Study On Synthesis And Application Of Oxide Based Nanoheterostructures

With the rapid development of the society,our demand for high-quality materials is also becoming more and more urgent.Correspondingly,the technical requirement of materials for different applications are also becoming more and more harsher.The traditional material field is undergoing a tremendous change.A single material is difficult to meet the diversification of modern device requirements,and composite materials with comprehensive performance advantages become increasingly hot.Composites are generally composed of two or more materials.The advantages of each material combine together to achieve the best overall performance.New composite material will be the most important research direction in material industry in future.Composite materials are normally composed of two parts,namely the carrier section and the active section.The carrier mainly provides the bearing function,which requires that the material must be chemically stable,its morphology being easily controlled,and its synthesis process being easy.Of course,the carrier material can also be the acceptor or donor to change the electronic energy band structure of the composite.The active layer refers to play a major role in the composite,which determines the main function of the composite.As we know,the chemical structure of oxides is stable and they possess excellent prospects.Titanium dioxide(TiO2)and zinc oxide(ZnO)are the two most widely used metal oxide semiconductors in our daily lives.The content of Ti and Zn in the earth is relatively high.That means TiO2 and ZnO are the cost-effective materials.Since it was commercialized in the early 20th century,TiO2 has become very common in our daily life and has been widely used in coatings,sunscreens and food additives.Since Fujisima and Honda first reported in the 1970s that TiO2 can be a photocatalysis to decompose water under ultraviolet light,the application of TiO2 has spreaded rapidly to many fields such as optoelectronics,photocatalysis,photo/electrochromism and gas sensing.ZnO has been studied as a promising candicate for light emmiting,laser,photocatalysis,and photodetector because of its excellent photoelectric property.These two oxides have good chemical stability and controlled morphology.They are good carrier materials and possess excellent photoelectric properties.Therefore,in this thesis we choose TiO2 and ZnO to study and use their energy band structure to design heterojunction structure,study the growth process of composite materials,as well as their applications in the gas and light detection.We successful design and prepare some excellent composite materials for gas detection or light detection.As we all know,the application of the car has facilitated our life,but it also emits some harmful gases.The new decoration materials are beautiful and cheap,but the residual harmful organic matter is endangering human health.Effective monitoring of these harmful gases,and then dealing with them are particularly important.However,most of the traditional gas-sensitive materials need to work at high temperature in order to get a better detection,and the response time is normally too long to achieve rapid detection.Schottky-junction based gas detectors can solve this problem.Their response time is short and they can work at room temperature condition for rapid gas detection.But most of Schottky-junction based gas detectors are fabricated using nanolithography,which is a barrier to mass production for practical applications.Although detectors based on nanostructures-powder coatings can reduce the cost of Schottky-junction based gas sensor,their response is relatively low.Considering these,we tried to synthesize a heterojunction structure with high gas detecting performance and fast response time,which will achieve rapid detection of the target gas at room temperature.Since the discovery of graphene,two-dimensional layered materials have attracted increasing attention due to their unique physical properties.In view of the novel physicochemical properties of 2D materials,we expect to combine them with TiO2 to synthesize a multi-heterojunction composite material,and then explore its novel physics and its application in gas detection.Some reports have shown that SnS2 has a good detection of humidity and can stably works at room temperature.Rough surface TiO2 nanobelts were first prepared by hydrothermal method and then SnS2 was combined with TiO2 nanobelts.The TiO2-SnS2 nanostructures were dispersed in water and dropped onto an alumina ceramic substrate to form a coating film.A thin Au interdigital electrode is deposited on the coating film to make a simple humidity detector.Saturated salt solution humidity generators are used to provide stable relative humidity environment and the humidity response of sensors based on TiO2,SnS2 and TiO2-SnS2 are characterized.TiO2-SnS2 based device shows a higher sensitivity in humidity detection than others.With the change of humidity,its resistance increases linearly.The resistance of TiO2-SnS2 is far lower than that of pure TiO2 and SnS2 at high relative humidity,which means that TiO2-SnS2 is an ideal humidity detection material.Sn3O4 has better NO2 detecting performance than SnO and SnO2.TiO2 also is an excellent material for NO2 gas detection.But their optimum working temperature is high,which is inconvenient for daily use.We combine Sn3O4 with TiO2 to get a TiO2-Sn3O4 heterojunction for NO2 detector.Firstly,TiO2 nanosheets were synthesized by hydrothermal method.Then,scaly TiO2-Sn3O4 were synthesized by hydrothermal co-deposition method.It is expected that the heterogeneous nature can be used to achieve a high NO2 gas detecting sensitivity at room temperature.A novel sensor based on Schottky-junction inducing avalanche breakdown effect in TiO2-Sn3O4 heterojunction is assembled.TiO2-Sn3O4 nanosheet composite was synthesized as the gas sensing layer and Schottky contact was formed by Au electrodes.The Schottky contact functions as a "gate" which can trigger the avalanche breakdown effect in the TiO2-Sn3O4 heterojunction.High sensitivity for NO2 gas sensing is demonstrated at room temperature.By tuning the Schottky barrier height through the responsive variation of the surface chemisorbed gas and the bias of device,NO2 at a concentration from 5 to 50 ppm can be detected with an average response time of 8s at room temperature.Maximum responsivity approximately 900%was obtained when exposed to 50 ppm NO2 at room temperature.This provides a new route to explore room temperature operating gas sensor.Ultraviolet and infrared light have important military and civilian applications due to its non-visible feature.Therefore,the emission and detection of UV and infrared light have attracted much attention all over the world.The photodetector based on metal-semiconductor-metal has been reported.But there are a lot of technology and cost barriers for using semiconductor epitaxial films.The technology for preparing semiconductor film is complex.And these detectors need an additional operating power supply.Self-powered photodetectors based on photochemical cells provide an effective solution.Nanomaterials and nanocomposite materials have been widely used as photoanode in self-powered photodetectors.Self-powered photodetectors based on PEC structure were normally fabricated using a liquid I-/I3-redox couple electrolyte.However,liquid I-/I3-redox couple electrolyte is not ideal for long-term operation:it is highly corrosive,volatile,and photoreactive,interacting with common metallic components and sealing materials.We have tried to to achieve efficient detection of UV and infrared light using friendly and harmless electrolytes.In view of its excellent photoelectrical response of TiO2 to UV light,we design a SnO2-TiO2 nano-array structure using TiO2 as active layer and SnO2 as carrier.TiO2-SnO2 heterojunction is used to accelerate the separation of photocarriers in TiO2 active layer.Efficient collection and transmission of electrons is obtained by the high electron mobility of SnO2,which improves the photodetector responsivity.A quasi-solid-state self-powered UV detector was designed based on the SnO2-TiO2 nano-array structure.The photodetector is constructed with SnO2 nanotube array wrapped with TiO2 as photoanode,organic polymer as electrolyte and Pt as counter electrode,which forms a photochemical cell structure.This array structure can greatly enhance the scattering of ultraviolet light,therein,improve the utilization of light and result a substantial high light response.And the quasi-solid electrolyte can avoid the leakage problem.The detector shows a long-term stability.The maximum light response of this quasi-solid-state detector is 0.153 A/W.At 340 nm,the IPCE is 55.8%,the response time is 0.14 s,and the recovery time is 0.06 s.It shows the highest responsivity among the reported solid-state self-powered UV detectors.In order to further improve the specific surface area and realize the efficient transfer of photo-generated carriers,we designed a TiO2-SnO2 nanomace arrays with rutile TiO2 single crystal nanorods as active layer and SnO2 nanotube array as the carrier.A solid-liquid self-powered UV detector was fabricated with this nanostructure.Water,the most environment-friendly liquid,was used as the electrolyte.The performance of the UV detector depends strongly on the growth time of TiO2 nanorods.The best photoelectric response performance was observed for device based on the 18h grown TiO2 nanorod.The self-powered UV photodetector based on optimized SnO2-TiO2 nanomace array exhibits a high responsivity of 0.145A/W,a fast rise time of 0.037s and a decay time of 0.015s,as well as excellent spectral selectivity.The responsivity of this water-based UV detector is even higher than that of some I-/I3-based UV detectors.Moreover,the electrolyte of this photodetector is water,which is low-cost,stable and environment friendly.In order to broaden the detecting spectrum of the photodetector,we used ZnO as the carrier and Ag2S nanoparticles as the light absorber to form a ZnO-Ag2S nano-heterostructure array and fabricated a solid-state self-powered wide-spectrum photodetector with P-type organic semiconductor Spiro-MeOTAD as the hole-conducting layer.The detector shows a good detecting performance over a wide spectral detection range(from 390 nm in the ultraviolet region to 1100 nm in the near-infrared region).The IPCE at 440 nm reaches a maximum of about 36.7%,and it is still very high in the near-infrared region from 800 nm to 1050 nm.The ZnO-Ag2S photodetector not only extends the detection range to the near-infrared region,but also reduces the response time to milliseconds.This kind of photodetector has a great potential for large-scale,high-sensitivity and high-speed photodetectors and photoelectronic devices in the visible to near IR range.