Synthesis and high sensing properties of a single Pd-doped SnO2 nanoribbon
© MA et al.; licensee Springer. 2014
Received: 4 July 2014
Accepted: 7 September 2014
Published: 16 September 2014
Monocrystal SnO2 and Pd-SnO2 nanoribbons have been successfully synthesized by thermal evaporation, and novel ethanol sensors based on a single Pd-SnO2 nanoribbon and a single SnO2 nanoribbon were fabricated. The sensing properties of SnO2 nanoribbon (SnO2 NB) and Pd-doped SnO2 nanoribbon (Pd-SnO2 NB) sensors were investigated. The results indicated that the SnO2 NB showed a high sensitivity to ethanol and the Pd-SnO2 NB has a much higher sensitivity of 4.3 at 1,000 ppm of ethanol at 230°C, which is the highest sensitivity for a SnO2-based NB. Pd-SnO2 NB can detect ethanol in a wide range of concentration (1 ~ 1,000 ppm) with a relatively quick response (recovery) time of 8 s (9 s) at a temperature from 100°C to 300°C. In the meantime, the sensing capabilities of the Pd-SnO2 NB under 1 ppm of ethanol at 230°C will help to promote the sensitivity of a single nanoribbon sensor. Excellent performances of such a sensor make it a promising candidate for a device design toward ever-shrinking dimensions because a single nanoribbon device is easily integrated in the electronic devices.
One-dimensional (1D) nanomaterials are attractive building blocks for future high-performance nanoscale devices and sensors [1–3]. With their unique structural characteristics and versatile physical properties, semiconductor nanowires and nanoribbons have been applied to photodetectors , nanolasers , surface-enhanced Raman scattering (SERS) , solar cells , sensors, and so on [8, 9]. It is well known that 1D nanomaterials possess high surface-to-volume ratio, which is crucial to show high sensitivity . Therefore, special attention has been focused on the application of 1D nanomaterials for detecting toxic, flammable, explosive gases and volatile organic compounds (VOCs). For instance, ZnO-CdS coaxial nanocables have shown to enhance sensitivity toward NH3. In2O3-ZnO core-shell nanowires  and Zn doping into the In2O3 nanowires  are found to possess better response toward CO, H2, and ethanol. Among these metal oxides and their derivatives, the 1D SnO2-based nanomaterial is regarded as a promising candidate for gas monitoring .
SnO2-based gas sensor has been studied for the detection of a variety of gases. As reported in the literature, the SnO2-CeO2 nanofiber composite with a Ce content of 6 mol.% exhibited the highest sensor response to ethanol at 250°C . The response to hydrogen of the Pt-decorated bead-like tin oxide nanowire device  is approximately 5.7 times higher than that of its undecorated counterpart. Bimetallic Pd/Pt nanoparticle-functionalized SnO2 nanowires  have a fast response and recovery time to NO2. Although 1D SnO2-based nanomaterial sensors have been proved to detect many kinds of gases and show high sensitivity to oxidizing and reducing gases, the major drawbacks for detecting VOCs are as follows: (1) They are not selective, i.e., They are not able to distinguish a specified VOC when they are exposed to a mixture of reducing gases [18, 19]; (2) the difficulties in using a single nanowire are attributed to the complicated fabrication processes, poor reproducibility, and high costs . In order to remedy these drawbacks or increase the selectivity effectively, various methods are used to improve the sensing properties. One of the best routes to enchance the sensor sensitivity and selectivity is to functionalize the surface of nanowires with rare earth/noble metals, such as Ag , Au , and Rh . Therefore, it is a good selection to dope 1D SnO2 nanomaterials with rare earth/noble metals to optimize their sensing properties.
In this communication, Pd-doped SnO2 nanoribbons were synthesized by thermal evaporation. The sensing properties of a single Pd-doped SnO2 nanoribbon (Pd-SnO2 NB) and its bare counterpart were investigated. It was found that the doping of Pd has a great influence on the electrical properties of the SnO2 nanoribbons (SnO2 NBs), and that the Pd-SnO2 NB sensor possesses reliable, highly sensitive, easily compact, and integrative properties. Our ongoing studies also revealed that doping SnO2 with noble or rare earth metals may be a simple and efficient way to improve sensitivity, selectivity as well as response time, and reduce operating temperature. Therefore, it will become an important research field to dope 1D SnO2-based nanomaterials with noble or rear earth metals for enchancing their sensing properties.
Synthesis of SnO2 nanoribbons and Pd-doped SnO2 nanoribbons
The SnO 2 NBs and Pd-SnO2 NBs were prepared in a horizontal alundum tube (outer diameter of 4.0 cm, length of 100 cm), which was mounted inside a high-temperature tube furnace. For synthesis of SnO 2 NBs, high-purity SnO 2 powders (>99.99 wt.%) were placed into a ceramic boat, which was then loaded into the central region of the alundum tube. A silicon substrate coated with about 10-nm-thick Au film was put into the tube at a distance of about 10 cm from the ceramic boat. After cleaning the tube several times with nitrogen gas, the tube was evacuated by a mechanical pump to a pressure of 1 to 5 Pa. The temperature at the center of the alundum tube was increased to 1,350°C at a rate of 10°C/min, and it was held at this temperature for 2 h. In the whole experiment, argon was flowed at 30 sccm and the pressure inside the tube was maintained at 125 Torr by continuous pumping. After the furnace was cooled to room temperature, white wool-like products were deposited on the silicon substrates.
For preparation of Pd-SnO2 NBs, the starting materials are the mixture of pure SnO2 powders (>99.99 wt.%) and Pd(O2CCH3)2 powders mixed in the weight ratio of 20:1, instead of pure SnO2 powders. The synthesis procedure is repeated for Pd-SnO2 NBs as the above mentioned. After finishing the experiment, Pd-SnO2 NBs were obtained.
The nanoribbons were characterized by X-ray diffraction (XRD, D/max-3B Rigaku, Tokyo, Japan) with Cu-Kα radiation (λ = 0.15406 nm), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) spectra (SEM Quanta 200 ESEM equipped with EDS from FEI Company, Hillsboro, OR, USA). The microstructures of the obtained samples were analyzed by transmission electron microscopy (TEM) and high-resolution electron microscopy (HRTEM) (JEOL 2010 HRTEM, JEOL, Tokyo, Japan).
The production of a single nanoribbon device
The measurement was processed with a static method. The sensor on a heating station with a precision temperature controller was put into a closed stainless steel chamber and the predetermined amount of solvent was injected into the chamber for the measurement of the sensing performance. The chamber was also equipped with a fan and an evaporator. The evaporator was used to accelerate the volatilization of the VOC liquid, and the fan was employed to obtain a homogeneous gas mixture in the chamber . The sensing properties were measured by the Keithley 4200 semiconductor testing system (Keithley Instruments, Inc., Cleveland, OH, USA). The testing bias voltage was 1 V and the testing interval was 3 min.
Results and discussion
Crystal structure and morphology
Further insight into the structure of NBs is obtained from TEM and HRTEM recorded on an individual Pd-SnO2 NB. The low-magnification TEM image of a typical Pd-doped SnO2 NB is presented in Figure 2c. It shows that the nanoribbon is straight with a uniform thickness. The HRTEM in Figure 2d exhibits good crystalline and continuous lattice fringes over a large area, which is acquired by enlarging Figure 2c. The interplanar spacing is 4.766 Å, corresponding to the d (100) interplanar spacing for the tetragonal structure SnO2. Its selected-area electron diffraction (SAED) pattern is shown in the inset of Figure 2d, which shows the lattice plane index and zone axis .
Analysis of gas sensing
where Ra is the sensor resistance in air (base resistance) and Rg is the resistance in a mixture of target gas and air. The response time and recovery time are defined as the time taken by the sensor to achieve 90% of the total resistance change in the case of adsorption and desorption, respectively .
Selectivity is a key parameter of gas sensors. Figure 5b shows the response bar diagram of the two devices to various target gases (1,000 ppm), including ethanol, ethanediol, and acetone at 230°C. It should be noted that the Pd-SnO2 NB exhibits an outstanding selectivity to ethanol at 230°C and its sensitivity is 4.3, which is 2.2 times as much as that of the SnO2 NB. Hence, the highest sensing response of the Pd-SnO2 NB sensor is to ethanol.
We provide more details of the response to 1 ~ 1,250 ppm of ethanol concentrations in Figure 6b. It is observed that the sensor response depended on the approximate linearity on ethanol vapor concentration in the ranges of 1 to 60 and 60 to 1,000 ppm. More importantly, the slope of the curve with concentrations varying from 1 to 60 ppm in the inset of Figure 6b is larger than that obtained from 60 to 1,000 ppm. This result indicates that the Pd-SnO2 NB sensor is suitable for quantitative detection for ethanol at low concentrations, greatly simplifying the use in practical terms. On the other hand, it was observed that the knee responds as the concentration of ethanol equals to 60 ppm. Many research literatures observed this phenomenon in the concentration range of 10 to 50 ppm [25, 27], but few people discussed this phenomenon. To the best of our knowledge, the turning point of alcohol concentration when it reaches 60 ppm on the Pd-SnO2 NB sensor is reported for the first time. The appearance of the inflection point may help us to establish theoretical systems and boost the effect in practical application. At the same time, the response of gas sensors based on metal oxide semiconductors is empirically represented as Rs = a[C] b + 1  at a certain working temperature, and then the above equation can be rewritten as log(Rs - 1) = b log(C) + loga, where a and b are constants and C is the concentration of the target gas. In this current work, log(Rs - 1) exhibits a linear relationship with log(C): Y = 0.6504X - 1.4328, enjoying a good relativity (R = 0.9957), as shown in Figure 6c. Therefore, practical ethanol sensors using these SnO2 NBs can be greatly simplified in detection, due to the linear relationship between S and C obtained in logarithmic forms.
For low temperature detection, the response of a single Pd-SnO2 NB is 1.056 as the concentration of ethanol vapor is 100 ppm at 100°C whereas pure SnO2 NB has no response, as shown in Figure 7b,c. It follows from our experimental results that the single Pd-SnO2 NB may work steadily at 100°C for 100 ppm ethanol at low temperature. Interestingly, the single Pd-SnO2 NB has a relatively stable response to ethanol of 100 ppm at 25°C and its sensitivity is nearly 1.02. For a low temperature, even around room temperature, effective gas detection such as ethanol gas has been a direction that the scientists are trying to conquer. However, a single Pd-SnO2 NB may achieve this goal.
The performance of two kinds of NB sensors
A single Pd-SnO2NB
A single pure SnO2NB
Response time (s)
Recovery time (s)
Response time (s)
Recovery time (s)
During the whole measurement process, similar measurements had been carried out every day in the first seven days, and the performance of the sensor is stable and distinguished. After that, we measured once every 10 days and the performances of the sensors were also stable and reliable. Up to date, it has been several months.
In summary, pure SnO2 NBs and Pd-SnO2 NBs have been successfully synthesized by thermal evaporation at 1,350°C, and a highly sensitive single Pd-SnO2 NB and SnO2 NB sensor devices have been developed. The sensing properties of the two devices were investigated systematically. It is found that the single pure SnO2 NB sensor shows a high sensitivity at 230°C to ethanol, and the Pd-SnO2 NB one exhibits a higher sensitivity of 4.3 to ethanol at the concentration of 1,000 ppm, which is the highest sensitivity for a single SnO2 NB. The Pd-SnO2 NB can detect C2H5OH from 25°C to 300°C for a wide range of concentration (1 ~ 1,000 ppm). Additionally, whether it is ethanol, ethanediol, or acetone, the Pd-SnO2 NB shows a better gas-sensing property than the pure SnO2 NB. Furthermore, both at low temperature and low concentration, the response of Pd-SnO2 NB is better than that of its counterpart.
The ethanol nanosensors described have low power consumption which can respond to as low as 1 ppm of ethanol at 100°C and are easily integrated because of its nanoscale size. In addition, a single nanoribbon/nanobelt or nanorod device with a single crystal structure has no composition segregation and other advantages such as reliable and sensitive properties, low weight, and relatively quick response and recovery time. Hence, it offers a unique route toward miniaturization of sensors while maintaining their functionality and opens up a new horizon for sensor design. Therefore, this kind of gas sensor can be applied to a variety of applications, including leak detection of hydrocarbon fuels, personal health monitoring and environmental monitoring.
The authors would like to acknowledge the help of my teachers and friends. This work was supported by the National Natural Science Foundation (Grant Nos. 10764005 and 11164034), Yunnan Province Natural Science Foundation (Grant No. 2010 DC053), the Key Applied Basic Research Program of Science and Technology Commission Foundation of Yunnan Province (Grant No. 2013FA035), the New Century Training Program Foundation for Talents from the Ministry of Education of China (Grant No. NCET-08-0926), and Innovative Talents of Science and Technology Plan Projects of Yunnan Province (Grant No. 2012HA007).
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