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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]

Nowadays, xylene is not only one major air pollutant which threats human health even if its concentration is lower than the human olfactory threshold of 470 ppb, but also one of the typical gases exhaled by the lung cancer patients with a criterion of 10–20 ppb. However, in situ detection of the ppb-level xylene for air quality monitoring and breath analysis remains challenging using the easily fabricated and low cost metal oxide semiconductor gas sensors. Herein, a synergetic p+n field effect transistor (FET) amplification circuit is designed to detect the ppb level xylene. By optimizing the load resistor ( RL ) and the p- and n-FET coupling effect, a magnification factor (~7.5) is obtained. This amplification circuit decreases the detection limit of TGS2602 sensor to ~10 ppb xylene with apparent response of about 2.3 and voltage change of >0.5 V, promising for air quality monitoring (the highest permissive limit of 42 ppb) and breath disease analysis (threshold of lung cancer 10–20 ppb). The mechanism is that the matched couple of p + n FETs work synchronically when their ( R\mathrm {L} + R\mathrm {FET}) - I curves nearly coincide with each other. All those results show the prospect of ppb level gas detection with MOX sensors using the synergetic p+n FET amplification circuit. [ABSTRACT FROM PUBLISHER]

Well-defined polyhedral Cu2O exposing different crystal facets are synthesized via a facile solution-phase method, and the facet-dependent gas sensing properties are investigated. The results show that cubic Cu2O exposing (100) facets has higher sensitivity to NO2, acetone and benzene than octahedral Cu2O with (111) facets. More importantly, the cubic Cu2O has high selectivity to NO2 and acetone at varied working temperatures by different sensing mechanisms of adsorption, chemical reaction, and intrinsic carrier excitation, as verified by the catalytic and electric property measurements. The higher sensitivity and selectivity of cubic Cu2O is explained by the relatively lower work function of (100) surfaces than that of (111), which induces a thicker surface hole accumulation layer and thus, a higher bulk hole activation energy (0.55 eV and 0.36 eV respectively). The synergistic effect of catalytic and electric properties gives rise to gas sensitivity decrease with the reduced percentage of (111) facets, which is beneficial to the gas sensing material design and also, is promising for the future highly sensitive and selective gas sensor fabrication. [ABSTRACT FROM AUTHOR]

Nowadays, detection of trace concentration gases is still challenging for portable sensors, especially for the low-cost and easily operated metal–oxide–semiconductor (MOX) gas sensors. In this paper, a widely applicable amplification circuit is designed and fabricated to evidently enhance the signal of the MOX sensors by adding a field effect transistor (FET) into the conventional circuits. By optimizing the FET parameters and the loading resistance, this amplification circuit enables the commercial Figaro TGS2602 toluene sensors response effectively to the highest permissive limit (0.26 ppm) of toluene in indoor air of cars, with the detection limit of ~0.1 ppm. Furthermore, this circuit can also make the commercial Hanwei MP502 acetone sensors and MQ3 ethanol sensors response to the 1–2-ppm acetone in breath of diabetes and 2-ppm ethanol for fast and effectively drinker driver screening. The mechanism is investigated to be the gate voltage induced resistance change of the FET, with the highest theoretically estimated and experimentally measured magnification factor of 5–6. This FET amplifier can effectively enable the ppm level commercial MOX sensors response to sub-ppm level gases, promising for MOX gas sensor integration and also for other kind of resistive sensors. [ABSTRACT FROM PUBLISHER]

Nanostructured ZnO materials including Zn@ZnO core-shell structure (CS), ZnO hollow sphere (HS), and hierarchically structured ZnO hollow microsphere with nanorods (HSNR) grown on the outer surface were prepared by a simple and versatile chemical vapor deposition process. Scanning electron microscopy, X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, and room temperature photoluminescence spectroscopy were used to characterize the morphology and crystalline structure of the samples. It was found that the gas-sensing property of the three structured ZnO had different responses in resistive mode using formaldehyde gas as the probe. In contrast with CS and HS samples, the HSNR exhibits better sensing property because it has more oxygen vacancies and more surface sites, which produce a higher resistance. However, the higher work function of Zn core than that of the ZnO sheath forms the ohmic contact, producing negative charge accumulating layers within ZnO side of Zn-ZnO junction, thus contributing a lower sensing response of CS model when exposed to the targeted formaldehyde gas. [ABSTRACT FROM AUTHOR]

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: Anodic aluminum oxide (AAO)/Al composite structure was prepared and used to fabricate metal oxide gas sensing film through electrophoretic deposition (EPD) process, and Ga-doped ZnO (GZO) and GZO post-treated in H2 (GZO-H) were used as high resistance and low resistance metal oxide models. The results showed that, in EPD process, Al (residual Al after anodic oxidation) plays as the electrode with the electric field penetrating the pores of AAO. And dispersant such as alcohols were preferred as H2 would be produced on the AAO/Al electrode in aqueous dispersion, which destroyed both the AAO layer and the GZO (or GZO-H) film. While in the gas sensing process, AAO (AAO layer) plays as the insulative layer between GZO (or GZO-H) film and Al substrate. The thickness of AAO layer should be properly tailored (i.e. >80μm) as to make its resistance far larger than that of the GZO film, which prevents the current leakage from the GZO film to the Al substrate. While the thickness can be 10μm when GZO-H film was used as its resistance is smaller. The AAO/Al composite structure made the EPD process feasible for gas sensing film fabrication, without any further treatment, which showed prospect in fabrication of metal oxide semiconductor gas sensors. [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]

Low concentration acetone gas detection is significantly important for diabetes diagnosis as 1.8–10 ppm of acetone exists in exhaled breath from diabetes patients. A new interlocking p+n field-effect transistor (FET) circuit has been proposed for Mn-doped ZnO nanoparticles (MZO) to detect the acetone gas at low concentration, especially close to 1.8 ppm. It is noteworthy that MZO in this interlocking amplification circuit shows a low voltage signal of <0.3 V to the acetone <2 ppm while it displays a transilient response with voltage signal >4.0 V to >2 ppm acetone. In other words, the response to acetone from 1 ppm to 2 ppm increases by ~1233%, which is competent to separate diabetic patients from healthy people. Moreover, the response to 2 ppm acetone is hardly influenced by high relative humidity of 85%. In the meanwhile, MZO in this interlocking circuit possesses a high acetone selectivity compared to formaldehyde, acetaldehyde, toluene and ethanol, suggesting a promising technology for the widespread qualitative screening of diabetes. Importantly, this interlocking circuit is also applicable to other types of metal oxide semiconductor gas sensors. The resistance jump of p- and n-FETs induced by the change of their gate voltages is deemed to make this interlocking circuit produce the transilient response. [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]

Abstract: In this paper, a simple way is developed for the synthesis of the ZnO micro-windbreak film (ZMW) based on layered basic zinc salt precursor. The ZnO products maintain the original morphologies of precursors without deformation. Scanning electron microscopy, transmission electron microscopy, infrared spectrum and X-ray diffraction are used to characterize the detailed structures of the as-prepared products. Because of better gas diffusion in single layer ordered flower arrays (highly exposed surfaces) and thin belt-like branches (high diffusion coefficient) than other hierarchical structures, ZMW exhibits the highest responses to benzene gas. High responses allow ZMW to be used for the detection of benzene at ppb-level. By simply sputtering the platinum on both faces of ZMW to enhance the surface reaction, the optimal operating temperature for benzene detection could be reduced to 350°C and the responses are significantly improved. [Copyright &y& Elsevier]

Graphical abstract Highlights • SnO 2 /MO x (M=Zn, Ga, W) nanotubes and nanofibers were fabricated by one-step electrospinning and subsequent calcination. • Selectivity was effectively changed and controlled by designing various SnO 2 /MO x heterostructures. • SnO 2 /MO x sensors were firstly used to compose sensor arrays to detect VOCs mixtures by means of matrix manipulation. Abstract It is a challenge to effectively detect the components of a gas mixture using metal oxide sensors which are easily fabricated and inexpensive. In this work, heterostructured SnO 2 /MO x (i.e. M = Zn, Ga and W) nanotubes (NTs) and nanofibers (NFs) are synthesized via a one-step electrospinning technology and subsequent calcination for use in gas sensors. In specific, compared with pure SnO 2 NTs, SnO 2 /ZnO NTs show an enhanced response to 100 ppm ethanol and acetone, while SnO 2 /Ga 2 O 3 NTs show an obviously higher response to 100 ppm ethanol and SnO 2 /WO 3 NFs show an optimal response to 100 ppm xylene. All these sensors are selective in the presence of typically interfering gases such as formaldehyde, benzene and toluene. As a proof-of-concept, sensor arrays constructed using these three sensors precisely detected the gas mixtures of ethanol, acetone and xylene by means of matrix manipulation, delivering a superior accuracy of <9% deviation for the gas concentration of 10 ppm and 20 ppm and <38% for the concentration of 5 ppm. These results show the possibility of improved selectivity in detecting gas mixtures using heterostructured gas-sensing materials for sensor arrays. [ABSTRACT FROM AUTHOR]

Highlights • MEMS compatible Au/SnO 2 :NiO films were fabricated using sputtering and self-assembly techniques. • The Au/SnO 2 :NiO film showed high sensitivity, selectivity and stability, as well as good consistency to NO 2. • The enhanced sensing property was attributed to catalytic role of the AuNPs and p-n junctions between Au, SnO 2 and NiO. Abstract Here, heterostructured Au/SnO 2 :NiO thin films are fabricated by sputtering SnO 2 :NiO target onto the self-assembled Au nanoparticle (NPs, diameter ∼ 9.5 nm) arrays and then activated by H 2 annealing. Compared with the SnO 2 and SnO 2 :NiO thin film counterparts, the 20 nm thick Au/SnO 2 :NiO film shows the highest NO 2 sensing performance after activated at optimized temperature of 500 °C, with high response of ∼185 to 5 ppm NO 2 , low detection limit of ∼50 ppb, high selectivity, good stability and also low sensor-to-sensor variation of <15%. The gas sensing property enhancement could not only be attributed to catalytic role of the AuNPs, but also to the effective Schottky barrier and p-n junction formation between Au, SnO 2 and NiO, which is verified by the relatively higher Fermi level of ∼4.56 eV compared with those of 4.1–4.3 eV as measured by the ultraviolet photoelectron spectroscopy. Furthermore, this method is of generality to effectively enhance the electron transfer in other sensing materials like pure SnO 2 and WO 3 , showing the promising potency of this Micro-Electrical-Mechanical System (MEMS) compatible heterostucture fabrication method in wafer-scale gas sensor production with high sensitivity, selectivity and good consistency towards low concentration gas detection such as ppb-level NO 2. [ABSTRACT FROM AUTHOR]

• Zooming p + n FETs was proposed to address the signal saturation of MOX gas sensors. • FETs endowed MOX sensors with 3.0 V/decade to 100–2000 ppm acetone. • This zooming technology was also suitable for detecting 1%–20% LEL methane. • The principle depended on the suppressing and amplifying roles of FETs. Detecting high-concentration gases is challenging by metal oxide semiconductor (MOX) gas sensors, because the voltage signal would become saturated. In order to solve this problem, the zooming p + n field-effect transistors (FETs) circuit has been designed, combining an n-type enhancement-mode FET (EMFET) and a p-type depletion-mode FET (DMFET). This designed zooming p + n FETs can endow MOX gas sensors with the high signal resolution of ~3.0 V/decade to the 100–2000 ppm (part per million) acetone gas, triple that of MOX gas sensors without FETs (~0.8 V/decade). Meanwhile, this zooming technology is also suitable for detecting other gases at high-concentration, such as 1%–20% LEL (lower explosion limit) methane. The principle of zooming p + n FETs is that with increasing the gas concentration, the suppressing role of the EMFET is firstly induced leading to a reduced signal resolution to the low-concentration target gas; then its suppressing effect becomes saturated and the DMFET starts to switch from ON state to OFF state in the high-concentration target gas, resulting in an amplifying effect herein and thus an enhanced signal resolution. This circuit is promising for the high-concentration gas detection as well as for the multi-functional gas detector design. [ABSTRACT FROM AUTHOR]