-assisted highly sensitive room temperature NO2 gas sensors based on low-temperature solution-processed ZnO/TiO2 nanorods decorated with plasmonic Au nanoparticles

Nanostructured semiconducting metal oxides such as SnO₂, ZnO, TiO₂, and CuO have been widely used to fabricate high performance gas sensors. To improve the sensitivity and stability of gas sensors, we developed NO₂ gas sensors composed of ZnO/TiO₂ core-shell nanorods (NRs) decorated with Au nanoparticles (NPs) synthesized via a simple low-temperature aqueous solution process, operated under ultraviolet irradiation to realize room temperature operation. The fabricated gas sensor with a 10 nm-thick TiO₂ shell layer shows 9 times higher gas sensitivity and faster response and recovery times than ZnO NR-based gas sensors. This high performance of the fabricated gas sensor can be ascribed to band bending between the ZnO and TiO₂ core-shell layers and the localized surface plasmon resonance effect of Au NPs with a sufficient Debye length of the TiO₂ shell layer.

TiO2/Cu2O/CuO Multi-Nanolayers as Sensors for H2 and Volatile Organic Compounds: An Experimental and Theoretical Investigation

TiO₂/Cu₂O/CuO multi-nanolayers highly sensitive toward volatile organic compounds (VOCs) and H₂ have been grown in various thicknesses by a cost-effective and reproducible combined spray-sputtering-annealing approach. The ultrathin TiO₂ films were deposited by spray pyrolysis on top of sputtered-annealed Cu₂O/CuO nanolayers to enhance their gas sensing performance and improve their protection against corrosion at high operating temperatures. The prepared heterostructures were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), and ultraviolet visible (UV-vis) and micro-Raman spectroscopy. The gas sensing properties were measured at several operating temperatures, where the nanolayered sensors with oxide thicknesses between 20 and 30 nm (Cu₂O/CuO nanolayers) exhibited a high response and an excellent selectivity to ethanol vapor after thermal annealing the samples at 420 °C. The results obtained at an operating temperature of 350 °C demonstrate that the CuO/Cu₂O nanolayers with thicknesses between 20 and 30 nm are sensitive mainly to ethanol vapor, with a response of ∼150. The response changes from ethanol vapors to hydrogen gas as the thickness of the CuO/Cu₂O nanolayers changes from 50 to 20 nm. Density functional theory-based calculations were carried out for the geometries of the CuO(1̅11)/Cu₂O(111) and TiO₂(111)/CuO(1̅11)/Cu₂O(111) heterostructures and their sensing mechanism toward alcohols of different chain lengths and molecular hydrogen. The reconstructed hexagonal Cu₂O(111) surface and the reconstructed monoclinic CuO(1̅11) and TiO₂(111) facets, all of which terminate in an O layer, lead to the lowest surface energies for each isolated material. We studied the formation of the binary and ternary heteroepitaxial interfaces for the surface planes with the best-matching lattices. Despite the impact of the Cu₂O(111) substrate in lowering the atomic charges of the CuO(1̅11) adlayer in the binary sensor, we found that it is the different surface structures of the CuO(1̅11)/Cu₂O(111) and TiO₂(111)/CuO(1̅11)/Cu₂O(111) devices that are fundamental in driving the change in the sensitivity response observed experimentally. The experimental data, supported by the computational results, are important in understanding the use of the multi-nanolayered films tested in this work as reliable, accurate, and selective sensor structures for the tracking of gases at low concentrations.

Robust Protection of III-V Nanowires in Water Splitting by a Thin Compact TiO2 Layer

Narrow-band-gap III-V semiconductor nanowires (NWs) with a suitable band structure and strong light-trapping ability are ideal for high-efficiency low-cost solar water-splitting systems. However, due to their nanoscale dimension, they suffer more severe corrosion by the electrolyte solution than the thin-film counterparts. Thus, short-term durability is the major obstacle for using these NWs for practical water-splitting applications. Here, we demonstrated for the first time that a thin layer (∼7 nm thick) of compact TiO₂ deposited by atomic layer deposition can provide robust protection to III-V NWs. The protected GaAs NWs maintain 91.4% of its photoluminescence intensity after 14 months of storage in ambient atmosphere, which suggests the TiO₂ layer is pinhole-free. Working as a photocathode for water splitting, they exhibited a 45% larger photocurrent density compared with unprotected counterparts and a high Faraday efficiency of 91% and can also maintain a record-long highly stable performance among narrow-band-gap III-V NW photoelectrodes; after 67 h photoelectrochemical stability test reaction in a strong acid electrolyte solution (pH = 1), they show no apparent indication of corrosion, which is in stark contrast to the unprotected NWs that fully failed after 35 h. These findings provide an effective way to enhance both stability and performance of III-V NW-based photoelectrodes, which are highly important for practical applications in solar-energy-based water-splitting systems.

Chemical vapour deposition (CVD) of nickel oxide using the novel nickel dialkylaminoalkoxide precursor [Ni(dmamp′)2] (dmamp′ = 2-dimethylamino-2-methyl-1-propanolate)

Nickel oxide (NiO) has good optical transparency and wide band-gap, and due to the particular alignment of valence and conduction band energies with typical current collector materials has been used in solar cells as an efficient hole transport-electron blocking layer, where it is most commonly deposited via sol–gel or directly deposited as nanoparticles. An attractive alternative approach is via vapour deposition. This paper describes the chemical vapour deposition of p-type nickel oxide (NiO) thin films using the new nickel CVD precursor [Ni(dmamp′)₂], which unlike previous examples in literature is synthesised using the readily commercially available dialkylaminoalkoxide ligand dmamp′ (2-dimethylamino-2-methyl-1-propanolate). The use of vapour deposited NiO as a blocking layer in a solar-cell device is presented, including benchmarking of performance and potential routes to improving performance to viable levels.

We describe CVD of nickel oxide (NiO) thin films using a new precursor [Ni(dmamp′)₂], synthesised using a readily commercially available dialkylaminoalkoxide ligand (dmamp′), which is applied to synthesis of a hole transport-electron blocking layer.

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

The continuous emission of nitrous oxides contributes to the overall air pollution and deterioration of air quality. In particular, an effective NO₂ sensor capable of low concentration detection for continuous monitoring is demanded for safety, health, and wellbeing. The sensing performance of a metal oxide-based sensor is predominantly influenced by the availability of surface area for O₂ adsorption and desorption, efficient charge transport, and size or thickness of the sensing layer. In this study, we utilized anodic one-dimensional (1D) TiO₂ nanotube layers of 5 μm thick which offer large surface area and unidirectional electron transport pathway as a platform to accommodate thin SnO₂ coatings as a sensing layer. Conformal and homogeneous SnO₂ coatings across the entire inner and outer TiO₂ nanotubes were achieved by atomic layer deposition with a controlled thickness of 4, 8, and 16 nm. The SnO₂-coated TiO₂ nanotube layers attained a higher sensing response than a reference Figaro SnO₂ sensor. Specifically, the 8 nm SnO₂-coated TiO₂ nanotube layer has recorded up to ten-fold enhancement in response as compared to the blank nanotubes for the detection of 1 ppm NO₂ at an operating temperature of 300 °C with 0.5 V applied bias. This is attributed to the SnO₂/TiO₂ heterojunction effect and controlled SnO₂ thickness within the range of the Debye length. We demonstrated in this work, a tailored large surface area platform based on 1D nanotubes with thin active coatings as an efficient approach for sensing applications and beyond.

Humidity-Tolerant Ultrathin NiO Gas-Sensing Films

When the gas sensor active layer film thickness is decreased, increased sensitivity to changes in the adsorbate concentration is expected when measuring the resistance of the layer, in particular when this thickness is on the order of the Debye length of the material (one-tens of nanometers); however, this is demonstrated only for a limited number of materials. Herein, ultrathin NiO films of different thicknesses (8-21 nm) have been deposited via chemical vapor deposition to fabricate gas sensor devices. Sensor performance for a range of NO₂ concentrations (800 part-per-billion to 7 part-per-million) was evaluated and an optimum operating temperature of 125 °C determined. The dependence of the potential relative changes with respect to the NO₂ concentration and of the sensor signal with respect to the geometrical parameters was qualitatively evaluated to derive a transduction model capable of fitting the experimental results. The selective sensitivity toward NO₂ was confirmed by the limited response for different reducing gases, CO, CH₄, NH₃, and SO₂, under optimum operating conditions, and the sensor signal toward NO₂ increased with decreasing thickness, demonstrating that the concept of a Debye length dependence of sensitivity is applicable for the p-type semiconductor NiO. In addition, these NiO sensors were exposed to different relative levels of humidity over a wide range of operating temperatures and were found to display humidity tolerance far superior to those in previous reports on SnO₂ materials.

Use of a New Non-Pyrophoric Liquid Aluminum Precursor for Atomic Layer Deposition

An Al₂O₃ thin film has been grown by vapor deposition using different Al precursors. The most commonly used precursor is trimethylaluminum, which is highly reactive and pyrophoric. In the purpose of searching for a more ideal Al source, the non-pyrophoric aluminum tri-sec-butoxide ([Al(O^sBu)₃], ATSB) was introduced as a novel precursor for atomic layer deposition (ALD). After demonstrating the deposition of Al₂O₃ via chemical vapor deposition (CVD) and ‘pulsed CVD’ routes, the use of ATSB in an atomic layer deposition (ALD)-like process was investigated and optimized to achieve self-limiting growth. The films were characterized using spectral reflectance, ellipsometry and UV-Vis before their composition was studied. The growth rate of Al₂O₃ via the ALD-like process was consistently 0.12 nm/cycle on glass, silicon and quartz substrates under the optimized conditions. Scanning electron microscopy and transmission electron microscopy images of the ALD-deposited Al₂O₃ films deposited on complex nanostructures demonstrated the conformity, uniformity and good thickness control of these films, suggesting a potential of being used as the protection layer in photoelectrochemical water splitting.

Gallium Phosphide photoanode coated with TiO2 and CoOx for stable photoelectrochemical water oxidation

Gallium Phosphide (GaP) has a band gap of 2.26 eV and a valance band edge that is more negative than the water oxidation level. Hence, it may be a promising material for photoelectrochemical water splitting. However, one thing GaP has in common with other III-V semiconductors is that it corrodes in photoelectrochemical reactions. Cobalt oxide (CoO_x) is a chemically stable and highly active oxygen evolution reaction co-catalyst. In this study, we protected a GaP photoanode by using a 20 nm TiO₂ as a protection layer and a 2 nm cobalt oxide co-catalyst layer, which were both deposited via atomic layer deposition (ALD). A GaP photoanode that was modified by CoO_x exhibited much higher photocurrent, potential, and photon-to-current efficiency than a bare GaP photoanode under AM1.5G illumination. A photoanode that was coated with both TiO₂ and CoO_x layers was stable for over 24 h during constant reaction in 1 M NaOH (pH 13.7) solution under one sun illumination.

Recent Progress in Magnetron Sputtering Technology Used on Fabrics

The applications of magnetron sputtering technology on the surface coating of fabrics have attracted more and more attention from researchers. Over the past 15 years, researches on magnetron sputtering coated fabrics have been mainly focused on electromagnetic shielding, bacterial resistance, hydrophilic and hydrophobic properties and structural color etc. In this review, recent progress of the technology is discussed in detail, and the common target materials, technologies and functions and characterization of coated fabrics are summarized and analyzed. Finally, the existing problems and future prospects of this developing field are briefly proposed and discussed.