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Though the chemical origin of a metal oxide gas sensor is widely accepted to be the surface reaction of detectants with ionsorbed oxygen, how the sensing material transduces the chemical reaction into an electrical signal (i.e., resistance change) is still not well-recognized. Herein, the single ZnO NW is used as a model to investigate the relationship between the microstructure and sensing performance. It is found that the acetone responses arrive at the maximum at the NW diameter ( D) of ∼110 nm at the D range of 80 to 400 nm, which is temperature independent in the temperature region of 200 °C-375 °C. The electrical properties of the single NW field effect transistors illustrate that the electron mobility decreases but electron concentration increases with the D ranging from ∼60 nm to ∼150 nm, inferring the good crystal quality of thinner ZnO NWs and the abundant crystal defects in thicker NWs. Subsequently, the surface charge layer ( L) is calculated to be a constant of 43.6 ± 3.7 nm at this D range, which cannot be explained by the conventional D- L model in which the gas-sensing maximum appears when D approximates 2 L. Furthermore, the crystal defects in the single ZnO NW are probed by employing the microphotoluminescence technique. The mechanism is proposed to be the compromise of the two kinds of crystal defects in ZnO (i.e., more donors and fewer acceptors favor the gas-sensing performance), which is again verified by the gas sensors based on the NW contacts.

Core–shell metal@ZnO nanoparticles including Au@ZnO, Pd@ZnO and Pt@ZnO were synthesized and utilized for sensing low-concentration benzene. Various techniques were used to characterize the compositional properties of the typical core@shell structure and analyze the relation between the sensing properties and the metal–ZnO hetero-interfaces. When applied as gas-sensing materials, all three core–shell metal@ZnO nanoparticles showed better sensing performance than pure ZnO nanoparticles towards low concentration benzene. In particular, the gas-sensing response of the Pt@ZnO core–shell nanoparticles was 7 times higher than that of pure ZnO towards 0.1 ppm benzene and 63 times higher towards 5 ppm benzene, which was more sensitive than most gas-sensing materials in previous literature. Furthermore, the Pt@ZnO core–shell nanoparticles presented an ultra-low detection limit of no less than 10 ppb, which was lower than those of most gas-sensing materials in previous literature. Besides, the Pt@ZnO core–shell nanoparticles showed high selectivity and long-term response stability with a response value of 2.7 ± 1.6% towards 1 ppm benzene after operating for a month. The enhanced gas-sensing performances of the metal@ZnO core–shell nanoparticles are well correlated to the work function differences between the contacted metal and ZnO within the metal–ZnO hetero-interfaces, which produce high Schottky energy barriers and modulate the electron transfer. [ABSTRACT FROM AUTHOR]

Ga‐doped ZnO nanoparticles are successfully prepared by a facile coprecipitation method and then are activated by H2 annealing. The as‐prepared ZnO powders are characterized by X‐ray diffractometer (XRD), Brunauer–Emmett–Teller (BET), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), X‐ray photoelectron spectroscopy, Raman and photoluminescence spectroscopies. Gas sensing property using NO2 as probe shows that the 1 at.% Ga‐ZnO powders activated in H2 at 540 °C showed high NO2 response of ≈50/ppm, high selectivity toward typical interferences such as ethanol, acetone, toluene, NO, SO2, C2H6, and C2H4 and high stability as measured at 200 °C. On the contrary, the pure ZnO and the Ga‐ZnO without H2 activation show far lower response, which illustrates the effect of Ga dopant activation by H2 annealing aimed for the high performance gas sensing materials. [ABSTRACT FROM AUTHOR]

The hollow ZnO/ZnFe 2 O 4 microspheres with heterogeneous structure are synthesized by direct pyrolysis of metal-organic frameworks. The as-prepared ZnO/ZnFe 2 O 4 microspheres have well-defined spherical morphology with ∼1.5 μm in diameter and multiple porous shells constructed by interpenetrated ZnO and ZnFe 2 O 4 heterogeneous nanoparticles. Interestingly, the hollow ZnO/ZnFe 2 O 4 microspheres based gas sensors show interestingly temperature-dependent n-p-n type conductivity transition in detecting low-concentration VOC gases including ethanol, acetone, toluene and benzene. This interestingly n-p-n transition phenomenon is mainly ascribed to the trade-off of highly separated electron-hole pairs originated from the staggered type-II band alignment of in-shell ZnO-ZnFe 2 O 4 hetero-interfaces, which is modulated by thermally-dependent ionization reaction of surface-absorbed oxygen molecules and extra electron injection due to surface reaction of reductive VOCs during gas-sensing process. This work presents a facile route to construct hollow nanostructures with heterogeneous feature and provides a new insight into sensing mechanism, exhibiting the potential application of ZnO/ZnFe 2 O 4 microspheres in developing highly sensitive and selective gas-sensing materials in detecting low-concentration VOCs. [ABSTRACT FROM AUTHOR]

A facile and controllable one-step hydrothermal method to synthesize nanodisks and nanorods of zinc oxide with exposed different crystal facets of (0001), ( 1\bar 100) , ( 01\bar 10) , and (10\bar 10) has been realized. It was found that the average percentage of exposed (0001) crystal facets follows the order of nanodisks > nanorods-1 > nanorods-2 > nanorods-3, while the average percentage of exposed ( 1\bar 100) , (01\bar 10) , and ( 10\bar 10) crystal facets decreases monotonically. The sensing tests to NO2 indicate that the nanodisks exhibit higher sensitivity. It has also been validated that the (0001) crystal facet has the highest surface energy and the lowest binding energy, which is considered to be the key factor of high gas-sensing by the calculations of density functional theory. This finding will open a new window to explore the relationship between gas-sensing and exposed facets and will be useful for designing gas-sensing materials with specifically exposed surfaces. [ABSTRACT FROM PUBLISHER]

Abstract: One of the challenges in realizing metal oxide semiconductor gas sensors is to enhance the sensitivity of active materials in order to respond to the low concentration of detecting gases effectively and efficiently. In this report, transition metals such as Mn, Ni, Cu, and Co are used as dopants for the synthesis of highly formaldehyde-sensitive ZnO nanorods prepared by plasma enhanced chemical vapor deposition (PECVD) method. All the doped ZnO nanorods show improved formaldehyde-sensitivity as compared to undoped ZnO nanorods, and a gas sensitivity maximum of ∼25/ppm was obtained by using 10mol% CdO activated 1.0mol% Mn doped ZnO nanorods. Moreover, the ZnO nanorods have a higher sensitivity as compared to ZnO nanomaterials prepared by other methods such as precipitation and hydrothermal, which can be attributed to the abundant crystal defects induced by the dopants in a short crystallization process in this PECVD method. [Copyright &y& Elsevier]

ZnO and Sn-, Ga- and Mn-doped ZnO nanoparticles were prepared by a coprecipitation method, and characterized by scanning electron microscopy (SEM), energy dispersive spectra (EDS), X-ray diffraction (XRD) and Raman spectra. The gas sensing properties were studied using formaldehyde, relative humidity, NH, toluene and CO as the probes. The results show that all particles have wurtzite ZnO phase, though Sn-ZnO has a relatively smaller particle (and crystallite) size than the other three samples. Gas sensing property tests reveal that the temperature where the gas sensing maximum is gained ( T) is changed by different dopants: Sn-ZnO and Mn-ZnO have relatively lower T (∼100°C lower) compared with that of pure ZnO, while Ga-ZnO has the same T as pure ZnO except in CO sensing. Thermoluminescence (TL) spectra were used to investigate the mechanism of T change. The peak positions of Ga-ZnO and ZnO are the same at 300-350°C, while that of Sn-ZnO shifts to 250-300°C, which might contribute to the same T of Ga-ZnO and pure ZnO and relatively lower T of Sn-ZnO. In the case of Mn-ZnO, the luminescence emission is evidently limited by its black color. [ABSTRACT FROM AUTHOR]

Abstract: Sn-, Ni-, Fe- and Al-doped ZnO and pure ZnO are prepared by coprecipitation method, and characterized by scanning electron microscope (SEM), energy diffraction spectra (EDS) and X-ray diffraction (XRD). Their formaldehyde gas sensing properties are evaluated and the results show that 2.2mol% Sn dopant can increase the response of ZnO by more than 2 folds, while other dopants increase little response or even decrease response. Further, CdO is used to activate ZnO based formaldehyde sensing material. It is demonstrated that 10mol% CdO activated 2.2mol% Sn-doped ZnO has the highest formaldehyde gas response, with a linear sensitivity of ∼10/ppm at lowered work temperature of 200°C than 400°C of pure ZnO, and high selectivity over toluene, CO and NH3, as well as good stability tested in 1 month. [Copyright &y& Elsevier]

Abstract: Zinc oxide nanoparticles are prepared by calcining zinc hydrocarbonate precursors at 300–700°C (ZnO300–700), and corresponding gas sensing property are tested at 300°C by using formaldehyde as the probe. Although the nanoparticle sizes are found to gradually increase with calcination temperature, the sensor measurements reveal the size-independent behavior that ZnO500 and ZnO300 have the highest and lowest responses, respectively. Spectroscopic characterization further reveals nonstoichiometric compositions of ZnO nanoparticles: ZnO300 has the largest excess oxygen (oxygen interstitial, Oi), whereas ZnO500 has the largest excess zinc (oxygen vacancy, VO and/or zinc interstitial, Zni). Accordingly, a new sensing mechanism is proposed for ZnO nanoparticle sensors. Excess zinc favors chemisorption of oxygen onto the nanoparticle surface, leading to reacting with more formaldehyde molecules to get a high signal. On the contrary, excess oxygen inhibits free oxygen to be chemisorbed onto the nanoparticle surface, and thus decreases the gas response. Finally, this new sensing mechanism is verified by testing gas response of ZnO500 nanoparticles annealed at different atmospheres. [Copyright &y& Elsevier]

Abstract: Fe-, Ti-, Sn-doped (1.8, 7.4 and 3.4mol% respectively) and pure ZnO are prepared by hydrothermal method using 10mol% precursors, whose gas sensing property is studied using formaldehyde as the probe. The results show that the maximum response of pure ZnO to 205ppm formaldehyde is ∼43 (at relative humidity 70±10%) at 400°C. But the gas response maxima shift to 300°C for Fe–ZnO (∼52–205ppm) and Sn–ZnO (∼140–205ppm), and to 200°C for Ti–ZnO (∼26–205ppm). 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. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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 SiO2 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. [Copyright &y& Elsevier]

Abstract: The colloidal stabilization of nano-sized zinc oxide suspensions with maleic anhydride sodium salt copolymer is examined to understand the primary mechanism of dispersion. FTIR spectroscopic data provided evidence in support of hydrogen bonding and chemical interaction in the case of the MA-Na copolymer/ZnO system. The suspension stability was characterized by sedimentation tests and TEM method. The copolymer was found to stabilize the suspension of ZnO nanoparticles through electrosteric repulsion. Influences of factors such as pH and the copolymer concentration on the colloidal stability of aqueous suspension were investigated. The copolymer at high pH of the solution provided more effective degree of sustained particle dispersion. The adsorbed MA-Na copolymer provided a good stabilization under the saturated adsorption amount, while the excess copolymer flocculated the dispersed system. [Copyright &y& Elsevier]

Abstract: The thermal degradation behaviour of polyacrylate and its zinc oxide composites were investigated by differential scanning calorimetry (DSC), thermogravimetry (TG) and Fourier transform infrared spectroscopy (FTIR). Reaction mechanisms of polymer degradation with and without ZnO particles were obtained. Filler-free polyacrylate exhibited one DSC peak, indicating that the polymer was degraded with only one stage of weight loss. Polyacrylate/ZnO composites underwent two minor weight losses as well as the major weight loss. ZnO stabilized or destabilized the polymer molecules according to the temperature region. Two mechanisms were proposed for the change of thermal properties of the nanocomposite. An increase of the stabilization or destabilization effect of ZnO nanoparticles was found in the nanocomposite with a higher content of the inorganic phase. Compared to micron ZnO particles, ZnO nanoparticles exhibited a greater effect due to their high specific area giving more sites for interactions. In addition, kinetics of thermal degradation of filler-free polyacrylate and polyacrylate/ZnO composite were quantitatively obtained by means of non-isothermal weight loss data analysis. [Copyright &y& Elsevier]

Gas response control through structural and defect engineering in metal oxides are critical approaches to realize high performance sensors. Herein, we present the fabrication of ZnO thin films containing nanosized structures and high concentration of zinc interstitials for NO 2 and ethanol monitoring. The target materials were fabricated by aerosol assisted chemical vapour deposition (AACVD) method on ceramic and glass substrates, and were characterized by complementary analytical techniques. The obtained ZnO nanoparticulate and nanopyramidal thin films yielded an outstanding response of ~41 to 1 ppm NO 2 and ~236 towards 100 ppm ethanol, respectively. The reasons accounting for such an enhancement were revealed by the surface-depletion model in ZnO and the chemisorption energies of oxygen molecules on Zn i -enriched crystal facets. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Preparation of ordered mesoporous WO 3 nanocrystals using KIT-6 as a hard template. • Preparation of mWO 3 /ZnO composites with n–n isotype heterojunction for NO 2 sensor. • 5 wt% ZnO/WO 3 sensor exhibits high response, good selectivity and stability. Abstract Ordered mesoporous WO 3 nanocrystals have been successfully synthesized by a hydrothermal method using mesoporous silica of KIT-6 as a template and phosphotungstic acid as a precursor of WO 3. The structure, morphology, and specific surface of WO 3 nanocrystals were systematically characterized by XRD, SAXS, HR-TEM, and BET. To improve the sensing properties of WO 3 to NO 2, a series of different ZnO amounts were loaded on the mesoporous WO 3 to construct nanocomposites with n–n heterojunction for the fabrication of NO 2 sensors. The gas-sensing behaviors show that the sensor based on WO 3 /5 wt% ZnO composite to 1 ppm of NO 2 not only exhibits the high response, but also has good selectivity and stability at operating temperature of 150 °C, which can be contributed to the large specific surface and porous channels provided by mesoporous structures, and the formation of n–n heterojunctions at interface between both oxides. [ABSTRACT FROM AUTHOR]

Nowadays, highly sensitive metal oxide semiconductor gas sensors are exerting a growing important influence on the detection of target gases. It is still challenging to get both high sensitivity and selectivity to effectively distinguish gas mixtures. In this study, Mn doped ZnO (MZO) is prepared by a facile co-precipitation method and then further modified by CdO addition. The results show that 2.2 mol% MZO has high responses to both acetone and ethanol, while 10 mol% CdO activated 1 mol% MZO exhibits excellent sensitivity to ethanol and neglectable response to acetone. This is explained by the enhanced alkalinity of ZnO by the CdO addition expelling acetone from adsorption onto the sensor surface, which is verified by temperature–programmed CO 2 desorption. Moreover, the CdO-MZO gas sensor has an optimized working temperature of 240 °C, far lower than 340 °C of MZO, due to the higher oxidativity as proved by the temperature-programmed H 2 reduction. The MZO and CdO-MZO gas sensors are then used as a sensor array to distinguish the ethanol and acetone concentrations in mixtures with varied ratios, showing the promise of the gas sensor property tailoring approach for future high performance gas sensors. [ABSTRACT FROM AUTHOR]

Noble metal@metal oxides have attracted extensive attention as gas-sensing materials due to the formation of Schottky junction, modulating surface physicochemical properties of oxide nanomaterials. Herein, a facile way was developed to construct core-shell Au@ZnO nanoparticles through direct pyrolysis of Au@Zn-based metal-organic framework (MOF). The obtained Au@ZnO nanoparticles have ~ 50 nm Au nanoparticles as cores and interpenetrated ZnO nanoparticles (~ 20 nm) as porous shells. When applied as gas-sensing materials, the unique core-shell Au@ZnO nanoparticles exhibited significantly enhanced gas-sensing response and selectivity towards sub-ppm level acetone at 300 °C. The response value of Au@ZnO nanoparticles was 11 times higher than that of singular ZnO nanoparticles towards 1 ppm of acetone. The enhanced gas-sensing properties are ascribed to the electronic and chemical sensitization of Au nanoparticles, due to the formation of Schottky junction within Au-ZnO hetero-interface of core-shell nanoparticles. [ABSTRACT FROM AUTHOR]

The design and synthesis of nanostructured ZnO with high chemical sensing properties, especially towards ppb or sub-ppm level VOC gases is still highly desired and challengeable. Herein, the hierarchical hollow ZnO nanocages were synthesized by a facile strategy through the simple and direct pyrolysis of Zn-based metal-organic framework. The as-synthesized hollow ZnO products present the typical hierarchical structures with hollow interiors enveloped by interpenetrated ZnO nanoparticles as porous shell, providing structurally combined meso-/macro-porous channels for facilitating the diffusion and surface reaction of gas molecules. The gas-sensing experiments demonstrate that, in contrast with singular ZnO nanoparticles, the ZnO nanocages show significantly enhanced chemical sensing sensitivity and selectivity towards low-concentration volatile organic compounds, typically, acetone and benzene. Furthermore, the ZnO hollow nanocages perform sub-ppm level sensitivity with 2.3 ppm −1 towards 0.1 ppm benzene, and ppb level sensitivity with 15.3 ppm −1 towards 50 ppb acetone, respectively. The enhanced sensing performance of the MOF-derived ZnO nanocages is ascribed to the unique hierarchical structure with high specific surface area and abundant exposed active sites with surface-adsorbed oxygen. [ABSTRACT FROM AUTHOR]

• One-step hydrothermal method synthesized a novel gas-sensing ZnO/CdO/rGO composite. • The composite exhibits excellent sensitivity, stability and repeatability to NO 2. • The enhancement is due to the formation of junction and the modification of rGO. The triadic composite of ZnO/CdO heterojunction decorated with reduced graphene oxide (rGO) was prepared using a one-step hydrothermal method. The characterizations of morphology, structure and composition to the composite were undertaken by XRD, Raman, SEM, TEM, XPS, UV–vis spectra. The sensing experimental data indicate that the highest response of the ZnO/CdO/rGO (1.0 wt%) composite to ppm-level NO 2 is 8 times and 2 times higher than pure ZnO and ZnO/CdO junction, respectively. The composite not only exhibits fast response time and recovery time, high response, but also reveals outstanding stability and repeatability at an operating temperature of 125 °C. The sensing mechanism also has been discussed in detail in the work. The enhancement in gas sensing properties is credited to the development of ZnO/CdO heterojunction and the decoration of rGO with high conductivity. The logarithm of sensitivity in the range of 0.4–2.4 ppm NO 2 shows good linear dependence, indicating that the composite based sensor can be used to quantificationally detect low concentration of NO 2. [ABSTRACT FROM AUTHOR]

• ZnO with multiple types of defects are synthesized by annealing and doping. • ZnO doping with Ga and Li and annealed in H 2 behaves high ozone removal performance. • There exists electron transfer between defects, O 3 and intermediate oxygen specie. • The deactivation of catalyst is accompanied by the filling out of the oxygen vacancy. Nowadays, it is necessary to discover the interactions of O 3 with metal oxides in order to synthesize active O 3 degradation catalysts. Herein, ZnO is adopted as a model and the crystal defects are tuned by annealing in H 2 and O 2 , and by doping with Ga and Li. The H 2 annealed ZnO behaves far higher activity towards 20 ppm O 3 than those as-obtained and O 2 annealed ones, and the activity can be further improved by doping with Ga donors and Li acceptors. The mechanism investigation shows that the oxygen vacancy induced by H 2 annealing favors the activity while the oxygen interstitials depress it, implying that there existed electron transfer between crystal defects, O 3 and the intermediate species. Furthermore, the deactivation of catalyst is attributed to the filling out of the oxygen vacancy. All these results provide fundamental understanding for further improvement of O 3 degradation catalysts. [ABSTRACT FROM AUTHOR]