Synthesis, Photoelectricity And Gas Sensing Properties Of Quasi-2D Nano/micro Heterostructure

Recent developments of nanoscience of effectively combining two or morechemically distinct components in one single nanostructure with multifunction or newproperties induced by the heterointerfaces have led to revolutionary new applicationsof nanomaterials in various areas, such as catalysis, photovoltaic devices, sensors, andso on. Conventional gas-sensing materials are used based on the change of carrierconcentration caused by adsorption and desorption, which are functions of elevatedoperating temperature. Many of them have no gas sensitivity at room temperature. Asa result, gas sensors often require a heating device to operate. Unfortunately, hightemperature is harmful to the durability of sensors. There have been considerableefforts to overcome the operating temperature limitation. Nevertheless, production ofsensors operating at room temperature with high sensitivity, high selectivity, and lowpower consumption at nanowatt （nW） level remains a challenging task. Theconductivity of the heterojunction material with multi heterojunction barrier dependson heterojunction barrier, which is sensitive to the carrier concentration, and thecarriers transport by resonant tunneling in this system. Hence, the resonant tunnelingmodulation requires a low level change of carrier concentration. The resonanttunneling modulation process is very rapid and easy to restore, even at roomtemperature. Therefore, room temperature gas sensitivity can be obtained by theapplication of heterojunction materials.Herein, we report a resonant tunneling system, quasi-2D Cu₂O/SnO₂p-nhorizontal-multi-layer heterostructure multi-layer film, prepared by electrochemicaldeposition in a quasi-2D ultra-thin liquid layer, which displays excellent sensitivity toH₂S at room temperature due to the resonant tunneling modulation. By applying aspecial half-sine deposition potential across the electrodes, Cu₂O and SnO₂selectivelyand periodically deposited according to their reduction potentials. The periodicstructure is proved by high-resolution transmission electron microscopy （HRTEM）,selected area electron diffraction （SAED） and Line-profile analysis. Furthermore, thelight sensitivity and H₂S sensitivity properties are characterized carefully. And it is found that the detection limit of sensor based on this heterostructure system can reachas little as sub-ppm to H₂S at room temperature. Notably, laser illumination alsoimproves the sensing performance and response behavior.Another resonant tunneling system, CuO/In₂O₃p-n heterojunctionnano/microstructure array, is also reported, which shows sensitivity to H₂below roomtemperature. It was synthesized by electrochemical deposition in quasi-2D ultra-thinliquid layer combining with annealing treatment. By applying a special squarepotential across the electrodes, Cu₂+and In3+were selectively and periodicallydeposited according to their reduction potentials. However, the pattern ofheterojunction array based on Cu₂O and In/In（OH）3was first prepared. When thegrowth process finished, the pattern were then subjected to a high-temperature annealin air. The array is well-ordered with strictly periodic arrangement in hundreds ofmicrons, and the periodic structure is proved by selected area electron diffraction（SAED） and high-resolution transmission electron microscopy （HRTEM）. Thesensitivity of the CuO/In₂O₃p-n heterojunction nano/microstructure array to H₂at andbelow room temperature are characterised carefully. The results confirm that theCuO/In₂O₃p-n heterojunction nano/microstructure array is sensitive to H₂at andbelow room temperature, and the sensing mechanism based on resonant tunnelingmodulation obtained explanation.The electrochemistry deposition process in the quasi-two-dimensional growthspace was studied in this paper. By applying the cyclical deposition potential withspecial waveform, the controllable preparations of quasi-two-dimensionalheterojunction nano/microstructure array were realized. Through the study ofelectrochemistry deposition process and deposition mechanism, the electrochemistryand formation of special morphology of quasi-two-dimensional heterojunctionnano/microstructure array were analyzed. And by changing the parameters ofdeposition potential, such as amplitude, waveform and frequency, the morphology andstructure of the quasi-two-dimensional heterojunction nano/microstructure array canbe designed and controlled. The conductivity of Cu₂O/SnO₂p-n horizontal-multi-layer heterostructure is studied, and the results prove that theconductivity of heterostructures at room temperature is controlled by theheterojunctions. The research of gas sensitivity of these two heterostructures pointsout that the room temperature gas sensing can be achieved by the application ofresonant tunneling modulation. Also the light sensitivity and gas sensitivity based ontunneling modulation mechanism of those two heterojunction nano/microstructurefunctional materials were characterized and discussed comprehensively, and also thesensitivity improvement caused by laser irritation. Furthermore, the application ofheterostructure act as resonant tunneling system is considered to be an effectivemethod to improve the room temperature gas sensing.