Colloidal oxide nanoparticle inks for micrometer-resolution additive manufacturing of three-dimensional gas sensors.

Micrometer-resolution 3D printing of functional oxides is of growing importance for the fabrication of micro-electromechanical systems (MEMSs) with customized 3D geometries. Compared to conventional microfabrication methods, additive manufacturing presents new opportunities for the low-cost, energy-saving, high-precision, and rapid manufacturing of electronics with complex 3D architectures. Despite these promises, methods for printable oxide inks are often hampered by challenges in achieving the printing resolution required by today's MEMS electronics and integration capabilities with various other electronic components. Here, a novel, facile ink design strategy is presented to overcome these challenges. Specifically, we first prepare a high-solid loading (∼78 wt%) colloidal suspension that contains polyethyleneimine (PEI)-coated stannic dioxide (SnO2) nanoparticles, followed by PEI desorption that is induced by nitric acid (HNO3) titration to optimize the rheological properties of the printable inks. Our achieved ∼3-5 µm printing resolution is at least an order of magnitude higher than those of other printed oxide studies employing nanoparticle ink-based printing methods demonstrated previously. Finally, various SnO2 structures were directly printed on a MEMS-based microelectrode for acetylene detection application. The gas sensitivity measurements reveal that the device performance is strongly dependent on the printed SnO2 structures. Specifically, the 3D structured SnO2 gas sensor exhibits the highest response of ∼ 29.9 to 100 ppm acetylene with the fastest total response time of ∼ 65.8 s. This work presents a general ink formulation and printing strategy for functional oxides, which further provides a pathway for the additive manufacturing of oxide-based MEMSs.

Observation of Switchable Dual-Conductive Channels and Related Nitric Oxide Gas-Sensing Properties in the N-rGO/ZnO Heterogeneous Structure.

Gas sensors based on hybrid materials of graphene oxide/metal oxide semiconductors are an effective way to improve sensor performance. In this paper, we demonstrate a high-performance nitric oxide (NO) gas sensor based on nitrogen-doped reduced graphene oxide/ZnO nanocrystals (N-rGO/ZnO) operating at a low work temperature. ZnO nanocrystals, with an average size of approximately 5 nm, can be uniformly and compactly anchored on the surface of N-rGO using a facile two-step hydrothermal synthesis with an appropriate amount of ammonia as the nitrogen source. The sensor based on the N-rGO/ZnO composite with 0.3 mL of ammonia (N-rGO/ZnO-0.3), in comparison with N-rGO/ZnO with different amounts of ammonia, N-rGO, and rGO/ZnO, exhibited a significantly higher sensitivity (S = Rg/Ra) at the parts per billion (ppb) level for NO gas at 90 °C. The maximum sensitivity at 800 ppb NO was approximately 22, with much faster response and recovery times. In addition, the N-rGO/ZnO-0.3 sensor revealed great stability, a low detection limit of 100 ppb, and an excellent selectivity toward NO versus other gases (NO2, H2, CO, NH3, and CH4), especially at the ppb level. More interestingly, when exposed to oxidizing and reducing gases, unlike conventional semiconductor sensitive materials with resistances that normally change in the opposite direction, only the increase in the resistance is surprisingly and incomprehensibly observed for the N-rGO/ZnO-0.3 sensor. The peculiar sensing behaviors cannot be explained by the conventional theory of the adsorption process, redox reactions on the surfaces, and the well-defined p-n junction between N-rGO and ZnO, originating from the chemical bonding of Zn-C. We propose here for the first time that switchable contribution from dual-conduction paths including the corresponding ZnO channel with the p-n junction and the corresponding N-rGO channel to the sensitivity may exist in the interaction between gases and N-rGO/ZnO-0.3 material. When an oxidizing gas (such as NO) is exposed to the N-rGO/ZnO-0.3 sensor, the contribution from the conductive channel of ZnO nanoparticles and the p-n junction to the sensitivity is dominant. On the contrary, as for a reducing gas (such as H2), the contribution alters to the N-rGO channel as the dominating mode for sensitivity. For gas-sensing behavior of the NGZ-0.1 and NGZ-0.5 sensors, there is only one conduction path from the N-rGO channel for the sensitivity. The model of switchable dual-conduction paths has addressed the mysterious response observed for different gases, which may be utilized to enlighten the understanding of other application problems in nanoscale hybrid materials with a heterogeneous structure.

A Highly Sensitive and Selective ppb-Level Acetone Sensor Based on a Pt-Doped 3D Porous SnO2 Hierarchical Structure.

In view of the low sensitivity, high operating temperature and poor selectivity of acetone measurements, in this paper much effort has been paid to improve the performance of acetone sensors from three aspects: increasing the surface area of the material, improving the surface activity and enhancing gas diffusion. A hierarchical flower-like Pt-doped (1 wt %) 3D porous SnO2 (3DPS) material was synthesized by a one-step hydrothermal method. The micropores of the material were constructed by subsequent annealing. The results of the experiments show that the 3DPS-based sensor's response is strongly dependent on temperature, exhibiting a mountain-like response curve. The maximum sensor sensitivity (Ra/Rg) was found to be as high as 505.7 at a heating temperature of 153 °C and with an exposure to 100 ppm acetone. Additionally, at 153 °C, the sensor still had a response of 2.1 when exposed to 50 ppb acetone gas. The 3DPS-based sensor also has an excellent selectivity for acetone detection. The high sensitivity can be explained by the increase in the specific surface area brought about by the hierarchical flower-like structure, the enhanced surface activity of the noble metal nanoparticles, and the rapid diffusion of free-gas and adsorbed gas molecules caused by the multiple channels of the microporous structure.

Highly Selective, ppb-Level Xylene Gas Detection by Sn2+-Doped NiO Flower-Like Microspheres Prepared by a One-Step Hydrothermal Method.

Detecting xylene gas is an important means of avoiding human harm from gas poisoning. A precise measurement demands that the gas sensor used must have high sensitivity, high selectivity, and low working temperature. To meet these requirements, in this study, Sn2+-doped NiO flower-like microspheres (SNM) with different amounts of Sn2+ synthesized by a one-step hydrothermal process were investigated. The responses of gas sensors based on different Sn2+-doped NiO materials for various targeting gases were fully characterized. It was found that all of the synthesized materials exhibited the best gas response at a working temperature of 180 degrees, which was much lower than the previously reported working temperature range of 300-500 degrees. When exposed to 10 ppm xylene, the 8 at% Sn2+-doped NiO sensor (mol ratio) exhibited the highest response, with a value of 30 (Rg/Ra). More significantly, the detection limit of the 8 at% Sn2+-doped NiO sensor for xylene is down in the ppb level. The Sn2+-doped NiO material also exhibits excellent selectivity for other gases with long-term stability and repeatability. The significant improvement in the response to xylene can theoretically be attributed to a decrease in the intrinsic hole carrier concentration, higher amounts of adsorbed oxygen and active sites.

Nitric Oxide Detector Based on WO3-1wt%In2O3-1wt%Nb2O5 with State-of-the-Art Selectivity and ppb-Level Sensitivity.

Fast, sensitive, and precise detection of nitric oxide (NO) is critical to many applications in environmental monitoring and early disease diagnosis via respiratory testing. An effective detection system requires a sensor to detect NO gas at the parts per billion (ppb) level, and this system should possess a high degree of anti-interference selectivity. To achieve these targets, a series of gas sensor thin films based on intrinsic WO3, one-additive-doped WO3 (prepared by doping In2O3 or Nb2O5), and two-additive-doped WO3 (synthesized by doping with In2O3 and Nb2O5) oxides were successfully grown. By analyzing the properties of sensitivity, selectivity, responsiveness, and recovery time of the gas sensors, we found that WO3-1wt%In2O3-1wt%Nb2O5 has overwhelming advantages over intrinsic WO3, WO3-In2O3, and WO3-Nb2O5. A sensing response value of 2.4 was observed for NO concentrations as low as 20 ppb from the WO3-1wt%In2O3-1wt%Nb2O5 sensor. With 100 ppb NO gas, the WO3-1wt%In2O3-1wt%Nb2O5 sensor achieved a high response of 56.1 at 70 °C, which is a state-of-the-art performance for NO detection at low working temperature settings. WO3-1wt%In2O3-1wt%Nb2O5 also yields significantly improved selectivity and stability over intrinsic WO3, WO3-In2O3, and WO3-Nb2O5. Studies on the sensing mechanism show that the grain size, rather than the n-n heterostructure effect, plays a dominant role in the observed results. By decreasing the grain size so that it is close to the thickness of the space-charge layer, the sensing response is enhanced. Although room remains to further improve the sensing properties, the performance of WO3-1wt%In2O3-1wt%Nb2O5 is sufficient for implementation in low-content NO detection devices.