Nanostructured metal oxides for highly selective VOC and gas sensing

Abstract

Gas and volatile organic compounds (VOCs) sensors are essential part of many intelligent systems for safety purpose. Their varied applications are prevalent in automotive industry (e.g., detection of vehicle emission gases), indoor air quality (e.g., detection of CO) monitoring, greenhouse gas (e.g., CO2 and methane) monitoring, medical application (e.g., detection of volatile organic compounds from exhaled human breath), and coal mine application (e.g., methane detection). Depending upon the detection mechanism, there are different types of gas and VOC sensors available in the market such as resistive, capacitive, calorimetric, colorimetric, and optical based sensors. However, among these semiconducting metal oxide (MOX) based conductometric/resistive type sensors are one of the most promising candidate due to their lowcost manufacturing, easy fabrication methodologies, user-friendly nature in terms of deployabality, and capability of integration of detection nodes for multiple gases/VOCs on a single device/cheap. Primary drawbacks of MOX-based sensors are their lack of selectivity towards a particular test gas/VOC and require high temperature for their sensing operation. Sensors with high sensitivity to multiple gases/VOCs or low selectivity to particular gas/VOC are not suitable for practical application, due to the fact that they can produce false alarm or can disrupt the function of the safety system. Thereby, it is necessary to have a gas/VOC sensor with high selectivity as well as high sensitivity. In the past few decades, research work has been carried out worldwide to improve the selectivity and sensitivity of such sensors working at low or room temperature. In principle, the improvement is carried out by four methods: (1) architecture tailoring of MOX nanostructures, (2) surface modification by noble metals such as Pd, Pt, and Au etc., (3) usage of MOX composites, and (4) organic-inorganic hybrids. However, the highest level of selectivity achieved so far by these methods are still inadequate for the realization of practical gas/VOC sensors. Thus, research work has been pursued in search for superior material, which can exhibit ideal gas/VOC sensing characteristics with high values of sensitivity as well as selectivity. Among many MOXs, ZnO is widely studied as a sensing material, due to its wide energy band gap (Eg ~ 3.37 eV), intrinsic n-type conductivity, wide range of conductance variability, high thermal and chemical stability, and large surface area. In addition, ZnO’s precursor can easily be combined with other MOX precursors or organic component, i.e., making ZnO based hybrids is an easy task. Therefore, ZnO is an ideal candidate for producing new kind of hybrid materials. Also, recently, MoO3 has attracted the attention as a gas/VOC sensing material due to its wide band gap (Eg ~ 2.4-3.8 eV), n-type conductivity, good thermal and chemical stability. The presence of structural anisotropy and variable oxidation states, by which it can form various nanostructures such as nanobelts, nanorods, nanoflakes and nanoflowers etc., which are useful for gas/VOC sensing application due to their high surface area and morphology dependent unique sensing properties. However, the use of α-MoO3 as a gas/VOC sensing material is not well explored. Exploration of α-MoO3 is necessary to yield new morphology dependent unique gas/VOC sensing properties.In this thesis research work, aforementioned two n-type wide bandgap based MOX material, i.e., ZnO and MoO3 has been used in its bare, composite, or hybrid system to increase the selectivity and sensitivity towards CO, CO2 gas and alcohol. A π-conjugated amine and ZnO based organic-inorganic hybrid material was synthesized for the detection of CO2 gas. Naphthalene based πconjugated amine (NBA) was synthesized via conventional chemical synthesis method. Next, NBA-ZnO nanohybrid was synthesized via insitu hydrothermal process on an interdigitated electrodes (IDEs) coated PDMS substrate. Material characterization techniques assured the growth of NBA-ZnO nanohybrids. A comprehensive room temperature CO2 sensing study revealed that nanohybrid sensor had a good sensitivity of ~ 9% to 500 ppm CO2 and almost negligible response to other gases viz. CO, NH3, and H2S, implying excellent selectivity (~ 91%) to CO2 by the nanohybrid sensor. Good mechanical stability under bending conditions, excellent repeatability, and low humidity effect on CO2 sensing properties of the nanohybrid sensor suggest that the nanohybrid sensor has great potential for developing room temperature wearable CO2 sensor. An organo-di-benzoic acidified ZnO hybrid material was developed for the detection of CO gas. Initially, an organo-di-benzoic acid (ODBA) was synthesized through conventional chemical synthesis method, later it was used in a single-pot hydrothermal method for the synthesis of ODBA-ZnO nanohybrids. Material characterization results revealed that as synthesized ODBA-ZnO nanohybrids were highly porous and comprised of net-like hierarchical structures. Optimum operating temperature of the nanohybrid sensor was 125 °C, which was supported by thermogravimetric analysis data. At 125 °C nanohybrid sensor exhibits highest response of 35% to 100 ppm CO with selectivity around 88%. Unprecedented selectivity to CO, low humidity effect, and low operating temperature of the ODBA-ZnO nanohybrid sensor suggest that ODBA-ZnO nanohybrids is a good candidate for developing low cost CO sensors. A rose-like composite consisting of ZnO microcubes and MoO3 micrograss-like structures was hydrothermally synthesized for methanol detection. Experimental characterization results revealed that pistil-like ZnO microcube was surrounded by petal-like MoO3 micrograss in the composite. The surface area dramatically increased in the composite as compared to bare ZnO microcubes and MoO3 micrograss material. VOCs sensing results revealed that composite sensor had excellent selectivity to methanol as compared ZnO microcubes-based and MoO3 micrograss-based sensor. Nearly 60% selectivity was achieved by the composite sensor to 500 ppm methanol at 200 °C. Thirty days stability study of the composite sensor exhibited nearly stable response (56% to 500 ppm methanol) throughout the entire period. Presence of n-n heterojunction, increased surface area, and unique rose-like structure were possible reasons for getting enhance methanol selectivity and sensitivity by rose-like ZnO/MoO3 composite sensor. A novel temperature modulation technique was adopted to alter architecture of MoO3 nanostructures from nanobelts to ultra-long nanofibers. Two sets of experiments were carried out to synthesize MoO3 nanostructures. In 1st set, hydrothermal reactions were carried out at 120 °C for different hours to optimize the growth time for the growth of uniform nanostructures. In 2 nd set, hydrothermal reactions were carried out at different frequency dependent pulsed temperature at 120 °C for the optimized time (48 h). Experimental characterization results revealed that MoO3 nanofibers grown by pulsed temperature method at low frequency (1/24 h-1 ) for 48 hours has higher surface defects and higher surface area as compared to MoO3 nanobelts grown by constant temperature method for 48 hours. Detailed VOCs sensing study of the fabricated sensors revealed that MoO3 nanofibers-based sensor has highest sensitivity of 53 (Ra/Rg) to 200 ppm ethanol at an optimum temperature of 275 °C. MoO3 nanofibers-based sensor also exhibited good selectivity (~ 67%) to ethanol with an excellent repeatability. Finally, with the aim of developing lab-based low-cost portable gas/VOC sensors, an interface circuit, including a low cost resistance readout circuit for monitoring sensor resistance and a temperature control circuit for controlling the temperature of the microheater was simulated through Proteus 8 software. Next, the resistance read out circuit was practically realized and the response of the MoO3 nanofibers sensor was monitored for 50 ppm ethanol with good accuracy. At last the printed circuit board (PCB) layout design of the resistance read out circuit and temperature control circuit was realized towards one step forward for achieving the aim of developing lab-based portable gas/VOC sensors.