High-precision gas sensors operated at room temperature are attractive for various real-time gas monitoring applications, with advantages including low energy consumption, cost effectiveness and device miniaturization/flexibility. Studies on sensing materials, which play a key role in good gas sensing performance, are currently focused extensively on semiconducting metal oxide nanostructures (SMONs) used in the conventional resistance type gas sensors. This topical review highlights the designs and mechanisms of different SMONs with various patterns (e.g. nanoparticles, nanowires, nanosheets, nanorods, nanotubes, nanofilms, etc.) for gas sensors to detect various hazardous gases at room temperature. The key topics include (1) single phase SMONs including both n-type and p-type ones; (2) noble metal nanoparticle and metal ion modified SMONs; (3) composite oxides of SMONs; (4) composites of SMONs with carbon nanomaterials. Enhancement of the sensing performance of SMONs at room temperature can also be realized using a photo-activation effect such as ultraviolet light. SMON based mechanically flexible and wearable room temperature gas sensors are also discussed. Various mechanisms have been discussed for the enhanced sensing performance, which include redox reactions, heterojunction generation, formation of metal sulfides and the spillover effect. Finally, major challenges and prospects for the SMON based room temperature gas sensors are highlighted.

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Advances in designs and mechanisms of semiconducting metal oxide nanostructures for high-precision gas sensors operated at room temperature

We report a procedure to prepare highly monodisperse copper telluride nanocubes, nanoplates, and nanorods. The procedure is based on the reaction of a copper salt with trioctylphosphine telluride in the presence of lithium bis(trimethylsilyl)amide and oleylamine. CuTe nanocrystals display a strong near-infrared optical absorption associated with localized surface plasmon resonances. We exploit this plasmon resonance for the design of surface-enhanced Raman scattering sensors for unconventional optical probes. Furthermore, we also report here our preliminary analysis of the use of CuTe nanocrystals as cytotoxic and photothermal agents.

CuTe Nanocrystals: Shape and Size Control, Plasmonic Properties, and Use as SERS Probes and Photothermal Agents

Xu-dong Wang*,† and Otto S. Wolfbeis*,‡ †Department of Chemistry, Fudan University, 200433 Shanghai, P. R. China ‡Institute of Analytical Chemistry, Chemoand Biosensors, University of Regensburg, D-93040 Regensburg, Germany ■ CONTENTS Books, Reviews, and Articles of General Interest 204 Sensors for (Dissolved) Gases and Vapors 204 Hydrogen 204 Hydrocarbons 206 Oxygen 206 Carbon Dioxide 208 Nitrogen Oxides 208 Other Gases 208 Ammonia 209 Ethanol 209 Other Volatile Organic Compounds (VOC)s 210 Sensors for Humidity, Water Fractions, Hydrogen Peroxide, and Hydrazine 211 Humidity 211 Water Fractions in Organic Solvents 212 Hydrogen Peroxide 213 Sensors for pH Values, Ions, and Salinity 213 pH Values 213 Ions 214 Salinity and Ionic Strength 215 Sensors for Organic Species 216 Glucose Sensing 216 Sucrose 217 Oils 217 Other Organics 217 Biosensors 218 Nucleic Acid-Based Biosensors (DNAand Aptamer-Based) 218 Immunosensors 218 Enzymatic Biosensors 219 Other Biosensors 220 New Schemes and Materials 220 New Sensing Schemes 220 Molecularly Imprinted Polymer (MIP)-Based Sensors 221 Photonic Crystals 223 Author Information 223 Corresponding Authors 223 Notes 223 Biographies 223 Acknowledgments 223 References 223

Fiber-Optic Chemical Sensors and Biosensors (2013-2015).

Recent development of fiber-optic chemical sensors and biosensors: Mechanisms, materials, micro/nano-fabrications and applications

High-quality optical fibers can be produced now at a low cost and large quantity, and this has further promoted the development of fiber optic (chemical) sensors. After over 30 years of innovation, fiber optic sensing technology has become mature because of acceptable costs, compact instrumentation, high accuracy and the capability of performing measurements at inaccessible sites, over large distances, in strong (electro)magnetic fields and in harsh environment. The technology is still proceeding quickly in terms of innovation, and respective applications have been found in highly diversified fields. This review covers work published in the time period between October 2015 and October 2019. It is written in continuation of previous reviews.

Fiber-Optic Chemical Sensors and Biosensors (2015-2019).

In this work, we report the influence of oxygen vacancy related defects on the magnetism and gas sensing of TiO2 nanoparticles doped with different concentrations of cobalt (1, 3 and 5 at.%) prepared by sol–gel method at room temperature. XRD and Raman studies confirmed the formation of anatase tetragonal TiO2 nanoparticles and the studies also clearly supported that the Co atoms go into the Ti site in TiO2. The obtained nanoparticles of size 6–20 nm was identified by HRTEM. The distribution of particles and the elemental composition present in the sample were determined by SEM with EDS. UV–Vis absorption spectra showed that the incorporation of Co into the TiO2 lattice leads to redshift in the optical response as well as lowering of band gap energy. The defect oriented emissions were seen from photoluminescence study. Functional groups of the samples were identified by FTIR. The magnetic result demonstrated that the prepared Co-doped TiO2 nanoparticles exhibit room temperature ferromagnetism due to oxygen vacancy related defects on the surface of the sample. The ethanol sensing studies conducted at room temperature showed that Co-doped (5 at.%) TiO2 with higher oxygen vacancies is more sensitive to ethanol compared with pure TiO2, and the results reveal that oxygen vacancies play a significant role in both magnetism and sensing properties.

Oxygen vacancies induced room temperature ferromagnetism and gas sensing properties of Co-doped TiO 2 nanoparticles

Undoped and Mn-doped (1 at.%) Co3O4 were synthesized by simple precipitation method. The morphology of the material was investigated by scanning electron microscopy which confirmed the formation of nanorods (NRs). The NRs were formed from interconnected nanoparticles. X-ray diffraction analysis confirmed the presence and crystalline nature of the Co3O4 NRs. FTIR studies revealed that the defects are present in undoped Co3O4. Raman spectra confirmed the presence of five Raman active modes and the doping of Mn shifts the Raman peaks to higher wave number. Spectral responses of clad modified fiber optic gas sensor are studied for various concentrations (0–500 ppm) of ethanol at room temperature, where the clad portion is replaced with undoped and Mn-doped Co3O4 and used as gas sensing material. Since the refractive index of the samples calculated from UV-Vis absorption spectra is greater than that of core, the fiber optic sensor works under leaky mode condition. The sensor showed linear variation in the spectral peak intensity with ethanol concentration. Undoped Co3O4 exhibits higher sensitivity compared to Mn-doped Co3O4 due to defects present in the undoped sample which offers more reactive sites for ethanol adsorption.

Undoped and Mn-doped Co3O4 nanorods for ethanol sensing

This paper describes a simple, low-temperature and cost effective chemical precipitation method in aqueous media to synthesis uniformly distributed zinc oxide (ZnO) microstructures for the fabrication of dye-sensitized solar cells (DSSCs). The size and morphology of the ZnO microstructures are systematically controlled by adjusting the concentration of the precursors, zinc acetate dihydrate and ammonium hydroxide. X-ray diffraction (XRD) and scanning electron microscopy are used for the structural characterizations and photoluminescence and Fourier transform infrared spectroscopy are used to characterize the optical properties of the ZnO, respectively. The results reveal that ZnO crystallites exhibit hexagonal wurtzite structure with preferential orientation along c-axis. The effect of ammonia concentration on the crystallinity, morphology and optical properties of ZnO microstructures and the concomitant effect on the efficiency of DSSCs is also quantified. The peanut-shaped ZnO microstructure, which was found to increase DSSCs performance over other microstructure, is studied in detail in order to develop a formation mechanism. A sandwich type eosin yellow sensitized solar cell is prepared using peanut-shaped ZnO microstructures, which showed an efficiency of 0.37%. Ammonia was found to play a crucial role in the evolution of ZnO morphologies. These results are promising and provide a path towards low-cost high-performance DSSCs based on peanut-shaped ZnO microstructures and produced with only relatively simple wet chemistry synthesis.

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Peanut shaped ZnO microstructures: controlled synthesis and nucleation growth toward low-cost dye sensitized solar cells

Thin films of cupric telluride (CuTe) of thickness from 50 to 200 nm have been prepared using a thermal evaporation technique onto well-cleaned glass substrates kept at 300 K under a vacuum better than 2 × 10–5 mbar. After annealing the deposited films at 375 K for an hour, the resistivity of these films and that of bulk CuTe were measured using the four-probe technique as a function of temperature from 80 to 310 K. It is found that the bulk sample of CuTe exhibits metallic behaviour and the thin films show semiconducting be- haviour. Activation energies of the films have been evaluated and are in the range 0.05 and 0.15 eV. The metallic behaviour of bulk CuTe down to 4 K indicates that the variable range hopping mechanism is absent in CuTe at least down to liquid helium temperature. X-ray diffraction analysis confirmed the composition and polycrystalline nature of the CuTe films. The grain size of CuTe thin films, estimated from atomic force microscopy measurements, is around 40 nm. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Low-temperature electrical resistivity of cupric telluride (CuTe) thin films

Semiconducting Ag2SeTe thin films were prepared with different thicknesses onto glass substrates at room temperature using thermal evaporation technique. The structural properties were determined as a function of thickness by XRD exhibiting no preferential orientation along any plane, however the films are found to have peaks corresponding to mixed phase. The XRD studies were used to calculate the crystallite size and microstrain of the Ag2SeTe films. The calculated microstructure parameters reveal that the crystallite size increases and micro strain decreases with increasing film thickness. The refractive index, dielectric constants and thereby the optical bandgap of the films were calculated from transmittance spectral data recorded in the range 400–1200 nm by UV–VIS-Spectrometer. The direct optical bandgap of the Ag2SeTe thin films deposited on glass substrates with different thicknesses 50–230 nm were found to be in the range 1.48–1.59 eV. The carrier density value is estimated to be around 9.8 × 1021 cm−1 for the film thickness of 150 nm. The compositions estimated from the optical band gap studies reveal a value of 0.75 for Tellurium concentration. These structural and optical parameters are found to be very sensitive to the thin film thickness.

N. Soundararajan

Papers

Oxygen vacancies induced room temperature ferromagnetism and gas sensing properties of Co-doped TiO 2 nanoparticles

Undoped and Mn-doped Co3O4 nanorods for ethanol sensing

Peanut shaped ZnO microstructures: controlled synthesis and nucleation growth toward low-cost dye sensitized solar cells

Low-temperature electrical resistivity of cupric telluride (CuTe) thin films

Structural and optical properties of Ag2SeTe nano thin films prepared by thermal evaporation