Humidity measurement is one of the most significant issues in various areas of applications such as instrumentation, automated systems, agriculture, climatology and GIS. Numerous sorts of humidity sensors fabricated and developed for industrial and laboratory applications are reviewed and presented in this article. The survey frequently concentrates on the RH sensors based upon their organic and inorganic functional materials, e.g., porous ceramics (semiconductors), polymers, ceramic/polymer and electrolytes, as well as conduction mechanism and fabrication technologies. A significant aim of this review is to provide a distinct categorization pursuant to state of the art humidity sensor types, principles of work, sensing substances, transduction mechanisms, and production technologies. Furthermore, performance characteristics of the different humidity sensors such as electrical and statistical data will be detailed and gives an added value to the report. By comparison of overall prospects of the sensors it was revealed that there are still drawbacks as to efficiency of sensing elements and conduction values. The flexibility offered by thick film and thin film processes either in the preparation of materials or in the choice of shape and size of the sensor structure provides advantages over other technologies. These ceramic sensors show faster response than other types.

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Humidity Sensors Principle, Mechanism, and Fabrication Technologies: A Comprehensive Review

Graphene, a single, one-atom-thick sheet of carbon atoms arranged in a honeycomb lattice and the two-dimensional building block for carbon materials, has attracted great interest for a wide range of applications. Due to its superior properties such as thermo-electric conduction, surface area and mechanical strength, graphene materials have inspired huge interest in sensing of various chemical species. In this timely review, we discuss the recent advancement in the field of graphene based gas sensors with emphasis on the use of modified graphene materials. Further, insights of theoretical and experimental aspects associated with such systems are also discussed with significance on the sensitivity and selectivity of graphene towards various gas molecules. The first section introduces graphene, its synthesis methods and its physico-chemical properties. The second part focuses on the theoretical approaches that discuss the structural improvisations of graphene for its effective use as gas sensing materials. The third section discusses the applications of pristine and modified graphene materials in gas sensing applications. Various graphene modification methods are discussed including using dopants and defects, decoration with metal/metal oxide nanoparticles, and functionalization with polymers. Finally, a discussion on the future challenges and perspectives of this enticing field of graphene sensors for gas detection is provided.

Recent advances in graphene based gas sensors

Meeting the growing global energy demand is one of the important challenges of the 21st century. Currently over 80% of the world's energy requirements are supplied by the combustion of fossil fuels, which promotes global warming and has deleterious effects on our environment. Moreover, fossil fuels are non-renewable energy and will eventually be exhausted due to the high consumption rate. A new type of alternative energy that is clean, renewable and inexpensive is urgently needed. Several candidates are currently available such as hydraulic power, wind force and nuclear power. Solar energy is particularly attractive because it is essentially clean and inexhaustible. A year's worth of sunlight would provide more than 100 times the energy of the world's entire known fossil fuel reserves. Photocatalysis and photovoltaics are two of the most important routes for the utilization of solar energy. However, environmental protection is also critical to realize a sustainable future, and water pollution is a serious problem of current society. Photocatalysis is also an essential route for the degradation of organic dyes in wastewater. A type of compound with the defined structure of perovskite (ABX3) was observed to play important roles in photocatalysis and photovoltaics. These materials can be used as photocatalysts for water splitting reaction for hydrogen production and photo-degradation of organic dyes in wastewater as well as for photoanodes in dye-sensitized solar cells and light absorbers in perovskite-based solar cells for electricity generation. In this review paper, the recent progress of perovskites for applications in these fields is comprehensively summarized. A description of the basic principles of the water splitting reaction, photo-degradation of organic dyes and solar cells as well as the requirements for efficient photocatalysts is first provided. Then, emphasis is placed on the designation and strategies for perovskite catalysts to improve their photocatalytic activity and/or light adsorption capability. Comments on current and future challenges are also provided. The main purpose of this review paper is to provide a current summary of recent progress in perovskite materials for use in these important areas and to provide some useful guidelines for future development in these hot research areas.

Research progress of perovskite materials in photocatalysis- and photovoltaics-related energy conversion and environmental treatment.

Quantum dots (QDs) have received great interest for diverse applications due to their distinct advantages, such as narrow and symmetric emission with tunable colors, broad and strong absorption, reasonable stability, and solution processibility Doped QDs not only potentially retain almost all of the above advantages, but also avoid the self-quenching problem due to their substantial ensemble Stokes shift Two obvious advantages of doped QDs, especially doped ZnS QDs, over typical CdSe@ZnS and CdTe QDs are longer dopant emission lifetime and potentially lower cytotoxicity The lifetime of dopant emission from transition-metal ion or lanthanide ion-doped QDs is generally longer than that of the bandgap or defect-related emission of host, and that of biological background fluorescence, providing great opportunities to eliminate background fluorescence for biosensing and bioimaging For bioimaging applications, fluorescent dopants may mitigate toxicity problems by producing visible or infrared emission in nanocrystals made from less-harmful elements than those currently used In this review, recent advances in utilizing doped QDs for chemo/biosensing and bioimaging are discussed, and the synthetic routes and optical properties of doped QDs that make them excellent probes for various strategies in chemo/biosensing and bioimaging are highlighted Moreover, perspectives on future exploration of doped QDs for chemo/biosensing and bioimaging are also given

Doped quantum dots for chemo/biosensing and bioimaging

Sensing of gas molecules is critical to environmental monitoring, control of chemical processes, agricultural, and medical applications In particular, the detection of industrial toxic gases such as CO, NO x , and NH 3 is very important for many industries Metal oxides have been widely studied for the sensitivity of their properties to gases even though they do have some limitations Recently, graphene has been considered as a promising material for gas sensing since its electronic properties are strongly affected by the adsorption of foreign molecules Intrinsic graphene has high sensitivity at low gas concentrations; but the sensor selectivity is poor which limits its use in many practical applications Hence, hybrid architectures formed by blending of nanoparticles of metal–oxides with graphene or its derivatives have been explored by several researchers which showed improved gas sensing ability, especially the sensitivity and selectivity at room temperature Here we review the state of the art of gas sensors based on graphene and metal oxide hybrid nanostructures for detection of various common toxic gases

Graphene–metal oxide nanohybrids for toxic gas sensor: A review

Graphene-based nanocomposites have proven to be very promising materials for gas sensing applications. In this paper, we present a general approach for the preparation of graphene‐WO3 nanocomposites. Graphene‐WO3 nanocomposite thin-layer sensors were prepared by drop coating the dispersed solution onto the alumina substrate. These nanocomposites were used for the detection of NO2 for the first time. TEM micrographs revealed that WO3 nanoparticles were well distributed on graphene nanosheets. Three different compositions (0.2, 0.5 and 0.1 wt%) of graphene with WO3 were used for the gas sensing measurements. It was observed that the sensor response to NO2 increased nearly three times in the case of graphene‐WO3 nanocomposite layer as compared to a pure WO3 layer at room temperature. The best response of the graphene‐WO3 nanocomposite was obtained at 250 C. (Some figures may appear in colour only in the online journal)

https://iopscience.iop.org/article/10.1088/0957-4484/23/20/205501/pdf

Faster response of NO 2 sensing in graphene-WO 3 nanocomposites

Photocatalytic degradation of organic dyes under UV–Visible light using capped ZnS nanoparticles

Highly sensitive ammonia gas sensors were fabricated using Multi-walled Carbon Nanotubes (MWCNTs) reinforced electrically conducting polymer composites following solution casting method. Two types of conducting polymers like poly(3,4-ethylenedioxythiophene)–polystyrene sulfonic acid (PEDOT:PSS) and polyaniline (PANI) were used and compared for their ammonia gas sensing properties at room temperature (RT). Both the sensors were found to exhibit excellent sensitivity and poor recovery for ammonia gas at room temperature, but as compared to PANI, PEDOT:PSS polymer composite was found to be more sensitive (with sensitivity of ∼16%) with less response time (∼15 min). In addition, thermal behavior of both the composites was investigated in detail, where MWCNT–PEDOT:PSS composite showed significantly better thermal stability than MWCNT–PANI composite. Sensor recovery posed a great problem at room temperature and a new approach is proposed to get complete recovery, exclusively tested in polymer–CNT composite sensor, to lessen the heat dependence and improve the cycling behavior. A trial experiment is conducted in combination of heat and DC electric field to optimize the complete recovery of the MWCNT–PEDOT:PSS composite based sensor where the recovery time was reduced from 48 h to 20 min. It is believed that such stimulation process provides sufficient energy to desorb chemisorbed ammonia from CNT surface completely.

MWCNT-conducting polymer composite based ammonia gas sensors: A new approach for complete recovery process

Results pertaining to the development of highly dispersible stand-alone particles of Ce3+-doped Y3Al5O12 nanophosphor and their luminescence are presented. The yellow light-emitting nanophosphor was produced as a result of a single-step auto-combustion process, which requires no auxiliary annealing treatments prior to any practical application. The structure, photoluminescence, and chromaticity of the powder nanophosphor samples with the compositions Y(3−x)Al5O12:xCe3+ (x=0.01–1) are featured in the letter. The nanophosphor absorbs light efficiently in the visible region of 400–500nm, and shows single broadband emission peaking at ∼560nm. The luminescence yield is almost analogous to the samples made from other conventional methods. Hence, it could be an apt candidate for generating white light when coupled to a blue light-emitting diode (λem=450nm).

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Enhanced luminescence of Y3Al5O12 : Ce3+ nanophosphor for white light-emitting diodes

Nickel ferrite nanoparticles of very small size were prepared by sol-gel combustion and co-precipitation techniques. At the same annealing temperature sol-gel derived particles had bigger crystallite size. In both methods, crystallite size of the particles increased with annealing temperature. Sol-gel derived nickel ferrite particles were found to be of almost spherical shape and moderate particle size with a narrow size distribution; while co-precipitation derived particles had irregular shape and very small particle size with a wide size distribution. Nickel ferrite particles produced by sol-gel method exhibited more purity. Sol-gel synthesized nanoparticles were found to be of high saturation magnetization and hysteresis. Co-precipitation derived nickel ferrite particles, annealed at 400°C exhibited superparamagnetic nature with small saturation magnetization. Saturation magnetization increased with annealing temperature in both the methods. At the annealing temperature of 600°C, co-precipitation derived particles also became ferrimagnetic.

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Sukhvir Singh

Papers

Faster response of NO 2 sensing in graphene-WO 3 nanocomposites

Photocatalytic degradation of organic dyes under UV–Visible light using capped ZnS nanoparticles

MWCNT-conducting polymer composite based ammonia gas sensors: A new approach for complete recovery process

Enhanced luminescence of Y3Al5O12 : Ce3+ nanophosphor for white light-emitting diodes

Influence of preparation method on structural and magnetic properties of nickel ferrite nanoparticles