Gas sensing properties at room temperature of a quartz crystal microbalance coated with ZnO nanorods

Gas sensors based on a quartz crystal microbalance (QCM) coated with ZnO nanorods were developed for detection of NH₃ at room temperature. Vertically well-aligned ZnO nanorods were synthesized by a novel wet chemical route at a low temperature of 90 °C, which was used to grow the ZnO nanorods directly on the QCM for the gas sensor application. The morphology of the ZnO nanorods was examined by field-emission scanning electron microscopy (FE-SEM). The diameter and length of the nanorods were 100 nm and 3 μm, respectively. The QCM coated with the ZnO nanorods gas sensor showed excellent performance to NH₃ gas. The frequency shift (Δf) to 50 ppm NH₃ at room temperature was about 9.1 Hz. It was found that the response and recovery times were varied with the ammonia concentration. The fabricated gas sensors showed good reproducibility and high stability. Moreover, the sensor showed a high selectivity to ammoniac gas over liquefied petroleum gas (LPG), nitrous oxide (N₂O), carbon monoxide (CO), nitrogen dioxide (NO₂), and carbon dioxide (CO₂).     

涂敷脲醛树脂的石英晶体微天平湿度传感器

在碱性条件下预先合成脲醛树脂单体,然后在酸性条件下于石英晶体的电极上原位合成脲醛树脂高分子薄膜,制备了石英晶体微天平湿度传感器。同时考察敏感膜厚度对检测灵敏度的影响以及传感器的响应特性、重复性和稳定性。试验结果表明:涂敷脲醛树脂薄膜的传感器频率随测试的相对湿度变化明显,传感器具有良好的重复性和稳定性。制作的湿度传感器可应用于实际的生产、生活中。     

基于QCM的湿度传感器等效电路模型研究

基于石英晶体微天平(QCM)的湿度传感器是一种以石英晶体为核心元件的新型高灵敏度传感器.根据气相中晶体振荡电路的起振和稳定条件,对晶体表面敏感薄膜吸附水分时等效电路参数的变化进行建模,得到了一种新型的基于QCM的湿度传感器等效电路模型.通过电路仿真软件PSPICE对模型进行仿真,并搭建QCM湿度传感器实验测量平台,结果验证了基于QCM的湿度传感器等效电路模型的有效性和正确性,对QCM湿度传感器的振荡电路的进一步设计和优化具有指导意义.     

Fabrication of gas sensing devices with ZnO nanostructure by the low-temperature oxidation of zinc particles

A method for low-temperature synthesis of a mixture of high-density ZnO nanoflakes and nanowires was developed to produce low-cost and high-efficiency gas sensors with ZnO nanostructures. ZnO nanoflakes and nanowires were grown on glass substrates by the RF sputter deposition of Zn particles and localized oxidation at a low temperature of 300 °C. The synthesized ZnO nanoflakes and nanowires were polycrystalline and had nanometer dimensions, as revealed by X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM) measuring. A gas sensor based on the mixture of ZnO nanoflakes/nanowires responded rapidly and sensitively to ethanol. The sensing properties of the ZnO nanostructure sensor were approximately 72% for 50 ppm ethanol gas at an operating temperature of 100 °C. The response to 10 ppm of ethanol gas was 42% at the same temperature.     

ZnO纳米棒修饰的QCM气体传感器检测NH_3研究

主要介绍了ZnO纳米棒修饰的石英晶体微天平(QCM)气体传感器的制备与测试。采用两步法在石英晶振片表面制备直径为100 nm的ZnO纳米棒敏感膜,构成QCM NH3传感器。检测系统为自主研发的基于LabVIEW平台的QCM气体传感器频率测试软件。检测NH3的体积分数为5×10-6~50×10-6,响应时间均在10 s以内,最大频差值为10.9 Hz,响应最大频差值与NH3体积分数呈现良好的线性关系。室温条件下,ZnO纳米棒敏感膜可以完全实现吸附解吸过程,具有可逆性。该传感器性能稳定,响应灵敏,具有重复性。     

基于LiCl湿敏涂层的QCM湿度传感特性的研究

对0.2%质量比LiCl薄膜涂层的QCM湿度传感单元湿敏特性进行了系统性的研究。研究结果表明:0.2%质量比LiCl—QCM的湿度传感单元的线性测量范围为13%～94%RH,克服了常规LiCl湿敏材料传感单元量程窄的缺点。另外,此湿度传感单元还具有重复性好、灵敏度高、湿滞小(小于10 Hz)、数字频率化输出等优点,因而,具有较好的潜在应用前景。     

基于微接触印刷的ZnO紫外传感器特性研究

采用微接触印刷技术和水热生长方法在硅基底上实现了ZnO种子层的图案化转移与纳米线阵列的可控制备。利用X射线衍射(XRD)、能量色散谱(EDS)和扫描电子显微镜(SEM)等测试手段对制备的ZnO纳米线晶体结构、化学组分以及表面形貌进行了表征,并对制备的ZnO纳米线传感器进行了紫外特性测试。测试结果表明:随着紫外光强度的增加,传感器的光暗电流比和光响应度也随之增加。当紫外传感器偏压在4.5 V时,其光暗电流比为80.8,响应度可达4.05 A/W。     

Formaldehyde sensors based on nanofibrous polyethyleneimine/bacterial cellulose membranes coated quartz crystal microbalance

A novel formaldehyde sensor based on nanofibrous polyethyleneimine (PEI)/bacterial cellulose (BC) membranes coated quartz crystal microbalance (QCM) has been successfully fabricated. The nanoporous three-dimensional PEI/BC membranes are composed of nanofibers with diameter of 30-60 nm. The sensor showed high sensitivity with good linearity and exhibited a good reversibility and repeatability towards formaldehyde in the concentration range of 1-100 ppm at room temperature. Moreover, the results showed that the sensing properties were mainly affected by the content of PEI component in nanofibrous membranes, concentration of formaldehyde and relative humidity. Additionally, the nanofibrous PEI/BC membrane coated QCM sensors exhibited a good selectivity to formaldehyde when tested with competing vapors. The simple and feasible method to prepare and coat the PEI/BC sensing membranes on QCM makes it promising for mass production at a low cost.     

Density-controlled growth of well-aligned ZnO nanowires using chemical vapor deposition

Well-aligned ZnO nanowires were grown on Si substrate by chemical vapor deposition.The experimental results showed that the density of nanowires was related to the heating process and growth temperature.High-density ZnO nanowires were obtained under optimal conditions.The growth mechanism of the ZnO nanowires was presented as well.     

氧化锌纳米线振动问题研究

采用连续介质理论与分子动力学模拟相结合的方法,研究了氧化锌纳米线的振动问题.建立了氧化锌纳米线核壳模型,解释其等效杨氏模量及压电常数的尺寸效应.通过连续介质理论求得氧化锌纳米线振动固有频率,并与分子动力学模拟得到的结果进行对比.研究表明,氧化锌纳米线在极化方向的等效拉伸杨氏模量随着横截面尺寸的增加而逐渐增大,且通过核壳模型分别求得核、壳拉伸杨氏模量.拟合得到的等效拉伸杨氏模量与分子动力学方法获得的等效拉伸杨氏模量符合得很好.根据连续介质理论得到等效弯曲杨氏模量,发现等效弯曲杨氏模量也随着横截面尺寸的增加而增大.氧化锌纳米线极化方向的压电耦合能力比一般压电陶瓷好,压电常数随着横截面尺寸的增加逐渐减小.氧化锌纳米线在不同温度条件下的振动频率没有明显变化,在不同外电场条件下的振动频率有显著变化.分子动力学模拟得到不同横截面尺寸的氧化锌纳米线振动频率不同.根据连续介质理论,求得悬臂Timoshenko梁模型相应尺寸的振动频率,发现横截面的尺寸越大,连续介质理论与分子动力学模拟得到的振动频率越接近.     

Olfactory receptor based piezoelectric biosensors for detection of alcohols related to food safety applications

Our major goal in developing intelligent quality sensors is to detect bacterial pathogens such as Salmonella in the packaged beef. Olfactory sensing of specific volatile organic compounds released by the bacterial pathogens is one of the unique ways for determining contamination in food products. This work aims at developing a biomimetic piezoelectric olfactory sensor for detecting specific gases (alcohols) at low concentrations.The computational simulation was used to determine the biomimetic peptide-based sensing material to be deposited on the quartz crystal microbalance (QCM) sensor. Tripos/Sybyl^®8.0 was used to predict the binding site of an olfactory receptor and determine the binding affinity as well as orientation of the selected ligands (specific molecules) to the olfactory receptor. The designed polypeptide sequence based on the simulation program was synthesized and used as a sensing layer in the QCM crystal. The developed QCM sensors were sensitive to 1-hexanol as well as 1-pentanol as predicted by the simulation algorithm. The estimated lower detection limits of the QCM sensors for detecting 1-hexanol and 1-pentanol were 2-3 ppm and 3-5 ppm, respectively. This study demonstrates the applicability of simulation-based peptide sequence that mimics the olfactory receptor for sensing specific gases.     

Pinecone-shaped ZnO nanostructures: Growth, optical and gas sensor properties.

Pinecone-shaped ZnO nanostructures have been fabricated on Si substrate by pulsed laser deposition. The scanning electron microscope images showed that pinecone-shaped ZnO nanostructure was 6-fold symmetry and has the rough surface on one end. X-ray diffraction, Raman spectra and X-ray photoelectron spectroscopy indicated that the ZnO nanostructures have high crystal quality and a large amount of surface states. Compared with ZnO nanowires and nanobelts, the oxygen gas sensor based on pinecone-shaped ZnO nanostructures has excellent selectivity, fast response and recover, and lower operating temperature. Meanwhile, the response properties are very stable over several circles.     

Synthesis of ZnO thin films and their application as humidity sensors

Humidity sensors were fabricated using ZnO thin films synthesized on a Si wafer substrate. The ZnO thin films were grown via a vapor solid (VS) approach at temperatures ranging from 400 to 700 °C. Experiments were executed to observe the relationships between the relative humidity (RH) and resistance of these devices fabricated under various VS temperatures. Experimental results show that the ZnO thin films grown at a temperature of 700 °C using the VS approach exhibits an optimum sensitivity to humidity. The measured sensor resistance ranges from 495 × 10⁶ to 46 × 10³ Ω for RH ranging from 11 to 95 % at room temperature. The variance of sensor resistance exceeds 10⁴ times, indicating that the proposed method can produce a highly sensitive humidity sensor.     

Detection of heavy metal ions in aqueous solution by P(MBTVBC-co-VIM)-coated QCM sensor

A novel copolymer P(MBTVBC-co-VIM) was designed and successfully synthesized for the fabrication of copolymer-coated QCM sensors to detect the heavy metal ions in aqueous solution. The copolymer P(MBTVBC-co-VIM) contains many nitrogen (N) and sulfur (S) atoms in the side groups as electron donors, which can easily form complexes with heavy metal ions. The strong interaction between the S atom and Au electrode of quartz crystal further assures the stability of copolymer thin films on the quartz crystal surface in aqueous media. The QCM results indicated that the P(MBTVBC-co-VIM)-coated sensor exhibited high sensitivity, stability and selectivity for the detection of Cu²⁺ in aqueous solution. The lowest detection limit can reach 10 ppm Cu²⁺ in aqueous solution, which resulted in the frequency shift of 3.0 Hz (ΔF₃/3). The P(MBTVBC-co-VIM)-coated QCM sensors had porous surface morphologies as revealed by AFM investigation. Such porous structures enhanced the surface areas of the copolymer thin films, which increased the contacting probability of N and S atoms with heavy metal ions in solution and improved the detection sensitivity of the copolymer-coated QCM sensors.     

Enantioselective molecular imprinting polymer coated QCM for the recognition of l-tryptophan

A new quartz crystal microbalance (QCM) sensor that provides enantioselectivity to tryptophan enantiomers, with a high selectivity and sensitivity, was fabricated by the use of the molecularly imprinted polymers (MIPs) as the artificial biomimetic recognition material. Non-covalent molecular imprinting polymers were synthesized using acrylamide (AM) and trimethylolpropane trimethacrylate (TRIM) as a functional monomer and a cross-linking agent, respectively. The preparation of the thin permeable film coatings on QCM surface is described as well as the results and discussion on the sensitivity and selectivity of the coatings to tryptophan enatiomers under different conditions. The sensor was able to discriminate between the l- and d-tryptophan enantiomers in citric acid buffer solutions owing to the enantioselectivity of the imprinted sites. The detection limit is 8.8 μM. The influence of the cross-linking agent concentration on the sensitivity and selectivity of the fabricated polymer films was investigated and optimized. The enantionmeric composition of l- and d-tryptophan enantiomer mixtures can be determined by measuring the frequency shifts of the sample. The characteristics of the thiol monolayer and MIP film on the surface of the QCM were studied by atomic force microscopy (AFM).     

Pd掺杂ZnO纳米线的乙醇气敏特性研究

研究以贵金属Pd作为掺杂剂的ZnO纳米线敏感元件的乙醇气敏性能。采用水热法在叉指电极上直接制备出具有c轴取向的ZnO多晶纳米线。用SEM,XRD和EDS分析手段观测了材料的微观结构,并对其乙醇气敏性能进行了测试。实验结果表明:对于体积分数为200×10-6的乙醇气体,Pd掺杂的ZnO纳米线在灵敏度(8.17)、工作温度(325℃)和响应恢复时间(15,8s)上优于纯的ZnO纳米线。最后,对Pd掺杂的气敏机理进行了讨论。     

Growth and gas sensing characteristics of p- and n-type ZnO nanostructures

ZnO nanoparticles (NPs) of 5-15 nm size and nanowires (NWs) of 50-100 nm dia., exhibiting p and n-type characteristics, respectively, have been synthesized using simple chemical process. ZnO NW-films exhibited good sensitivity and selectivity towards H₂S in ppm range with fast response and recovery times. Interestingly, ZnO NP-films showed p-type conductivity that has been obtained for the first time without intentional doping while NW-films showed n-type conduction as has also been reported in various earlier studies. The p- and n-type conductivities in NP- and NW-films have been confirmed using hot probe and Kelvin probe measurements. The n-type behavior of NW-films is attributed to oxygen vacancies, whereas the p-type nature of NP-films is attributed to the zinc vacancy, surface acceptor levels created by the adsorbed oxygen and/or the unintentional carbon doping in ZnO.     

Highly stable and sensitive humidity sensors based on quartz crystal microbalance coated with bacterial cellulose membrane

A novel highly stable and sensitive humidity sensor based on bacterial cellulose (BC) coated quartz crystal microbalance (QCM) has been successfully fabricated. The results showed that the sensors possessed good sensing characteristics by increasing more than two orders of magnitude with increasing relative humidity (RH) from 5 to 97%, and the Log(Δf) showed good linearity (20-97% RH). The sensitivity of sensors coated with BC membranes was four times higher than that of the corresponding cellulose membranes at 97% RH. In addition, the sensor sensitivity is greatly enhanced by increasing the coating load of the BC membranes with more absorption sites in the sensing membranes. Moreover, the experimental results prove that the resultant sensors exhibited a good reversible behavior and good long term stability. Herein, not only a novel and low-cost humidity sensor material was exploited, but also a new application area for BC nanofibrous membranes was opened up.     

Synthesis of quantum size ZnO crystals and their gas sensing properties for NO2

Quantum size ZnO crystals have been synthesized successfully by a room temperature sol-gel process. Oleic acid (OA) has been used as capping agent to control the particle size of ZnO. The crystal structure and size of the ZnO are characterized by the X-ray diffraction (XRD) and transmission electron microscope (TEM). The XRD results show the as-synthesized ZnO has hexagonal wurtzite structure and the average crystallite size is 5.7 nm which is little less than TEM result. It is testified by photoluminescence (PL) and Raman spectra that the quantum size ZnO keeps the crystal structure of the bulk ZnO and possesses more surface defects. The quantum size ZnO has the highest response of 280 to NO₂ and the highest selectivity of 31 and 49 corresponding to CO and CH₄ at operating temperature of 290 °C. The effect of calcination temperatures on sensing property and transient response of the ZnO sensor are also investigated.     

阻抗分析中激励电平对石英晶体微天平响应性能的影响

阻抗分析法是表征石英晶体微天平(QCM)响应特性的主要手段之一,能够给出QCM的谐振频率及其等效电路参数.在阻抗计分析法诸多测定仪器参数中,施加在石英晶体上的激励电位水平对测定结果的影响少见文献讨论.最近有文献报道称激励电平是一个很关键的测量条件,对QCM的粘度响应模式存在明显的影响.该文系统测定了激励电平对QCM的等效电路参数值以及QCM对表面质量、溶液粘度的影响,结果表明,在5～1 000 mV范围内改变施加在QCM上的激励电平,对QCM的响应特性影响很小,为QCM的阻抗分析中的参数设置提出不同的见解.