Ultra-sensitive and selective NH3 room temperature gas sensing induced by manganese-doped titanium dioxide nanoparticles

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.

A New Analytical Method to Quantify Ammonia in Freshwater with a Bulk Acoustic Wave Sensor

A new method to analyse ammonia in freshwater, based on a piezoelectric quartz crystal coated with the metalloporphyrin chloro[5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato] manganese(III) is presented. A 9 MHz quartz crystal coated on both faces with an amount of porphyrin produced a frequency decrease of 21.4 kHz, which allowed ammonia in a 10.00 mL sample to be quantified in concentrations between 5 and 70 µg L^-1, with a sensitivity of 0.60 Hz L µg^-1, over a period of at least eight months. The proposed method has several advantages over the officially recommended indophenol spectrophotometric method: sample volume was reduced by a factor of 2.5, toxic reagents (phenol and sodium nitroprusside) were eliminated, analysing turbid samples presented no difficulty, and there was not only a significant time saving in solution preparation, but also in sample analysis time, which was reduced from 1 h to 2 min. No statistically significant differences (α = 0.05) were found both in the mean and precision of the results obtained for ammonia in water samples collected from domestic wells, analysed by this new method and by the indophenol spectrophotometric method. Furthermore, the proposed method would allow the individual quantification, with similar sensitivity, of amines and ammonia within a single analytical run.

Nanostructured ZnO/Ag Film Prepared by Magnetron Sputtering Method for Fast Response of Ammonia Gas Detection

Possessing a large surface-to-volume ratio is significant to the sensitive gas detection of semiconductor nanostructures. Here, we propose a fast-response ammonia gas sensor based on porous nanostructured zinc oxide (ZnO) film, which is fabricated through physical vapor deposition and subsequent thermal annealing. In general, an extremely thin silver (Ag) layer (1, 3, 5 nm) and a 100 nm ZnO film are sequentially deposited on the SiO₂/Si substrate by a magnetron sputtering method. The porous nanostructure of ZnO film is formed after thermal annealing contributed by the diffusion of Ag among ZnO crystal grains and the expansion of the ZnO film. Different thicknesses of the Ag layer help the formation of different sizes and quantities of hollows uniformly distributed in the ZnO film, which is demonstrated to hold superior gas sensing abilities than the compact ZnO film. The responses of the different porous ZnO films were also investigated in the ammonia concentration range of 10 to 300 ppm. Experimental results demonstrate that the ZnO/Ag(3 nm) sensor possesses a good electrical resistance variation of 85.74% after exposing the sample to 300 ppm ammonia gas for 310 s. Interestingly, a fast response of 61.18% in 60 s for 300 ppm ammonia gas has been achieved from the ZnO/Ag(5 nm) sensor, which costs only 6 s for the response increase to 10%. Therefore, this controllable, porous, nanostructured ZnO film maintaining a sensitive gas response, fabricated by the physical deposition approach, will be of great interest to the gas-sensing community.

Preparation of silver-loaded titanium dioxide hedgehog-like architecture composed of hundreds of nanorods and its fast response to xylene

Hedgehog-like titanium dioxide (TiO₂) architectures composed of hundreds of one-dimensional (1D) nanorods and silver (Ag) loaded TiO₂ with different amounts (0.2 at%, 0.5 at% and 1 at%) were successfully prepared by facile hydrothermal process and simple isometric impregnation route. The high electron mobility of 1D nanorods on the surface of TiO₂ and the high porosity of Ag loaded hedgehog-like TiO₂ architectures enable the sensor with fast responsive and recovered properties. TiO₂ loaded with 0.5 at% Ag exhibited the highest response to xylene with low response/recovery time at the operating temperature of 375 °C. In addition, the sensitivity and selectivity of the TiO₂ sensor were enhanced markedly with Ag loading.

A dual ammonia-responsive sponge sensor: preparation, transition mechanism and sensitivity

PDMS-PU (polydimethylsiloxane-polyurethane) sponge decorated with In(OH)3 (indium hydroxide) and BCP (bromocresol purple) particles is shown to be a room-temperature ammonia sensor with high sensitivity and excellent reproducibility; it can accomplish real-time detection and monitoring of ammonia in the surrounding environment. The superhydrophobic and yellowish In(OH)3-BCP-TiO2-based ammonia-responsive (IBT-AR) sponge changes to a purple superhydrophilic one when exposed to ammonia. Notably, after reacting with ammonia, the sponge can recover its original wettability and color after heating in air. The wettability, color and absorption signal of IBT-AR sponge have been measured for sensing ammonia using the water contact angle, macroscopic observation and UV-vis absorption spectrometry, respectively. The minimum ammonia concentrations that can be detected by the sponge wettability, color and absorption signal are 0.5%, 1.4 ppm and 50 ppb, respectively. This kind of sponge with smart wettability and color is a promising new ammonia detector.

Aero-gel assisted synthesis of anatase TiO2 nanoparticles for humidity sensing application

The aero-gel based one-pot synthesis of anatase phase TiO₂ nanoparticles having a high surface area of 125 m² g⁻¹ has been reported in this work.

Catalytic oxidation of dibromomethane over Ti-modified Co3O4 catalysts: Structure, activity and mechanism

Ti-modified Co₃O₄ catalysts with various Co/Ti ratios were synthesized using the co-precipitation method and were used in catalytic oxidation of dibromomethane (CH₂Br₂), which was selected as the model molecule for brominated volatile organic compounds (BVOCs). Addition of Ti distorted the crystal structure and led to the formation of a Co-O-Ti solid solution. Co₄Ti₁ (Co/Ti molar ratio was 4) achieved higher catalytic activity with a T₉₀ (the temperature needed for 90% conversion) of approximately 245°C for CH₂Br₂ oxidation and higher selectivity to CO₂ at a low temperature than the other investigated catalysts. In addition, Co₄Ti₁ was stable for at least 30h at 500ppm CH₂Br₂, 0 or 2vol% H₂O, 0 or 500ppm p-xylene (PX), and 10% O₂ at a gas hourly space velocity of 60,000h^-1. The final products were CO_x, Br₂, and HBr, without the formation of other Br-containing organic byproducts. The high catalytic activity was attributed to the high Co³⁺/Co²⁺ ratio and high surface acidity. Additionally, the synergistic effect of Co and Ti made it superior for CH₂Br₂ oxidation. Furthermore, based on the analysis of products and in situ DRIFTs studies, a receivable reaction mechanism for CH₂Br₂ oxidation over Ti-modified Co₃O₄ catalysts was proposed.