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thorn-sphere-like tungsten oxide composites
Accepted Manuscript Title: Enhanced NO2 sensing characteristics of Au modified porous silicon/thorn-sphere-like tungsten oxide composites Author: Lin Yuan Ming Hu Yulong Wei Wenfeng Ma PII: DOI: Reference:
S0169-4332(16)31504-5 http://dx.doi.org/doi:10.1016/j.apsusc.2016.07.068 APSUSC 33636
To appear in:
APSUSC
Received date: Revised date: Accepted date:
3-2-2016 5-7-2016 10-7-2016
Please cite this article as: Lin Yuan, Ming Hu, Yulong Wei, Wenfeng Ma, Enhanced NO2 sensing characteristics of Au modified porous silicon/thorn-sphere-like tungsten oxide composites, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.07.068 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced NO2 sensing characteristics of Au modified porous silicon/thorn-sphere-like tungsten oxide composites Lin Yuan, Ming Hu＊, Yulong Wei, Wenfeng Ma School of Electronic Information Engineering, Tianjin University, Tianjin 300072, PR China
Corresponding author: Ming Hu Address: School of Electronic Information Engineering, Tianjin University, Tianjin 300072, PR China E-mail address: [email protected] (M. Hu) Tel.: +86 22 27402372; fax: +86 22 27401233
Highlights 1. The thorn-sphere-like WO3 is synthesized through hydrothermal method on the Au-decorated porous silicon (PS) substrates with seed-layer induction. 2. The morphology of WO3 hierarchical nanostructure depends on the Au-sputtering time and hydrothermal reaction time. 3. The results show excellent response characteristics at room temperature even to ppb-level NO2. 4. The hierarchical nanostructure and heterostructure in the sensor play important roles on the enhanced NO2 response.
Abstract The thorn-sphere-like tungsten oxide (WO3) made up by 1D nanorods has been successfully synthesized through hydrothermal method on the Au-modified porous silicon (PS) substrates with seed-layer induction. By using XRD, EDS, FESEM and TEM techniques, we tested and verified that the crystal structure and morphology evolution of WO3 hierarchical nanostructure on the Au-modified PS strongly depend on the Au-sputtering time and hydrothermal reaction time. In addition, by comparing the NO2-sensing properties of the prepared products, we found that the 10 s-Au decorated PS/WO3-3 h (sputtering Au for 10 s and hydrothermal reaction for 3 h) composites sensor behaving as a typical p-type semiconductor and operating at room temperature (RT) exhibits high sensitivity and response characteristics even to ppb-level NO2, which makes this kind of sensor a competitive candidate for NO2-sensing applications. Moreover, the enhanced response may not only due to the high specific surface area but the Au nanoparticles acting as promoters for the spillover effect and forming metal-semiconductor heterojunctions with the PS and WO3. The transmission of electrons and holes in the heterogeneous interface generated among PS, WO3 and Au is proposed to illustrate the p-type response mechanism. Keywords: Tungsten oxide; Au modified PS; Gas sensor; Hierarchical structure; Hydrothermal synthesis; Heterointerface.
1. Introduction Nitrogen dioxide (NO2), which plays a major role in the formation of ozone and acid rain, is one of the most dangerous air pollutants. It may cause illness in children even at extremely low concentrations of sub-ppm level [1]. Hence preparation of a rapid response, good performance NO2 gas sensor is urgently needed. Recently, oxide-semiconductor nanomaterials as gas sensor materials have reflected the excellent performance in terms of NO2 gas detection [2-5]. Small size effect [6], quantum size effect [7] of the low-dimensional nanomaterials lead to their large specific surface area, surface atom number, surface energy and surface tension so that they can show a variety of unique gas sensing properties. Among these oxides, tungsten oxide, an important n-type semiconductor with small band gap and stable physicochemical properties, is regarded as a promising sensing material for NO2 monitoring, considering its excellent sensitivity and selectivity [8, 9]. However, despite the different shapes and sizes of WO3 (nanoparticles, nanotubes, nanorods, nanowires, nanopieces etc.) materials have been used in the detection of NO2 researches, WO3 still has the disadvantage of high working temperature (200–250 °C) in practical application [10-12]. The high-temperature operation will cause energy waste and device instability, aggravate the problems of the components designing complexity and influence the widespread use of WO3 gas sensors. In recent studies, the researches of developing new advanced hierarchical nanomaterials by making some nanometer structure units assemble in a certain order or a self-assembly way and extend in three-dimensional (3D) space have attracted much attention. Applying hierarchical nanostructure oxide materials especially coexist with heterojunction to fabricating gas sensors is an effective method to improve the gas sensitive performance. Li et al. prepared hierarchical self-assembled WO3 nanosheets hollow spheres exhibiting the response as high as 18 to 100 ppb NO2 at 140 °C [13]. Qi et al. synthesized a three-dimensional WO3 hierarchical structure via a hydrothermal process and the repeatable operating temperature was 110 °C [14]. Sharma et al. prepared WO3 nanoclusters-SnO2 heterostructural film gas sensor for detecting low concentration of NO2 gas (10 ppm) at a low operating temperature (100 °C) [15]. The improved response properties exhibited by the devices above are considered because that the hierarchical nanostructures can provide a relatively more active surface and interface, which promotes the gas molecules’ adsorption and desorption from the sensor [16]. Moreover, for the heterogeneous structure, by creating intimate electrical contact at the interface between different components, the Fermi levels across the interface can equilibrate to the same energy, usually resulting in charge transfer that can lead to enhanced sensor performances [17]. Porous silicon, which has gas sensitive properties at room temperature and is easy to be combined with integrated circuit, has been promisingly used as gas response material or substrate. But pure porous silicon gas sensors still have the shortcomings of low sensitivity, poor recovery and selectivity [18, 19]. At present, by modifying the porous silicon, such as decorating metal particles and special treating its surface or compounding with other low-dimensional materials, we can make its overall
performance improved [20-22]. However, few researchers attempt to combine WO3 hierarchical structure with metal modified PS to fabricate gas sensors. The sensors have both the hierarchical structure and heterogeneous structure, which can complement with each other to realize the low response temperature and high gas sensibility. In this work, we will report a facile hydrothermal approach to in situ grow WO3 nanorods microspheres on the PS surface modified by gold nanoparticles through sputtering system. The thorn-sphere-like WO3 gas sensor showed ppb-level (50 ppb) NO2 detection at RT with a fast response characteristic, meanwhile, good sensitivity and selectivity are also achieved. Furthermore, we mainly investigated how the sputtering time and hydrothermal time affected the morphology and NO2-sensing properties of the sensors prepared under different conditions. The sensing mechanism of the Au-decorated PS/WO3 sensors to NO2 was also discussed.
2. Experimental 2.1 Synthesis and characterization of Au-modified PS/WO3 composite The preparation conditions of PS substrates via a galvanostatic electrochemical etching method in a double-tank cell were shown in detail in our previous work [23, 24]. In brief, the p-type (100) silicon wafers with the resistivity of 10-15 Ω cm and 400 μm thicknesses were taken into the electrolyte (1:2 volume mixture of 40 wt.% hydrofluoric acid and 99.5 wt.% N, N-dimethyl formamide (DMF)) etching under the current density of 64 mA/cm2 for 8 min to form the PS. Modification with Au nanoparticles was achieved through KYKY SBC-12 ion sputter apparatus and the sputtering time was varied as 10, 30 and 50 s, respectively. The WO3 nanostructure was in situ synthesized by a two-step process on the Au-modified PS substrate, including the WO3 seed layers growth and the hydrothermal reaction process [25]. In a typical process, 1.65 g sodium tungstate dihydrate (Na2WO4 2H2O) was dissolved into 100 ml DI water and kept stirring for 2 h, then added 3 mol L-1 HCl solution into the solution above until no more precipitation formed. The product was centrifuged and dissolved in 26 ml H2O2 (30 wt%) to yield a homogeneous and stable colloid as the WO3 seed precursor, and subjected to five droping/spin-coating/drying cycles to remove WO3 seed precursor on the Au-decorated PS. Then the substrates were annealed at 650 °C for 2 h in air. After this process, substrates covered with WO3 seed layers were obtained. In the second step, 8.25 g Na2WO4 2H2O was dissolved into 25 ml DI water under magnetic stirring, after adding 3 mol L-1 HCl solution to adjust the pH value to 2.0, we continued to dilute the solution to 250 ml and then the pH value was adjusted to 2.5 using oxalic acid (1 mol L-1). The above 60 ml solution dissolved with 0.878 g sodium chloride (NaCl) was transferred into the Teflon-lined autoclave in which the Au-decorated PS substrate with seed layer was positioned horizontally. The hydrothermal reaction was maintained at 180 °C for 2, 3 and 4 h, respectively. The as-prepared substrates were washed with DI water and dried at 60 °C for 12 h. The formation and growth process of WO3 nanorods in the process of hydrothermal reaction can be described as follows [10]:
WO4 2
2H
H2 WO4 nH2O
WO3 (nuclei)
nH 2O
H 2 WO4 nH 2O
WO3 (nuclei) (n+1)H 2O
WO3 (nanorods)
(1) (2) (3)
Accordingly, these as-developed samples were coded as x s-Au-decorated PS/WO3x h. The morphology and crystalline structures of as-obtained products were characterized by field emission scanning electron microscope (FESEM, Hitachi S-4800), X-ray diffractometer (XRD, RIGAKU D/MAX-2500 V/PC, Cu Ka radiation), field emission transmission electron microscope (FETEM, TECNAI G2F-20), and energy dispersive espectroscopy (EDS) attached on the FETEM. 2.2 Gas sensor fabrication and measurement The gas sensors were fabricated by depositing two Pt electrodes of dimensions 3 mm×3 mm on the top of Au-modified PS/WO3 composite by RF magnetron sputtering with the aid of a shadow mask [26] and the schematic diagram of as-prepared sensor is depicted in Fig.1. The sizes of the silicon substrate and electrodes we adopted and the distribution of the composite materials which are the gas sensitive areas are shown in the picture. In the comparison process, as all of the sensors in contrast experiments used Pt electrodes, the results are based on ignoring the influence of Pt on the catalytic/sensing behavior of the sensors. The gas sensing characteristics were measured in a homemade gas sensing testing system consisting of a glass test chamber (30 L), a flat heating plate and two sets of data acquisition systems, which has been reported in our previous works [27-29]. In the gas response measurement, appropriate volume of target gas was directly injected into the closed test chamber by a microinjector (measuring range is 1/10/100/500 L), and the electrical resistance change of the sensor was continuously monitored and recorded by a personal computer during the gas injecting and exhausting process at the temperature from 25 °C to 200 °C. The gas sensing response or sensitivity was defined as the ratio (S = Ra/Rg) of the electrical resistance in air (Ra) to that in a target gas (Rg). The response time is defined as follows: starting from the gas sensor contacting with a certain concentration of measured gas, the total time needed from Ra changing to Ra–90%(Ra–Rg), and signified with tres. Conversely, the recovery time is calculated from the target gas separating from the gas sensitive element, the time needed from Rg changing to Rg+90%(Ra–Rg), and signified with trec.
3. Results and discussion 3.1 Structural and morphological characteristics The influence of gold content in the surface layer of PS on the morphology of WO3 nanostructures has been studied. FESEM images of the products with Au sputtered for different times (0 s, 10 s, 30 s, 50 s) are presented in Fig. 2 (a–d), and they are all prepared after undergoing hydrothermal reaction for 2 h at 180 °C. As observed in Fig. 2 (a), the dense WO3 nanorods with the diameter about 0.15 μm are grown on the PS
that the pores are distributed orderly and uniformly. Among these nanorods, it appears that some WO3 nanorods grown on the Au free PS surface overlap each other, constituting a stack structure. The WO3 hierarchical structure can be preliminary formed on the PS surface with less Au-sputtering time. Fig. 2 (b) gives the low magnification and high magnification images of the product on PS after sputtering gold nanoparticles for 10 s, which is composed of WO3 nanorods-cluster with the length about 1 μm. However, if continue to increase the density of gold nanoparticles, the density of WO3 nanorods will decrease. Fig. 2 (c) indicates there was a sharp decrease in the density of nanorods when the sputtering time was 30 s, and in Fig. 2 (d), sputtering time was 50 s, the acquired WO3 spindle-like nanorod bundles were already not able to grow out of the holes of PS and they grew parallel to the PS surface inside the holes. Varying with the Au sputtering time, the density of WO3 nanorods changed obviously, but the diameters of the nanorods had no obvious change. Fig. 3 demonstrates XRD patterns to identify the crystalline structures of the x s-Au-decorated PS/WO3-2 h composite. No matter whether mixing gold, for the diffraction peaks of tungsten oxide, pure hexagonal phase WO3 are obtained as the diffraction peaks which can be well assigned to JCPDS # 33-1387, and diffraction peaks of Si can be indexed with the hexagonal silicon, as referred to the JCPDS # 47-1186. These peaks are strong and narrow, indicating good crystallinity of the samples. In Fig. 3 (b)–(d), the peaks represent gold can be observed, corresponding to the cubic phase Au (JCPDS # 65-2870), and the diffraction peaks are relatively low, which can be ascribed to the low amount and low diffraction intensity of Au compared to the characteristic peaks of WO3 between 20o and 50o. From Fig. 3 (a)–(d), it is worth mentioning that the WO3 diffraction peaks relative intensity ratios of the (001) peak over the (200) peak (I001/I200) are 2.127, 2.767, 2.44 and 0.802, respectively, and with gold contents increasing, I001/I200 present the tendency of first increasing to reach maximum when Au-sputtering time is 10 s and then decreasing, which means the quantity of Au nanoparticals on PS surface have a great effect on the orientations of the as-prepared products. This trend coincides with the FESEM patterns showed in Fig. 2 (a)–(d) that the WO3 nanorods in sample of 10 s-Au-decorated PS/WO3-2 h are more inclined to grow perpendicular to the substrate surface possessing the preferential growth in the [001] direction as well as in sample of 50 s-Au-decorated PS/WO3-2 h, the nanorods grow parallel to the PS surface along the [200] direction. Furthermore, the crystallite size of WO3 estimated using the Debye–Scherrer equation based on the (001) ((a)–(c)) and (200) ((d)) peak is found to be 75.8 nm, 78.3 nm, 70.14 nm, 74.11 nm, respectively. Therefore, the crystallite size of the sample sputtered Au for 10 s is the largest. In order to obtain the WO3 hierarchical nanostructures with more stable construction and more uniform distribution on PS, after sputtering the gold particles for 10 s, we altered the hydrothermal reaction time to observe the morphology changes of the samples prepared at 180 °C. In this process, we found that through 3 h of reaction could directly synthesize the 3D thorn-sphere-like WO3 nanostructure on the Au-modified PS substrate. This unique morphology is illustrated with different
magnifications in Fig. 4 (a, b), it can be noticed that these dense distributed microspheres are composed of a crowd of 1D well-oriented individual nanorods as stack units. Fig. 4 (c) is the image of the hierarchical microgroup formed with 4 h of hydrothermal reaction, the WO3 nanorods begin to gather together like spindles lateral growing on the substrate surface and almost entirely cover the gold nanoparticles and holes of PS, which will greatly reduce the specific surface area and harm the entrance and exit of the gas molecules under test [25]. The structural features of WO3 nanorods and Au nanoparticle as constitutional units of the as-prepared thorn-sphere-like hierarchical nanostructures are further investigated using TEM in Fig. 4 (d–e). Fig. 4 (d) gives us a clearer vision that the WO3 nanorod is about 80–150 nm in diameter and up to 1–1.5μm in length, and the diameter of Au nanoparticle is about 90 nm. The WO3 nanorod has a single-crystal structure whose the lattice spacing is measured to be 0.393 nm, along with the (001) plane of the hexagonal WO3, corresponding to the XRD results. The spacing of the lattice fringe of Au nanoparticle is measured to be 0.203 nm and 0.228 nm indexed as the (200) and (111) plane of the cubic gold, respectively. Moreover, the corresponding EDS spectra of the WO3 nanorods and Au nanoparticles are shown in Fig. 4 (f). It shows the atomic concentrations of W, O and Au to be 23.39%, 70.34% and 4.27%, respectively, indicating the atomic ratio between W and O is close to 1:3. Based on the results above, the possible growth mechanism of Au-modified PS/WO3 composites is illustrated in Fig. 5, the Au-sputtering time and the hydrothermal time have significant effects on the morphology of the as-synthesized samples. The reason of the morphology differences in Fig. 5 (a–d) may be as follows: after coating WO3 seed solution on the Au-modified PS, as the Au nanoparticles have occupied a certain surface area of PS, the area left to attach seed layer become smaller, so that in the process of hydrothermal reaction, the plane for the WO3 nuclei growing into nanorods will shrunk, too [30]. Under the condition of less gold, the nucleus on the active sites of PS substrate assemble into large spheres and grow into nanorods along the [001] direction as “leading orientation” with the presence of NaCl [31], forming the thorn-sphere-like WO3 hierarchical nanostructures with dense distribution. When too many gold nanoparticles exist, WO3 nucleus are concentrated on the hole wall of PS and grow perpendicular to it, causing the phenomenon that WO3 nanorods grow parallel to the PS surface and these nanorods pile up in together forming spindle-like nanorod bundles. According to the Fig. 2 (b) and Fig. 4 (a–c), thorn-sphere-like WO3 prepared after hydrothermal reaction for 3 h is more perfect than the 2 h, but for 4 h, the hierarchical nanostructure changed from clusters to bundles. By increasing hydrothermal time, the WO3 nucleus continue to deposit on the as-prepared WO3 nanorods randomly, generating the angle between the [001] direction, resulting in the nanorods falling down and closely packing together to form bundles parallel to the PS substrate as shown in Fig. 4 (c) [32]. 3.2 Gas-sensing properties The response towards 1 ppm NO2 of the x s-Au-decorated PS/WO3-2 h sensors as a function of operating temperature is shown in Fig. 6 (a). It is obvious that the quantity of Au on PS surface plays a crucial role in the sensing mechanism.
Maximum sensitivity is in all cases obtained at 25 ºC, even for 0 s Au sputtering time, hence it was selected as their optimum operating temperature. Notably, the sensor response for 10 s-Au modified sample reveals the best NO2 response and according to the gas sensitive response/recovery process contrasting figures of the four cases shown in Fig. 6 (b–e). The 10 s-Au sample, like 30 s-Au sample, has the highest sensitivity and the shortest response and recovery time at RT. Meanwhile, the resistance of the sensors decreases dramatically at the time injecting NO2, indicating the p-type response of the composite gas sensor. As the significant improvement in the NO2 detection, we will focus on the gas-sensing properties of 10 s-Au-decorated PS/WO3 sensors. Fig. 7 (a) and (b) show typical response curves recorded for 10 s-Au-decorated PS/WO3 with 3, 4 h of hydrothermal time samples respectively towards 1 ppm NO2 at 25 °C. The response and recovery times are found to be 28 s and 721 s when reacting for 3 h, as well as 44 s and 2519 s for 4 h reaction. Fig. 7 (c) shows the sensitivities comparation among 10 s-Au-decorated PS/WO3-2, 3, 4 h sensors to 1 ppm NO2 at RT. The 10 s-Au-decorated PS/thorn-sphere-like WO3 composite prepared through 3 h hydrothermal reaction shows the best gas sensitive properties. Fig. 8 clearly shows the dynamic responses properties of the 10 s-Au-decorated PS/thorn-sphere-like WO3 composite sensor prepared after 3 h of hydrothermal time to different concentrations of NO2 at RT, and the relationship between NO2 concentration and the gas response. The sensor is sensitive to very small amounts of NO2 (50 ppb), indicating that this type of WO3 sensor is indeed capable of NO2 detection at ppb level even under RT, which has always been a challenge in environmental sensing materials [22]. It is also noteworthy that the sensor resistances could return to its initial value after gas removal, exhibiting very good dynamic response-recovery characteristics. Even after five response-recovery cycles to NO2 in the concentration range of 0.05–3 ppm, the recovered resistances and initial resistance are found to be similar, which confirms the reversibility and stability of the sensor. The dependence of gas response and response-recovery times to 1 ppm NO2 of Au-decorated PS/thorn-sphere-like WO3 gas sensor compared with pure WO3 and PS on working temperature were also studied. Fig. 9 (a) shows the relationship of the gas response and operating temperature for the three sensors. The p-type gas response of PS and Au-decorated PS/thorn-sphere-like WO3 gas sensors decreased with the increasing temperature. However, WO3 shows the opposite sensing behavior. The ln(tres) and ln(trec) as a function of the reverse of absolute temperature are shown in Fig. 9 (b) and (c), respectively. Based on the well-known thermal activation function [33], we can calculate the response (∆Eres) and recovery (∆Erec) barrier heights according to the obtained slopes of the lines. As deduced, for the WO3, PS and the compound gas sensors, the ∆Eres are determined to be 108, 68, 34.81 meV, and ∆Erec are found to be 68, 67.4, 36 meV, respectively. The lower barrier height is helpful to set off the response and recovery reaction, distinctly, the composite material gas sensor has the lowest barrier height so that the response and recovery reactions are more likely to happen than the pure sensors. Lastly, the gas response of Au modified PS/thorn-sphere-like WO3 sensor is
compared to those of other NO2 sensors based on nanostructured WO3 reported previously, as is shown in Table 1. It is noteworthy that the sensor in this work exhibits the moderate operating temperature and relative high sensitivity, which verifies the feasibility of its application in practice. 3.3 Selectivity For the practical application, a sensor should have high selectivity. Therefore, the gas responses of Au-modified PS/thorn-sphere-like WO3 composite sensor to the possible interferential gases (NH3, acetone, methanol, ethanol, isopropanol) at RT are shown in Fig. 10. What is attractive is that the sensor shows a weak p-type response to these gases, as their resistance increases under gas exposure. The values mean the gas responses when the sensor is exposed to oxidizing gases and reductive gases, respectively, which are defined as Ra/Rg and Rg/Ra. The results show that the as-developed sensor is insensitive to NH3 and other four reductive organic gases (100 ppm) with evidence of very low response values of 1.05–1.34, which are significantly lower than that to 1 ppm NO2 (5.16). With an enhanced response to NO2 and a decreased response to the other interferential gases, the Au-modified PS/thorn-sphere-like WO3 composite shows high sensitivity and good selectivity to NO2. 3.4. Gas-sensing mechanisms The gas absorption-reaction mechanism of the Au-modified PS/thorn-sphere-like WO3 sensor were taken into consideration, as illustrated in Fig. 11. On the basis of our results, the enhanced NO2 response of the Au-decorated PS/thorn-sphere-like WO3 sensor is mainly attributed to the two key factors: the existence of the gold nanoparticles and the formed special WO 3 hierarchical nanostructures. In particular, for the gold, it embodies two roles in the sensor: acting as the active sensitive points and forming the metal-semiconductor heterointerfaces. Au as the active sensitive points is also known as “spillover effect” to promote the catalytic dissociation of molecular oxygen species [39]. When the composite structure gas sensors is modified with Au and exposed to air, the oxygen atoms migrate onto the sensor surface and form chemisorbed oxygen species as O2- (<100 °C), O- (100–300 °C) and O2(>300 °C) by capturing the free electrons. At the optimum working temperature (RT), the reaction kinematics may be explained by the following reactions (see eqs (4)–(6)) [18]: O2 (gas)
O2 (ads)
O2 (ads) e O2 (ads) e
O2 (ads) 2O (ads)
(4) (5) (6)
With the adsorption of oxygen molecules, an electron depleted space-charge layer is formed on the surface regions of the sensor (Fig. 11 A). Au modification significantly increases the quantity of O- and creates additional active sites, leading to the formation of a deeper depletion region in comparison to that of pure sensors [40].
When the sensor contacts with NO2, NO2 will not only capture the electrons from the gas sensor surface due to its higher electrophilic property but also react with the adsorbed oxygen ions forming the NO2- (ads), as described in Fig. 11 B. The process of the reactions can be described as follows (eqs (7)–(8)) [10, 18]: NO2 (gas) e
NO2 (ads)
NO2 (gas) O2 (ads) 2e
(7) NO2 (ads) 2O (ads)
(8)
The above reactions will decrease the electron concentration on the material surface. For the Au-modified PS/thorn-sphere-like WO3 sensor, the Au nanoparticles form heterojunctions at the interface with p-type PS and n-type WO3, which offers the electronic transport corridor to improve the separation of electron-hole pairs contributing to the enhancement of surface reactions with the adsorbed oxygen so as to improve the gas-sensing properties. To be specific, the band gaps of PS and WO3 are 1.12 eV and 3.3 eV, respectively, and as the work function of the p-type silicon we used (WPS=4.946 eV), Au nanopaticles (WAu=5.2 eV) and tungsten oxide (WWO=6.8 eV) are different from each other, in the PS-Au-WO3 composite materials, the Ohmic contacts will be formed across the interfaces of Au contacting with the two kinds of semiconductor materials. The corresponding energy bands are schematically shown in Fig. 12 (a). When the three materials are in contact, the Fermi levels of them are aligned together. Because WPSWAu, the electrons will flow from PS to Au then from Au to WO3, leading to the valence band of PS bending upward, while the conduction band of WO3 bend downward at the interface with Au. This process causes the holes accumulation in the PS and the electrons accumulation in the WO3, resulting in the so-called charge-separation [41], which forms the built-in electric field in the internal of gold. The efficient charge separation makes the electrons and holes hard to composite together, thus prolonging the service lifetime of the charge carriers as well as promoting the reaction with the adsorbed oxygen [42]. When the Au-modified PS/thorn-sphere-like WO3 sensor is placed in the air, as shown in Fig. 12 (b), because the oxygen has strong electrophilic characteristics and the electrons are mainly concentrated in WO3, the oxygen adsorption is mainly happened in the WO3 nanorods sphere, which leads to the substantial reduction of the electrons in the heterointerface and internal of WO3 nanorods. Meanwhile, in order to maintain the charge carriers balance, the electrons located in the interface will continuously diffuse into the internal of WO3 and recombine with the holes. Based on the above reasons, the electron-depleted layer appears in the heterointerface, such as the blue line in Fig. 12 (b), the conduction band of WO3 bends upward. However, as the Au nanoparticles acting as active sites and thorn-sphere-like WO3 providing high surface areas, the increased quantity of oxygen adsorption causes the formation of an inversion layer (the intrinsic Fermi level Ei lies above the Fermi level EF), as shown in Fig. 12 (b), the conduction band and Ei bend upward as the red
lines, which lead to the greater concentration of the holes than the electrons at the interface. Only a small amount of oxygen molecules adsorbed on the PS, thus making its valence band further bent upward. At this time, for the sensor, the main charge carriers both in PS and WO3 are the holes, thus the conduction type is similar to the p-type semiconductor. When the sensor is exposed in the NO2, the NO2 molecules capture electrons, causing the concentration of holes in the sensor increase, the energy bands change as the green lines shown in Fig.12 (b), which dwindle the resistance of the composite materials sensor. The Au-modified PS/thorn-sphere-like WO3 sensor shows the p-type response to NO2, indicating that the PS plays the leading role in the NO2 sensing. The activity of PS in the compound sensor increases because of the suitable contact area and distribution way of the individual materials in the sensor [43]. As the previous discussion above, the PS in the heterogenous structure dominates the flow direction of electrons, and the Au acts as the facilitator, which urges the electrons-holes separation prompting oxygen molecules mainly adsorbed on the WO3 nanorods, which enhances the response and recovery properties. This can be further proved by the lower ∆Eres and ∆Erec values of the Au-decorated PS/thorn-sphere-like WO3 sensor. Apart from the important role of metal-semiconductor heterointerfaces on gas sensitivity promotion, the special WO3 hierarchical nanostructure is also an important factor for the excellent response capability of the sensor. The globular structure formed by 1D WO3 nanorods and the porous structure of silicon will offer the large active area interacting with the gas molecules. Meanwhile, the well-aligned arrays and uniform distribution of the hierarchical nanostructures make NO2 molecules diffuse agilely in the sensing layer. These features are expected and assumed to result in the excellent gas sensing properties.
Conclusions The thorn-sphere-like (WO3) hierarchical nanostructure has been successfully grown in situ on the PS sputtered by Au for 10 s through hydrothermal reaction at 180 °C for 3 h. Modifying appropriate amount of Au and controlling the hydrothermal reaction time have great effect on the formation of this unique gas sensor. The content of Au nanoparticles and the time of hydrothermal reaction affect the adhering and growing tendency of WO3 nanorods on porous silicon, meanwhile, they will affect the NO2 response performance of the gas sensors. Au-modified PS/thorn-sphere-like WO3 sensor containing both the hierarchical structure and heterogeneous structure realizes the request of RT response and low concentration response. These results demonstrate a promising approach in the development and realization of the competitive materials for using in NO2 sensors.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 61274074, 61271070), Tianjin Key Research Program of Application Foundation and Advanced Technology, China (No. 11JCZDJC15300).
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Table Captions Table 1 Comparisons of the operating temperature and gas responses of the WO3 nanostructured NO2 sensors reported previously.
Figure Captions Fig. 1. The schematic diagram of Au-modified PS/ thorn-sphere-like WO3 sensor. Fig. 2. FESEM images of WO3 nanostructures grew with 2 h-hydrothermal-reaction on PS sputtered with Au for different time periods: (a) 0 s (inset: the PS substrate) (b) 10 s (inset: the high magnification FESEM image of a single WO3 microcluster ) (c) 30 s, (d) 50 s. Fig. 3. XRD patterns of the as-prepared x s-Au-decorated PS/WO3-2 h products: (a) 0 s, (b) 10 s, (c) 30 s, (d) 50 s. Fig. 4. (a) Low magnification and (b) high magnification FESEM images of 10 s-Au-decorated PS/WO3-3 h. (c) 10 s-Au-decorated PS/WO3-4 h (d) TEM image of the WO3 nanorods and Au nanoparticle composite. (e) High magnification TEM image of (d) and (f) the corresponding EDS image. Fig. 5. Supposed growth mechanism of Au-modified PS/WO3 composites Fig. 6. (a) The gas responses toward 1 ppm NO2 of the x s-Au-decorated PS/WO3-2 h sensors with operating temperatures ranging from RT to 200 °C, and (b)-(e) are the dynamic response resistances of the sensors at RT. Fig. 7. The gas responses toward 1 ppm NO2 of the 10 s-Au-decorated PS/WO3 sensors for different hydrothermal time periods: (a) 3 h, (b) 4 h and (c) is the comparation of 10 s-Au-decorated PS/WO3-2, 3, 4 h sensitivities variation curves to 1 ppm NO 2 at RT. Fig. 8. (a) Dynamic responses curves and (b) the gas response of the Au-decorated PS/thorn-sphere-like WO3 sensor as a function of NO2 concentrations at RT. Fig. 9. (a) The gas responses of WO3, PS and Au-decorated PS/WO3 sensors toward 1 ppm NO2 with operating temperatures ranging from RT to 200 °C and the Logarithm of (b) the response time constant and (c) recovery time (both in s) constants versus the reverse of temperature. The dots are experimental data and the lines are the linear fitting functions. Fig. 10. Gas response of the Au-modified PS/thorn-sphere-like WO3 sensor to various gases at RT. Fig. 11. Adsorption and reaction model of the sensing process on the surface of Au-modified PS/thorn-sphere-like WO3 sensor. Fig. 12. Schematical energy bands of (a) the Au-modified PS/thorn-sphere-like WO3 sensor and (b) the energy state changes on exposure to air and NO2. EF denotes Fermi level, Ei—intrinsic Fermi level, Eg—Forbidden band width, W—Work function, Ev, Ec —the valence and the conduction band edges.
Table 1 Comparisons of the operating temperature and gas responses of the WO3 nanostructured NO2 sensors reported previously. Sensing material
Optimal temp. (°C)
NO2 conc. (ppm)
Gas response
Ref.
Au-doped PS/ thorn-sphere-like WO3
RT
1
5.16
Present work
WO3 nanorods
250
20
4.0
[34]
PS/WO3 nanorods
RT
1
3.38
[25]
WO3 hierarchical structure film
RT
1
3.0
[35]
Au-decorated WO3 cross-linked nanodomes
250
1
74.23
[36]
networked WO3 nanowires
250
1
20
[37]
100
0.05
100
[38]
multi-shelled WO3 yolk–shell spheres
S0169-4332(16)31504-5 http://dx.doi.org/doi:10.1016/j.apsusc.2016.07.068 APSUSC 33636
To appear in:
APSUSC
Received date: Revised date: Accepted date:
3-2-2016 5-7-2016 10-7-2016
Please cite this article as: Lin Yuan, Ming Hu, Yulong Wei, Wenfeng Ma, Enhanced NO2 sensing characteristics of Au modified porous silicon/thorn-sphere-like tungsten oxide composites, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.07.068 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced NO2 sensing characteristics of Au modified porous silicon/thorn-sphere-like tungsten oxide composites Lin Yuan, Ming Hu＊, Yulong Wei, Wenfeng Ma School of Electronic Information Engineering, Tianjin University, Tianjin 300072, PR China
Corresponding author: Ming Hu Address: School of Electronic Information Engineering, Tianjin University, Tianjin 300072, PR China E-mail address: [email protected] (M. Hu) Tel.: +86 22 27402372; fax: +86 22 27401233
Highlights 1. The thorn-sphere-like WO3 is synthesized through hydrothermal method on the Au-decorated porous silicon (PS) substrates with seed-layer induction. 2. The morphology of WO3 hierarchical nanostructure depends on the Au-sputtering time and hydrothermal reaction time. 3. The results show excellent response characteristics at room temperature even to ppb-level NO2. 4. The hierarchical nanostructure and heterostructure in the sensor play important roles on the enhanced NO2 response.
Abstract The thorn-sphere-like tungsten oxide (WO3) made up by 1D nanorods has been successfully synthesized through hydrothermal method on the Au-modified porous silicon (PS) substrates with seed-layer induction. By using XRD, EDS, FESEM and TEM techniques, we tested and verified that the crystal structure and morphology evolution of WO3 hierarchical nanostructure on the Au-modified PS strongly depend on the Au-sputtering time and hydrothermal reaction time. In addition, by comparing the NO2-sensing properties of the prepared products, we found that the 10 s-Au decorated PS/WO3-3 h (sputtering Au for 10 s and hydrothermal reaction for 3 h) composites sensor behaving as a typical p-type semiconductor and operating at room temperature (RT) exhibits high sensitivity and response characteristics even to ppb-level NO2, which makes this kind of sensor a competitive candidate for NO2-sensing applications. Moreover, the enhanced response may not only due to the high specific surface area but the Au nanoparticles acting as promoters for the spillover effect and forming metal-semiconductor heterojunctions with the PS and WO3. The transmission of electrons and holes in the heterogeneous interface generated among PS, WO3 and Au is proposed to illustrate the p-type response mechanism. Keywords: Tungsten oxide; Au modified PS; Gas sensor; Hierarchical structure; Hydrothermal synthesis; Heterointerface.
1. Introduction Nitrogen dioxide (NO2), which plays a major role in the formation of ozone and acid rain, is one of the most dangerous air pollutants. It may cause illness in children even at extremely low concentrations of sub-ppm level [1]. Hence preparation of a rapid response, good performance NO2 gas sensor is urgently needed. Recently, oxide-semiconductor nanomaterials as gas sensor materials have reflected the excellent performance in terms of NO2 gas detection [2-5]. Small size effect [6], quantum size effect [7] of the low-dimensional nanomaterials lead to their large specific surface area, surface atom number, surface energy and surface tension so that they can show a variety of unique gas sensing properties. Among these oxides, tungsten oxide, an important n-type semiconductor with small band gap and stable physicochemical properties, is regarded as a promising sensing material for NO2 monitoring, considering its excellent sensitivity and selectivity [8, 9]. However, despite the different shapes and sizes of WO3 (nanoparticles, nanotubes, nanorods, nanowires, nanopieces etc.) materials have been used in the detection of NO2 researches, WO3 still has the disadvantage of high working temperature (200–250 °C) in practical application [10-12]. The high-temperature operation will cause energy waste and device instability, aggravate the problems of the components designing complexity and influence the widespread use of WO3 gas sensors. In recent studies, the researches of developing new advanced hierarchical nanomaterials by making some nanometer structure units assemble in a certain order or a self-assembly way and extend in three-dimensional (3D) space have attracted much attention. Applying hierarchical nanostructure oxide materials especially coexist with heterojunction to fabricating gas sensors is an effective method to improve the gas sensitive performance. Li et al. prepared hierarchical self-assembled WO3 nanosheets hollow spheres exhibiting the response as high as 18 to 100 ppb NO2 at 140 °C [13]. Qi et al. synthesized a three-dimensional WO3 hierarchical structure via a hydrothermal process and the repeatable operating temperature was 110 °C [14]. Sharma et al. prepared WO3 nanoclusters-SnO2 heterostructural film gas sensor for detecting low concentration of NO2 gas (10 ppm) at a low operating temperature (100 °C) [15]. The improved response properties exhibited by the devices above are considered because that the hierarchical nanostructures can provide a relatively more active surface and interface, which promotes the gas molecules’ adsorption and desorption from the sensor [16]. Moreover, for the heterogeneous structure, by creating intimate electrical contact at the interface between different components, the Fermi levels across the interface can equilibrate to the same energy, usually resulting in charge transfer that can lead to enhanced sensor performances [17]. Porous silicon, which has gas sensitive properties at room temperature and is easy to be combined with integrated circuit, has been promisingly used as gas response material or substrate. But pure porous silicon gas sensors still have the shortcomings of low sensitivity, poor recovery and selectivity [18, 19]. At present, by modifying the porous silicon, such as decorating metal particles and special treating its surface or compounding with other low-dimensional materials, we can make its overall
performance improved [20-22]. However, few researchers attempt to combine WO3 hierarchical structure with metal modified PS to fabricate gas sensors. The sensors have both the hierarchical structure and heterogeneous structure, which can complement with each other to realize the low response temperature and high gas sensibility. In this work, we will report a facile hydrothermal approach to in situ grow WO3 nanorods microspheres on the PS surface modified by gold nanoparticles through sputtering system. The thorn-sphere-like WO3 gas sensor showed ppb-level (50 ppb) NO2 detection at RT with a fast response characteristic, meanwhile, good sensitivity and selectivity are also achieved. Furthermore, we mainly investigated how the sputtering time and hydrothermal time affected the morphology and NO2-sensing properties of the sensors prepared under different conditions. The sensing mechanism of the Au-decorated PS/WO3 sensors to NO2 was also discussed.
2. Experimental 2.1 Synthesis and characterization of Au-modified PS/WO3 composite The preparation conditions of PS substrates via a galvanostatic electrochemical etching method in a double-tank cell were shown in detail in our previous work [23, 24]. In brief, the p-type (100) silicon wafers with the resistivity of 10-15 Ω cm and 400 μm thicknesses were taken into the electrolyte (1:2 volume mixture of 40 wt.% hydrofluoric acid and 99.5 wt.% N, N-dimethyl formamide (DMF)) etching under the current density of 64 mA/cm2 for 8 min to form the PS. Modification with Au nanoparticles was achieved through KYKY SBC-12 ion sputter apparatus and the sputtering time was varied as 10, 30 and 50 s, respectively. The WO3 nanostructure was in situ synthesized by a two-step process on the Au-modified PS substrate, including the WO3 seed layers growth and the hydrothermal reaction process [25]. In a typical process, 1.65 g sodium tungstate dihydrate (Na2WO4 2H2O) was dissolved into 100 ml DI water and kept stirring for 2 h, then added 3 mol L-1 HCl solution into the solution above until no more precipitation formed. The product was centrifuged and dissolved in 26 ml H2O2 (30 wt%) to yield a homogeneous and stable colloid as the WO3 seed precursor, and subjected to five droping/spin-coating/drying cycles to remove WO3 seed precursor on the Au-decorated PS. Then the substrates were annealed at 650 °C for 2 h in air. After this process, substrates covered with WO3 seed layers were obtained. In the second step, 8.25 g Na2WO4 2H2O was dissolved into 25 ml DI water under magnetic stirring, after adding 3 mol L-1 HCl solution to adjust the pH value to 2.0, we continued to dilute the solution to 250 ml and then the pH value was adjusted to 2.5 using oxalic acid (1 mol L-1). The above 60 ml solution dissolved with 0.878 g sodium chloride (NaCl) was transferred into the Teflon-lined autoclave in which the Au-decorated PS substrate with seed layer was positioned horizontally. The hydrothermal reaction was maintained at 180 °C for 2, 3 and 4 h, respectively. The as-prepared substrates were washed with DI water and dried at 60 °C for 12 h. The formation and growth process of WO3 nanorods in the process of hydrothermal reaction can be described as follows [10]:
WO4 2
2H
H2 WO4 nH2O
WO3 (nuclei)
nH 2O
H 2 WO4 nH 2O
WO3 (nuclei) (n+1)H 2O
WO3 (nanorods)
(1) (2) (3)
Accordingly, these as-developed samples were coded as x s-Au-decorated PS/WO3x h. The morphology and crystalline structures of as-obtained products were characterized by field emission scanning electron microscope (FESEM, Hitachi S-4800), X-ray diffractometer (XRD, RIGAKU D/MAX-2500 V/PC, Cu Ka radiation), field emission transmission electron microscope (FETEM, TECNAI G2F-20), and energy dispersive espectroscopy (EDS) attached on the FETEM. 2.2 Gas sensor fabrication and measurement The gas sensors were fabricated by depositing two Pt electrodes of dimensions 3 mm×3 mm on the top of Au-modified PS/WO3 composite by RF magnetron sputtering with the aid of a shadow mask [26] and the schematic diagram of as-prepared sensor is depicted in Fig.1. The sizes of the silicon substrate and electrodes we adopted and the distribution of the composite materials which are the gas sensitive areas are shown in the picture. In the comparison process, as all of the sensors in contrast experiments used Pt electrodes, the results are based on ignoring the influence of Pt on the catalytic/sensing behavior of the sensors. The gas sensing characteristics were measured in a homemade gas sensing testing system consisting of a glass test chamber (30 L), a flat heating plate and two sets of data acquisition systems, which has been reported in our previous works [27-29]. In the gas response measurement, appropriate volume of target gas was directly injected into the closed test chamber by a microinjector (measuring range is 1/10/100/500 L), and the electrical resistance change of the sensor was continuously monitored and recorded by a personal computer during the gas injecting and exhausting process at the temperature from 25 °C to 200 °C. The gas sensing response or sensitivity was defined as the ratio (S = Ra/Rg) of the electrical resistance in air (Ra) to that in a target gas (Rg). The response time is defined as follows: starting from the gas sensor contacting with a certain concentration of measured gas, the total time needed from Ra changing to Ra–90%(Ra–Rg), and signified with tres. Conversely, the recovery time is calculated from the target gas separating from the gas sensitive element, the time needed from Rg changing to Rg+90%(Ra–Rg), and signified with trec.
3. Results and discussion 3.1 Structural and morphological characteristics The influence of gold content in the surface layer of PS on the morphology of WO3 nanostructures has been studied. FESEM images of the products with Au sputtered for different times (0 s, 10 s, 30 s, 50 s) are presented in Fig. 2 (a–d), and they are all prepared after undergoing hydrothermal reaction for 2 h at 180 °C. As observed in Fig. 2 (a), the dense WO3 nanorods with the diameter about 0.15 μm are grown on the PS
that the pores are distributed orderly and uniformly. Among these nanorods, it appears that some WO3 nanorods grown on the Au free PS surface overlap each other, constituting a stack structure. The WO3 hierarchical structure can be preliminary formed on the PS surface with less Au-sputtering time. Fig. 2 (b) gives the low magnification and high magnification images of the product on PS after sputtering gold nanoparticles for 10 s, which is composed of WO3 nanorods-cluster with the length about 1 μm. However, if continue to increase the density of gold nanoparticles, the density of WO3 nanorods will decrease. Fig. 2 (c) indicates there was a sharp decrease in the density of nanorods when the sputtering time was 30 s, and in Fig. 2 (d), sputtering time was 50 s, the acquired WO3 spindle-like nanorod bundles were already not able to grow out of the holes of PS and they grew parallel to the PS surface inside the holes. Varying with the Au sputtering time, the density of WO3 nanorods changed obviously, but the diameters of the nanorods had no obvious change. Fig. 3 demonstrates XRD patterns to identify the crystalline structures of the x s-Au-decorated PS/WO3-2 h composite. No matter whether mixing gold, for the diffraction peaks of tungsten oxide, pure hexagonal phase WO3 are obtained as the diffraction peaks which can be well assigned to JCPDS # 33-1387, and diffraction peaks of Si can be indexed with the hexagonal silicon, as referred to the JCPDS # 47-1186. These peaks are strong and narrow, indicating good crystallinity of the samples. In Fig. 3 (b)–(d), the peaks represent gold can be observed, corresponding to the cubic phase Au (JCPDS # 65-2870), and the diffraction peaks are relatively low, which can be ascribed to the low amount and low diffraction intensity of Au compared to the characteristic peaks of WO3 between 20o and 50o. From Fig. 3 (a)–(d), it is worth mentioning that the WO3 diffraction peaks relative intensity ratios of the (001) peak over the (200) peak (I001/I200) are 2.127, 2.767, 2.44 and 0.802, respectively, and with gold contents increasing, I001/I200 present the tendency of first increasing to reach maximum when Au-sputtering time is 10 s and then decreasing, which means the quantity of Au nanoparticals on PS surface have a great effect on the orientations of the as-prepared products. This trend coincides with the FESEM patterns showed in Fig. 2 (a)–(d) that the WO3 nanorods in sample of 10 s-Au-decorated PS/WO3-2 h are more inclined to grow perpendicular to the substrate surface possessing the preferential growth in the [001] direction as well as in sample of 50 s-Au-decorated PS/WO3-2 h, the nanorods grow parallel to the PS surface along the [200] direction. Furthermore, the crystallite size of WO3 estimated using the Debye–Scherrer equation based on the (001) ((a)–(c)) and (200) ((d)) peak is found to be 75.8 nm, 78.3 nm, 70.14 nm, 74.11 nm, respectively. Therefore, the crystallite size of the sample sputtered Au for 10 s is the largest. In order to obtain the WO3 hierarchical nanostructures with more stable construction and more uniform distribution on PS, after sputtering the gold particles for 10 s, we altered the hydrothermal reaction time to observe the morphology changes of the samples prepared at 180 °C. In this process, we found that through 3 h of reaction could directly synthesize the 3D thorn-sphere-like WO3 nanostructure on the Au-modified PS substrate. This unique morphology is illustrated with different
magnifications in Fig. 4 (a, b), it can be noticed that these dense distributed microspheres are composed of a crowd of 1D well-oriented individual nanorods as stack units. Fig. 4 (c) is the image of the hierarchical microgroup formed with 4 h of hydrothermal reaction, the WO3 nanorods begin to gather together like spindles lateral growing on the substrate surface and almost entirely cover the gold nanoparticles and holes of PS, which will greatly reduce the specific surface area and harm the entrance and exit of the gas molecules under test [25]. The structural features of WO3 nanorods and Au nanoparticle as constitutional units of the as-prepared thorn-sphere-like hierarchical nanostructures are further investigated using TEM in Fig. 4 (d–e). Fig. 4 (d) gives us a clearer vision that the WO3 nanorod is about 80–150 nm in diameter and up to 1–1.5μm in length, and the diameter of Au nanoparticle is about 90 nm. The WO3 nanorod has a single-crystal structure whose the lattice spacing is measured to be 0.393 nm, along with the (001) plane of the hexagonal WO3, corresponding to the XRD results. The spacing of the lattice fringe of Au nanoparticle is measured to be 0.203 nm and 0.228 nm indexed as the (200) and (111) plane of the cubic gold, respectively. Moreover, the corresponding EDS spectra of the WO3 nanorods and Au nanoparticles are shown in Fig. 4 (f). It shows the atomic concentrations of W, O and Au to be 23.39%, 70.34% and 4.27%, respectively, indicating the atomic ratio between W and O is close to 1:3. Based on the results above, the possible growth mechanism of Au-modified PS/WO3 composites is illustrated in Fig. 5, the Au-sputtering time and the hydrothermal time have significant effects on the morphology of the as-synthesized samples. The reason of the morphology differences in Fig. 5 (a–d) may be as follows: after coating WO3 seed solution on the Au-modified PS, as the Au nanoparticles have occupied a certain surface area of PS, the area left to attach seed layer become smaller, so that in the process of hydrothermal reaction, the plane for the WO3 nuclei growing into nanorods will shrunk, too [30]. Under the condition of less gold, the nucleus on the active sites of PS substrate assemble into large spheres and grow into nanorods along the [001] direction as “leading orientation” with the presence of NaCl [31], forming the thorn-sphere-like WO3 hierarchical nanostructures with dense distribution. When too many gold nanoparticles exist, WO3 nucleus are concentrated on the hole wall of PS and grow perpendicular to it, causing the phenomenon that WO3 nanorods grow parallel to the PS surface and these nanorods pile up in together forming spindle-like nanorod bundles. According to the Fig. 2 (b) and Fig. 4 (a–c), thorn-sphere-like WO3 prepared after hydrothermal reaction for 3 h is more perfect than the 2 h, but for 4 h, the hierarchical nanostructure changed from clusters to bundles. By increasing hydrothermal time, the WO3 nucleus continue to deposit on the as-prepared WO3 nanorods randomly, generating the angle between the [001] direction, resulting in the nanorods falling down and closely packing together to form bundles parallel to the PS substrate as shown in Fig. 4 (c) [32]. 3.2 Gas-sensing properties The response towards 1 ppm NO2 of the x s-Au-decorated PS/WO3-2 h sensors as a function of operating temperature is shown in Fig. 6 (a). It is obvious that the quantity of Au on PS surface plays a crucial role in the sensing mechanism.
Maximum sensitivity is in all cases obtained at 25 ºC, even for 0 s Au sputtering time, hence it was selected as their optimum operating temperature. Notably, the sensor response for 10 s-Au modified sample reveals the best NO2 response and according to the gas sensitive response/recovery process contrasting figures of the four cases shown in Fig. 6 (b–e). The 10 s-Au sample, like 30 s-Au sample, has the highest sensitivity and the shortest response and recovery time at RT. Meanwhile, the resistance of the sensors decreases dramatically at the time injecting NO2, indicating the p-type response of the composite gas sensor. As the significant improvement in the NO2 detection, we will focus on the gas-sensing properties of 10 s-Au-decorated PS/WO3 sensors. Fig. 7 (a) and (b) show typical response curves recorded for 10 s-Au-decorated PS/WO3 with 3, 4 h of hydrothermal time samples respectively towards 1 ppm NO2 at 25 °C. The response and recovery times are found to be 28 s and 721 s when reacting for 3 h, as well as 44 s and 2519 s for 4 h reaction. Fig. 7 (c) shows the sensitivities comparation among 10 s-Au-decorated PS/WO3-2, 3, 4 h sensors to 1 ppm NO2 at RT. The 10 s-Au-decorated PS/thorn-sphere-like WO3 composite prepared through 3 h hydrothermal reaction shows the best gas sensitive properties. Fig. 8 clearly shows the dynamic responses properties of the 10 s-Au-decorated PS/thorn-sphere-like WO3 composite sensor prepared after 3 h of hydrothermal time to different concentrations of NO2 at RT, and the relationship between NO2 concentration and the gas response. The sensor is sensitive to very small amounts of NO2 (50 ppb), indicating that this type of WO3 sensor is indeed capable of NO2 detection at ppb level even under RT, which has always been a challenge in environmental sensing materials [22]. It is also noteworthy that the sensor resistances could return to its initial value after gas removal, exhibiting very good dynamic response-recovery characteristics. Even after five response-recovery cycles to NO2 in the concentration range of 0.05–3 ppm, the recovered resistances and initial resistance are found to be similar, which confirms the reversibility and stability of the sensor. The dependence of gas response and response-recovery times to 1 ppm NO2 of Au-decorated PS/thorn-sphere-like WO3 gas sensor compared with pure WO3 and PS on working temperature were also studied. Fig. 9 (a) shows the relationship of the gas response and operating temperature for the three sensors. The p-type gas response of PS and Au-decorated PS/thorn-sphere-like WO3 gas sensors decreased with the increasing temperature. However, WO3 shows the opposite sensing behavior. The ln(tres) and ln(trec) as a function of the reverse of absolute temperature are shown in Fig. 9 (b) and (c), respectively. Based on the well-known thermal activation function [33], we can calculate the response (∆Eres) and recovery (∆Erec) barrier heights according to the obtained slopes of the lines. As deduced, for the WO3, PS and the compound gas sensors, the ∆Eres are determined to be 108, 68, 34.81 meV, and ∆Erec are found to be 68, 67.4, 36 meV, respectively. The lower barrier height is helpful to set off the response and recovery reaction, distinctly, the composite material gas sensor has the lowest barrier height so that the response and recovery reactions are more likely to happen than the pure sensors. Lastly, the gas response of Au modified PS/thorn-sphere-like WO3 sensor is
compared to those of other NO2 sensors based on nanostructured WO3 reported previously, as is shown in Table 1. It is noteworthy that the sensor in this work exhibits the moderate operating temperature and relative high sensitivity, which verifies the feasibility of its application in practice. 3.3 Selectivity For the practical application, a sensor should have high selectivity. Therefore, the gas responses of Au-modified PS/thorn-sphere-like WO3 composite sensor to the possible interferential gases (NH3, acetone, methanol, ethanol, isopropanol) at RT are shown in Fig. 10. What is attractive is that the sensor shows a weak p-type response to these gases, as their resistance increases under gas exposure. The values mean the gas responses when the sensor is exposed to oxidizing gases and reductive gases, respectively, which are defined as Ra/Rg and Rg/Ra. The results show that the as-developed sensor is insensitive to NH3 and other four reductive organic gases (100 ppm) with evidence of very low response values of 1.05–1.34, which are significantly lower than that to 1 ppm NO2 (5.16). With an enhanced response to NO2 and a decreased response to the other interferential gases, the Au-modified PS/thorn-sphere-like WO3 composite shows high sensitivity and good selectivity to NO2. 3.4. Gas-sensing mechanisms The gas absorption-reaction mechanism of the Au-modified PS/thorn-sphere-like WO3 sensor were taken into consideration, as illustrated in Fig. 11. On the basis of our results, the enhanced NO2 response of the Au-decorated PS/thorn-sphere-like WO3 sensor is mainly attributed to the two key factors: the existence of the gold nanoparticles and the formed special WO 3 hierarchical nanostructures. In particular, for the gold, it embodies two roles in the sensor: acting as the active sensitive points and forming the metal-semiconductor heterointerfaces. Au as the active sensitive points is also known as “spillover effect” to promote the catalytic dissociation of molecular oxygen species [39]. When the composite structure gas sensors is modified with Au and exposed to air, the oxygen atoms migrate onto the sensor surface and form chemisorbed oxygen species as O2- (<100 °C), O- (100–300 °C) and O2(>300 °C) by capturing the free electrons. At the optimum working temperature (RT), the reaction kinematics may be explained by the following reactions (see eqs (4)–(6)) [18]: O2 (gas)
O2 (ads)
O2 (ads) e O2 (ads) e
O2 (ads) 2O (ads)
(4) (5) (6)
With the adsorption of oxygen molecules, an electron depleted space-charge layer is formed on the surface regions of the sensor (Fig. 11 A). Au modification significantly increases the quantity of O- and creates additional active sites, leading to the formation of a deeper depletion region in comparison to that of pure sensors [40].
When the sensor contacts with NO2, NO2 will not only capture the electrons from the gas sensor surface due to its higher electrophilic property but also react with the adsorbed oxygen ions forming the NO2- (ads), as described in Fig. 11 B. The process of the reactions can be described as follows (eqs (7)–(8)) [10, 18]: NO2 (gas) e
NO2 (ads)
NO2 (gas) O2 (ads) 2e
(7) NO2 (ads) 2O (ads)
(8)
The above reactions will decrease the electron concentration on the material surface. For the Au-modified PS/thorn-sphere-like WO3 sensor, the Au nanoparticles form heterojunctions at the interface with p-type PS and n-type WO3, which offers the electronic transport corridor to improve the separation of electron-hole pairs contributing to the enhancement of surface reactions with the adsorbed oxygen so as to improve the gas-sensing properties. To be specific, the band gaps of PS and WO3 are 1.12 eV and 3.3 eV, respectively, and as the work function of the p-type silicon we used (WPS=4.946 eV), Au nanopaticles (WAu=5.2 eV) and tungsten oxide (WWO=6.8 eV) are different from each other, in the PS-Au-WO3 composite materials, the Ohmic contacts will be formed across the interfaces of Au contacting with the two kinds of semiconductor materials. The corresponding energy bands are schematically shown in Fig. 12 (a). When the three materials are in contact, the Fermi levels of them are aligned together. Because WPS
lines, which lead to the greater concentration of the holes than the electrons at the interface. Only a small amount of oxygen molecules adsorbed on the PS, thus making its valence band further bent upward. At this time, for the sensor, the main charge carriers both in PS and WO3 are the holes, thus the conduction type is similar to the p-type semiconductor. When the sensor is exposed in the NO2, the NO2 molecules capture electrons, causing the concentration of holes in the sensor increase, the energy bands change as the green lines shown in Fig.12 (b), which dwindle the resistance of the composite materials sensor. The Au-modified PS/thorn-sphere-like WO3 sensor shows the p-type response to NO2, indicating that the PS plays the leading role in the NO2 sensing. The activity of PS in the compound sensor increases because of the suitable contact area and distribution way of the individual materials in the sensor [43]. As the previous discussion above, the PS in the heterogenous structure dominates the flow direction of electrons, and the Au acts as the facilitator, which urges the electrons-holes separation prompting oxygen molecules mainly adsorbed on the WO3 nanorods, which enhances the response and recovery properties. This can be further proved by the lower ∆Eres and ∆Erec values of the Au-decorated PS/thorn-sphere-like WO3 sensor. Apart from the important role of metal-semiconductor heterointerfaces on gas sensitivity promotion, the special WO3 hierarchical nanostructure is also an important factor for the excellent response capability of the sensor. The globular structure formed by 1D WO3 nanorods and the porous structure of silicon will offer the large active area interacting with the gas molecules. Meanwhile, the well-aligned arrays and uniform distribution of the hierarchical nanostructures make NO2 molecules diffuse agilely in the sensing layer. These features are expected and assumed to result in the excellent gas sensing properties.
Conclusions The thorn-sphere-like (WO3) hierarchical nanostructure has been successfully grown in situ on the PS sputtered by Au for 10 s through hydrothermal reaction at 180 °C for 3 h. Modifying appropriate amount of Au and controlling the hydrothermal reaction time have great effect on the formation of this unique gas sensor. The content of Au nanoparticles and the time of hydrothermal reaction affect the adhering and growing tendency of WO3 nanorods on porous silicon, meanwhile, they will affect the NO2 response performance of the gas sensors. Au-modified PS/thorn-sphere-like WO3 sensor containing both the hierarchical structure and heterogeneous structure realizes the request of RT response and low concentration response. These results demonstrate a promising approach in the development and realization of the competitive materials for using in NO2 sensors.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 61274074, 61271070), Tianjin Key Research Program of Application Foundation and Advanced Technology, China (No. 11JCZDJC15300).
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Table Captions Table 1 Comparisons of the operating temperature and gas responses of the WO3 nanostructured NO2 sensors reported previously.
Figure Captions Fig. 1. The schematic diagram of Au-modified PS/ thorn-sphere-like WO3 sensor. Fig. 2. FESEM images of WO3 nanostructures grew with 2 h-hydrothermal-reaction on PS sputtered with Au for different time periods: (a) 0 s (inset: the PS substrate) (b) 10 s (inset: the high magnification FESEM image of a single WO3 microcluster ) (c) 30 s, (d) 50 s. Fig. 3. XRD patterns of the as-prepared x s-Au-decorated PS/WO3-2 h products: (a) 0 s, (b) 10 s, (c) 30 s, (d) 50 s. Fig. 4. (a) Low magnification and (b) high magnification FESEM images of 10 s-Au-decorated PS/WO3-3 h. (c) 10 s-Au-decorated PS/WO3-4 h (d) TEM image of the WO3 nanorods and Au nanoparticle composite. (e) High magnification TEM image of (d) and (f) the corresponding EDS image. Fig. 5. Supposed growth mechanism of Au-modified PS/WO3 composites Fig. 6. (a) The gas responses toward 1 ppm NO2 of the x s-Au-decorated PS/WO3-2 h sensors with operating temperatures ranging from RT to 200 °C, and (b)-(e) are the dynamic response resistances of the sensors at RT. Fig. 7. The gas responses toward 1 ppm NO2 of the 10 s-Au-decorated PS/WO3 sensors for different hydrothermal time periods: (a) 3 h, (b) 4 h and (c) is the comparation of 10 s-Au-decorated PS/WO3-2, 3, 4 h sensitivities variation curves to 1 ppm NO 2 at RT. Fig. 8. (a) Dynamic responses curves and (b) the gas response of the Au-decorated PS/thorn-sphere-like WO3 sensor as a function of NO2 concentrations at RT. Fig. 9. (a) The gas responses of WO3, PS and Au-decorated PS/WO3 sensors toward 1 ppm NO2 with operating temperatures ranging from RT to 200 °C and the Logarithm of (b) the response time constant and (c) recovery time (both in s) constants versus the reverse of temperature. The dots are experimental data and the lines are the linear fitting functions. Fig. 10. Gas response of the Au-modified PS/thorn-sphere-like WO3 sensor to various gases at RT. Fig. 11. Adsorption and reaction model of the sensing process on the surface of Au-modified PS/thorn-sphere-like WO3 sensor. Fig. 12. Schematical energy bands of (a) the Au-modified PS/thorn-sphere-like WO3 sensor and (b) the energy state changes on exposure to air and NO2. EF denotes Fermi level, Ei—intrinsic Fermi level, Eg—Forbidden band width, W—Work function, Ev, Ec —the valence and the conduction band edges.
Table 1 Comparisons of the operating temperature and gas responses of the WO3 nanostructured NO2 sensors reported previously. Sensing material
Optimal temp. (°C)
NO2 conc. (ppm)
Gas response
Ref.
Au-doped PS/ thorn-sphere-like WO3
RT
1
5.16
Present work
WO3 nanorods
250
20
4.0
[34]
PS/WO3 nanorods
RT
1
3.38
[25]
WO3 hierarchical structure film
RT
1
3.0
[35]
Au-decorated WO3 cross-linked nanodomes
250
1
74.23
[36]
networked WO3 nanowires
250
1
20
[37]
100
0.05
100
[38]
multi-shelled WO3 yolk–shell spheres