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Low power consumption gas sensor created from silicon nanowires/TiO2 core-shell heterojunctions Dong Liu, Leimiao Lin, Qiaofen Chen, Hongzhi Zhou, and Jianmin Wu ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.7b00459 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 13, 2017
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Low power consumption gas sensor created from silicon nanowires/TiO2 core-shell heterojunctions Dong Liu, Leimiao Lin, Qiaofen Chen, Hongzhi Zhou, and Jianmin Wu* Institute of Analytical System, Department of Chemistry, Zhejiang University, Hangzhou, 310058, China KEYWORDS: Silicon nanowires, Titanium dioxide, Core-shell nanostructure, Heterojunction, Methane sensing
ABSTRACT: Silicon nanowires/TiO2 (SiNWs/TiO2) array with core-shell nanostructure was created by sol-gel and dropcasting methods. The hybrid material displayed excellent sensing performance for CH4 detection at room temperature. The chemiresistor sensor has a linear response towards CH4 gas in 30 ~ 120 ppm range with a detection limit of 20 ppm, which is well below most of CH4 sensors reported before. The enhanced gas sensing performance at room temperature was attributed to the creation of heterojunctions that form a depletion layer at the interface of SiNWs and TiO2 layer. Adsorption of oxygen and corresponding gas analyte on TiO2 layer could induce the change of depletion layer thickness and consequently the width of SiNWs conductive channel, leading to a sensitive conductive response towards gas analyte. Compared to conventional metal oxide gas sensor, the room temperature gas sensor constructed from SiNWs/TiO2 do not need additional heating device and work at power of µW level. The low power consumption feature is of great importance for sensing devices, if they are widely deployed and connected to Internet of Things. The innovation of room temperature sensing materials may push forward the integration of gas sensing element with wireless device.
With the advance of smart phone technology, various physical and chemical sensors are being integrated into a cell phone, whose functions not only limit to personal communication, but also expand to healthcare, environmental, food monitoring, and public safety. While optical camera acting as “electronic eye” has played an important role in smartphone technology, gas sensors with the role of “e-nose” still have not been integrated into a commercialized smartphone product. Up to now, the shifts of application from industrial sensor to portable sensor still face some challenges. There are some basic requirements on sensing materials if a gas sensor would be integrated into a smartphone device. First, sensing chip should be small enough without sacrificing its performance, such as sensitivity and repeatability. Second, the sensing materials should be low-power consumption, which means that they can work at room temperature without additional heating device. Miniaturized gas sensors fabricated by various nanomaterials and nanostructures are of intense scientific and technological interest in environmental monitoring and air-quality control [1, 2]. Semiconductor nanowires (NWs) with a large surface-to-volume ratio and high crystalline are exceedingly attractive building blocks for developing high performance gas sensing devices. Compared with membrane and particle-based sensing materials, NWs allow gases to diffuse rapidly and effectively through its network, thereby increasing the availability of surface area participating in gas sensing [3].
Among all types of nanowire materials, silicon nanowires (SiNWs) are promising candidates for gas sensing applications due to their intrinsic advantages. For example, SiNWs are highly compatible with standard CMOS (Complementary Metal Oxide Semiconductor) technology, which made it possible to integrate with electronic devices. More importantly, SiNWs have a relatively large carrier mobility due to its narrow band gap (Si, Eg=1.12 eV) [4], thereby showing good conductivity at room temperature. In addition, various types of heterojunctions and homojuctions can be constructed from SiNWs with different doping types and doping levels [5]. Up to now, SiNWs-based sensors have been applied in the detection of various gas analytes (eg.NH3, NO2, H2O, H2 and VOCs) [6]. However, in a chemiresistor format, SiNWs can only detect gases with high electron withdrawing or donating ability, because only those gases can significantly change the charge carrier density of SiNWs upon their adsorption. For the detection of organic vapors or other types of relative inert gases, Field Effect Transistor (FET) devices are always employed [7], since FET devices are more sensitive than that of chemiresistors. But nanowire-based FET devices require complicated micro-fabrication procedure. Metal oxide semiconductor gas sensors are predominant solid-state gas sensors that have been widely commercialized owing to its low cost, high sensitivity, fast response/recovery, and simple electronic interface [8]. Metal
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oxide gas sensors in chemiresistor format have ability to measure and monitor trace amounts of hazardous gases like NOx, COx, NH3, CH4, H2S, SO2 and so on [9]. But most of metal oxide sensors need to be operated at high temperature or under UV irradiation because of their larger bandgap. Metal oxide usually displays superior catalytic properties towards the oxidation of reducing gas or volatile organic compounds (VOCs) at high temperature, leading to the change of depletion layer thickness on metal oxide surface. For example, TiO2, a typical wide band-gap (3.0~3.2ev) n-type semiconductor has been used in the detection of VOCs at high temperature [10]. To increase the sensitivity of metal oxide sensors, binary metal oxide materials have been studied. For instance, Choi and co-workers [11] investigated the gas sensing behaviors of n-type SnO2 NWs functionalized with p-type Cr2O3 NPs. The sensing performance on the composite materials was greatly improved, owing to the electronic sensitization occurred at p−n heterojunctions and additional expansion of the electron-depletion layer of n-type SnO2 NWs. However, the operation of gas sensors constructed from the binary metal oxide materials still required elevated temperature, which prevented them from being integrated into a smartphone device.
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(China). The Titanium (IV) butoxide was purchased from Sigma-Aldrich. The standard CH4 gas sample with concentration of 1000 ppm (diluted in dry air) was purchased from Nanjing special gas factory Co. (China). The morphologies of the as-prepared SiNWs/TiO2 coreshell nanostructure were investigated using field-emission scanning electron microscopy (FE-SEM, Hitachi SU 8010) and transmission electron microscopy (TEM, Hitachi HT 7700). Elemental mapping was measured by energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS, VG ESCALAB MKII) equipped with an Mg Kα X-ray radiation source. The phase purity of the sample was determined by X-ray diffraction (XRD, Rigaku Ultima IV). Preparation of SiNWs/TiO2 array. Si wafers were cut into pieces with dimension of 5×10 mm2. The silicon nanowires (SiNWs) were prepared by metal assisted chemical etching (MACE) similar to the method as described previously [18]. Briefly, the Si wafers were immersed in 0.004 M AgNO3 and 4.8 M HF for 2 min at room temperature. After the Ag nanoparticles formed, the wafers were washed with deionized water to remove the remained Ag+ ions, and then etched in 4.8 M HF and 0.4 M H2O2 for 2 h under room temperature. Then the wafers were washed repeatedly with deionized water and immersed in the HNO3 solution (1: 1 V/V) for 30 min to dissolve the residue Ag particles. Finally, the SiNWs were subsequently cleaned with deionized water and ethanol before drying under N2 stream.
In the present work, SiNWs with room temperature conductivity were combined with TiO2 layer. To meet this end, we prepared SiNWs/TiO2 array with core-shell structure, in which the SiNWs not only act as a template to grow TiO2 along its surface, but also provide a pervasive substrate to form different heterojunctions between Si and metal oxide, since these two materials have large difference in bandgaps level. To test its gas sensing properties, CH4 was chosen as the target gas because it is highly flammable and explosive [12]. Although metal oxide-based CH4 sensors have been widely commercialized [13-17], most of them need to work at high temperature, imposing a dangerous factor in detecting explosive gases because high temperature might trigger the explosions of CH4. In this way, a room temperature CH4 sensor will offer great advantages, such as reduced energy consumption, simplified device structure, and convenience of deploying such sensors in explosive environments where high temperature is undesirable. The SiNWs/TiO2 core-shell nanostructure presented in this work displayed excellent sensing performance for CH4 detection at room temperature without need of UV light irradiation. Its detection limit is far below the lowest explosion limit (LEL) of CH4. Future integration of room-temperature methane gas sensor into a device will undoubtedly help to ensure the public and family safety.
Scheme 1 Schematic pictures of the fabrication process of SiNWs/TiO2 array and its use for gas sensing. The overall schematic process for the preparation of SiNWs/TiO2 is illustrated in Scheme 1. First, the TiO2 sol was prepared according to the following steps: 5 ml isopropyl alcohol, 0.805 ml H2O and 1.0 ml acetic acid were mixed thoroughly. Then 0.18 ml Titanium (IV) butoxide was added into the above mixture gradually under mechanical stirring at room temperature for 1.5 h until the formation of opalescent TiO2 sol, which was then placed in an ultrasonic bath for 30 min to disperse TiO2 sol uniformly. SiNWs/TiO2 coreshell array was prepared by a drop-diffuse method. Around 20 μL of TiO2 sol was dropped onto the surface of SiNWs array. The chip was then placed in air for
EXPERIMENTAL SECTION Materials and Characterization. Silicon wafers (1 0 0 crystal, p-type, boron-doped, 5-10 Ω cm) was purchased from Beijing XiHe Ruida Technology Co. (China). Si (1 0 0) wafers (n-type, phosphor-doped, 1-10 Ω cm) was purchased from Zhejiang Lijin Co. (China). HF (40%), H2O2 (30%), AgNO3 (>99.9%), HNO3 (65-68%), isopropyl alcohol (iPrOH, ≥99.7%), ethanol (≥99.7%), acetic acid (≥99.5%) were purchased from Sinopharm Chemical Reagent Co.
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several minutes allowing the diffusion of TiO2 sol through the whole surface of vertical SiNWs array which was then dried in ambient air at 60℃. By repeating the above procedure, the thickness of TiO2 layer can be adjusted. In this case, the TiO2 coating step was repeated for four cycles. Finally, the SiNWs array coated with TiO2 layer was calcined in air at 450℃ for 2 h (heating rate 5℃/min).
200 nm (Fig. 1a), although some of SiNWs were slightly bended, forming a congregate bundle structure. The crosssectional SEM image shows that the most of SiNWs with length of ~35 μm are still vertical to the silicon wafer substrate (Fig. 1b). After coating of TiO2 layer, the vertical nanowire structure still remained. Compared with bare SiNWs, the diameter of the SiNWs/TiO2 increased obviously and the surface of nanowires appeared more roughly due to the coverage of TiO2 layer. EDS spectrum shows the existence of Ti element on SiNWs surface (Fig. S1, S2, ESM). The core-shell nanostructure of SiNWs/TiO2 was further studied by TEM, which clearly indicated that the diameter of the SiNW/TiO2 increase to ~400 nm while the bare SiNWs have an average diameter of ~200 nm (Fig. 1c and f). Accordingly, the thickness of TiO2 layer on the surface of SiNWs is around 100 nm. The TEM image observed on the root part of SiNW/TiO2 nanowire also proves that the surface of SiNWs is covered by TiO2 layer (Fig. S3, ESM).
Conductivity measurement for gas sensing. The conductivity change of SiNWs/TiO2 array in response to methane gas was measured in a chemiresistive format at room temperature (~25℃). The concentration of methane gas sample was adjusted by mixing the standard methane sample (1000 ppm) with dry air at a specific ratio controlled by an automatic gas mixing apparatus (National Institute of Metrology, China). The total flow rate of the mixture was kept at 200 mL•min-1. The diluted sample was then admitted to a chamber containing the SiNWs/TiO2 sensing chip, which was contacted with two elastic metal electrodes. A bias voltage applied onto the two electrodes was set at 1.0 V, and the electrical current flowing through the sensing material was measured with a picoamperometer (Keithley 6487, USA). Real-time conductivity change (or current change) was recorded with Labview 8.6 software. The electrical signal in response to methane gas was normalized using the equation: G = (I - I0)/I0, where G is the relative conductive response after exposing the senor to analyte, I0 is the baseline electrical current measured in air stream, and I is the current measured after exposing the sensor to gas analytes. Response and recovery time (tres and trec) are defined as the time to reach 90% of steady-state response after dosing or removing gas analytes, respectively.
Figure 2. XPS high resolution spectra of (a) Si 2p peaks and (b) Ti 2p peaks observed in SiNWs/TiO2 structure (oxidized at 450℃).
RESULTS AND DISCUSSION
The surface chemistry of different nanowire samples was characterized by X-ray photoelectron spectrometer (XPS). As shown in high-resolution XPS, all samples have Si–Si peak at 99.15 eV and Si–O peak at 102.72 eV, which is derived from SiO2 layer formed during thermal treatment (Fig. 2a) [23]. In SiNWs oxide sample, oxygen peak (O1s, 532.67 eV) and silicon peak constitute the major peaks in XPS spectrum. Low intensity of carbon peak (C1s, 285.6 eV) is found, probably due to the carbon contamination (Fig. S4, ESM). In contrast, Titanium peaks can be found either in p-type or n-type SiNWs/TiO2 samples. XPS peaks at 458.2 and 463.6 eV are corresponding to Ti 2p3/2 and Ti 2p1/2 respectively (Fig. 2b), indicating the presence of TiO2 on nanowire surface [24]. High-resolution XPS of SiNWs, SiNWs oxide and SiNWs/TiO 2 samples also confirmed the formation of TiO2 in SiNWs/TiO2 sample (Fig. S5, ESM). The crystallographic structures of pure TiO2 and p-type SiNWs/TiO2 samples were measured by X-Ray Diffraction (XRD). The TiO2 sample is clearly identified as anatase (1 0 1) due to the strong reflection at 2θ=25.4° (Fig. 3 black trace). SiNWs/TiO2 sample shows high intense peaks at 2θ=28.5°, 47.4° and 56.3° corresponding to Si (1 1 1), (2 2 0) and (3 1 1) reflections, respectively (Fig. 3 red trace). Compared with pure TiO2 powder, the peak locations of TiO2 diffraction on SiNW/TiO2 sample are the same, but the
Structural and morphological characteristics. Methods for preparing SiNWs/TiO2 array include chemical vapour deposition (CVD) [19], atomic layer deposition (ALD) [20], co-precipitation [21], sol-gel [22] and so on. In the present work, a sol-gel method was combined with drop-casting procedure to simplify the whole process.
Figure 1. Characterization of SiNWs and SiNWs/TiO2 arrays (a, b) SEM images of SINWs before and (d, e) after TiO2 coating, (a, d) top view images, and (b, e) cross-sectional images， (c) TEM image of an individual SINW before and (f) after being coated with TiO2.
Morphology of the as-prepared SiNWs/TiO2 array is shown in Fig. 1. The top view of SEM image shows that the diameter of individual p type SiNWs is in the range from 100 to
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peak intensities are significantly weaker due to the low content of TiO2 in the composite sample.
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of TiO2 layer was adjusted by changing the times of dropcasting. The results showed that the conductive response increased with the increasing of TiO2 layer thickness until reach a maximal response. Thereafter, response decreased with increase of TiO2 layer thickness (Fig. S6, ESM). Oxygen adsorption on TiO2 can create a depletion layer due to electron transfer. Initial increase in the thickness of TiO2 layer (L) could enhance gas adsorption ability and increase the thickness of the depletion layer (DL). When L < DL, the width of SiNWs conductive channel can be significantly changed, thereby producing sensitive response to gas adsorption. However, when L > DL, the conductive channel of SiNWs will be insensitive to gas adsorption. The length of SiNWs also exerts remarkable influence on the gas sensing performance of the composite materials. With the increasing of SiNWs length, the conductive response towards CH4 significantly increased (Fig. S7, ESM). The results confirm that the SiNWs act as main conduction path in the coreshell structure whereas the TiO2 layer may help to increase the sensitivity. To further investigate the influence of temperature on the sensitivity of semiconductor nanowire gas sensors, the conductive response of the SiNWs/TiO2 to 100 ppm of CH4 was measured at different temperatures in the range from 30 to 150℃. In general, the SiNWs/TiO2 sensor displayed high sensitivity in low temperature range, while the conductive response remarkably decreased with the increasing temperature (Fig. 4b). The temperature-dependent behavior shown in the SiNWs/TiO2 sensor is obviously different from conventional metal oxide-based sensors, which are sensitive only within high temperature range. The observed reduction in the sensor response could be originated from the following reasons. As known, silicon has a high thermal conductivity and most of the heat in Si is carried by phonons with mean free path larger than 300 nm [26]. When the temperature increases, the phonon assisted tunneling will enhanced and results in the decrease of resistance (Fig. S8, ESM) [27], which means the thickness of space charge layer will decrease at the junction of SiNWs-TiO2. In addition, the amount of adsorbed gas will also decrease with the increase of temperature. Both factors lead to the decrease in sensitivity upon elevating temperature. Comparison of gas sensing behavior between the p-type and n-type SiNWs/TiO2 may help to clarify the sensing mechanism. Therefore, the conductive responses to different concentration of CH4 were measured on both types of sensing materials. As shown in the results, there are remarkable differences in responding behavior between p-type and n-type SiNWs/TiO2 sensors. The p-type SiNWs/TiO2 sensors produced negative conductive response to CH4 gas (Fig. 5a), whereas the n-type SiNWs/TiO2 sensor generated positive conductive response in the same concentration range (Fig. 5c). The detection limit for methane is estimated to be 20 ppm (S/N=3), which is far below the LEL of CH4 gas, inferring that the equipment of such a sensor can effectively provide an early warning service.
Figure 3. X-ray diffraction pattern of TiO2 powders (black line) and TiO2 decorated SiNWs (red line) annealed at 450℃. Both lattice layers indexes indicate that TiO2 presented as polycrystalline anatase.
Gas sensing performance. To confirm whether the TiO2 layer plays an important role in methane gas sensing, the conductive responses on three types of samples were measured at room temperature, respectively. Among them, SiNWs/TiO2 acted as the testing sample, whereas SiNWs with and without thermal oxidation was employed as the control samples. Before exposing to CH4, the chamber containing a sensing chip was equilibrated with pure air sample until the baseline became stable. Upon exposing to 200 ppm CH4 gas, the p-type SiNWs/TiO2 generate a large negative conductive response, while the two control samples only produce very small positive response (Fig. 4a). The results indicated that the TiO2 layer exerts a tremendous effect on the conductive response to CH4. It should
Figure 4. The normalized relative response of (a) raw SiNWs (blue), SiNWs oxidized at 450℃ (black), and SiNWs/TiO2 (red) for both p and n type SiNWs towards 200 ppm CH4 at room temperature; (b) the conductive response of n-SiNWs/TiO2 sensor to 100 ppm CH4 at different temperatures.
be noted that all detection was operated at room temperature, at which pure TiO2 with a large bandgap theoretically can’t produce such a significant response because of weak electron transfer property [25]. Therefore, we speculated that there must be a synergic effect caused by the interplay between SiNWs and TiO2 layer. To confirm this, the thickness Table 1 Brief summary of results reported on various CH4 sensors.
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Conc. (ppm)
Response
Temp. (oC)
Reference
SnO2–Ni2O3
200
127%
400
[13]
ZnO nanowalls
100
220%
300
[30]
Porous SnO2 nanorods
125
160%
100
[17]
Cu2O
2500
N
140
[31]
Graphitic carbon nanofibers
5000
99% (=R/R0)
RT
[32]
n-SiNWs/TiO2
120
182%
RT
This work
p-SiNWs/TiO2
120
-50%
RT
This work
*RT: room temperature
in six cycles, displaying good repeatability during the cyclic gas exposure. To test its reproducibility and long-term stability, the sensing chip was stored in ambient air. Repeated dose-response test and its conductive response to 100 ppm CH4 gas was measured at different time, respectively. The results show that the sensor display good reproducibility and stability (Fig. S11 and Fig. S12, ESM). Compared with other types of CH4 sensors (Table 1), the SiNWs/TiO2 sensor fabricated in this work showed excellent sensing performance towards CH4 either in sensitivity or working temperature. However, the sensitivity of both sensors was affected by humidity. With the increase of humidity in air sample, the conductive response towards CH4 decreased (Fig. S13, ESM). In the range of 0~ 60% RH (Relative Humidity), the dose-response curve still follows linear relationship. Accordingly, a humidity sensor need be integrated into the sensor device, so that the results can be rectified. However, if the RH exceeds 60%, a desiccator tube or a moisture resistance membrane should be attached to the gas inlet to eliminate humidity interference. In addition, the sensor could also response to volatile organic compound (VOC) such as acetone, ethanol (Fig. S14, ESM). The responses to the two kinds of VOC were higher than that of CH4, indicating that both compounds have better adsorptive ability on SiNWs/TiO2.
Figure 5. Real-time conductive response of (a) p-type SiNWs/TiO2 and (c) n-type SiNWs/TiO2; (b, d) represent the linear response of p-type and n-type SiNWs/TiO2 sensors to the concentrations of CH4 from 30-200 ppm at room temperature, respectively.
At room temperature, the response (tres) and recovery time (trec) are ~75 s and 191 s for the n-type SiNWs/TiO2 sensor, while for the p-type SiNWs/TiO2, the data is ~90 s and 125 s, respectively (Fig. S9, ESM). Both sensors produce reversible conductivity response towards CH4 gas within several minutes, although the speed of response is not as fast as that of high temperature metal oxide semiconductor sensor. The normalized conductive response of p-type and n-type SiNWs/TiO2 have a linear relationship (R=0.99) with the concentration of CH4 gas in the range from 30~120 ppm (Fig. 5b, d). When the concentration of CH4 exceeds 150 ppm, both sensors tends to be saturated, thereby displaying a non-linear relationship. To test the reproducibility of the SiNWs/TiO2 gas sensor, both sensors were periodically exposed to 100 ppm CH4. Non-steady state response was measured during cyclic dosing, because the sensor need long time to reach a steady-state. The measurement of non-steady state response is reasonable since the initial response is also closely related with the concentration of gas analyte (Fig. S10, ESM). Results shown in Fig. 6a and b indicated that the relative standard deviation (RSD) of conductive response measured on p-type and ntype SiNWs/TiO2 sensor was ~4.0% and 5.0% respectively
Figure 6. Reproducibility checked for (a) p-SiNWs/TiO2 and (b) n-SiNWs/TiO2 to 100 ppm CH4 at room temperature, the blue column represented the time window for dosing of CH4.
Gas sensing mechanism. The sensing mechanisms of p-type and n-type SiNWs/TiO2 arrays are illustrated in Scheme 2a and b, respectively. The junctions formed between the SiNWs and the n-type TiO2 play a vital role in modulating the sensing behavior of the nanowire arrays. As shown in Fig. 5a, the sensing behavior of p-SiNWs/TiO2
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First, oxygen is adsorbed on the 𝑇i𝑂2/𝑆𝑖𝑁𝑊𝑠 surface followed by an electron transfer process. When temperature is lower than 100℃, O2-ad is the dominant specie on TiO2 surface [28]. Upon exposing to CH4, the adsorbed O2-ad may form a weakly bound complex [𝑂2⋯𝐶𝐻4]-, which is an intermediate with lower energy level. The unstable intermediate eventually turns back into O2 and CH4 accompanying by the release of electron to TiO2 [29]. This weak chemisorption mechanism can well explain why the sensor can work under room temperature with a reversible and relative quick adsorption and desorption process. This possible mechanism needs to be further confirmed by in-situ spectroscopy methods in future studies. As a side proof to the sensing mechanism, the conductive response of p-type SiNWs/TiO2 towards NH3 and NO2 were also measured, respectively (Fig. S15, ESM). The NH3 and NO2 were diluted with N2 carrier gas, and did not need the participation of oxygen. This evidence means that both gas can directly exchange electrons with the nanowire. However, the direct electron exchange usually involves strong chemisorption. Consequently, their desorption process was much slower and even couldn’t restore to the original baseline. The gas sensing behavior to both types of gases is well in accordance with the proposed sensing mechanism. For the n-type SiNWs/TiO2 sensor, band bending can also occur at n–n heterojunction [10]. As shown in Scheme 2b, the Fermi level of TiO2 is more positive than that of n-SiNWs, electrons will flow from the conduction band of n-SiNWs to the conduction band of TiO2 and accumulate at the TiO2 side, thereby forming a depletion layer at the n-type SiNWs side. Adsorption of O2 will further increase the thickness of depletion layer at the interface and reduce the conductivity of n-type SiNWs. Once the nanostructure exposed to CH4, it will replace the adsorbed oxygen and reduce the depletion layer of SiNWs, thus leading to enlarge the conducting core and increase the conductivity of SiNWs. From the baseline shown in Fig. 5, the initial current of p-type and n-type SiNWs/TiO2 in air are ~1.4 and 0.8 μA respectively, which can also prove that the adsorption of O2 can increase the conduction path of p-type SiNWs and decrease the conduction path for the n-type SiNWs. According to the proposed mechanism, any reducing gas that can form weak intermediate complex with adsorbed O2- or compete with oxygen adsorption may affect the thickness of depletion layer at the nanowire interface. As shown in supporting information, the n-type SiNWs/TiO2 could even response to the N2 gas mixed with air sample (Fig. S16b, ESM). Nevertheless, the sensitivity to N2 is far lower than that to CH4 (Fig. S16a, ESM), indicating that N2 gas is only physically adsorbed whereas the CH4 gas follow a weak chemisorption process as indicated in our mechanism. The overall gas sensing mechanism of SiNWs/TiO2 is somewhat similar with that of conventional metal oxide semiconductor, except that the adsorption of O2 and gas analyte on SiNWs/TiO2 surface occurred at room temperature. In contrast, metal oxide sensors need high temperature to initiate strong chemical adsorption process, which involve the electron transfer at semiconductor surface and adsorption of O- ion in lattice [28].
sensor is in accordance with that of conventional p-type semiconductor sensor upon exposing to CH4. The results mean that the dominant conductive path in the core–shell nanostructures should be the p-type SiNWs. As illustrated in Scheme 2a, diffusion of charge carrier will take place at the interface of SiNWs and TiO2 owing to the difference in the Fermi level between p-type SiNW and TiO2. Consequently, a depletion region with inner electrical field from TiO2 side to SiNWs side is created at the interface. When O2 molecules adsorb at the TiO2 surface, electron transfer from TiO2 to O2 may take place due to the electron withdrawing ability of O2 molecules. Therefore, the partial positive charged TiO2 layer hindered the diffusion of hole from Si-
Scheme 2. The sensing mechanism of (a) p-SiNWs/TiO2 and (b) n-SiNWs/TiO2 array for CH4 detection at room temperature.
NWs side to TiO2 side, leading to the reduction of depletion layer thickness. Upon exposure to CH4 gas, portion of adsorbed O2- can be replaced by reducing gas and released the trapping electron back to the TiO2 layer, resulting in an increase of depletion layer thickness and decrease in the conductivity of p-type SiNWs owing to narrowed conduction path. However, neither oxygen nor CH4 is active at room temperature, the sensing mechanism under such conditions can’t follow the oxidization pathway, in which the CH4 is oxidized by oxygen. Therefore, the possible mechanism for room-temperature gas sensing behavior can been represented by the following equations: TiO2 /SiNWs + O2 (𝑔) + e− ⇔ O−2(𝑎𝑑−𝑇𝑖𝑂2 /𝑆𝑖𝑁𝑊𝑠) O−2(𝑎𝑑−𝑇𝑖𝑂2 /𝑆𝑖𝑁𝑊𝑠) + CH4 (𝑔) ⇔ [O2 ⋯CH4 ]−𝑎𝑑−𝑇𝑖𝑂2 /𝑆𝑖𝑁𝑊𝑠 ⇔ O2 (𝑔) + CH4(𝑎𝑑−𝑇𝑖𝑂2 /𝑆𝑖𝑁𝑊𝑠) + e−
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Power consumption of the gas sensor. As shown in our experiment data, the sensor can be operated at 1 V DC voltage with an output current of ~10-6 A, so the power consumption of the sensing material was at ~μW level. It means that the senor can continuously work for ~106 h just driven by a conventional cell phone battery, if the power consumption of electronic circuit is not taken into consideration. In contrast, the power consumption of the commercialized metal oxide sensor with a heating device is usually at ~ hundreds of mW level. To demonstrate the feasibility of real application in wireless sensing device, the SiNWs/TiO2 chip was integrated with a prototype of electronic circuit, which can transmit sensing data to cell phone app through a blue tooth. The whole sensing device also includes a micro-pump, humidity and temperature sensor and control module. As shown in Fig. 7, gas sensor network can be established by connecting multiple sensing devices with a mobile phone. Thus, gas leakage or air pollution at multiple sites can be monitored in real time.
port various types of metal oxide semiconductors. Therefore, a wide variety of room temperature gas sensors can be created by appropriately selecting the type of metal oxide. The room-temperature gas sensing technology may push forward the integration of gas sensing element on smartphone or mobile device for daily life security.
ASSOCIATED CONTENT Supporting Information. Further details of the characterization and properties and more gas sensing test is available. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. /
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This work was supported by National Science Foundation of China (No. 21575127), and Zhejiang Provincial Natural Science Foundation of China (No. Z15B050001).
REFERENCES [1] Kauffman, D. R.; Star, A. Carbon nanotube gas and vapor sensors. Angewandte Chemie International Edition 2008, 47, 65506570.
Figure 7. The gas sensor network connected by a mobile phone installed with an Andriod application software, the integrated circuit and the sensing trace shown on cell phone figure are the pictures of real products.
[2] Demami, F.; Ni, L.; Rogel, R.; Salaun, A.-C.; Pichon, L. Silicon nanowires based resistors as gas sensors. Sensors and Actuators B: Chemical 2012, 170, 158-162. [3] Hwang, I.-S.; Lee, E.-B.; Kim, S.-J.; Choi, J.-K.; Cha, J.-H.; Lee, H.-J.; Ju, B.-K.; Lee, J.-H. Gas sensing properties of SnO2 nanowires on micro-heater. Sensors and Actuators B: Chemical 2011, 154, 295300.
CONCLUSION In summary, SiNWs/TiO2 with core-shell nanostructure could be conveniently prepared by coating TiO2 sol on different types of SiNWs, following with thermal treatment. The resulting materials showed ultra-sensitive to CH4 gas at room temperature. In addition, the core-shell structure nanosensor displayed reversible and reproducible response toward CH4. The room temperature gas sensing property was attributed to the creation of heterojunctions which formed a depletion layer at the interface between SiNWs and surface coated TiO2. Adsorption of oxygen and corresponding gas analyte could induce the change of depletion layer thickness, leading to the change of conduction of SiNWs core. We believe that the nanostructure reported in this work is pervasive, since SiNWs core can sup-
[4] Cui, Y.; Zhong, Z.; Wang, D.; Wang, W. U.; Lieber, C. M. High performance silicon nanowire field effect transistors. Nano letters 2003, 3, 149-152. [5] Cui, Y.; Duan, X.; Hu, J.; Lieber, C. M. Doping and electrical transport in silicon nanowires. The Journal of Physical Chemistry B 2000, 104, 5213-5216. [6] Cao, A.; Sudhölter, E. J.; de Smet, L. C. Silicon nanowire based devices for gas-phase sensing. Sensors 2013, 14, 245-271. [7] Shehada, N.; Brönstrup, G.; Funka, K.; Christiansen, S.; Leja, M.; Haick, H. Ultrasensitive silicon nanowire for real-world gas sensing: Noninvasive diagnosis of cancer from breath volatolome. Nano letters 2014, 15, 1288-1295. [8] Choi, J.-K.; Hwang, I.-S.; Kim, S.-J.; Park, J.-S.; Park, S.-S.; Jeong, U.; Kang, Y. C.; Lee, J.-H. Design of selective gas sensors
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using electrospun pd-doped SnO2 hollow nanofibers. Sensors and Actuators B: Chemical 2010, 150, 191-199.
array cathode. Environmental Science & Technology 2009, 43, 7849-55.
[9] Wetchakun, K.; Samerjai, T.; Tamaekong, N.; Liewhiran, C.; Siriwong, C.; Kruefu, V.; Wisitsoraat, A.; Tuantranont, A.; Phanichphant, S. Semiconducting metal oxides as sensors for environmentally hazardous gases. Sensors and Actuators B: Chemical 2011, 160, 580-591.
[20] Yun, J. H.; Boukai, A.; Yang, P. High density n-Si/n-TiO2 core/shell nanowire arrays with enhanced photoactivity. Nano letters 2009, 9, 410-5. [21] Rasool, K.; Rafiq, M. A.; Ahmad, M.; Imran, Z. TiO2 nanoparticles and silicon nanowires hybrid device: Role of interface on electrical, dielectric, and photodetection properties. Applied Physics Letters 2012, 101, 253104-253104-5.
[10] Zeng, W.; Liu, T.; Wang, Z. Sensitivity improvement of TiO2doped SnO2 to volatile organic compounds. Physica E: Low-dimensional Systems and Nanostructures 2010, 43, 633-638.
[22] Tao, B.; Miao, F.; Chu, P. K. Fabrication and photoelectrochemical study of vertically oriented TiO2 /Ag/SiNWs arrays. Journal of Alloys & Compounds 2015, 635, 112-117.
[11] Choi, S.-W.; Katoch, A.; Kim, J.-H.; Kim, S. S. Prominent reducing gas-sensing performances of n-SnO2 nanowires by local creation of p–n heterojunctions by functionalization with pCr2O3 nanoparticles. ACS applied materials & interfaces 2014, 6, 17723-17729.
[23] Errien, N.; Vellutini, L.; Louarn, G.; Froyer, G. Surface characterization of porous silicon after pore opening processes inducing chemical modifications. Applied surface science 2007, 253, 72657271.
[12] De Smedt, G.; De Corte, F.; Notele, R.; Berghmans, J. Comparison of two standard test methods for determining explosion limits of gases at atmospheric conditions. Journal of hazardous materials 1999, 70, 105-113.
[24] McCurdy, P. R.; Sturgess, L. J.; Kohli, S.; Fisher, E. R. Investigation of the PECVD TiO2–Si (100) interface. Applied surface science 2004, 233, 69-79.
[13] Vuong, N. M.; Hieu, N. M.; Hieu, H. N.; Yi, H.; Kim, D.; Han, Y.-S.; Kim, M. Ni2O3-decorated SnO2 particulate films for methane gas sensors. Sensors and Actuators B: Chemical 2014, 192, 327-333.
[25] Eranna, G.; Joshi, B. C.; Runthala, D. P.; Gupta, R. P. Oxide materials for development of integrated gas sensors - a comprehensive review. Crit. Rev. Solid State Mat. Sci. 2004, 29, 111-188. [26] Feser, J. P.; Sadhu, J. S.; Azeredo, B. P.; Hsu, K. H.; Ma, J.; Kim, J.; Seong, M.; Fang, N. X.; Li, X; Ferreira, P. M.; et al. Thermal conductivity of silicon nanowire arrays with controlled roughness. Journal of Applied Physics, 2012, 112, 114306.
[14] Haridas, D.; Gupta, V. Enhanced response characteristics of SnO2 thin film based sensors loaded with Pd clusters for methane detection. Sensors and Actuators B: Chemical 2012, 166, 156-164. [15] Waitz, T.; Wagner, T.; Sauerwald, T.; Kohl, C. D.; Tiemann, M. Ordered mesoporous In2O3: Synthesis by structure replication and application as a methane gas sensor. Advanced Functional Materials 2009, 19, 653-661.
[27] Yu J Y, Chung S W, Heath J R. Silicon nanowires: preparation,
[16] Bhattacharyya, P.; Basu, P.; Saha, H.; Basu, S. Fast response methane sensor using nanocrystalline zinc oxide thin films derived by sol–gel method. Sensors and Actuators B: Chemical 2007, 124, 62-67.
[28] Barsan, N.; Weimar, U. Conduction model of metal oxide gas sensors. Journal of Electroceramics 2001, 7, 143-167.
device fabrication, and transport properties. The Journal of Physical Chemistry B, 2000, 104, 11864-11870.
[29] Borchert, H.; Baerns, M. The effect of oxygen-anion conductivity of metal-oxide doped lanthanum oxide catalysts on hydrocarbon selectivity in the oxidative coupling of methane. J. Catal. 1997, 168, 315-320.
[17] Biaggi-Labiosa, A.; Solá, F.; Lebrón-Colón, M.; Evans, L.; Xu, J.; Hunter, G.; Berger, G.; González, J. A novel methane sensor based on porous SnO2 nanorods: Room temperature to high temperature detection. Nanotechnology 2012, 23, 455501.
[30] Chen, T.-P.; Chang, S.-P.; Hung, F.-Y.; Chang, S.-J.; Hu, Z.-S.; Chen, K.-J. Simple fabrication process for 2d ZnO nanowalls and their potential application as a methane sensor. Sensors 2013, 13, 3941-3950.
[18] Zhang, M.-L.; Peng, K.-Q.; Fan, X.; Jie, J.-S.; Zhang, R.-Q.; Lee, S.-T.; Wong, N.-B. Preparation of large-area uniform silicon nanowires arrays through metal-assisted chemical etching. The Journal of Physical Chemistry C 2008, 112, 4444-4450.
[31] Cheng, Q.; Yan, W.; Randeniya, L.; Zhang, F.; Ostrikov, K. K. Plasma-produced phase-pure cuprous oxide nanowires for methane gas sensing. Journal of Applied Physics 2014, 115, 124310.
[19] Yu, H.; Li, X.; Quan, X.; Chen, S.; Zhang, Y. Effective utilization of visible light (including lambda > 600 nm) in phenol degradation with p-silicon nanowire/TiO2 core/shell heterojunction
[32] Li, W.; Zhang, L.-S.; Wang, Q.; Yu, Y.; Chen, Z.; Cao, C.-Y.; Song, W.-G. Low-cost synthesis of graphitic carbon nanofibers as excellent room temperature sensors for explosive gases. Journal of Materials Chemistry 2012, 22, 15342-15347.
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