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Hydrogen sensors based on Pt-decorated SnO2 nanorods with fast and sensitive room-temperature sensing performance
Journal of Alloys and Compounds 811 (2019) 152086
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Hydrogen sensors based on Pt-decorated SnO2 nanorods with fast and sensitive room-temperature sensing performance Zihui Chen a, Keyang Hu a, Piaoyun Yang a, Xingxing Fu a, Zhao Wang a, *, Shulin Yang b, Juan Xiong a, Xianghui Zhang a, Yongming Hu a, Haoshuang Gu a a Hubei Key Laboratory of Ferro- & Piezo-electric Materials and Devices, Faculty of Physics and Electronic Sciences, Hubei University, Wuhan, 430062, PR China b School of Physics and Electronic Information, Hubei Key Laboratory for Processing and Application of Catalytic Materials, Huanggang Normal University, Huanggang, 438000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 June 2019 Received in revised form 27 August 2019 Accepted 28 August 2019 Available online 29 August 2019
Hydrogen energy has been regarded as one of the most promising green and renewable energy in future society. The development and application of fast and cost-effective hydrogen sensors are of great significant for the safe application of hydrogen energy. In this work, high-performance semiconductor hydrogen sensors with fast room-temperature (RT) hydrogen response were fabricated by using Ptdecorated SnO2 nanorods. The SnO2 nanorods with tetragonal phase were synthesized through a hydrothermal method, and then decorated by Pt nanoparticles by a photochemical reduction method under UV irradiation. The hydrogen sensing performance of the SnO2 nanorods at RT were greatly enhanced after the Pt-decoration. The sensor response was gradually increased from 27.10% to 87.35% with Pt/Sn mol ratio increasing from 0 to 3.63%. Meanwhile, the response and recovery process were also accelerated with increasing Pt loading amount. The room-temperature response and recovery time of the sensor with Pt/Sn ratio of 3.63% was down to 0.33 and 29.60 s, respectively. The sensor also exhibited outstanding repeatability and selectivity against CO and CH4. Moreover, the impact of humidity on the sensor performance were also investigated. The sensor response was decreased with increasing environmental humidity, which could be partially recovered after the humidity was decreased. The remarkably accelerated sensor performance could be attributed to the catalytic property of Pt nanoparticles with spillover effect and the contribution of Pt-induced metal-semiconductor contact effect to the hydrogen response of the sensors. © 2019 Elsevier B.V. All rights reserved.
Keywords: Hydrogen sensor Semiconductor Tin dioxide Pt Nanorods Surface decoration
1. Introduction Hydrogen has been regarded as one of the most promising clean and renewable energy sources owing to its numerous advantages such as high calorific value of combustion, fast ignition rate and wide flammable range, etc. [1] However, the high permeability, highly flammable and explosive property of hydrogen gas may lead to serious potential safety hazard during the production, application, transportation and storage process. The application of reliable hydrogen sensors and on-line monitoring smart systems for the monitoring of hydrogen concentration and instant alarming of hydrogen leakage are of great significance for the widespread
* Corresponding author. E-mail address: [email protected] (Z. Wang). https://doi.org/10.1016/j.jallcom.2019.152086 0925-8388/© 2019 Elsevier B.V. All rights reserved.
application of hydrogen energy [2]. Chemiresistor hydrogen sensors based on semiconductor oxides have attracted great attention because of their high sensitivity, low production cost and good compatibility to IC technique [3e5]. The electrical resistance of such sensors will be changed after the exposure towards the atmosphere containing the analytic gases. However, the typical working temperature of those sensors are up to 200e300 C, which may lead to high power consumption, poor gas selectivity, and potential safety hazards during high-temperature operating process. Recently, one-dimensional (1-D) semiconductor nanostructures such as nanowires (NWs), nanotubes (NTs) and nanorods (NRs) have exhibited great potential in building high-performance hydrogen sensors which could operate at lower temperatures [6]. The ultra-high specific surface area and nano-scaled diameter could enhance the sensitivity of the 1-D electrical transportation
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behavior to the surface reaction of the materials, and therefore improve the gas sensitivity and response rate [7]. Recently, a series of chemiresistor-type hydrogen sensor based on SnO2, ZnO, MoO3, and WO3 NWs and NRs have been demonstrated by researchers, which could realize highly sensitive hydrogen detecting at roomtemperature (RT) [8e12]. Among them, SnO2 exhibited outstanding hydrogen sensing performance as well as good physicochemical stability. The simple and low-cost fabrication process is also favorable for practical applications. However, the response and recovery time of most SnO2-based RT hydrogen sensors are always up to several tens of seconds, which are still unsatisfactory in view of the fast diffusion rate and ignition behavior of hydrogen gas. As reported, the surface decoration of the semiconductor oxides by using noble metal nanoparticles (NPs) such as Pt and Pd is a highly efficiency method for improving their hydrogen sensing properties [13,14]. With the presence of Pd or Pt NPs, the hydrogen molecules could be split up into dissociated hydrogen atoms and then transferred onto the surface of the semiconductor oxides due to so-called spillover effect, thus increase the reaction rate of hydrogen with the host materials. For instance, Pd-decorated SnO2 nanowires exhibited enhanced hydrogen response and selectivity at 300 C [8]. Moreover, O. Lupan et al. also reported the enhanced hydrogen sensing behavior of ZnO nanowire with ultrafast response behavior after Pd surface functionalization [10]. However, the hydrogen brittleness of Pd may lead to the drop of sensor performance during long-term operation process [15]. Comparing with Pd, Pt is more stable in hydrogen due to the much lower hydrogen solubility, which is beneficial for improving the stability of the gas sensors [16]. For example, Huang et al. reported the gas sensor based on Pt decorated SnO2 nanorod arrays which were integrated on interdigital electrodes. The SnO2 nanorod arrays exhibited remarkably enhanced sensing performance to both H2 and CO at 300e400 C after the surface decoration of a Pt layer with 2 nm in thickness. However, the low-temperature sensing performance of the decorated nanorod arrays were still non-satisfactory for practical applications. In this work, SnO2 nanorods were synthesized by a simple hydrothermal method, and then decorated by Pt NPs through UVirradiated photochemical reduction method. The nanorods were laterally integrated by the interdigital electrodes for building the semiconductor gas sensors. The samples exhibited outstanding RT hydrogen sensing performance with response time lower than 0.4 s to 1000 ppm of hydrogen at an optimal composition. Moreover, the humidity-related sensing behavior and sensing mechanism were also discussed in detail. 2. Experimental details 2.1. Materials The raw materials of SnCl4$5H2O, NaOH, H2PtCl6$6H2O (37%) and absolute ethanol used in this work for synthesizing SnO2 nanorods were purchased from the Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All of the reagents were analytical grade and used as received without further purification. 2.2. Hydrothermal growth of SnO2 nanorods The SnO2 nanorods were synthesized by using a one-step hydrothermal method, which was reported in our previous work [12]. Firstly, 1.33 g of SnCl4$5H2O was dissolved into 20 mL of NaOH solution with concentration of 1.26 M. After further stirred for 10 min at RT, 20 mL absolute ethanol was added into the reaction system and further stirred to obtain a white translucent suspended solution. After that, the product solution was transferred into a
stainless-steel autoclave with volume of 60 mL. The system was sealed and then heated in an electric oven at 200 C for 72 h. After naturally cooled to RT, the product was washed for several times with DI water and then collected after vacuum suction filtration. The as-prepared products were dried at 70 C for 12 h. 2.3. Pt decoration In a typical experimental procedure, 0.15 g of the as-synthesized SnO2 nanorods were dispersed into 5 mL of absolute ethanol in a quartz glass bottle by magnetic stirring. Then, 0.1 g of H2PtCl6$6H2O was dissolved into 19.3 mL of absolute ethanol to obtain a brown solution (100 mM). 2 mL of H2PtCl6 solution was added into the SnO2 dispersion. After stirred in dark condition for 30 min, the product solution was transferred into a black-box UV analyzer (ZF7A, Shanghai Qinke) with light wavelength of 254 and 365 nm for UV photochemical reduction. After UV irradiation and stirring for 12 h, the product was collected and washed by DI water for several times, and then dried at 80 C for 6 h. In order to prepare the products with different Pt loading amount, the relative mol ratio between Pt and Sn in the precursors for photochemical reduction was fixed as 1:20, 1:15, 1:5 and 1:3 by changing the amount of relative amount of SnO2 nanorods and H2PtCl6. Those samples were labeled as SePt1 to SePt4 in sequence as the increase of Pt loading amount. 2.4. Characterization The crystal structures of the products were detected by X-ray diffraction spectrum (XRD, Bruker D8A25, CuKa, l ¼ 1.5406 Å). The surface morphology and microstructure were characterized by the field-emission scanning electron microscopy (FESEM, JEOL JSM7100F) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM 2010). The composition and element valence state of the samples were measured by using X-ray photoelectron spectroscopy (XPS) with a VG ESCALAB-MK electron spectrometer using CuKa radiation. 2.5. Sensor fabrication and measurement Firstly, 0.2 g of Pt-decorated SnO2 NRs were dispersed into 5 mL of absolute ethanol with magnetic stirring. Then a piece of quartz glass substrate with size of 2 2 cm2 and thickness of 1.5 mm was used as the insulating substrate. After the ultrasonic clean of the substrate by ethanol, acetone and DI water, the Pt-decorated SnO2 NRs were transferred onto the substrate by dropping the asprepared dispersion. After dried at 70 C, the Pt/Ti interdigital electrodes (IDEs) with electrode spacing of 100 mm and thickness of 120 nm were deposited on the top surface of the substrate by DC sputtering method. The gas sensing performance of the sensors were measured in a homemade automatic gas sensor testing system with static testing method as reported in our previous works at room temperature of 27 C [17]. During the testing process, the analyte gas with various hydrogen concentration was prepared by the computer-controlled gas mixers. Firstly, the atmosphere of the testing chamber was replaced by dry air. Then, by controlling the flow rate and injection time by the mass flow controllers and electromagnetic valves, different volume of pure hydrogen gas were injected into the testing chamber for obtaining the analyte gas with different concentration. During this process, the sensor resistance was monitored by using Keithley 2400 source meter at a DC voltage of 5 V. The application of 5 V DC voltage could decrease the impact of interface barrier between the electrodes and nanorods on the sensing performance. Therefore, the sensing performance of the
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hydrogen gas was swept out by dry air for testing the recovery performance of the devices. The testing of sensor performance under different humidity condition was realized by injecting the hydrogen gas through a wetting chamber before it was injected into the testing chamber. The humidity was monitored by a commercial humidity sensor (Honeywell HIH-4000). The sensor response factor (S) was defined as S ¼ (Ra-Rg)/Ra 100%, where Ra and Rg is the steady electrical resistances of gas sensors in air and analyte gas, respectively. The response time (tres) and recovery time (trec) was defined as the time taken for 90% response and recovery, respectively. 3. Results and discussions
Fig. 1. The XRD patterns of the pristine and Pt-decorated SnO2 NRs.
devices were mainly attributed to the gas-solid reaction on the Ptdecorated SnO2 nanorods for investigating the intrinsic sensing behavior of the materials. After the response was saturated, the
Fig. 1 shows the XRD patterns of the pristine and the Ptdecorated samples. All curves match well with the JCPDS Card No. 99e0024, indicating that the products are consisted of tetragonal phase of SnO2 materials. There is no diffraction peak belonging to Pt or related compounds in the diffraction curves of SePt1 to 4, which may be attributed to the much low content of Pt-related decoration layer in the samples. Fig. 2(a) shows the SEM images of the as-synthesized pristine SnO2 NRs through hydrothermal method. The low-magnified SEM image shown in the inset picture
Fig. 2. The SEM images of the pristine and Pt-decorated SnO2 NRs. (a) Pristine SnO2 NRs, the inset picture is a low-magnified SEM image; (b) SePt1; (c) SePt2; (d) SePt3; (e) SePt4.
Fig. 3. The TEM results of Pt-decorated SnO2 NRs in sample SePt4. (a,b) TEM images; (c) HRTEM image.
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Fig. 4. The XPS results of the Pt-decorated SnO2 NRs. (a) Pt 4f; (b) O 1s.
Fig. 5. The sensor response of pristine and Pt-decorated SnO2 NRs towards 1000 ppm of hydrogen at 27 C.
confirmed the formation of sphere-like microparticles consisting of several NRs with 100e200 nm in width. After the UV photochemical reduction treatment, the size and morphology of the NRs remain unchanged. As shown in Fig. 2(bee), a few small nanoparticles could be found on the surface of the NRs, which could be confirmed to be consisted of Pt element according to the EDS mapping results shown in Fig. S1. In order to further confirm the crystallinity and valence state of the surface-decorated NPs, the sample was further characterized by HRTEM and XPS. Fig. 3 shows the HRTEM characterization results of an individual particle in SePt4. As shown in Fig. 3(b), large amounts of small NPs could be found on the surface of the SnO2 NRs. The lattice fringe shown in Fig. 3(c) could be indexed into the (220) facets of Pt with interplanar distance of 0.28 nm, which confirmed the formation of metal Pt NPs after the UV reduction process. However, a thin amorphous layer could be found on the surface of the SnO2 NRs and the Pt NPs in this sample, which might be due to
Fig. 6. The comparison of response, recovery time and sensor response of the sensors based on pristine and Pt-decorated SnO2 NRs with different Pt loading amount.
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Table 1 Comparison of hydrogen sensing performance of the sensors based on semiconductor 1-D nanomaterials. Sensing Materials
H2 conc. (ppm)
Temp. (oC)
S (%)a
tres (s)
trec (s)
Ref.
ZnO single nanowire PdeZnO single nanowire ZnO nanorod arrays ZnO nanowires PteZnO nanowires WO3-decorated ZnO nanowires Nb2O5 nanorod arrays Nb2O5 nanowires Nb2O5 nanorod arrays TiO2 nanotube arrays Pd-modified TiO2 nanorods SnO2 single nanowire SnO2 nanorods Pd-doped SnO2 nanowires Pd-doped SnO2 nanowires PdeSnO2 nanowires PdeSnO2 nanofibers Pd-coated SnO2 nanorod arrays Pt-doped SnO2 nanowires Pt-decorated SnO2 nanorods
1000 100 1000 1000 1000 1000 1000 1000 1000 100 1000 20000 1000 1000 1000 100 100 10000 1000 1000
RT RT RT 150 150 200 180 RT RT RT RT RT RT 150 RT 300 385 RT 100 RT
65.74 99.99 99.80 97.70 99.90 86.01 94.12 97.42 37.86 99.58 96.77 50.00 80.20 99.60 97.77 96.42 98.28 99.05 99.20 87.35
e 6.4 176 54 42 102 240 129.6 30.6 45 >300 220 62 40 138 49 0.8 15 <2400 0.33
e 7.4 116 5 4 81 1800
[20] [10] [21] [22] [22] [23] [24] [25] [26] [27] [28] [29] [12] [30] [31] [8] [32] [33] [34] This work
a
e >300 500 200 e 143 31.8 e e 29.6
The value of sensor response S ¼ (Ra - Rg)/Rg were recalculated according to the original value in the literatures for comparison.
the incomplete reduction of H2PtCl6. Fig. 4 and Fig. S2 shows the XPS results of the Pt-decorated SnO2 NRs. As shown in Fig. S2, all products were consisted of Sn, O and Pt elements together with carbon element which was introduced during the sample preparing process. The Pt 4f spectra shown in Fig. 4(a) confirmed the existence of Pt element in all surface-decorated samples. Among them, a weak peak belongs to the Pt4þ (PtO2) could be found at 78.5 eV in the curve of SePt4, which did not show up in the XPS curves of other samples. The presence of Pt4þ in SePt4 further confirmed
that the H2PtCl6 were not completely reduced after the UV reduction process [18]. According to the XPS results, the mol ratio between metallic Pt (Pt0) and Sn in sample SePt1 to SePt4 could be calculated as 0.96%, 3.01%, 3.63% and 5.875%, which increased with the amount of H2PtCl6 added in the precursors. Moreover, the O 1s spectra in Fig. 4(b) shows two distinguished peaks which belongs to the lattice oxygen (Olat) at ~531 eV and the surface adsorbed oxygen (Oads) at ~ 532.5 eV. According to the fitting results, the relative content of Oads was increased with the Pt load amount,
Fig. 7. The detailed sensor response of sample SePt3. (a) The response curve towards different concentration of hydrogen; (b) The relationship between the full sensor response and the hydrogen concentration; (c) Five continuous response cycle towards 1000 ppm of hydrogen; (d) The sensor response towards different kind of analyte gas with concentration of 1000 ppm.
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indicating that the adsorbed oxygen might be related to both the physically adsorbed oxygen (O 2 ) on SnO2 and the chemically adsorbed oxygen on Pt NPs [16,19]. Fig. 5 and Fig. S3 show the RT sensor response towards 1000 ppm hydrogen of the sensors consisted of pristine and Ptdecorated SnO2 NRs with different Pt loading amount. All samples except SePt4 exhibited n-type sensor response with sensor resistance decreased after the exposure to hydrogen. As shown in Fig. S3, the resistance of SePt4 gradually increased when hydrogen gas was introduced into the testing chamber, and then recovered when hydrogen was swept out. Meanwhile, the original resistance in air of SeP4 was much higher than the sensor based on pristine SnO2 NRs [12]. The abnormal electrical and hydrogen sensing behavior of SePt4 might be attributed to the coating layer on the surface of the SnO2 NRs shown in the HRTEM images, which blocked the electron transferring between the adjacent NRs and changed the surface reaction of the NRs with hydrogen. Moreover, the sensor response of other samples (pristine, SePt1 to SePt3) gradually increased with the Pt loading amount. As shown in Fig. 6, the full sensor response was increased with the Pt loading amount, while both the response and recovery time was decreased. For sensor SePt3 with Pt0/Sn mol ratio of 3.63%, the full sensor response towards 1000 ppm of hydrogen at RT was up to 87.35%, with ultra-fast response (tres ¼ 0.33 s) rate. The recovery time trec of the sensors were also decreased to ~29.6 s, which could meet the requirements of practical application. Table 1 listed the comparison of sensor performance of the recent reported hydrogen sensors based on semiconductor oxides 1-D nanomaterials in the literatures. The SePt3 sensor reported in this work shows much shorter response time and considerable sensor response and recovery time than the other reported sensors. Fig. 7 shows the detailed sensor performance of SePt3. As shown in Fig. 7(a), the sensor shows sensitive response towards hydrogen gas with concentration varying from 100 to 1000 ppm, which exhibited good linearity between the full sensor response and hydrogen concentration from 100 to 800 ppm (as shown in Fig. 7(b)). The deviation of operation curve at 1000 ppm should be attributed to the saturation of surface reaction. Fig. S4 shows the variation of response time with the increase of hydrogen concentration. The response time is decreased with the increase hydrogen concentration, which indicated the enhanced response rate at higher concentration of hydrogen. Although the response time at relatively lower concentration is longer than that for 1000 ppm of hydrogen, it is still much faster than the response time of the reported SnO2 nanowires or nanorods (listed in Table 1) in references at a similar concentration level at room temperature. Moreover, the sensor also exhibited good repeatability and selectivity, as shown in Fig. 7(c and d). The full sensor response towards CH4 and CO with concentration of 1000 ppm is ~30.74% and 33.78%, which is much lower than the hydrogen response. In order to evaluate the influence of environmental humidity to the hydrogen sensing performance, we measured the sensor response toward 1000 ppm of hydrogen at different humidity condition. As shown in Fig. 8(a), the sensor response obviously decreased when the relative humidity of the testing chamber was increased during the humidification process. The full sensor response at 83.50 %RH was decreased to ~26.58%, which was less than a third of the full response at dry air (21.88 % RH). Moreover, the humidification process also resulted in the drop of sensor performance even though the humidity was decreased to the original level. As shown in Fig. 9(b), the full sensor response was decreased to 78.2% when the relative humidity was recovered to 21.85%RH. Meanwhile, the increase of humidity also slowed down the response process besides the decrease of response magnitude, as shown in Fig. 9(c). Moreover,
Fig. 8. The sensor response of SePt3 towards 1000 ppm of hydrogen at different humidity. (a) Sensor response curves; The variation of (b) full sensor response and (c) response time during humidification and dehumidification process.
the dehumidification process could not lead to the recovery of the fast response performance of the sensors. As reported, water molecules could be firstly physically adsorbed and then chemically adsorbed on the surface of the semiconductor oxides at higher humidity level [35]. The adsorption of water molecules will block the reaction site on the surface of the nanorods, which lead to the performance drop on the hydrogen sensing performance. Fig. 9 illustrated the possible hydrogen sensing mechanism of the Pt-decorated SnO2 NRs at RT, which could be interpreted as the reaction formulas listed below.
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Fig. 9. The schematic diagram for the hydrogen response of Pt-decorated SnO2 NRs. (a) Schematic diagram in air; (b) schematic diagram in hydrogen containing atmosphere; (ceh) The variation of energy band diagram for (c,d) surface depletion layer, (e,f) SnO2/SnO2 interface and (g,h) Pt/SnO2 interface.
O2 (gas) / O2 (ads)
(1)
O2 (ads) þ e / O 2 (ads)
(2)
O 2 (ads) þ H2 (gas) / H2O (gas) þ e
(3)
Firstly, the oxygen molecules could be physically adsorbed on the surface of SnO2 NRs at RT and trapped the electrons from the conduction band of SnO2, leading to the formation of O 2 ions [36]. Therefore, the charge carrier density at the surface area of the SnO2 NRs would be decreased, which might lead to the formation of surface depletion region as illustrated by Fig. 9(a) and (c). Moreover, the surface adsorbed O 2 ions could also diffused into the interface between the adjacent NRs due to the surface concentration gradient [37]. As shown in Fig. 9(e), the diffusion of O 2 could further increase the Schottky barrier height at the interface between the NRs. Moreover, the difference on the work function between Pt (~5.56 eV) and SnO2 (~4.7 eV) could also lead to the formation of a depletion region in SnO2 [38]. As reported, the adsorbed oxygen on Pt may increase the work function of Pt, and further increase the energy barrier height and width of depletion region [16]. As a result, the resistivity of the sensing layer consisted of Pt-decorated SnO2 NRs will maintained at the relatively higher level in air due to the above-mentioned three mechanisms. Once the hydrogen gas was introduced into the testing chamber, the surface adsorbed oxygen species including the O 2 ions adsorbed on surface and interface regions and the oxygen atoms adsorbed on Pt NPs could reacted with hydrogen under the assistance of Pt NPs due to the spillover effect, leading to the formation of water molecules [39]. As illustrated in Fig. 9 (b,d,f), those redox reaction could lead to the release of trapped electrons to the conduction band of SnO2 NRs, which could eliminate the depletion region at the surface and the SnO2/SnO2 interfaces [4]. The reaction between the adsorbed oxygen on Pt and hydrogen could also lead to the decrease of work function of Pt. Furthermore, hydrogen atoms could also be adsorbed into Pt NPs and diffused to the interface region of Pt/SnO2, which could further decrease the work function of Pt and greatly decrease the energy barrier height of the metal-
semiconductor (M S) interfaces (Fig. 9(h)) [25]. All of the abovementioned process could lead to the increase of charge carrier density in the SnO2 NRs, which consequentially resulted in the decrease of resistivity of the sensing layers during the response process. After the hydrogen gas was swept out from the testing chamber, the adsorption of oxygen species due to the introduction of air could result in the re-construction of the depletion regions, and thus lead to the recovery of sensor resistivity to higher level. In this work, the surface-decorated Pt NPs could act as the catalyst with spillover effect towards hydrogen, which could enhance th reaction and fasten the reaction process. Meanwhile, Pt decoration also provided additional contribution to the hydrogen response by forming the M S contact. Therefore, the improvement of hydrogen sensing performance with the increase of Pt loading content could be attributed to the both the enhanced catalytic performance and the increased number of M S interfaces with higher Pt loading amount. 4. Conclusions In summary, SnO2 nanorods decorated by Pt nanoparticles were synthesized through hydrothermal and UV-assisted photochemical reduction process. The surface-decoration enhanced the RT hydrogen sensing performance of the SnO2 nanorods. With Pt0/Sn mol ratio of 3.63%, the sensor response towards 1000 ppm of hydrogen gas in air was increased to 87.35%, together with the remarkably accelerated response and recovery rate. The response time was lower than 0.4 s, which could realize fast detection and instant alarming of hydrogen leakage accident. Moreover, the sensor also exhibited outstanding gas selectivity against CO and CH4. However, the performance drift would be induced by the change of environmental humidity, which indicated the necessity of humidity-drift compensation in practical applications. Acknowledgement This work was financially supported by the National Natural
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Science Foundation of China (NSFC Project Nos. 51972102, 51802109) and the Major technical innovation project of Hubei Province in China (No. 2016AAA002). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152086. References Gallacho ir, The role of hydrogen in low carbon [1] E.S. Hanley, J. Deane, B.O. energy futureseA review of existing perspectives, Renew. Sustain. Energy Rev. 82 (2018) 3027e3045. [2] W.J. Buttner, M.B. Post, R. Burgess, C. Rivkin, An overview of hydrogen safety sensors and requirements, Int. J. Hydrogen Energy 36 (2011) 2462e2470. [3] B. Sharma, A. Sharma, J.-S. Kim, Recent advances on H2 sensor technologies based on MOX and FET devices: a review, Sens. Actuators B Chem. 262 (2018) 758e770. [4] Z. Li, Z. Yao, A.A. Haidry, T. Plecenik, L. Xie, L. Sun, Q. Fatima, Resistive-type hydrogen gas sensor based on TiO2: a review, Int. J. Hydrogen Energy 43 (2018) 21114e21132. [5] A. Dey, Semiconductor metal oxide gas sensors: a review, Mater. Sci. Eng., B 229 (2018) 206e217. [6] Z. Li, H. Li, Z. Wu, M. Wang, J. Luo, H. Torun, P. Hu, C. Yang, M. Grundmann, X. Liu, Y. Fu, Advances in designs and mechanisms of semiconducting metal oxide nanostructures for high-precision gas sensors operated at room temperature, Mater. Horiz. 6 (2019) 470e506. [7] E. Comini, Metal oxide nanowire chemical sensors: innovation and quality of life, Mater. Today 19 (2016) 559e567. [8] J.-H. Kim, A. Mirzaei, H.W. Kim, S.S. Kim, Improving the hydrogen sensing properties of SnO2 nanowire-based conductometric sensors by Pd-decoration, Sens. Actuators B Chem. 285 (2019) 358e367. [9] S. Yang, Z. Wang, Y. Hu, Y. Cai, R. Huang, X. Li, Z. Huang, Z. Lan, W. Chen, H. Gu, Defect-original room-temperature hydrogen sensing of MoO3 nanoribbon: experimental and theoretical studies, Sens. Actuators B Chem. 260 (2018) 21e32. , R. Adelung, Ultra-sensitive [10] O. Lupan, V. Postica, F. Labat, I. Ciofini, T. Pauporte and selective hydrogen nanosensor with fast response at room temperature based on a single Pd/ZnO nanowire, Sens. Actuators B Chem. 254 (2018) 1259e1270. €klin, N. Halonen, A. Shchukarev, Z. Konya, [11] J. Kukkola, M. Mohl, A.-R. Leino, J. Ma H. Jantunen, K. Kordas, Room temperature hydrogen sensors based on metal decorated WO3 nanowires, Sens. Actuators B Chem. 186 (2013) 90e95. [12] G. Liu, Z. Wang, Z. Chen, S. Yang, X. Fu, R. Huang, X. Li, J. Xiong, Y. Hu, H. Gu, Remarkably enhanced room-temperature hydrogen sensing of SnO2 nanoflowers via vacuum annealing treatment, Sensors 18 (2018) 949. [13] M. Weber, J.-H. Kim, J.-H. Lee, J.-Y. Kim, I. Iatsunskyi, E. Coy, M. Drobek, A. Julbe, M. Bechelany, S.S. Kim, High-performance nanowire hydrogen sensors by exploiting the synergistic effect of Pd nanoparticles and metaleorganic framework membranes, ACS Appl. Mater. Interfaces 10 (2018) 34765e34773. [14] J.M. Walker, S.A. Akbar, P.A. Morris, Synergistic effects in gas sensing semiconducting oxide nano-heterostructures: a review, Sens. Actuators B Chem. 286 (2019) 624e640. [15] B.D. Adams, A. Chen, The role of palladium in a hydrogen economy, Mater. Today 14 (2011) 282e289. [16] F. Cao, P. Zhao, Z. Wang, X. Zhang, H. Zheng, J. Wang, D. Zhou, Y. Hu, H. Gu, An ultrasensitive and ultraselective hydrogen sensor based on defect-dominated electron scattering in Pt nanowire arrays, Adv. Mater. Interfaces 6 (2019) 1801304. [17] S. Yang, Z. Wang, Y. Zou, X. Luo, X. Pan, X. Zhang, Y. Hu, K. Chen, Z. Huang, S. Wang, K. Zhang, H. Gu, Remarkably accelerated room-temperature hydrogen sensing of MoO3 nanoribbon/graphene composites by suppressing the nanojunction effects, Sens. Actuators B Chem. 248 (2017) 160e168.
[18] H. Tian, H. Fan, J. Ma, Z. Liu, L. Ma, S. Lei, J. Fang, C. Long, Pt-decorated zinc oxide nanorod arrays with graphitic carbon nitride nanosheets for highly efficient dual-functional gas sensing, J. Hazard Mater. 341 (2018) 102e111. [19] K. Suematsu, M. Sasaki, N. Ma, M. Yuasa, K. Shimanoe, Antimony-doped tin dioxide gas sensors exhibiting high stability in the sensitivity to humidity changes, ACS Sens. 1 (2016) 913e920. [20] S.N. Das, J.P. Kar, J.-H. Choi, T.I. Lee, K.-J. Moon, J.-M. Myoung, Fabrication and characterization of ZnO single nanowire-based hydrogen sensor, J. Phys. Chem. C 114 (2010) 1689e1693. [21] J.J. Hassan, M.A. Mahdi, C.W. Chin, H. Abu-Hassan, Z. Hassan, A high-sensitivity room-temperature hydrogen gas sensor based on oblique and vertical ZnO nanorod arrays, Sens. Actuators B Chem. 176 (2013) 360e367. [22] C.S. Rout, S. Hari Krishna, S.R.C. Vivekchand, A. Govindaraj, C.N.R. Rao, Hydrogen and ethanol sensors based on ZnO nanorods, nanowires and nanotubes, Chem. Phys. Lett. 418 (2006) 586e590. [23] S. Park, Enhancement of hydrogen sensing response of ZnO nanowires for the decoration of WO3 nanoparticles, Mater. Lett. 234 (2019) 315e318. [24] Z. Pytlicek, M. Bendova, J. Prasek, A. Mozalev, On-chip sensor solution for hydrogen gas detection with the anodic niobium-oxide nanorod arrays, Sens. Actuators B Chem. 284 (2019) 723e735. [25] Z. Wang, Y.M. Hu, W. Wang, X. Zhang, B.X. Wang, H.Y. Tian, Y. Wang, J.G. Guan, H.S. Gu, Fast and highly-sensitive hydrogen sensing of Nb2O5 nanowires at room temperature, Int. J. Hydrogen Energy 37 (2012) 4526e4532. [26] Y. Zou, J. He, Y. Hu, R. Huang, Z. Wang, Q. Gu, Room-temperature hydrogen sensing performance of Nb2O5 nanorod arrays, RSC Adv. 8 (2018) 16897e16901. [27] K.S. Chen, K. Xie, X.R. Feng, S.F. Wang, R. Hu, H.S. Gu, Y. Li, An excellent roomtemperature hydrogen sensor based on titania nanotube-arrays, Int. J. Hydrogen Energy 37 (2012) 13602e13609. € [28] E. S¸ennik, O. Alev, Z.Z. Oztürk, The effect of Pd on the H2 and VOC sensing properties of TiO2 nanorods, Sens. Actuators B Chem. 229 (2016) 692e700. [29] L. Fields, J. Zheng, Y. Cheng, P. Xiong, Room-temperature low-power hydrogen sensor based on a single tin dioxide nanobelt, Appl. Phys. Lett. 88 (2006) 263102. [30] Y. Shen, T. Yamazaki, Z. Liu, D. Meng, T. Kikuta, N. Nakatani, M. Saito, M. Mori, Microstructure and H2 gas sensing properties of undoped and Pd-doped SnO2 nanowires, Sens. Actuators B Chem. 135 (2009) 524e529. [31] Y. Shen, D. Wei, M. Li, W. Liu, S. Gao, C. Han, B. Cui, Microstructure and roomtemperature H2 sensing properties of undoped and impurity-doped SnO2 nanowires, Chem. Lett. 42 (2013) 492e494. [32] J.-K. Choi, I.-S. Hwang, S.-J. Kim, J.-S. Park, S.-S. Park, U. Jeong, Y.C. Kang, J.H. Lee, Design of selective gas sensors using electrospun Pd-doped SnO2 hollow nanofibers, Sens. Actuators B Chem. 150 (2010) 191e199. [33] M.H. Kim, B. Jang, W. Kim, W. Lee, Enhanced hydrogen sensing properties of Pd-coated SnO2 nanorod arrays in nitrogen and transformer oil, Sens. Actuators B Chem. 283 (2019) 890e896. [34] Y. Shen, T. Yamazaki, Z. Liu, D. Meng, T. Kikuta, Hydrogen sensors made of undoped and Pt-doped SnO2 nanowires, J. Alloy. Comp. 488 (2009) L21eL25. [35] S. Kotresh, Y.T. Ravikiran, H.G. Raj Prakash, C.V.V. Ramana, S.C. Vijayakumari, S. Thomas, Humidity sensing performance of spin coated polyanilineecarboxymethyl cellulose composite at room temperature, Cellulose 23 (2016) 3177e3186. [36] K. Suematsu, M. Yuasa, T. Kida, N. Yamazoe, K. Shimanoe, Determination of oxygen adsorption species on SnO2: exact analysis of gas sensing properties using a sample gas pretreatment system, J. Electrochem. Soc. 161 (2014) B123eB128. [37] R. Khan, H.-W. Ra, J. Kim, W. Jang, D. Sharma, Y. Im, Nanojunction effects in multiple ZnO nanowire gas sensor, Sens. Actuators B Chem. 150 (2010) 389e393. [38] L. Tien, P. Sadik, D. Norton, L. Voss, S. Pearton, H. Wang, B. Kang, F. Ren, J. Jun, J. Lin, Hydrogen sensing at room temperature with Pt-coated ZnO thin films and nanorods, Appl. Phys. Lett. 87 (2005) 222106. [39] M. Horprathum, T. Srichaiyaperk, B. Samransuksamer, A. Wisitsoraat, P. Eiamchai, S. Limwichean, C. Chananonnawathorn, K. Aiempanakit, N. Nuntawong, V. Patthanasettakul, Ultrasensitive hydrogen sensor based on Pt-decorated WO3 nanorods prepared by glancing-angle dc magnetron sputtering, ACS Appl. Mater. Interfaces 6 (2014) 22051e22060.
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Hydrogen sensors based on Pt-decorated SnO2 nanorods with fast and sensitive room-temperature sensing performance Zihui Chen a, Keyang Hu a, Piaoyun Yang a, Xingxing Fu a, Zhao Wang a, *, Shulin Yang b, Juan Xiong a, Xianghui Zhang a, Yongming Hu a, Haoshuang Gu a a Hubei Key Laboratory of Ferro- & Piezo-electric Materials and Devices, Faculty of Physics and Electronic Sciences, Hubei University, Wuhan, 430062, PR China b School of Physics and Electronic Information, Hubei Key Laboratory for Processing and Application of Catalytic Materials, Huanggang Normal University, Huanggang, 438000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 June 2019 Received in revised form 27 August 2019 Accepted 28 August 2019 Available online 29 August 2019
Hydrogen energy has been regarded as one of the most promising green and renewable energy in future society. The development and application of fast and cost-effective hydrogen sensors are of great significant for the safe application of hydrogen energy. In this work, high-performance semiconductor hydrogen sensors with fast room-temperature (RT) hydrogen response were fabricated by using Ptdecorated SnO2 nanorods. The SnO2 nanorods with tetragonal phase were synthesized through a hydrothermal method, and then decorated by Pt nanoparticles by a photochemical reduction method under UV irradiation. The hydrogen sensing performance of the SnO2 nanorods at RT were greatly enhanced after the Pt-decoration. The sensor response was gradually increased from 27.10% to 87.35% with Pt/Sn mol ratio increasing from 0 to 3.63%. Meanwhile, the response and recovery process were also accelerated with increasing Pt loading amount. The room-temperature response and recovery time of the sensor with Pt/Sn ratio of 3.63% was down to 0.33 and 29.60 s, respectively. The sensor also exhibited outstanding repeatability and selectivity against CO and CH4. Moreover, the impact of humidity on the sensor performance were also investigated. The sensor response was decreased with increasing environmental humidity, which could be partially recovered after the humidity was decreased. The remarkably accelerated sensor performance could be attributed to the catalytic property of Pt nanoparticles with spillover effect and the contribution of Pt-induced metal-semiconductor contact effect to the hydrogen response of the sensors. © 2019 Elsevier B.V. All rights reserved.
Keywords: Hydrogen sensor Semiconductor Tin dioxide Pt Nanorods Surface decoration
1. Introduction Hydrogen has been regarded as one of the most promising clean and renewable energy sources owing to its numerous advantages such as high calorific value of combustion, fast ignition rate and wide flammable range, etc. [1] However, the high permeability, highly flammable and explosive property of hydrogen gas may lead to serious potential safety hazard during the production, application, transportation and storage process. The application of reliable hydrogen sensors and on-line monitoring smart systems for the monitoring of hydrogen concentration and instant alarming of hydrogen leakage are of great significance for the widespread
* Corresponding author. E-mail address: [email protected] (Z. Wang). https://doi.org/10.1016/j.jallcom.2019.152086 0925-8388/© 2019 Elsevier B.V. All rights reserved.
application of hydrogen energy [2]. Chemiresistor hydrogen sensors based on semiconductor oxides have attracted great attention because of their high sensitivity, low production cost and good compatibility to IC technique [3e5]. The electrical resistance of such sensors will be changed after the exposure towards the atmosphere containing the analytic gases. However, the typical working temperature of those sensors are up to 200e300 C, which may lead to high power consumption, poor gas selectivity, and potential safety hazards during high-temperature operating process. Recently, one-dimensional (1-D) semiconductor nanostructures such as nanowires (NWs), nanotubes (NTs) and nanorods (NRs) have exhibited great potential in building high-performance hydrogen sensors which could operate at lower temperatures [6]. The ultra-high specific surface area and nano-scaled diameter could enhance the sensitivity of the 1-D electrical transportation
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behavior to the surface reaction of the materials, and therefore improve the gas sensitivity and response rate [7]. Recently, a series of chemiresistor-type hydrogen sensor based on SnO2, ZnO, MoO3, and WO3 NWs and NRs have been demonstrated by researchers, which could realize highly sensitive hydrogen detecting at roomtemperature (RT) [8e12]. Among them, SnO2 exhibited outstanding hydrogen sensing performance as well as good physicochemical stability. The simple and low-cost fabrication process is also favorable for practical applications. However, the response and recovery time of most SnO2-based RT hydrogen sensors are always up to several tens of seconds, which are still unsatisfactory in view of the fast diffusion rate and ignition behavior of hydrogen gas. As reported, the surface decoration of the semiconductor oxides by using noble metal nanoparticles (NPs) such as Pt and Pd is a highly efficiency method for improving their hydrogen sensing properties [13,14]. With the presence of Pd or Pt NPs, the hydrogen molecules could be split up into dissociated hydrogen atoms and then transferred onto the surface of the semiconductor oxides due to so-called spillover effect, thus increase the reaction rate of hydrogen with the host materials. For instance, Pd-decorated SnO2 nanowires exhibited enhanced hydrogen response and selectivity at 300 C [8]. Moreover, O. Lupan et al. also reported the enhanced hydrogen sensing behavior of ZnO nanowire with ultrafast response behavior after Pd surface functionalization [10]. However, the hydrogen brittleness of Pd may lead to the drop of sensor performance during long-term operation process [15]. Comparing with Pd, Pt is more stable in hydrogen due to the much lower hydrogen solubility, which is beneficial for improving the stability of the gas sensors [16]. For example, Huang et al. reported the gas sensor based on Pt decorated SnO2 nanorod arrays which were integrated on interdigital electrodes. The SnO2 nanorod arrays exhibited remarkably enhanced sensing performance to both H2 and CO at 300e400 C after the surface decoration of a Pt layer with 2 nm in thickness. However, the low-temperature sensing performance of the decorated nanorod arrays were still non-satisfactory for practical applications. In this work, SnO2 nanorods were synthesized by a simple hydrothermal method, and then decorated by Pt NPs through UVirradiated photochemical reduction method. The nanorods were laterally integrated by the interdigital electrodes for building the semiconductor gas sensors. The samples exhibited outstanding RT hydrogen sensing performance with response time lower than 0.4 s to 1000 ppm of hydrogen at an optimal composition. Moreover, the humidity-related sensing behavior and sensing mechanism were also discussed in detail. 2. Experimental details 2.1. Materials The raw materials of SnCl4$5H2O, NaOH, H2PtCl6$6H2O (37%) and absolute ethanol used in this work for synthesizing SnO2 nanorods were purchased from the Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All of the reagents were analytical grade and used as received without further purification. 2.2. Hydrothermal growth of SnO2 nanorods The SnO2 nanorods were synthesized by using a one-step hydrothermal method, which was reported in our previous work [12]. Firstly, 1.33 g of SnCl4$5H2O was dissolved into 20 mL of NaOH solution with concentration of 1.26 M. After further stirred for 10 min at RT, 20 mL absolute ethanol was added into the reaction system and further stirred to obtain a white translucent suspended solution. After that, the product solution was transferred into a
stainless-steel autoclave with volume of 60 mL. The system was sealed and then heated in an electric oven at 200 C for 72 h. After naturally cooled to RT, the product was washed for several times with DI water and then collected after vacuum suction filtration. The as-prepared products were dried at 70 C for 12 h. 2.3. Pt decoration In a typical experimental procedure, 0.15 g of the as-synthesized SnO2 nanorods were dispersed into 5 mL of absolute ethanol in a quartz glass bottle by magnetic stirring. Then, 0.1 g of H2PtCl6$6H2O was dissolved into 19.3 mL of absolute ethanol to obtain a brown solution (100 mM). 2 mL of H2PtCl6 solution was added into the SnO2 dispersion. After stirred in dark condition for 30 min, the product solution was transferred into a black-box UV analyzer (ZF7A, Shanghai Qinke) with light wavelength of 254 and 365 nm for UV photochemical reduction. After UV irradiation and stirring for 12 h, the product was collected and washed by DI water for several times, and then dried at 80 C for 6 h. In order to prepare the products with different Pt loading amount, the relative mol ratio between Pt and Sn in the precursors for photochemical reduction was fixed as 1:20, 1:15, 1:5 and 1:3 by changing the amount of relative amount of SnO2 nanorods and H2PtCl6. Those samples were labeled as SePt1 to SePt4 in sequence as the increase of Pt loading amount. 2.4. Characterization The crystal structures of the products were detected by X-ray diffraction spectrum (XRD, Bruker D8A25, CuKa, l ¼ 1.5406 Å). The surface morphology and microstructure were characterized by the field-emission scanning electron microscopy (FESEM, JEOL JSM7100F) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM 2010). The composition and element valence state of the samples were measured by using X-ray photoelectron spectroscopy (XPS) with a VG ESCALAB-MK electron spectrometer using CuKa radiation. 2.5. Sensor fabrication and measurement Firstly, 0.2 g of Pt-decorated SnO2 NRs were dispersed into 5 mL of absolute ethanol with magnetic stirring. Then a piece of quartz glass substrate with size of 2 2 cm2 and thickness of 1.5 mm was used as the insulating substrate. After the ultrasonic clean of the substrate by ethanol, acetone and DI water, the Pt-decorated SnO2 NRs were transferred onto the substrate by dropping the asprepared dispersion. After dried at 70 C, the Pt/Ti interdigital electrodes (IDEs) with electrode spacing of 100 mm and thickness of 120 nm were deposited on the top surface of the substrate by DC sputtering method. The gas sensing performance of the sensors were measured in a homemade automatic gas sensor testing system with static testing method as reported in our previous works at room temperature of 27 C [17]. During the testing process, the analyte gas with various hydrogen concentration was prepared by the computer-controlled gas mixers. Firstly, the atmosphere of the testing chamber was replaced by dry air. Then, by controlling the flow rate and injection time by the mass flow controllers and electromagnetic valves, different volume of pure hydrogen gas were injected into the testing chamber for obtaining the analyte gas with different concentration. During this process, the sensor resistance was monitored by using Keithley 2400 source meter at a DC voltage of 5 V. The application of 5 V DC voltage could decrease the impact of interface barrier between the electrodes and nanorods on the sensing performance. Therefore, the sensing performance of the
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hydrogen gas was swept out by dry air for testing the recovery performance of the devices. The testing of sensor performance under different humidity condition was realized by injecting the hydrogen gas through a wetting chamber before it was injected into the testing chamber. The humidity was monitored by a commercial humidity sensor (Honeywell HIH-4000). The sensor response factor (S) was defined as S ¼ (Ra-Rg)/Ra 100%, where Ra and Rg is the steady electrical resistances of gas sensors in air and analyte gas, respectively. The response time (tres) and recovery time (trec) was defined as the time taken for 90% response and recovery, respectively. 3. Results and discussions
Fig. 1. The XRD patterns of the pristine and Pt-decorated SnO2 NRs.
devices were mainly attributed to the gas-solid reaction on the Ptdecorated SnO2 nanorods for investigating the intrinsic sensing behavior of the materials. After the response was saturated, the
Fig. 1 shows the XRD patterns of the pristine and the Ptdecorated samples. All curves match well with the JCPDS Card No. 99e0024, indicating that the products are consisted of tetragonal phase of SnO2 materials. There is no diffraction peak belonging to Pt or related compounds in the diffraction curves of SePt1 to 4, which may be attributed to the much low content of Pt-related decoration layer in the samples. Fig. 2(a) shows the SEM images of the as-synthesized pristine SnO2 NRs through hydrothermal method. The low-magnified SEM image shown in the inset picture
Fig. 2. The SEM images of the pristine and Pt-decorated SnO2 NRs. (a) Pristine SnO2 NRs, the inset picture is a low-magnified SEM image; (b) SePt1; (c) SePt2; (d) SePt3; (e) SePt4.
Fig. 3. The TEM results of Pt-decorated SnO2 NRs in sample SePt4. (a,b) TEM images; (c) HRTEM image.
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Fig. 4. The XPS results of the Pt-decorated SnO2 NRs. (a) Pt 4f; (b) O 1s.
Fig. 5. The sensor response of pristine and Pt-decorated SnO2 NRs towards 1000 ppm of hydrogen at 27 C.
confirmed the formation of sphere-like microparticles consisting of several NRs with 100e200 nm in width. After the UV photochemical reduction treatment, the size and morphology of the NRs remain unchanged. As shown in Fig. 2(bee), a few small nanoparticles could be found on the surface of the NRs, which could be confirmed to be consisted of Pt element according to the EDS mapping results shown in Fig. S1. In order to further confirm the crystallinity and valence state of the surface-decorated NPs, the sample was further characterized by HRTEM and XPS. Fig. 3 shows the HRTEM characterization results of an individual particle in SePt4. As shown in Fig. 3(b), large amounts of small NPs could be found on the surface of the SnO2 NRs. The lattice fringe shown in Fig. 3(c) could be indexed into the (220) facets of Pt with interplanar distance of 0.28 nm, which confirmed the formation of metal Pt NPs after the UV reduction process. However, a thin amorphous layer could be found on the surface of the SnO2 NRs and the Pt NPs in this sample, which might be due to
Fig. 6. The comparison of response, recovery time and sensor response of the sensors based on pristine and Pt-decorated SnO2 NRs with different Pt loading amount.
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Table 1 Comparison of hydrogen sensing performance of the sensors based on semiconductor 1-D nanomaterials. Sensing Materials
H2 conc. (ppm)
Temp. (oC)
S (%)a
tres (s)
trec (s)
Ref.
ZnO single nanowire PdeZnO single nanowire ZnO nanorod arrays ZnO nanowires PteZnO nanowires WO3-decorated ZnO nanowires Nb2O5 nanorod arrays Nb2O5 nanowires Nb2O5 nanorod arrays TiO2 nanotube arrays Pd-modified TiO2 nanorods SnO2 single nanowire SnO2 nanorods Pd-doped SnO2 nanowires Pd-doped SnO2 nanowires PdeSnO2 nanowires PdeSnO2 nanofibers Pd-coated SnO2 nanorod arrays Pt-doped SnO2 nanowires Pt-decorated SnO2 nanorods
1000 100 1000 1000 1000 1000 1000 1000 1000 100 1000 20000 1000 1000 1000 100 100 10000 1000 1000
RT RT RT 150 150 200 180 RT RT RT RT RT RT 150 RT 300 385 RT 100 RT
65.74 99.99 99.80 97.70 99.90 86.01 94.12 97.42 37.86 99.58 96.77 50.00 80.20 99.60 97.77 96.42 98.28 99.05 99.20 87.35
e 6.4 176 54 42 102 240 129.6 30.6 45 >300 220 62 40 138 49 0.8 15 <2400 0.33
e 7.4 116 5 4 81 1800
[20] [10] [21] [22] [22] [23] [24] [25] [26] [27] [28] [29] [12] [30] [31] [8] [32] [33] [34] This work
a
e >300 500 200 e 143 31.8 e e 29.6
The value of sensor response S ¼ (Ra - Rg)/Rg were recalculated according to the original value in the literatures for comparison.
the incomplete reduction of H2PtCl6. Fig. 4 and Fig. S2 shows the XPS results of the Pt-decorated SnO2 NRs. As shown in Fig. S2, all products were consisted of Sn, O and Pt elements together with carbon element which was introduced during the sample preparing process. The Pt 4f spectra shown in Fig. 4(a) confirmed the existence of Pt element in all surface-decorated samples. Among them, a weak peak belongs to the Pt4þ (PtO2) could be found at 78.5 eV in the curve of SePt4, which did not show up in the XPS curves of other samples. The presence of Pt4þ in SePt4 further confirmed
that the H2PtCl6 were not completely reduced after the UV reduction process [18]. According to the XPS results, the mol ratio between metallic Pt (Pt0) and Sn in sample SePt1 to SePt4 could be calculated as 0.96%, 3.01%, 3.63% and 5.875%, which increased with the amount of H2PtCl6 added in the precursors. Moreover, the O 1s spectra in Fig. 4(b) shows two distinguished peaks which belongs to the lattice oxygen (Olat) at ~531 eV and the surface adsorbed oxygen (Oads) at ~ 532.5 eV. According to the fitting results, the relative content of Oads was increased with the Pt load amount,
Fig. 7. The detailed sensor response of sample SePt3. (a) The response curve towards different concentration of hydrogen; (b) The relationship between the full sensor response and the hydrogen concentration; (c) Five continuous response cycle towards 1000 ppm of hydrogen; (d) The sensor response towards different kind of analyte gas with concentration of 1000 ppm.
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indicating that the adsorbed oxygen might be related to both the physically adsorbed oxygen (O 2 ) on SnO2 and the chemically adsorbed oxygen on Pt NPs [16,19]. Fig. 5 and Fig. S3 show the RT sensor response towards 1000 ppm hydrogen of the sensors consisted of pristine and Ptdecorated SnO2 NRs with different Pt loading amount. All samples except SePt4 exhibited n-type sensor response with sensor resistance decreased after the exposure to hydrogen. As shown in Fig. S3, the resistance of SePt4 gradually increased when hydrogen gas was introduced into the testing chamber, and then recovered when hydrogen was swept out. Meanwhile, the original resistance in air of SeP4 was much higher than the sensor based on pristine SnO2 NRs [12]. The abnormal electrical and hydrogen sensing behavior of SePt4 might be attributed to the coating layer on the surface of the SnO2 NRs shown in the HRTEM images, which blocked the electron transferring between the adjacent NRs and changed the surface reaction of the NRs with hydrogen. Moreover, the sensor response of other samples (pristine, SePt1 to SePt3) gradually increased with the Pt loading amount. As shown in Fig. 6, the full sensor response was increased with the Pt loading amount, while both the response and recovery time was decreased. For sensor SePt3 with Pt0/Sn mol ratio of 3.63%, the full sensor response towards 1000 ppm of hydrogen at RT was up to 87.35%, with ultra-fast response (tres ¼ 0.33 s) rate. The recovery time trec of the sensors were also decreased to ~29.6 s, which could meet the requirements of practical application. Table 1 listed the comparison of sensor performance of the recent reported hydrogen sensors based on semiconductor oxides 1-D nanomaterials in the literatures. The SePt3 sensor reported in this work shows much shorter response time and considerable sensor response and recovery time than the other reported sensors. Fig. 7 shows the detailed sensor performance of SePt3. As shown in Fig. 7(a), the sensor shows sensitive response towards hydrogen gas with concentration varying from 100 to 1000 ppm, which exhibited good linearity between the full sensor response and hydrogen concentration from 100 to 800 ppm (as shown in Fig. 7(b)). The deviation of operation curve at 1000 ppm should be attributed to the saturation of surface reaction. Fig. S4 shows the variation of response time with the increase of hydrogen concentration. The response time is decreased with the increase hydrogen concentration, which indicated the enhanced response rate at higher concentration of hydrogen. Although the response time at relatively lower concentration is longer than that for 1000 ppm of hydrogen, it is still much faster than the response time of the reported SnO2 nanowires or nanorods (listed in Table 1) in references at a similar concentration level at room temperature. Moreover, the sensor also exhibited good repeatability and selectivity, as shown in Fig. 7(c and d). The full sensor response towards CH4 and CO with concentration of 1000 ppm is ~30.74% and 33.78%, which is much lower than the hydrogen response. In order to evaluate the influence of environmental humidity to the hydrogen sensing performance, we measured the sensor response toward 1000 ppm of hydrogen at different humidity condition. As shown in Fig. 8(a), the sensor response obviously decreased when the relative humidity of the testing chamber was increased during the humidification process. The full sensor response at 83.50 %RH was decreased to ~26.58%, which was less than a third of the full response at dry air (21.88 % RH). Moreover, the humidification process also resulted in the drop of sensor performance even though the humidity was decreased to the original level. As shown in Fig. 9(b), the full sensor response was decreased to 78.2% when the relative humidity was recovered to 21.85%RH. Meanwhile, the increase of humidity also slowed down the response process besides the decrease of response magnitude, as shown in Fig. 9(c). Moreover,
Fig. 8. The sensor response of SePt3 towards 1000 ppm of hydrogen at different humidity. (a) Sensor response curves; The variation of (b) full sensor response and (c) response time during humidification and dehumidification process.
the dehumidification process could not lead to the recovery of the fast response performance of the sensors. As reported, water molecules could be firstly physically adsorbed and then chemically adsorbed on the surface of the semiconductor oxides at higher humidity level [35]. The adsorption of water molecules will block the reaction site on the surface of the nanorods, which lead to the performance drop on the hydrogen sensing performance. Fig. 9 illustrated the possible hydrogen sensing mechanism of the Pt-decorated SnO2 NRs at RT, which could be interpreted as the reaction formulas listed below.
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Fig. 9. The schematic diagram for the hydrogen response of Pt-decorated SnO2 NRs. (a) Schematic diagram in air; (b) schematic diagram in hydrogen containing atmosphere; (ceh) The variation of energy band diagram for (c,d) surface depletion layer, (e,f) SnO2/SnO2 interface and (g,h) Pt/SnO2 interface.
O2 (gas) / O2 (ads)
(1)
O2 (ads) þ e / O 2 (ads)
(2)
O 2 (ads) þ H2 (gas) / H2O (gas) þ e
(3)
Firstly, the oxygen molecules could be physically adsorbed on the surface of SnO2 NRs at RT and trapped the electrons from the conduction band of SnO2, leading to the formation of O 2 ions [36]. Therefore, the charge carrier density at the surface area of the SnO2 NRs would be decreased, which might lead to the formation of surface depletion region as illustrated by Fig. 9(a) and (c). Moreover, the surface adsorbed O 2 ions could also diffused into the interface between the adjacent NRs due to the surface concentration gradient [37]. As shown in Fig. 9(e), the diffusion of O 2 could further increase the Schottky barrier height at the interface between the NRs. Moreover, the difference on the work function between Pt (~5.56 eV) and SnO2 (~4.7 eV) could also lead to the formation of a depletion region in SnO2 [38]. As reported, the adsorbed oxygen on Pt may increase the work function of Pt, and further increase the energy barrier height and width of depletion region [16]. As a result, the resistivity of the sensing layer consisted of Pt-decorated SnO2 NRs will maintained at the relatively higher level in air due to the above-mentioned three mechanisms. Once the hydrogen gas was introduced into the testing chamber, the surface adsorbed oxygen species including the O 2 ions adsorbed on surface and interface regions and the oxygen atoms adsorbed on Pt NPs could reacted with hydrogen under the assistance of Pt NPs due to the spillover effect, leading to the formation of water molecules [39]. As illustrated in Fig. 9 (b,d,f), those redox reaction could lead to the release of trapped electrons to the conduction band of SnO2 NRs, which could eliminate the depletion region at the surface and the SnO2/SnO2 interfaces [4]. The reaction between the adsorbed oxygen on Pt and hydrogen could also lead to the decrease of work function of Pt. Furthermore, hydrogen atoms could also be adsorbed into Pt NPs and diffused to the interface region of Pt/SnO2, which could further decrease the work function of Pt and greatly decrease the energy barrier height of the metal-
semiconductor (M S) interfaces (Fig. 9(h)) [25]. All of the abovementioned process could lead to the increase of charge carrier density in the SnO2 NRs, which consequentially resulted in the decrease of resistivity of the sensing layers during the response process. After the hydrogen gas was swept out from the testing chamber, the adsorption of oxygen species due to the introduction of air could result in the re-construction of the depletion regions, and thus lead to the recovery of sensor resistivity to higher level. In this work, the surface-decorated Pt NPs could act as the catalyst with spillover effect towards hydrogen, which could enhance th reaction and fasten the reaction process. Meanwhile, Pt decoration also provided additional contribution to the hydrogen response by forming the M S contact. Therefore, the improvement of hydrogen sensing performance with the increase of Pt loading content could be attributed to the both the enhanced catalytic performance and the increased number of M S interfaces with higher Pt loading amount. 4. Conclusions In summary, SnO2 nanorods decorated by Pt nanoparticles were synthesized through hydrothermal and UV-assisted photochemical reduction process. The surface-decoration enhanced the RT hydrogen sensing performance of the SnO2 nanorods. With Pt0/Sn mol ratio of 3.63%, the sensor response towards 1000 ppm of hydrogen gas in air was increased to 87.35%, together with the remarkably accelerated response and recovery rate. The response time was lower than 0.4 s, which could realize fast detection and instant alarming of hydrogen leakage accident. Moreover, the sensor also exhibited outstanding gas selectivity against CO and CH4. However, the performance drift would be induced by the change of environmental humidity, which indicated the necessity of humidity-drift compensation in practical applications. Acknowledgement This work was financially supported by the National Natural
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