High-performance gas sensors prepared using p-type oxide semiconductors such as NiO, CuO, Cr2O3, Co3O4, and Mn3O4 were reviewed. The ionized adsorption of oxygen on p-type oxide semiconductors leads to the formation of hole-accumulation layers (HALs), and conduction occurs mainly along the near-surface HAL. Thus, the chemoresistive variations of undoped p-type oxide semiconductors are lower than those induced at the electron-depletion layers of n-type oxide semiconductors. However, highly sensitive and selective p-type oxide-semiconductor-based gas sensors can be designed either by controlling the carrier concentration through aliovalent doping or by promoting the sensing reaction of a specific gas through doping/loading the sensor material with oxide or noble metal catalysts. The junction between p- and n-type oxide semiconductors fabricated with different contact configurations can provide new strategies for designing gas sensors. p-Type oxide semiconductors with distinctive surface reactivity and oxygen adsorption are also advantageous for enhancing gas selectivity, decreasing the humidity dependence of sensor signals to negligible levels, and improving recovery speed. Accordingly, p-type oxide semiconductors are excellent materials not only for fabricating highly sensitive and selective gas sensors but also valuable additives that provide new functionality in gas sensors, which will enable the development of high-performance gas sensors.

Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview

Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: A review

Sorption is an effective, reliable, and environmentally friendly treatment process for the removal of phosphorus from wastewater sources which otherwise can cause eutrophication of receiving waters. Phosphorus in wastewater, if economically recovered, can partly overcome the future scarcity of phosphorus resulting from exhaustion of natural phosphate rock reserves. The authors present a comprehensive and critical review of the literature on the effectiveness of a number of sorbents, especially some novel ones that have recently emerged, in removing and recovering phosphate. Mechanisms and thermodynamics of sorption, as well as regeneration of sorbents for reuse using acids, bases, and salts, are critically examined.

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Removal and Recovery of Phosphate From Water Using Sorption

Synthesis of hierarchical Ni(OH)(2) and NiO nanosheets and their adsorption kinetics and isotherms to Congo red in water.

A simple, mild, and effective self-templated approach has been developed to directly convert solid SiO2 microspheres into hollow structures. The reaction involves initial partial dissolution of silica cores in a NaBH4 solution and subsequent shell formation due to the redeposition of the silicate species back onto the colloid surfaces. The increasing concentration of NaBO2 as the result of the slow decomposition of NaBH4 in water is found to be responsible for the regrowth of the silica shell. This method allows the production of hollow silica spheres with sizes ranging from ∼70 nanometers to several micrometers, largely determined by the size of the starting silica colloids. The solid-to-hollow transformation mechanism is investigated in detail by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier Transform Infrared (FTIR) spectrometry, X-ray absorption spectroscopy (XAS), N2 adsorption−desorption, and X-ray diffraction (XRD). We also study the reaction conditions that al...

A Self-Templated Route to Hollow Silica Microspheres

A two-sensor array was fabricated to improve gas selectivity, an intrinsic disadvantage of mono-metal oxide semiconductor gas sensor. The sensors comprising the array were made of Ga-doped ZnO (GZO) annealed at different temperatures, and the selectivity over relative humidity (RH 0-70 +/- 10%) was greatly improved in formaldehyde gas sensing. The physical and mathematical principle for high selectivity is based on the different responses of the two sensors to analyte (formaldehyde) and to interferent (RH), that is. one sensor has larger response to formaldehyde than the other but the same response to RH. But under the same fabrication process for ZnO without Ga dopant, one sensor was insensitive either to formaldehyde or to RH due to the reaction of ZnO with the SiO(2) substrate. Consequently, such a sensor array cannot be fabricated using ZnO sensors without Ga dopant. Possible mechanism concerning interactions between GZO and the two gases (formaldehyde and water vapor) was discussed. (C) 2009 Elsevier B.V. All rights reserved.

Improving humidity selectivity in formaldehyde gas sensing by a two-sensor array made of Ga-doped ZnO

Gas sensing property of ZnO nanorods prepared by plasma-enhanced chemical vapor deposition (CVD) method is studied using formaldehyde as the probe gas, and the intrinsic defects are investigated by photoluminescence (PL). The results show that high ratio of visible to ultra-violet luminescence cannot account for high gas response. The PL spectra are Gaussian decomposed to subpeaks according to their origination, which are separated into donor-(DL) and acceptor-related (AL) ones. A conclusion is derived that where the content of DL is high and that of AL is low, the gas response is high. This conclusion is further confirmed by tuning the PL spectra and gas sensing property through annealing in different atmospheres. (C) 2009 Elsevier B.V. All rights reserved.

Photoluminescence investigation on the gas sensing property of ZnO nanorods prepared by plasma-enhanced CVD method

Fe-, Ti-, Sn-doped (1.8, 7.4 and 3.4 mol% respectively) and pure ZnO are prepared by hydrothermal method using 10 mol% precursors, whose gas sensing property is studied using formaldehyde as the probe. The results show that the maximum response of pure ZnO to 205 ppm formaldehyde is similar to 43 (at relative humidity 70 +/- 10%) at 400 degrees C. But the gas response maxima shift to 300 degrees C for Fe-ZnO (similar to 52-205 ppm) and Sn-ZnO (similar to 140-205 ppm), and to 200 degrees C for Ti-ZnO (similar to 26-205 ppm). Beyond the maxima, the response of Fe-ZnO is lower than that of pure ZnO, while that of Sn-ZnO is always higher. The morphology, crystal structure, vibration modes, bandgap and crystal defects are studied to investigate the different doping effects on gas sensing property of ZnO. The results show that the secondary phase identified by X-ray diffraction and Raman spectra, and the crystal defects detected by photoluminescence might account for the different sensing behaviors. (C) 2010 Elsevier B.V. All rights reserved.

Evaluating the doping effect of Fe, Ti and Sn on gas sensing property of ZnO

Raspberry-like hollow SiO2 spheres were synthesized successfully through a dual latex−surfactant templating route. In this approach, cationic cetyltrimethylammonium (CTA+) micelles and microscale polystyrene (PS) latex were employed as structure-directing templates for constructing the inorganic hollow SiO2 spheres with hierarchical structures. The final product has been analyzed scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), single-angle X-ray diffraction (SAXRD), Brunauer−Emmett−Teller (BET) measurements, and Hg porosimetry measurements. The as-obtained hollow SiO2 spheres present a novel raspberry-like protruding surface morphology and possess hierarchical porous shells. Moreover, the factors that could impact the surface morphology and hierarchical porous structure are discussed, and the corresponding mechanism was proposed accordingly. This dual templating route is expected to be an effective means to pattern multiscale structures in the shell of the SiO...

Yajun Tian

Papers

Improving humidity selectivity in formaldehyde gas sensing by a two-sensor array made of Ga-doped ZnO

Photoluminescence investigation on the gas sensing property of ZnO nanorods prepared by plasma-enhanced CVD method

Evaluating the doping effect of Fe, Ti and Sn on gas sensing property of ZnO

Raspberry-like silica hollow spheres: Hierarchical structures by dual latex-surfactant templating route

Preparation of a new sorbent with hydrated lime and blast furnace slag for phosphorus removal from aqueous solution