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

Polymer nanocomposites based on functionalized carbon nanotubes

Metal oxide-based resistive-type gas sensors are solid-state devices which are widely used in a number of applications from health and safety to energy efficiency and emission control. Nanomaterials such as nanowires, nanorods, and nanoparticles have dominated the research focus in this field due to their large number of surface sites facilitating surface reactions. Previous studies have shown that incorporating two or more metal oxides to form a heterojunction interface can have drastic effects on gas sensor performance, especially the selectivity. Recently, these effects have been amplified by designing heterojunctions on the nano-scale. These designs have evolved from mixed commercial powders and bi-layer films to finely-tuned core–shell and hierarchical brush-like nanocomposites. This review details the various morphological classes currently available for nanostructured metal-oxide based heterojunctions and then presents the dominant electronic and chemical mechanisms that influence the performance of these materials as resistive-type gas sensors. Mechanisms explored include p–n and n–n potential barrier manipulation, n–p–n response type inversions, spill-over effects, synergistic catalytic behavior, and microstructure enhancement. Tables are presented summarizing these works specifically for SnO2, ZnO, TiO2, In2O3, Fe2O3, MoO3, Co3O4, and CdO-based nanocomposites. Recent developments are highlighted and likely future trends are explored.

Nanoscale metal oxide-based heterojunctions for gas sensing: A review

Co3O4 nanotubes, nanorods, and nanoparticles are used as the anode materials of lithium-ion batteries. The results show that the Co3O4 nanotubes prepared by a porous-alumina-template method display high discharge capacity and superior cycling reversibility. Furthermore, Co3O4 nanotubes exhibit excellent sensitivity to hydrogen and alcohol, owing to their hollow, nanostructured character.

Co3O4 nanomaterials in lithium-ion batteries and gas sensors

Polymers in sensor applications

A new hybrid SWCNTs/SnO 2 gas sensor is developed by adding single-walled carbon nanotubes (SWCNTs) into a SnO 2 substrate. The fabrication involved heat-treating the SWCNTs/SnO 2 layer, which was fabricated by spin coating using an organometallic solution of dispersed with SWCNTs. The hybrid sensor is utilized to detect NO 2 concentrations in flowing air or N 2 , by considering alterations in electrical properties. The tests are performed at room temperature. Two hybrid sensors with different concentrations of SWCNTs are examined to elucidate the effect of a concentration of SWCNTs on the sensing properties of the sensors. A SnO 2 sensor without SWCNTs, a blank sample, is also examined for comparison. Comparative gas sensing results reveal that the prepared hybrid SWCNTs/SnO 2 sensors exhibit much higher sensing sensitivity and recovery property in detecting NO 2 gas at room temperature than the blank SnO 2 sensor. This finding shows that doping with SWCNTs improves the sensitivity of hybrid sensors. Microstructural observations revealed that SWCNT bundles are embedded in the SnO 2 matrix. Therefore, a model is presented to relate potential barriers to electronic conduction in the hybrid material. This model suggests that the high sensitivity is associated with the stretching of the depletion layers at the grain boundaries of SnO 2 and the SWCNTs/SnO 2 interfaces when detected gases are adsorbed.

A novel SnO2 gas sensor doped with carbon nanotubes operating at room temperature

Composite humidity sensing films were made from carbon nanotube (CNT) and Nafion. The sensing films were prepared using the spin-coating method. Sensing films coated on the electrode of a Quartz Crystal Microbalance (QCM) were used for low humidity measurement. This low humidity system was set up with a divided flow generator and calibrated by an optical chilled dew-point hygrometer in the measuring range from 8.50 to 15,300 ppm v . The detection limit of this sensing system was about 15.76 ppm v . In this low humidity sensing system, the CNT/Nafion sensing material coated on the gold electrode of QCM showed excellent sensitivity, linearity (Rsqr = 0.995), short response and recovery time (less than 5 s, at testing point of 15.76 ppm v ).

The application of CNT/Nafion composite material to low humidity sensing measurement

A novel sensing graphene/polypyrrole (PPy) material was prepared by a chemical oxidative polymerization method with various weight percentages of graphene. The structure and morphology were characterized by XRD (X-ray diffraction), FTIR (Fourier transform infrared), TEM (transmission electron microscopy), UV–vis spectrometry and TGA (thermo-gravimetric analysis). The humidity sensing properties were measured by using an LCR (inductance, capacitance and resistance) analyzer. The results indicate that the sample with 10% graphene/PPy had better humidity sensing properties than the other samples at relative humidity (RH) in the range 12–90%. The response and recovery times were approximately 15 s and 20 s, respectively. The prepared sensor has a higher sensitivity (S = 138) than those developed elsewhere and the humidity hysteresis value was very small at all humidities (<0.16%). The results herein demonstrate the potential of graphene/PPy composite for use in fabricating high-performance humidity sensors.

Applied novel sensing material graphene/polypyrrole for humidity sensor

Homogeneous TiO2 nanowires were fabricated by hydrothermal method. SEM pictures proved the yield of nanowires to be more than 90%. Composite humidity sensing films were made by using TiO2 nanowires, TEOS and Nafion. FTIR absorption spectroscopy was used as a semi-quantitative method to get information about the protonation. The sensing films were prepared by a dip-coating method. The composite films coated on a pair of gold electrodes were tested for humidity sensors of resistance type. The measurement was carried out at five fixed humidity points in the range of 12–97% relative humidity, which were controlled by employing five different salt solutions. Resistance changes were about three orders of magnitude. The nanowires-based humidity sensors showed moderate sensitivity, short response and recovery time (<2 min) at relative humidity less than 76%, and good long-term stability.

Composite of TiO2 nanowires and Nafion as humidity sensor material

This paper describes the fabrication of a Au@SnO2 core–shell structure using the sol gel method to manufacture the sensing materials. The properties of the sensing material were analyzed with UV–vis spectra, Fourier transform infrared spectroscopy, and transmission electron microscopy. The SnO2 material showed no response, whereas 1 wt% Au/SnO2 exhibited a 2.4 sensor response. The response t90 (for a result equaling 90% of the equilibrium signal) and recovery tr90 (to adjust the signal to 90% background signal) times of the 1 wt% Au/SnO2 material measured 240 s and 210 s, respectively. Au@SnO2 further enhanced the sensor response to 2.9 while reducing the response and recovery times to 80 s and 62 s. This study also demonstrates a formaldehyde gas-sensing mechanism involving HCHO adsorption and surface reaction and product desorption.

Ren-Jang Wu

Papers

A novel SnO2 gas sensor doped with carbon nanotubes operating at room temperature

The application of CNT/Nafion composite material to low humidity sensing measurement

Applied novel sensing material graphene/polypyrrole for humidity sensor

Composite of TiO2 nanowires and Nafion as humidity sensor material

Fabrication of a Au@SnO2 core–shell structure for gaseous formaldehyde sensing at room temperature