Advances in Noble Metal-Decorated Metal Oxide Nanomaterials for Chemiresistive Gas Sensors:Overview

Highly sensitive gas sensors with remarkably low detection limits are attractive for diverse practical application fields including real-time environmental monitoring,exhaled breath diagnosis,and food freshness analysis.Among various chemiresistive sensing materials,noble metal-decorated semiconducting metal oxides(SMOs)have currently aroused extensive attention by virtue of the unique electronic and catalytic properties of noble metals.This review highlights the research progress on the designs and applications of different noble metal-decorated SMOs with diverse nanostructures(e.g.,nanoparticles,nanowires,nanorods,nanosheets,nanoflowers,and microspheres)for high-performance gas sensors with higher response,faster response/recovery speed,lower operating temperature,and ultra-low detection limits.The key topics include Pt,Pd,Au,other noble metals(e.g.,Ag,Ru,and Rh.),and bimetals-decorated SMOs containing ZnO,SnO_(2),WO_(3),other SMOs(e.g.,In_(2)O_(3),Fe_(2)O_(3),and CuO),and heterostructured SMOs.In addition to conventional devices,the innovative applications like photo-assisted room temperature gas sensors and mechanically flexible smart wearable devices are also discussed.Moreover,the relevant mechanisms for the sensing performance improvement caused by noble metal decoration,including the electronic sensitization effect and the chemical sensitization effect,have also been summarized in detail.Finally,major challenges and future perspectives towards noble metal-decorated SMOs-based chemiresistive gas sensors are proposed.

Ferroelectric perovskite-type films with robust in-plane polarization toward efficient room-temperature chemiresistive sensing

Ferroelectric materials have become key components for versatile device applications,and their thin films are highly desirable for integrating the miniaturized devices.Despite substantial endeavors,it is still challenging to achieve effective chemiresistive sensing in the ferroelectric films.Here,for the first time,we have exploited ferroelectric thin films of 2D hybrid perovskite BA_(2)EA_(2)Pb_(3)I_(10)(1),to fabricate the high-performance chemiresistor gas sensors.The spin-coated films of 1 exhibit high orientation and good crystallinity,thus preserving robust in-plane spontaneous polarization(P_(s)~2.0μC/cm^(2))and low electric coercivity.Notably,such ferroelectric filmbased sensors after electric poling enable the dramatic room-temperature sensing responses to NO_(2) gas,including high sensitivity(0.05 ppm^(-1)),extremely low detection limit(1 ppm)and fast responding rate(~6 s).Besides,the chemiresistive responses are remarkably enhanced by threefold(up to 320%)through electric poling.It is proposed that this behavior closely involves with strong in-plane ferroelectric polarization of 1 that generates a built-in electric field inhibiting the recombination of charge carriers.As far as we know,this ferroelectric-based film chemiresisor is one of the best room-temperature sensors for NO_(2) gas among all the existing candidate materials.These findings highlight great potential of ferroelectrics toward effective chemiresistive performances,and also establish a bright direction to explore their future device applications.

A new 3D cupric coordination polymer as chemiresistor humidity sensor:narrow hysteresis,high sensitivity,fast response and recovery

A new three-dimensional coordination polymer composed of Cu^(2+) centres and semiquinoid linkers(dhbq^(2-)) was synthesized which was composed by two independent,enantiomeric,interpenetrated[Cu_2(dhbq)_3]^(2-) networks with(10,3)-a topology.The compound has good water stability and typical behaviors of semiconductor,whose conductivity increases along with raising temperature.The chemiresistive humidity sensor made from this material shows good properties including linear sensitivity,high response,fast response and recovery,and particularly narrow hysteresis during humidity adsorption and

Orientation controlled preparation of nanoporous carbon nitride fibers and related composite for gas sensing under ambient conditions

Creating pores in suprastructures of two-dimensional （2D） materials while controlling the orientation of the 2D building blocks is important in achieving large specific surface areas and tuning the anisotropic properties of the obtained functional hierarchical structures. In this contribution, we report that arranging graphitic carbon nitride （g-C3N4） nanosheets into one-dimensional （1D） architectures with controlled orientation has been achieved by using 1D oriented melem hydrate fibers as the synthetic precursor via a polycondensation process, during which the removal of water molecules and release of ammonia gas led to the creation of pores without destroying the 1D morphology of the oriented structures. The resulting porous g-C3N4 fibers with both meso- and micro-sized pores and largely exposed edges exhibited good sensing sensitivity and selectivity towards NO2. The sensing performance was further improved by hybridization of the porous fibers with Au nanoparticles （Au NPs）, leading to a detection limit of 60 ppb under ambient conditions. Our results suggest that the highly porous g-C3N4 fibers and the related hybrid structures with largely exposed graphitic layer edges are excellent sensing platforms and may also show promise in other electronic and electrochemical applications.

Selective and self-validating breath-level detection of hydrogen sulfide in humid air by gold nanoparticle-functionalized nanotube arrays

We demonstrate the selective detection of hydrogen sulfide at breath concentration levels under humid airflow,using a self-validating 64-channel sensor array based on semiconducting single-walled carbon nanotubes(sc-SWCNTs).The reproducible sensor fabrication process is based on a multiplexed and controlled dielectrophoretic deposition of sc-SWCNTs.The sensing area is functionalized with gold nanoparticles to address the detection at room temperature by exploiting the affinity between gold and sulfur atoms of the gas.Sensing devices functionalized with an optimized distribution of nanoparticles show a sensitivity of 0.122%/part per billion(ppb)and a calculated limit of detection(LOD)of 3 ppb.Beyond the self-validation,our sensors show increased stability and higher response levels compared to some commercially available electrochemical sensors.The cross-sensitivity to breath gases NH3 and NO is addressed demonstrating the high selectivity to H2S.Finally,mathematical models of sensors’electrical characteristics and sensing responses are developed to enhance the differentiation capabilities of the platform to be used in breath analysis applications.

Atomically Thin 2D TiO_(2) Nanosheets with Ligand Modified Surface for Ultra-sensitive Humidity Sensor

As a kind of two-dimensional(2D)nanostructured materials,metal oxide nanosheets(MONS)are attractive and promising humidity sensing materials due to their considerable surface area,good charge carrier transportation,and designable surface functional groups properties.Nevertheless,the ultra-thin MONS modified with active functional groups for humidity sensing are still rare.As a proof of concept,the atomically thin TiO_(2)nanosheets with high surface area and electron-donating amino groups are prepared by a structure-maintained post-ligand modification strategy.The fabricated TiO_(2)-based sensors demonstrate superior humidity sensing performance with high response,short response time,narrow hysteresis,and ultra-low theoretical limit of detection of about 15 ppm.Additionally,the possible mechanism is proposed from the AC complex impedance measurements and DC instantaneous reverse polarity experiments.This work provides a possible path for developing the high-performance 2D nanostructured metal oxides-based humidity materials through the surface chemical method.

High-humidity Sensor of a New Trinuclear Ti_(3)-Oxo Cluster

Crystalline polyoxo-titanium clusters(PTCs),as a molecular model of TiO_(2)nanomaterials,have attracted unprecedented attention due to their designable structure,tunable band gap,catalysis,and photochromic properties.A new trinuclear Ti3-oxo cluster,[Ti3(μ2-O)(μ3-O)(abz)_(6)(OiPr)_(2)]·CH_(3)CN·H_(2)O(Ti3),was synthesized by solvothermal method with a yield of 60%by using 4-aminobenzoic acid as ligand.Single-crystal X-ray diffraction shows that it has a[Ti3(μ2-O)(μ3-O)(abz)_(6)(OiPr)_(2)]trinuclear cluster structure.Ti3 crystallizes in monoclinic space group P21/c with a=11.091(1),b=22.837(2),c=22.754(1)Å,β=90.580(6)°,V=5763.0(6)Å3,Z=4,Dc=1.345 g·cm-3,F(000)=2412,μ=2.743 mm^(−1),R=0.0796,and wR=0.2260(I>2σ(I)).Ti3 shows typical semiconductive behavior determined by temperature-dependent conductivity test.The chemiresistive humidity sensor fabricated by Ti3 showed good performance,including high response(four orders of magnitude current change from 0 to 100%RH)and fast response time(160 s)and recovery time(26 s).