TiO2 nanostructures with different crystal phases for sensitive acetone gas sensors

Tunable structure of TiO2 deposited by DC magnetron sputtering to adsorb Cr (VI) and Fe (III) from water

TiO2 thin films with mixtures of the anatase, rutile, and brookite phases were deposited on glass substrates via magnetron sputtering. Based on XRD and Raman results, the TiO2-0.47 and TiO2-3.47 films principally contained the brookite phase, while the TiO2-1.27 and TiO2-2.13 films were primarily anatase. The capacities of the TiO2 films to adsorb heavy metals were tested with Cr(VI) and Fe(III) solutions, and the maximum Cr(VI) and Fe(III) adsorption capacities were realized with the TiO2-0.47 film (334.5 mg/g) and TiO2-3.47 film (271.3 mg/g), respectively. SEM‒EDS results revealed the presence of Cr and Fe on the surfaces of the films, thus corroborating the ability of the TiO2 films to adsorb and remove heavy metals. They are strong candidates for use in wastewater treatment plants.

The Challenges and Opportunities for TiO2 Nanostructures in Gas Sensing

Chemiresistive gas sensors based on metal oxides have been widely applied in industrial monitoring, medical diagnosis, environmental pollutant detection, and food safety. To further enhance the gas sensing performance, researchers have worked to modify the structure and function of the material so that it can adapt to different gas types and environmental conditions. Among the numerous gas-sensitive materials, n-type TiO2 semiconductors are a focus of attention for their high stability, excellent biosafety, controllable carrier concentration, and low manufacturing cost. This Perspective first introduces the sensing mechanism of TiO2 nanostructures and composite TiO2-based nanomaterials and then analyzes the relationship between their gas-sensitive properties and their structure and composition, focusing also on technical issues such as doping, heterojunctions, and functional applications. The applications and challenges of TiO2-based nanostructured gas sensors in food safety, medical diagnosis, environmental detection, and other fields are also summarized in detail. Finally, in the context of their practical application challenges, future development technologies and new sensing concepts are explored, providing new ideas and directions for the development of multifunctional intelligent gas sensors in various application fields.

Electronic structure analysis of NiO quantum dot-modified jackfruit-shaped ZnO sensors and sensing properties investigation of their highly sensitive and selective for butyl acetate

Detection of flammable, explosive and toxic butyl acetate helps to avoid accidents and protect health in industrial production. However, there are few reports on butyl acetate sensors, especially highly sensitive, low detection limit and highly selective ones. In this work, density functional theory (DFT) analyzes the electronic structure of sensing materials and the adsorption energy of butyl acetate. The effects of Ni element doping, oxygen vacancy constructions, and NiO quantum dot modifications on the modulation of the electronic structure of ZnO and on the adsorption energy of butyl acetate are investigated in detail. Based on the DFT analysis, the NiO quantum dot modified jackfruit-shaped ZnO is synthesized via thermal solvent method reduction. The NiO/ZnO sensor has a response 502.5 for 100 ppm butyl acetate with 100 ppb detection limit, and the response for 100 ppm butyl acetate is at least 6.2 times higher than 100 ppm methanol, 100 ppm benzene, 100 ppm triethylamine, 100 ppm isopropanol, 100 ppm ethyl acetate and 100 ppm formic acid. X-ray photoelectron spectroscopy (XPS) explores the change of oxygen vacancies in sensor accompanied with the addition of Ni element and reveales the reason for the change of oxygen vacancies.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Ultrahigh-acetone-sensitivity sensor based on Pt-loaded TiO2porous nanoparticles synthesized via a facile hydrothermal method

A simple hydrothermal method based on an orthogonal experimental design was used to synthesis Pt-loaded TiO2mesoporous nanoparticles in one step. The successful synthesis of Pt-loaded TiO2nanoparticles was demonstrated by various characterization methods. The effects of the modification of Pt and its explanation are described in detail by means of the test results. Through systematic gas-sensing tests, we found that the Pt-loaded TiO2nanoparticles outperform pure TiO2nanoparticles, with a high response value (S= 42.5) to 200 ppm acetone at 260 °C and with a film thickness of 0.45 mm, far superior to that of pure TiO2. The response time (8 s) and recovery time (11 s) of the material are also relatively good with excellent selectivity and long-term stability (30 days). The frequent use of acetone as an organic solution in factories and laboratories, as well as the possibility of making a preliminary diagnosis of diabetes by detecting acetone levels in exhaled gas, make this work promising for environmental monitoring and medical diagnosis.

Facile and template-free fabrication of hierarchical coral spheres for acetone gas sensors

In this paper, highly dispersed hierarchical coral spheres of CeO2/ZnO have been constructed through a facile template-free hydrothermal strategy, followed by an annealing treatment. The resulting coral spheres exhibit enhanced activity for acetone sensing compared with CeO2 or ZnO as well as excellent cyclability and long-term stability. At the optimum working temperature of 245 °C, by controlling the ratio of Ce/Zn, the highest response of the coral spheres towards 100 ppm acetone is up to 145, which is about 5.5 times that of ZnO coral spheres. The significantly improved gas sensing activity may be ascribed to the well-dispersed and assembled CeO2 nanoparticles on the surface of ZnO coral spheres and the heterojunctions between CeO2 and ZnO, which produced abundant oxygen vacancies in the CeO2/ZnO coral spheres.

Enhanced Acetone-Sensing Properties of Pt-Decorated In2O3 Hollow Microspheres Derived from Pt-Embedded Template

Pt-decorated In2O3 hollow microspheres were prepared using a template and reflux method. The size of the prepared carbon templates was adjusted from 200 nm to 1.3 μm by introducing chloroplatinic acid during the hydrothermal process. At the same time, Pt nanoparticles inside the carbon layer were protected from oxidation and agglomeration. Also, the folds created on the surface of the hollow sphere during shrinkage led to a substantial increase in specific surface area. The response of the In2O3-based sensor toward acetone was significantly enhanced by the addition of Pt decoration. This improvement can be attributed to the increased availability of active sites for the target gas and the consequential alteration of the energy band structure. In addition, high response sensitivity, rapid dynamic processes, long-term reliability, and selectivity have all been achieved. The detectable limit is less than 1 ppm, which might satisfy the 1.8 ppm threshold value in the exhaled breath of patients with diabetes. Consequently, the proposed sensor has great sensitivity and can detect low-concentration of acetone, making it an ideal choice for applications such as monitoring daily dietary intake, managing diabetes, and inspecting industrial production processes.

Recent Advancements in TiO2 Nanostructures: Sustainable Synthesis and Gas Sensing

The search for sustainable technology-driven advancements in material synthesis is a new norm, which ensures a low impact on the environment, production cost, and workers' health. In this context, non-toxic, non-hazardous, and low-cost materials and their synthesis methods are integrated to compete with existing physical and chemical methods. From this perspective, titanium oxide (TiO₂) is one of the fascinating materials because of its non-toxicity, biocompatibility, and potential of growing by sustainable methods. Accordingly, TiO₂ is extensively used in gas-sensing devices. Yet, many TiO₂ nanostructures are still synthesized with a lack of mindfulness of environmental impact and sustainable methods, which results in a serious burden on practical commercialization. This review provides a general outline of the advantages and disadvantages of conventional and sustainable methods of TiO₂ preparation. Additionally, a detailed discussion on sustainable growth methods for green synthesis is included. Furthermore, gas-sensing applications and approaches to improve the key functionality of sensors, including response time, recovery time, repeatability, and stability, are discussed in detail in the latter parts of the review. At the end, a concluding discussion is included to provide guidelines for the selection of sustainable synthesis methods and techniques to improve the gas-sensing properties of TiO₂.

Interaction of 5-Fluorouracil on the Surfaces of Pristine and Functionalized Ca12O12 Nanocages: An Intuition from DFT

The utilization of nanostructured materials for several biomedical applications has tremendously increased over the last few decades owing to their nanosizes, porosity, large surface area, sensitivity, and efficiency as drug delivery systems. Thus, the incorporation of functionalized and pristine nanostructures for cancer therapy offers substantial prospects to curb the persistent problems of ineffective drug administration and delivery to target sites. The potential of pristine (Ca₁₂O₁₂) and formyl (-CHO)- and amino (-NH₂)-functionalized (Ca₁₂O₁₂-CHO and Ca₁₂O₁₂-NH₂) derivatives as efficient nanocarriers for 5-fluorouracil (5FU) was studied at the B3LYP-GD3(BJ)/6-311++G(d,p) theoretical level in two electronic media (gas and solvent). To effectively account for all adsorption interactions of the drug on the investigated surfaces, electronic studies as well as topological analysis based on the quantum theory of atoms in molecules (QTAIM) and noncovalent interactions were exhaustively utilized. Interestingly, the obtained results divulged that the 5FU drug interacted favorably with both Ca₁₂O₁₂ and its functionalized derivatives. The adsorption energies of pristine and functionalized nanostructures were calculated to be -133.4, -96.9, and -175.6 kcal/mol, respectively, for Ca₁₂O₁₂, Ca₁₂O₁₂-CHO, and Ca₁₂O₁₂-NH2. Also, both topological analysis and NBO stabilization analysis revealed the presence of interactions among O₃-H₃₂, O₂₇-C₂₄, O₁₀-C₂₇, and N₂₄-H₃₂ atoms of the drug and the surface. However, 5FU@Ca₁₂O₁₂-CHO molecules portrayed the least adsorption energy due to considerable destabilization of the molecular complex as revealed by the computed deformation energy. Therefore, 5FU@Ca₁₂O₁₂ and 5FU@Ca₁₂O₁₂-NH₂ acted as better nanovehicles for 5FU.

A Review on the Progress of Optoelectronic Devices Based on TiO2 Thin Films and Nanomaterials

Titanium dioxide (TiO₂) is a kind of wide-bandgap semiconductor. Nano-TiO₂ devices exhibit size-dependent and novel photoelectric performance due to their quantum limiting effect, high absorption coefficient, high surface-volume ratio, adjustable band gap, etc. Due to their excellent electronic performance, abundant presence, and high cost performance, they are widely used in various application fields such as memory, sensors, and photodiodes. This article provides an overview of the most recent developments in the application of nanostructured TiO₂-based optoelectronic devices. Various complex devices are considered, such as sensors, photodetectors, light-emitting diodes (LEDs), storage applications, and field-effect transistors (FETs). This review of recent discoveries in TiO₂-based optoelectronic devices, along with summary reviews and predictions, has important implications for the development of transitional metal oxides in optoelectronic applications for researchers.

Investigation of High-Sensitivity NO2 Gas Sensors with Ga2O3 Nanorod Sensing Membrane Grown by Hydrothermal Synthesis Method

In this work, Ga₂O₃ nanorods were converted from GaOOH nanorods grown using the hydrothermal synthesis method as the sensing membranes of NO₂ gas sensors. Since a sensing membrane with a high surface-to-volume ratio is a very important issue for gas sensors, the thickness of the seed layer and the concentrations of the hydrothermal precursor gallium nitrate nonahydrate (Ga(NO₃)₃·9H₂O) and hexamethylenetetramine (HMT) were optimized to achieve a high surface-to-volume ratio in the GaOOH nanorods. The results showed that the largest surface-to-volume ratio of the GaOOH nanorods could be obtained using the 50-nm-thick SnO₂ seed layer and the Ga(NO₃)₃·9H₂O/HMT concentration of 12 mM/10 mM. In addition, the GaOOH nanorods were converted to Ga₂O₃ nanorods by thermal annealing in a pure N₂ ambient atmosphere for 2 h at various temperatures of 300 °C, 400 °C, and 500 °C, respectively. Compared with the Ga₂O₃ nanorod sensing membranes annealed at 300 °C and 500 °C, the NO₂ gas sensors using the 400 °C-annealed Ga₂O₃ nanorod sensing membrane exhibited optimal responsivity of 1184.6%, a response time of 63.6 s, and a recovery time of 135.7 s at a NO₂ concentration of 10 ppm. The low NO₂ concentration of 100 ppb could be detected by the Ga₂O₃ nanorod-structured NO₂ gas sensors and the achieved responsivity was 34.2%.

High Sensing Performance Toward Acetone Vapor Using TiO2 Flower-Like Nanomaterials

For real-application gas sensors, high performances (response, selectivity, response/recovery time and stability) are demanded. An effective strategy is applying nanomaterials in gas sensors. In this study, the anatase TiO₂ flower-like nanomaterials (FLNMs) are prepared through a one-step hydrothermal method which exhibit high-performance toward acetone vapor. TiO₂ FLNMs sensors property are characterized at optimal working temperature of 330 °C with selectivity (acetone), response (S = 33.72 toward 250 ppm acetone), linear dependence (R² = 0.9913), response/recovery time (46/24 s toward 250 ppm acetone) and long-term stability (30 days). These demonstrate that TiO₂ FLNMs get a high performance for acetone sensor. Moreover, the limit of detection of acetone is 0.65 ppm which is lower than that of exhaled air for diabetes (0.8 ppm), indicating that TiO₂ FLNMs gas sensor gets potential application in medical diagnosis.

A novel route to the synthesis of α-Fe2O3@C@SiO2/TiO2 nanocomposite from the metal-organic framework as a photocatalyst for water treatment

In this study, an attempt was made to synthesize metal-organic frameworks (MOFs) based magnetic iron particles as photocatalysts for textile dye wastewater. Improvement strategy was a novel two-step dry method without using conventional methods to eliminate the consumption of chemical reagents. First, the heterogeneous photocatalyst of Fe-MOFs derived magnetic carbon nanocomposite with carboxylic acid surface functional groups (Fe@C-COOH) was achieved. Next, the α-Fe₂O₃@C@SiO₂/TiO₂ was successfully synthesized followed by a sol-gel method to coat the SiO₂ shell and a solvothermal method to coat the surface of the intermediate TiO₂ particles. The as-synthesized nanocomposite materials were characterized and physicochemical analytical equipment. Further, the investigation on magnetic photocatalytic nanocomposite α-Fe₂O₃@C@SiO₂/TiO₂ performance of dye degradation and photocatalytic activity on Reactive yellow 145 (RY145), using as an indicator was conducted. The as-synthesized nanocomposite particles were characterized using X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FT-IR), vibrating sample magnetometer (VSM), X-ray energy dispersive spectroscopy (EDX), and scanning electron microscopy (SEM) techniques. The structural characterization of the as-synthesized materials proved that these methods generate oxygen-containing functional groups, such as, -OH, -CO, and -COOH, which increases the polarity and hydrophilicity of the photocatalyst. The photocatalytic oxidation of RY145 dye under UVc light was discussed by the apparent first-order reaction rate and the kinetic model of the Langmuir-Hinshelwood followed a better fitting. The optimal performance of the composite is at pH = 2, 15 mg/100 mL of photocatalyst dose, 150 mg/L concentration of the dye RY145 at 25 °C temperature under UVc lamp irradiation for 90 min, and with the apparent reaction rate constant was 0.0165 min^-1. The thermodynamic analysis of activation parameters computed by the Eyring model and based on transition state theory (TST), an endothermic reaction with a positive value for Δ^‡H^o (50.16 kJ mol^-1) and a negative value for Δ^‡S^o (-153 J/mol K) both contribute toward achieving positive values for Δ^‡G^o and a nonspontaneous process. The proposed α-Fe₂O₃@C@SiO₂/TiO₂ demonstrated a high capability of photocatalytic degradation up to 97% after five successive cycles at the optimal condition compared to that of Fe₃O₄@C (18.74%) and Fe@C-COOH (77.9%) without reusability.

Hydrothermal Synthesis of Hierarchical SnO2 Nanostructures for Improved Formaldehyde Gas Sensing

The indoor environment of buildings affects people’s daily life. Indoor harmful gases include volatile organic gas and greenhouse gas. Therefore, the detection of harmful gas by gas sensors is a key method for developing green buildings. The reasonable design of SnO₂-sensing materials with excellent structures is an ideal choice for gas sensors. In this study, three types of hierarchical SnO₂ microspheres assembled with one-dimensional nanorods, including urchin-like microspheres (SN-1), flower-like microspheres (SN-2), and hydrangea-like microspheres (SN-3), are prepared by a simple hydrothermal method and further applied as gas-sensing materials for an indoor formaldehyde (HCHO) gas-sensing test. The SN-1 sample-based gas sensor demonstrates improved HCHO gas-sensing performance, especially demonstrating greater sensor responses and faster response/recovery speeds than SN-2- and SN-3-based gas sensors. The improved HCHO gas-sensing properties could be mainly attributed to the structural difference of smaller nanorods. These results further indicate the uniqueness of the structure of the SN-1 sample and its suitability as HCHO- sensing material.

Rationally Designed TiO2 Nanostructures of Continuous Pore Network for Fast-Responding and Highly Sensitive Acetone Sensor

For the last several years, indoor air quality monitoring has been a significant issue due to the increasing time portion of indoor human activities. Especially, the early detection of volatile organic compounds potentially harmful to the human body by the prolonged exposure is the primary concern for public human health, and such technology is imperatively desired. In this study, highly porous and periodic 3D TiO₂ nanostructures are designed and studied for this concern. Specifically, extremely high gas molecule accessibility throughout the whole nanostructures and precisely controlled internecks of 3D TiO₂ nanostructures can achieve an unprecedented gas response of 299 to 50 ppm CH₃ COCH₃ with an extremely fast response time of less than 1s. The systematic approach to utilize the whole inner and outer surfaces of the gas sensing materials and periodically formed internecks to localize the current paths in this study can provide highly promising perspectives to advance the development of chemoresistive gas sensors using metal oxide nanostructures for the Internet of Everything application.