TiO2-Based Nanomaterials for Gas Sensing-Influence of Anatase and Rutile Contributions

Nanomaterial Gas Sensors for Biosensing Applications: A Review

BACKGROUND

Nanomaterial is one of the most used materials for various gas sensing applications to detect toxic gases, human breath, and other specific gas sensing. One of the most important applications of nanomaterial based gas sensors is biosensing applications. In this review article, the gas sensors for biosensing are discussed on the basis of crystalline structure and different categories of nanomaterial.

METHODS

In this paper, firstly, rigorous efforts have been made to find out research questions by going through a structured and systematic survey of available peer reviewed high quality articles in this field. The papers related to nanomaterial based biosensors are then reviewed qualitatively to provide substantive findings from the recent developments in this field.

RESULTS

In this mini-review article, firstly, classifications of nanomaterial gas sensors have been presented on the basis of the crystalline structure of nanomaterial and different types of nanomaterial available for biosensing applications. Further, the gas sensors based on nanomaterial for biosensing applications are collected and reviewed in terms of their performance parameters such as sensing material used, target gas component, detection ranges (ppm-ppb), response time, operating temperature and method of detection, etc. The different nanomaterials possess slightly different sensing and morphological properties due to their structure; therefore, it can be said that a nanomaterial must be selected carefully for a particular application. The 1D nanomaterials show the best selectivity and sensitivity for gases available in low concentration ranges due to their miniaturised structure compared to 2D and 3D nanomaterials. However, these 2D and 3D nanomaterials also so good sensing properties compared to bulk semiconductor materials. The polymer and nanocomposites which are also discussed in this patent article have opened the door for future research and have great potential for new generation gas sensors for detecting biomolecules.

CONCLUSION

These nanomaterials extend great properties towards sensing the application of different gases for a lower concentration of particular gas particles. Nano polymer and nanocomposites have great potential to be used as gas sensors for the detection of biomolecules.

Gas Sensors Based on Localized Surface Plasmon Resonances: Synthesis of Oxide Films with Embedded Metal Nanoparticles, Theory and Simulation, and Sensitivity Enhancement Strategies

This work presents a comprehensive review on gas sensors based on localized surface plasmon resonance (LSPR) phenomenon, including the theory of LSPR, the synthesis of nanoparticle-embedded oxide thin films, and strategies to enhance the sensitivity of these optical sensors, supported by simulations of the electromagnetic properties. The LSPR phenomenon is known to be responsible for the unique colour effects observed in the ancient Roman Lycurgus Cup and at the windows of the medieval cathedrals. In both cases, the optical effects result from the interaction of the visible light (scattering and absorption) with the conduction band electrons of noble metal nanoparticles (gold, silver, and gold–silver alloys). These nanoparticles are dispersed in a dielectric matrix with a relatively high refractive index in order to push the resonance to the visible spectral range. At the same time, they have to be located at the surface to make LSPR sensitive to changes in the local dielectric environment, the property that is very attractive for sensing applications. Hence, an overview of gas sensors is presented, including electronic-nose systems, followed by a description of the surface plasmons that arise in noble metal thin films and nanoparticles. Afterwards, metal oxides are explored as robust and sensitive materials to host nanoparticles, followed by preparation methods of nanocomposite plasmonic thin films with sustainable techniques. Finally, several optical properties simulation methods are described, and the optical LSPR sensitivity of gold nanoparticles with different shapes, sensing volumes, and surroundings is calculated using the discrete dipole approximation method.

Room Temperature Operation of UV Photocatalytic Functionalized AlGaN/GaN Heterostructure Hydrogen Sensor

An AlGaN/GaN heterostructure based hydrogen sensor was fabricated using a dual catalyst layer with ZnO-nanoparticles (NPs) atop of Pd catalyst film. The ZnO-NPs were synthesized to have an average diameter of ~10 nm and spin coated on the Pd catalyst layer. Unlike the conventional catalytic reaction, the fabricated sensors exhibited room temperature operation without heating owing to the photocatalytic reaction of the ZnO-NPs with ultraviolet illumination at 280 nm. A sensing response of 25% was achieved for a hydrogen concentration of 4% at room temperature with fast response and recovery times; a response time of 8 s and a recovery time of 11 s.

Influence of Solvents' Polarity on the Physicochemical Properties and Photocatalytic Activity of Titania Synthesized Using Deinbollia pinnata Leaves

Nanotechnology is one of the most interesting areas of research due to its flexibility to improve or form new products from nanoparticles (NPs), and as a fast, greener, more eco-friendly and sustainable solution to technological and environmental challenges. Among metal oxides of photocatalytic performance, the use of titania (TiO₂) as photocatalyst is most popular due to its unique optical and electronic properties. Despite the wide utilization, the synthesis of TiO₂ NPs bears many disadvantages: it utilizes various less environmental-friendly chemicals, high cost, requires high pressure and energy, and potentially hazardous physical and chemical methods. Hence, the development of green synthesis approach with eco-friendly natural products can be used to overcome these adverse effects. In this work, TiO₂ NPs have been prepared by using Deinbollia pinnata leaves extracts, obtained by different solvents (n-hexane, ethyl acetate, and ethanol) with different polarities. The extracts acted as the reducing agent, while titanium isopropoxide as the precursor and water as the solvent. X-ray diffraction (XRD) pattern confirmed the synthesized TiO₂ consist of anatase phase in high purity, with average crystallite size in the range of 19–21 nm. Characterization by using field emission scanning electron microscopy (FESEM) showed the TiO₂ NPs possess a uniform semi-spherical shape in the size range of 33–48 nm. The energy dispersive X-ray (EDX) spectra of green TiO₂ NPs showed two peaks for the main elements of Ti (61 Wt.%) and O (35 Wt.%). The band-gap energy of 3.2 eV was determined using UV-Vis spectroscopy. From the nitrogen sorption analysis, type V isotherm of the material was obtained, with BET surface area of 31.77 m²/g. The photocatalytic activity of synthesized TiO₂ was evaluated for photodegradation of methyl orange (MO) under UV light irradiation. Based on the results, it is shown that TiO₂ NPs synthesized with D. pinnata leaves extracted using ethyl acetate showed the most effective photodegradation performance, achieving 98.7% of MO conversion within 150 min. It can be concluded that the use of plant extracts in synthesis with TiO₂ managed to produce highly crystalline anatase TiO₂ with superior photocatalytic activity in the photodegradation of organic dye.

SnO2/TiO2 Thin Film n-n Heterostructures of Improved Sensitivity to NO2

Thin-film n-n nanoheterostructures of SnO₂/TiO₂, highly sensitive to NO₂, were obtained in a two-step process: (i) magnetron sputtering, MS followed by (ii) Langmuir-Blodgett, L–B, technique. Thick (200 nm) SnO₂ base layers were deposited by MS and subsequently overcoated with a thin and discontinuous TiO₂ film by means of L–B. Rutile nanopowder spread over the ethanol/chloroform/water formed a suspension, which was used as a source in L–B method. The morphology, crystallographic and electronic properties of the prepared sensors were studied by scanning electron microscopy, SEM, X-ray diffraction, XRD in glancing incidence geometry, GID, X-ray photoemission spectroscopy, XPS, and uv-vis-nir spectrophotometry, respectively. It was found that amorphous SnO₂ films responded to relatively low concentrations of NO₂ of about 200 ppb. A change of more than two orders of magnitude in the electrical resistivity upon exposure to NO₂ was further enhanced in SnO₂/TiO₂ n-n nanoheterostructures. The best sensor responses R_NO2/R₀ were obtained at the lowest operating temperatures of about 120 °C, which is typical for nanomaterials. Response (recovery) times to 400 ppb NO₂ were determined as a function of the operating temperature and indicated a significant decrease from 62 (42) s at 123 °C to 12 (19) s at 385 °C A much smaller sensitivity to H₂ was observed, which might be advantageous for selective detection of nitrogen oxides. The influence of humidity on the NO₂ response was demonstrated to be significantly below 150 °C and systematically decreased upon increase in the operating temperature up to 400 °C.

Challenges to rutile-based geoscientific tools: low-temperature polymorphic TiO2 transformations and corresponding reactive pathways

Rutile, a common accessory mineral in a wide variety of rocks, is the most stable naturally occurring TiO₂ polymorph. The relationship between its trace element composition and formation conditions has provided geoscientists with discriminant tools for fingerprinting geological processes, such as magmatic evolution and subduction zone metamorphism, alongside applications to the study of sediment provenance. In the present work, volcaniclastic rock samples belonging to Fara and Saiq Formations, outcropping in Jebel Akhdar mountains, Oman, are studied with Raman spectroscopy and Electron Microprobe (EMP) aiming: of (i) the identification of different naturally-occurring TiO₂ polymorphs, (ii) the evaluation of their trace element contents in relation with hydrothermal alteration features, and (iii) the analysis of the mineral reactive pathways behind the observed textural relationships. Raman investigations demonstrated that interstitial, fine-grained TiO₂ corresponds to anatase, whereas rutile occurs as isolated single grains. EMP determinations further revealed that an identified Nb-enrichment in anatase is coupled with a corresponding Nb-depletion in rutile. The combination of the obtained results with petrographic observations enabled unravelling the TiO₂ reactive pathways affecting the studied samples. Thus, a coupled polymorphic dissolution-precipitation reaction assisting rutile-to-anatase conversion has been defined, together with the role of Nb in further stabilizing the structure of the lower temperature polymorph. Semi-quantitative thermometric considerations suggest that rutile substrates are likely of magmatic origin, whereas anatase formation is clearly associated with a lower temperature aqueous environment. The gathered results raise fundamental questions concerning the application of commonly used rutile-based geochemical and thermometric tools.

Volatile organic compound gas sensors based on methylammonium lead iodide perovskite operating at room temperature

Methylammonium lead iodide (MAPbI₃) perovskite thin film has been successfully applied to a volatile organic compound (VOC) gas sensor that can operate at room temperature. In this study, ∼100 nm-thick MAPbI₃ film shows good reversibility and repeatability as a VOC gas sensor. The resistance of the MAPbI₃ film substantially decreases when it is exposed to VOC vapour and recovers back to high resistance when the VOC gas is removed. Adsorption of VOC gas molecules to vacancies in MAPbI₃ film might lead to charge trap passivation. The VOC sensor based on perovskite thin film is tested in terms of film thickness, applied bias voltage, and polarity of VOC. We expect that our VOC gas sensor based on solution-processed MAPbI₃ operating at room temperature has potential to be developed as a low cost and low power smart gas sensor.

At room temperature, conductivity of methylammonium lead iodide perovskite was increased in the presence of volatile organic compound (VOC) gas, which was interpreted in the context of charge trap passivation mechanism.

Surface-Controlled Photocatalysis and Chemical Sensing of TiO2, α-Fe2O3, and Cu2O Nanocrystals

A relatively new approach to the design of photocatalytic and gas sensing materials is to use the shape-controlled nanocrystals with well-defined facets exposed to light or gas molecules. An abrupt increase in a number of papers on the synthesis and characterization of metal oxide semiconductors such as a TiO2, α-Fe2O3, Cu2O of low-dimensionality, applied to surface-controlled photocatalysis and gas sensing, has been recently observed. The aim of this paper is to review the work performed in this field of research. Here, the focus is on the mechanism and processes that affect the growth of nanocrystals, their morphological, electrical, and optical properties and finally their photocatalytic as well as gas sensing performance.

Highly Sensitive, Temperature-Independent Oxygen Gas Sensor Based on Anatase TiO2 Nanoparticle Grafted, 2D Mixed Valent VO x Nanoflakelets

Herein, we report a facile approach for the synthesis of TiO₂ nanoparticles tethered on 2D mixed valent vanadium oxide (VO _x/TiO₂) nanoflakelets using a thermal decomposition assisted hydrothermal method and investigation of its temperature-independent performance enhancement in oxygen-sensing properties. The material was structurally characterized using XRD, TEM, Raman, DSC, and XPS analysis. The presence of mixed valent states, such as V₂O₅ and VO₂ in VO _x, and the metastable properties of VO₂ have been found to play crucial roles in the temperature-independent electrical conductivity of VO _x/TiO₂ nanoflakelets. Though pristine VO _x exhibited characteristic semiconductor-to-metal transition of monoclinic VO₂, pure VO _x nanoflakelets exhibited poor sensitivity toward sensing oxygen. VO _x/TiO₂ nanoflakelets showed a very low temperature coefficient of resistance above 150 °C with improved sensitivity (35 times higher than VO _x for 100 ppm) toward oxygen gas. VO _x/TiO₂ nanoflakelets exhibited much higher response, faster adsorption and desorption toward oxygen as compared to pristine VO _x beyond 100 °C, which endowed the sensor with excellent temperature-independent sensor properties within 150-500 °C. The faster adsorption and desorption after 100 °C led to shorter response time (3-5 s) and recovery time (7-9 s). The results suggest that 2D VO _x/TiO₂ can be a promising candidate for temperature-independent oxygen sensor applications.

Hybrid Noble-Metals/Metal-Oxide Bifunctional Nano-Heterostructure Displaying Outperforming Gas-Sensing and Photochromic Performances

As nanomaterials are dominating 21st century's scene, multiple functionality in a single (nano)structure is becoming very appealing. Inspired by the Land of the Rising Sun, we designed a bifunctional (gas-sensor/photochromic) nanomaterial, made with TiO₂ whose surface was simultaneously decorated with copper and silver (the Cu/Ag molar ratio being 3:1). This nanomaterial outperformed previous state-of-the-art TiO₂-based sensors for the detection of acetone, as well as the Cu-TiO₂-based photochromic material. It indeed possessed splendid sensitivity toward acetone (detection limit of 100 ppb, 5 times lower than previous state-of-the-art TiO₂-based acetone sensors), as well as reduced response/recovery times at very low working temperature, 150 °C, for acetone sensing. Still, the same material showed itself to be able to (reversibly) change in color when stimulated by both UV-A and, most remarkably, visible light. Indeed, the visible-light photochromic performance was almost 3 times faster compared to the standard Cu-TiO₂ photochromic material-that is, 4.0 min versus 10.8 min, respectively. It was eventually proposed that the photochromic behavior was triggered by different mechanisms, depending on the light source used.

Synergistic Cooperation of Rutile TiO2 {002}, {101}, and {110} Facets for Hydrogen Sensing

An oriented TiO₂ thin film-based hydrogen sensor has been demonstrated to have excellent sensing properties at room temperature. The exposed high energy surface offers a low energy barrier for H₂ adsorption and dissociation. In this work, rutile TiO₂ with {101} and {002} facets exposed was controllably synthesized by adjusting the ethanol content of the hydrothermal solvent. The crystalline structure, morphologies, and H₂ sensing performance of the samples varied with the relative ratios of {002} and {101} facets. By increasing the ethanol content, the (002) orientation growth was enhanced and the (101) orientation growth was restrained, the size of the nanorods composing the thin film was reduced and the density of the film was increased. All of the prepared TiO₂ nanorod array film-based hydrogen sensors performed very well at room temperature. The TiO₂ hydrogen sensor with both {110} and {002} facets exposed gave a faster response, as well as better repeatability and stability than those with only {002} facets. Density functional theory simulations have been adopted to reveal the surface interaction of H₂ and the TiO₂ surface. The results suggested that H₂ tended to be adsorbed and dissociated on the (002) and (101) surface. There is a very small active barrier for atomic H to recombine into H₂ molecules on the (110) surface. Thin films with lower density, where more (110) surface is exposed, offered more space for H₂ regeneration, leading to shorter response and recovery times as well as higher sensitivity. The (002), (101), and (110) surfaces of rutile TiO₂ synergistically cooperated to complete the whole H₂ sensing process.

Porous TiO₂-Based Gas Sensors for Cyber Chemical Systems to Provide Security and Medical Diagnosis

Gas sensors play an important role in our life, providing control and security of technical processes, environment, transportation and healthcare. Consequently, the development of high performance gas sensor devices is the subject of intense research. TiO₂, with its excellent physical and chemical properties, is a very attractive material for the fabrication of chemical sensors. Meanwhile, the emerging technologies are focused on the fabrication of more flexible and smart systems for precise monitoring and diagnosis in real-time. The proposed cyber chemical systems in this paper are based on the integration of cyber elements with the chemical sensor devices. These systems may have a crucial effect on the environmental and industrial safety, control of carriage of dangerous goods and medicine. This review highlights the recent developments on fabrication of porous TiO₂-based chemical gas sensors for their application in cyber chemical system showing the convenience and feasibility of such a model to provide the security and to perform the diagnostics. The most of reports have demonstrated that the fabrication of doped, mixed and composite structures based on porous TiO₂ may drastically improve its sensing performance. In addition, each component has its unique effect on the sensing properties of material.

Enhanced photovoltaic properties of perovskite solar cells by TiO2 homogeneous hybrid structure

In this paper, we fabricated a TiO₂ homogeneous hybrid structure for application in perovskite solar cells (PSCs) under ambient conditions. Under the standard air mass 1.5 global (AM 1.5G) illumination, PSCs based on homogeneous hybrid structure present a maximum power conversion efficiency of 5.39% which is higher than that of pure TiO₂ nanosheets. The enhanced properties can be explained by the better contact of TiO₂ nanosheets/nanoparticles with CH₃NH₃PbI₃ and fewer pinholes in electron transport materials. The advent of such unique structure opens up new avenues for the future development of high-efficiency photovoltaic cells.

One-step preparation of TiO2particles with controllable phase and morphology by plasma electrolysis

TiO₂particles with controllable phase and morphology were prepared by one-step plasma electrolysis at room temperature.