Fanli Meng

Metal Oxide Nanostructures and Their Gas Sensing Properties: A Review

Metal oxide gas sensors are predominant solid-state gas detecting devices for domestic, commercial and industrial applications, which have many advantages such as low cost, easy production, and compact size. However, the performance of such sensors is significantly influenced by the morphology and structure of sensing materials, resulting in a great obstacle for gas sensors based on bulk materials or dense films to achieve highly-sensitive properties. Lots of metal oxide nanostructures have been developed to improve the gas sensing properties such as sensitivity, selectivity, response speed, and so on. Here, we provide a brief overview of metal oxide nanostructures and their gas sensing properties from the aspects of particle size, morphology and doping. When the particle size of metal oxide is close to or less than double thickness of the space-charge layer, the sensitivity of the sensor will increase remarkably, which would be called “small size effect”, yet small size of metal oxide nanoparticles will be compactly sintered together during the film coating process which is disadvantage for gas diffusion in them. In view of those reasons, nanostructures with many kinds of shapes such as porous nanotubes, porous nanospheres and so on have been investigated, that not only possessed large surface area and relatively mass reactive sites, but also formed relatively loose film structures which is an advantage for gas diffusion. Besides, doping is also an effective method to decrease particle size and improve gas sensing properties. Therefore, the gas sensing properties of metal oxide nanostructures assembled by nanoparticles are reviewed in this article. The effect of doping is also summarized and finally the perspectives of metal oxide gas sensor are given.

UV irradiation synthesis of an Au–graphene nanocomposite with enhanced electrochemical sensing properties

A facile and environmentally friendly strategy using UV irradiation has been successfully developed for the preparation of an Au–reduced graphene oxide (Au–RGO) nanocomposite. The as-prepared nanocomposite was analysed and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, UV-Vis absorption spectroscopy and X-ray photoelectron spectroscopy (XPS). The results demonstrated that the simultaneous reduction of graphene oxide and formation of Au nanoparticles were achieved. The obtained Au nanoparticles with an average diameter of 25.7 nm were uniformly dispersed on graphene sheets. The introduction of Au nanoparticles has efficiently maximized the electroactive surface area of catalysts and the conductivity of the Au–RGO nanocomposite. The graphene sheets not only provided the nucleation sites but also prevented the Au nanoparticles from agglomerating. Moreover, in order to illuminate the advantages of the Au–RGO nanocomposite, its electrochemical sensing performance toward TNT as an example has been further investigated by cyclic voltammetry (CV) and linear scan voltammetry (LSV). The results confirmed that the Au–RGO nanocomposite exhibited much better electrocatalytic activity toward TNT than RGO and holds great promise for developing as an electrochemical sensor.

Parts per billion-level detection of benzene using SnO2/graphene nanocomposite composed of sub-6 nm SnO2 nanoparticles

In the present work, the SnO(2)/graphene nanocomposite composed of 4-5 nm SnO(2) nanoparticles was synthesized using a simple wet chemical method for ppb-level detection of benzene. The formation mechanism of the nanocomposite was investigated systematically by means of simultaneous thermogravimetry analysis, X-ray diffraction, and X-ray photoelectron spectroscopy cooperated with transmission electron microscopy observations. The SnO(2)/graphene nanocomposite showed a very attractive improved sensitivity to toxic volatile organic compounds, especially to benzene, compared to a traditional SnO(2). The responses of the nanocomposite to benzene were a little higher than those to ethanol and the detection limit reached 5 ppb to benzene which is, to our best knowledge, far lower than those reported previously.

Preparation of a leaf-like CdS micro-/nanostructure and its enhanced gas-sensing properties for detecting volatile organic compounds

A novel leaf-like CdS micro-/nanostructure was prepared via a hydrothermal method using dimethyl sulfoxide as the growth template. The as-synthesized hierarchical micro-/nanostructures were characterized by field emission scanning electronic microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectrometry (XPS). The elemental mapping and the line scans were also performed in the high-angle annular dark-field (HAADF) mode on the same TEM. The growth mechanism was demonstrated from the aspects of time-dependent growth processes and different influencing factors including temperatures and templates. In gas-sensing measurements, typical volatile organic compounds at a series of concentrations were employed as the target analytes, including ether, methanol, acetone and isopropanol. We found that the gas sensor based on the special leaf-like CdS micro-/nanostructures exhibited a fascinating performance including high response, short response/recovery times and good recognition ability towards different analytes. The detecting limits towards ether, methanol, acetone and isopropanol could be lower than 25, 50, 50, and 10 ppb, respectively. By comparing with a conventional spherical structure, the leaf-like micro-/nanostructure was revealed to possess an improved ability for diffusion and adsorption/desorption. The structure could be generally significant for developing some other novel materials which possess a morphology inspired from nature for specific applications, such as sensors, energy transfer and storage devices, and catalysts.

Carboxylation multi-walled carbon nanotubes modified with LiClO4 for water vapour detection

In this paper, the humidity sensing properties of multi-walled carbon nanotubes modified with LiClO4 are investigated. The new sensing material is characterized by FESEM, FTIR, Raman, ICP and XPS. For comparison, the humidity sensing characteristics of the four conventional materials MnWO4 (hubnerite), BaTiO3 (perovskite), NiWO4 (huberite), and ZnCr2O4 (spinel) are also discussed. The selectivity of the new humidity sensing material is studied. Finally, the response to water vapour of the new sensing material is discussed by using a dynamic testing method.

Performance of novel hydroxyapatite nanowires in treatment of fluoride contaminated water

Novel ultralong hydroxyapatite (HAP) nanowires were successfully prepared for fluoride removal for the first time. The fluoride adsorption on the HAP nanowires was studied on a batch mode. The results revealed that the adsorption data could be well described by the Freundlich model, and the adsorption kinetic followed the pseudo-second-order model. The maximum of adsorption capacity was 40.65 mg/g at pH 7.0 when the fluoride concentration is 200mg/L. The thermodynamic parameters suggested that the adsorption of fluoride was a spontaneous endothermic process. The FT-IR, XPS and Zeta potential analysis revealed that both anion exchange and electrostatic interactions were involved in the adsorption of fluoride. Furthermore, the HAP nanowires were made into HAP membrane through a simple process of suction filtration. Membrane filtration experiments revealed that the fluoride removal capabilities depended on the membrane thickness, flow rate and initial concentration of fluoride. The as-prepared membrane could remove fluoride efficiently through continues filtration. The filtered water amount could reach 350, 192, and 64 L/m(2) when the fluoride concentrations were 4, 5 and 8 ppm, respectively, using the HAP membrane with only 150 μm thickness. The as-synthesized ultralong HAP nanowires were thus demonstrated to be very effective and biocompatible adsorbents for fluoride removal from contaminated water.

Nanomaterial-Assisted Signal Enhancement of Hybridization for DNA Biosensors: A Review

Detection of DNA sequences has received broad attention due to its potential applications in a variety of fields. As sensitivity of DNA biosensors is determined by signal variation of hybridization events, the signal enhancement is of great significance for improving the sensitivity in DNA detection, which still remains a great challenge. Nanomaterials, which possess some unique chemical and physical properties caused by nanoscale effects, provide a new opportunity for developing novel nanomaterial-based signal-enhancers for DNA biosensors. In this review, recent progress concerning this field, including some newly-developed signal enhancement approaches using quantum-dots, carbon nanotubes and their composites reported by our group and other researchers are comprehensively summarized. Reports on signal enhancement of DNA biosensors by non-nanomaterials, such as enzymes and polymer reagents, are also reviewed for comparison. Furthermore, the prospects for developing DNA biosensors using nanomaterials as signal-enhancers in future are also indicated.

Wide pH range for fluoride removal from water by MHS-MgO/MgCO3 adsorbent: Kinetic, thermodynamic and mechanism studies

A novel environment friendly adsorbent, micro-nano hierarchical structured flower-like MgO/MgCO3 (MHS-MgO/MgCO3), was developed for fluoride removal from water. The adsorbent was characterized and its defluoridation properties were investigated. Adsorption kinetics fitted well the pseudo-second-order model. Kinetic data revealed that the fluoride adsorption was rapid, more than 83-90% of fluoride could be removed within 30 min, and the adsorption equilibrium was achieved in the following 4 h. The fluoride adsorption isotherm was well described by Freundlich model. The maximum adsorption capacity was about 300 mg/g at pH=7. Moreover, this adsorbent possessed a very wide available pH range of 5-11, and the fluoride removal efficiencies even reached up to 86.2%, 83.2% and 76.5% at pH=11 for initial fluoride concentrations of 10, 20 and 30 mg/L, respectively. The effects of co-existing anions indicated that the anions had less effect on adsorption of fluoride except phosphate. In addition, the adsorption mechanism analysis revealed that the wide available pH range toward fluoride was mainly resulted from the exchange of the carbonate and hydroxyl groups on the surface of the MHS-MgO/MgCO3 with fluoride anions.

Novel hybridized SWCNT–PCD: synthesis and host–guest inclusion for electrical sensing recognition of persistent organic pollutants

Fabrication of a hybridized single-walled carbon nanotube (SWCNT) device based on novel sensing material mono-6-deoxy-6-(p-aminophenylamino)-β-cyclodextrin (PCD) via a facile approach has been reported for the first time; moreover, the hybridized material PCD-decorated SWCNTs (SWCNT–PCD) can be characterized by ultraviolet–visible–near infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis, field-emission scanning electron microscopy and high resolution transmission electron microscopy. The PCD-decorated SWCNT devices have been employed to detect Persistent Organic Pollutants (POPs) on the basis of formation of an inclusion complex with guest molecules between SWCNT–PCD and POPs, such as 2,4,5-trichlorobiphenyl (TCB), etc. The significant variation of the electrical conductance of the SWCNT–PCD hybrid corresponding to the presence of several POPs with different adsorption efficiencies on the surface indicates that the hybrid is highly sensitive to some specific POP molecules, implying that hybridized SWCNT–PCD has great potential in application as an environmental monitor. In order to make a quantitative assessment of the inclusion complexation behavior of PCD with these POP molecules, microcalorimetric titration experiments and discussions have been carried out.

Performance of a novelly-defined zirconium metal-organic frameworks adsorption membrane in fluoride removal.

A novelly-defined adsorption membrane for rapid removal of fluoride from drinking water was prepared. Both zirconium metal-organic frameworks (Zr-MOFs) adsorbent and membrane with large specific surface area of 740.28m2/g were used for fluoride removal for the first time. For adsorption technique, fluoride adsorption on Zr-MOFs was studied on a batch mode. The adsorption data could be well described by Langmuir isotherm model while the adsorption kinetic followed pseudo-second-order model. The maximum of adsorption capacity was 102.40mg/g at pH 7.0 when the initial fluoride concentration was 200mg/L. The FT-IR and XPS analyses of Zr-MOFs revealed that both surface hydroxyl groups and Zr(IV) active sites played important roles in fluoride adsorption process. The as-prepared Zr-MOFs adsorbent was suitable for practical treatment of drinking water and regeneration by sodium hydroxide solution (3wt%). For membrane experiments, Zr-MOFs membrane supported on Alumina substrate could remove fluoride efficiently through dynamic filtration. The fluoride removal capability of Zr-MOFs membrane depended on flow rate and initial concentration of fluoride. The fluoride removal abilities of Zr-MOFs membrane with 20μm thickness could reach 5510, 5173, and 4664L/m2 when fluoride concentrations were 5, 8 and 10mg/L, respectively. This study indicated that Zr-MOFs membrane could be developed into a very viable technology for highly effective removal of fluoride from drinking water.