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Si multilayers at room temperature
Sensors and Actuators B 205 (2014) 255–260
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
High hydrogen response of Pd/TiO2 /SiO2 /Si multilayers at room temperature Cuicui Ling a,b , Qingzhong Xue a,b,∗ , Zhide Han b , Zhongyang Zhang b , Yonggang Du a,b , Yanmin Liu a , Zifeng Yan a,∗ a b
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, Shandong, People’s Republic of China College of Science, China University of Petroleum, Qingdao 266580, Shandong, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 1 April 2014 Received in revised form 29 July 2014 Accepted 26 August 2014 Available online 3 September 2014 Keywords: Hydrogen response Pd/TiO2 /SiO2 /Si multilayers Current–voltage curve
a b s t r a c t A series of Pd/TiO2 /SiO2 /Si multilayers were produced using magnetron sputtering method. It is found that H2 molecules have dramatic effect on the current–voltage (I–V) characteristics of the Pd/TiO2 /SiO2 /Si multilayers at room temperature (RT). When Pd/TiO2 /SiO2 /Si multilayer is exposed to H2 , the Pd film quickly reacts with H2 and forms palladium hydride which results in transferring more electrons from the Pd film to TiO2 film. Therefore, the I–V characteristic of Pd/TiO2 /SiO2 /Si multilayer was greatly changed when exposed to H2 . For example, a Pd/TiO2 /SiO2 /p-Si multilayer can show a high response (∼2431%) to 1% H2 with appreciable short response time of 13 s and recovery time of 4 s at RT. Besides, it is demonstrated that Si substrate has a great effect on the H2 response of Pd/TiO2 /SiO2 /Si multilayers. When exposed to H2 the current of Pd/TiO2 /SiO2 /p-Si multilayer at −0.5 V greatly decreases while the current of Pd/TiO2 /SiO2 /n-Si multilayer greatly increase, which can be understood by their energy band structures. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen (H2 ) as a kind of new-type and clean fuel has become one of the best choice to replace fossil energy in future. However, H2 is colorless, odorless, explosive and extremely flammable with lower explosive limit of 4% in air. Consequently, a reliable sensor is needed to detect leakage from the storage and transportation of H2 as well as to monitor its concentration over a wide range. However, the shortcomings of current H2 sensors, such as big volume, expensive price, working at high temperature, greatly limit their performance. Therefore, the development of new-type H2 sensor material possesses extremely important scientific significance [1,2]. The nanometer oxide semiconductor gas sensor with high response, light stability, corrosion resistance and simple measurement has attracted much attention [3]. For example, nano zinc oxide (ZnO) gas sensors, nano tin oxide gas sensors and nano titanium oxide (TiO2 ) etc. gas sensors show high response to polar gases [4–6] and can detect gases in harsh environments [7].
∗ Corresponding authors at: China University of Petroleum, State Key Laboratory of Heavy Oil Processing, Qingdao 266580, Shandong, People’s Republic of China. Tel.: +86 532 86981169; fax: +86 532 86981169. E-mail address: [email protected] (Q. Xue). http://dx.doi.org/10.1016/j.snb.2014.08.072 0925-4005/© 2014 Elsevier B.V. All rights reserved.
Recently, it was found that the TiO2 nanotubes and nanowires not only can detect the polar gases, but also show certain response to non-polar gases such as H2 , carbon dioxide [8,9]. TiO2 nanotubes or nanowires arrays sensors show high response (∼1000%) to 0.1% H2 [8,10], its response time and recovery time in H2 gas can reach to 13 s and 120 s, respectively [11,12]. In addition, it is found that the precious metals such as palladium (Pd) etc. can further improve the sensing performance of TiO2 nanotubes and nanowires [13–15]. Though gas sensing performance of TiO2 nanotubes and nanowires is excellent, they possess several shortcomings such as strict preparation, complex processing and high cost, which will make them difficult to realize industrialization. In addition, these gas sensors tend to obtain better gas response at high temperatures [16]. Compared with TiO2 nanotubes and nanowires, TiO2 film can be simply prepared using many low-cost methods. Moreover, the electronic structure of TiO2 thin film can be modulated by compounded with other semiconductor materials [17], doped with nonmetallic element [18,19] or modified by precious metal [20–23]. Recent studies demonstrated that TiO2 thin film is also sensitive to H2 . For example, TiO2 film doped with Pd shows fast response time (5 s) in H2 , its gas response in 0.05–0.1% H2 is more than 40% at 300 ◦ C [23,24]. It is also found that TiO2 film showed good response (270%) at 200 ◦ C in 0.1% H2 [25] and response time (1 min) in 1% H2 [26]. In addition, it is demonstrated that TiO2 film sensor shows response (∼55%) to H2 with a short response time (2 s) at 175 ◦ C in 1% H2
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with N2 as carrier gas [27]. In a word, compared with TiO2 nanotubes and nanowires the gas response of TiO2 film is very low. How to further improve the gas response of TiO2 film? Recently, it is found that the interfacial effect of the heterojunction is similar to an excellent “amplifier” and at room temperature (RT) can make the gas response of amorphous carbon film increase more than 2–5 orders of magnitude [28–32]. Based on the above discussion, we put forward an idea to improve the H2 gas response of TiO2 film. Herein, we use the interfacial amplification effect of TiO2 film/SiO2 /Si heterojunction to enhance the H2 sensing properties of TiO2 film. It is found that a Pd/TiO2 /SiO2 /p-Si multilayer can show a high response (∼2431%) to 1% H2 with appreciable short response time of 13 s and recovery time of 4 s at RT. In other words, the gas response and response time of Pd/TiO2 /SiO2 /Si heterojunction are comparable to that of TiO2 nanotubes array. 2. Materials and experiments 2.1. Synthesis and characterization of Pd/TiO2 /SiO2 /Si structure The fabrication of Pd/TiO2 /SiO2 /Si sensors can be briefly described as follows: first, a TiO2 film was grown on Si 1 0 0 substrate (10 mm × 10 mm) with native oxide layer (1.2 nm) [33] using RF magnetron sputtering method. The resistivities of p-Si or nSi substrates (the 46 Institute of Ministry of Electronics Industry, Tianjin, China) are 0.1–1 cm and 1–10 cm, respectively. The Si substrates were successively cleaned in ethanol and acetone solution using ultrasonic (Tianjin Kermel Chemical Reagent Co., Ltd., Tianjin, China) for 5 min, in the cleaning process no etching solution was used. TiO2 target is purchased at Beijing Jinyan Zhong new material Co., Ltd., Beijing, China. Before depositing TiO2 film, the sputtering chamber was pumped below 2 × 10−4 Pa and Si substrate was kept at RT. The working gas during deposition was mixed gas of argon and oxygen, two kinds of gas proportion is 1:1. The total gas pressure is 5 Pa, both oxygen and argon partial pressures are 2.5 Pa. In the deposition process, gas pressure, deposition power and time were 5 Pa, 90 W and 90 s, respectively. Second, the Pd film was deposited on the TiO2 /SiO2 /Si structure utilizing a metal mask from a Pd target (Beijing Mountain Technical Development Center, Beijing, China) using DC magnetron sputtering. In the deposition process, argon gas pressure, deposition power and time were 3 Pa, 40 W and 2 min, respectively. The substrate was still kept at RT. Thus the Pd/TiO2 /SiO2 /Si was prepared and its structure is illustrated in Fig. 1. The size of Pd film is 5.0 mm × 5.0 mm. The Pd film was used as sensitive layer. Moreover, it should be noted that the electrodes on the Pd film and Si substrate were made by In solder. Ohmic contact can be formed between In and Pd film or In
Fig. 1. The schematic illustrations of the I–V measurement of Pd/TiO2 /SiO2 /Si multilayers.
and Si substrate. The interface barriers are very little or no contact barrier even, which have little influence on I–V characteristics of the Pd/TiO2 /SiO2 /Si junction. The thicknesses of the TiO2 film and Pd film are about 15 nm and 16 nm, respectively, which are measured using Scanning Electron Microscopy. Finally, the crystal phases was observed with X-ray diffraction (XRD) on X’Pert Pro MPD XRD sys˚ It is demonstrated that the TiO2 thin film tem (CuK␣1, = 1.5406 A). is amorphous. 2.2. Hydrogen sensing measurement All the H2 sensing measurements were conducted in a chamber by exposing the multilayers to different concentrations of H2 (Qingdao Tianyuan Gas Production Limited Company, Qingdao, China) in air at RT (24 ± 1 ◦ C). The relative humidity in air is 30% and the air pressure is 1.0 × 105 Pa (normal pressure). The I–V characteristics of Pd/TiO2 /SiO2 /Si multilayers were measured using two-probe method. The two electrodes of each sensor were mounted on a probe holder and connected to Keithley 2400 source-meter (Keithley Instruments Inc., Cleveland, U.S.) controlled by a computer. 3. Results and discussions 3.1. I–V characteristics of Pd/TiO2 /SiO2 /Si multilayers Fig. 2a shows the I–V curves of the as-fabricated Pd/TiO2 / SiO2 /p-Si (In contact on p-Si is anode) and Pd/TiO2 /SiO2 /n-Si (In contact on n-Si is cathode) multilayers in air and pure H2 at RT. The I–V curve of the Pd/TiO2 /SiO2 /p-Si multilayer in air shows that Pd/TiO2 /SiO2 /p-Si is conductive when it is loaded positive or
Fig. 2. Plots of measured I–V curves of (a) the Pd/TiO2 /SiO2 /p-Si and (b) Pd/TiO2 /SiO2 /n-Si multilayers in air and after exposure to pure H2 at RT.
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Fig. 3. The sketch maps of energy-band structures of the Pd/TiO2 /SiO2 /p-Si multilayer in air (a) and in H2 (b).
Fig. 4. The sketch maps of energy-band structures of the Pd/TiO2 /SiO2 /n-Si multilayer in air (a) and in H2 (b).
negative bias voltage. However, the I–V curve of the Pd/TiO2 /SiO2 /pSi in pure H2 indicates that it shows an evident unidirectional conductive property [34,35], which indicates that the multilayer can be regarded as a diode [27]. In addition, it is found that the I–V properties of the Pd/TiO2 /SiO2 /n-Si multilayer in air and pure H2 at RT are completely contrary to that of the Pd/TiO2 /SiO2 /p-Si multilayer in air and pure H2 , as shown in Fig. 2b. In other words, the I–V curve of Pd/TiO2 /SiO2 /n-Si multilayer in air shows an evident unidirectional conductive property, while Pd/TiO2 /SiO2 /n-Si multilayer in pure H2 is conductive when it is loaded positive or negative bias voltage. In order to reveal the physical mechanism of H2 sensing properties of Pd/TiO2 /SiO2 /Si multilayers, we analyze the Fermi level of multilayers and draw the sketch map of energy-band structures of Pd/TiO2 /SiO2 /Si multilayers in air, as shown in Fig. 3a and Fig. 4a. Wherein EF , EC , EV , EVac are Fermi energy, conductanceband energy, valence-band energy and vacuum level, respectively. The subscripts 0, 1 and 2 denote Pd film, TiO2 film and Si, respectively. It is well known that the work function of Pd film is about 5.12 eV [36]. For amorphous TiO2 film, it is demonstrated that its value of energy band gap is 3.28 eV [37] which is close to 3.2 eV of anatase-structured TiO2 . According to energy band structure of anatase-structured TiO2 , EC is about located at −4.2 eV and EV is about located at −7.4 eV. Moreover, the work functions of anatasestructured TiO2 are calculated to be about 5.1 eV [38]. Therefore, we can deduce that the TiO2 film is weakly n-type in air.
Herein, the resistivities of p-Si or n-Si substrates are 0.1–1 cm and 1–10 cm, respectively. According to Physics of Semiconductor Devices [39], we can obtain the average concentration Nd (9.6 × 1014 cm3 ) and Na (2.60 × 1016 cm3 ) of n-Si or p-Si. Further, we calculated the relations among EF , EC , EV of p- or n-Si by = 1/ = 1/nqn , n = Nc exp EF − Ec /kT , EF = Si + Eg − (EF − Ec ), where , , si , and Eg are the p- or n-Si resistivity, conductivity, electron affinity and energy band gap, respectively. n , Nc , k0 and T are electron mobility, effective conduction band density of states, Boltzmann constant and absolute temperature, respectively. It is well known that band gap of Si is about 1.1 eV. Based on above discussion, the sketch maps of energy structure of p-Si or n-Si are shown in Figs. 3 and 4. As shown in Fig. 3a, due to the Fermi Levels of the TiO2 film and p-Si substrate are similar, the interfacial barrier between the TiO2 film and p-Si substrate is almost zero. Therefore, when exposed to air, the values of the current will increase with increasing absolute voltage. On one hand, When exposed to H2 , the Pd film may quickly reacts with H2 and forms palladium hydride (PdHx ) which possesses lower work function than pure Pd [40–42], resulting in transferring more electrons from the Pd film to TiO2 film. On the other hand, when H2 is absorbed on the TiO2 surface and may react with the adsorbed oxygen on the oxide surface free electrons are released. Meanwhile, H2 is dissociated on the Pd surface to H atoms [43] that get dissolved into the Pd bulk, which decrease the work
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Table 1 Response, response time and recovery time of the Pd/TiO2 /SiO2 /p-Si H2 sensor. H2 concentration in air (%)
0.05%
0.5%
1.0%
Response (S): (%) Response time (Tres ): (s) Recovery time (Trec ): (s)
821% 200.125 11.125
1541% 28.5 6.75
2431% 13 4
function of Pd [13]. These electrons trap and neutralize the holecarriers in TiO2 film [44], which lowers the hole-carrier concentration of the TiO2 film and enhances the Fermi level of the TiO2 film accordingly [27], as shown in Fig. 3b. Therefore, when exposed to H2 , the electrons will diffuse from the TiO2 film to the p-Si substrate, and finally an internal electrical field from the TiO2 film to the p-Si substrate will be built, which acts an interfacial barrier to inhibit the current form the TiO2 film to the p-Si substrate when multilayer is at the given negative bias voltage. Therefore, the Pd/TiO2 /SiO2 /p-Si multilayer can be regarded as a diode in pure H2 , as shown in Fig. 2a. The sketch map of energy-band structures of the Pd/TiO2 /SiO2 /n-Si structure is shown in Fig. 4. When exposed to air (Fig. 4a), due to the Fermi level of the TiO2 film is lower than that of the n-Si substrate, the electrons will diffuse from the n-Si substrate to the TiO2 film, and finally an internal electrical field from the n-Si substrate to the TiO2 film will be built, which acts an interfacial barrier to inhibit the current from the n-Si substrate to the TiO2 film when multilayer is at the given negative bias voltage. Therefore, the Pd/TiO2 /SiO2 /n-Si multilayer in air can be regarded as a diode, as shown in Fig. 2b. When exposed to H2 gas (Fig. 4b), the TiO2 film can obtain enough electrons from the Pd film and the Fermi level of the TiO2 film is enhanced to the vicinity of Fermi level of the n-Si substrate so that the interfacial barrier between the TiO2 film and the n-Si substrate was reduced to a smaller value. Now, the Pd/TiO2 /SiO2 /n-Si is conductive when it is loaded positive or negative bias voltage. In other words, the values of the current will increase with increasing absolute voltage.
Fig. 5. The variations of absolute value of current of (a) the Pd/TiO2 /SiO2 /p-Si or (b) n-Si multilayers with 0.05%, 0.5% and 1.0% H2 at −0.5 V and at RT.
3.2. Room-temperature hydrogen sensing performance The hydrogen sensing tests were conducted in ambient air. Usually, response time, recovery time and response are selected to evaluate and compare the device performance for H2 sensing [42]. The response time, Tres , is defined as the time required for the electrical current to reach 90% of the total change, while the recovery time, Trec , is defined as the time necessary for the electrical current to return 90% of the total change (the reference initial and final junction electrical current value are obtained from one single cycle from the sensing device). The time dependence of current of the Pd/TiO2 /SiO2 /p-Si structure at various concentrations of H2 is shown in Fig. 5a. As shown in this figure, the structure shows strong responses to 0.05% H2 , 0.5% H2 and 1.0% H2 at RT at the same given reverse bias voltage of −0.5 V. The H2 sensing parameters of the Pd/TiO2 /SiO2 /p-Si structure have been listed in Table 1. The structure exhibits good H2 sensing properties such as fast response time (∼13 s). Moreover, when the gas flow was switched from hydrogen to air, the current of the sample can reverted to its original value, and its recovery time is ∼4 s for 1.0% H2 in air at RT. Before defining the response (S), we investigate the work principle of H2 sensing device (Pd/TiO2 /SiO2 /p-Si structure) first. Fig. 6a shows the semi-logarithm plot for the I–V curves of the asfabricated Pd/TiO2 /SiO2 /p-Si structure (In contact on p-Si is anode) in air, 0.05%, 0.5%, 1.0% at RT. In view of the contacts between In and Pd or In and Si being ohmic, the I–V curves essentially reflect the characteristics of junction composted of Pd/TiO2 /SiO2 and p-Si. As observed in Fig. 6a, it is demonstrated that H2 concentration has apparent effect on the I–V curves of the Pd/TiO2 /SiO2 /p-Si
multilayer. When the Pd/TiO2 /SiO2 /p-Si structure is exposed to H2 in air, the currents decrease with increasing H2 concentration. Moreover, it is found that the response of the Pd/TiO2 /SiO2 /p-Si structure to H2 is higher under the negative bias −0.5 V. For example, the absolute values of measured current are 0.0937 (0.355), 0.0102 (0.269), 0.00571 (0.272), 0.00370 (0.273) mA at bias voltage −0.5 (+0.5) V when the Pd/TiO2 /SiO2 /p-Si structure is subjected to air, 0.05%, 0.5%, 1.0% at RT, respectively. When Pd/TiO2 /SiO2 /p-Si structure is applied negative bias −0.5 V in air or H2 , the conductivity of Pd/TiO2 /SiO2 /p-Si changes from conductive to nonconductive due to built-in electric field from TiO2 film to p-Si as Pd/TiO2 /SiO2 /pSi in H2 as shown in Fig. 3, and at this moment H2 response is much larger. Therefore, the effect of H2 molecules on the I–V characteristics of Pd/TiO2 /SiO2 /p-Si structure at bias voltage −0.5 V could serve as the work basis for the Pd/TiO2 /SiO2 /p-Si H2 sensor. As shown in Fig. 6a, the absolute value of the current of Pd/TiO2 /SiO2 /p-Si structure decrease with increasing H2 concentration at bias voltage −0.5 V. Herein, S of the sensor Pd/TiO2 /SiO2 /p-Si can be calculated according to following relationship: S=
Iair − IH2 × 100% IH2
(1)
where IH2 and Iair are the current values measured in ambient air and different concentrations of H2 in air at a given bias voltage −0.5 V at RT. Table 1 shows the S values of the sensor when exposing it to different concentrations of H2 in air at RT. As shown in Table 1, the sensor exhibits excellent H2 sensing properties. The S value
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current of Pd/TiO2 /SiO2 /n-Si structure in 1.0% H2 is 5.4 times higher than that of Pd/TiO2 /SiO2 /n-Si structure in 1.0% air. 3.3. The effects of native oxide It has been demonstrated that intrinsic SiOx layer may provide to a large number of nonradiative centers at the ZnO/Si interface and gives rise to the lattice mismatch between ZnO and Si substrate so that produce poor crystallinity in the ZnO film [45]. These lattice mismatch may cause interface state which can pin the barrier height of junction so that potential barrier between the film and Si substrate is not well responsive to the change of the Fermi level of film, resulting in a relative low gas response of film/Si junction. However, herein TiO2 film is amorphous. There is no lattice mismatch between TiO2 and Si substrate. In other words, it is thought that the very thin SiO2 has little influence on the movement of the carrier. Therefore, effect of SiO2 insulting layer in the band diagram construct was not considered and Pd/TiO2 /SiO2 /Si structure can exhibit a high H2 response as discussed above. In addition, we found that Pd/TiO2 /SiO2 /p-Si shows certain humidity response, while Pd/TiO2 /SiO2 /n-Si shows an obvious humidity response. Therefore, it needs to be calibrated before the Pd/TiO2 /SiO2 /Si structure being used to detect H2 gas. In the coming research, we will systematically study humidity response of Pd/TiO2 /SiO2 /Si multilayers and the effect of humidity on their H2 response. As is well known, H2 is very dangerous after being mixed with air over 4% (Lower Flammable Limit, LFL) concentration due to its explosive and flammable characteristics [46]. In Europe, it is state of the art that a H2 sensor should give a fast and reliable indication of danger at value of 20% of the LFL (0.8%) and a final alarm at value of 40% LFL (1.6%) [47]. Therefore, both Pd/TiO2 /SiO2 /p-Si and Pd/TiO2 /SiO2 /n-Si multilayers have a great potential in H2 detection for its high H2 response at RT. Fig. 6. The semi-logarithm plot of measured I–V curves of (a) the Pd/TiO2 /SiO2 /p-Si and (b) Pd/TiO2 /SiO2 /n-Si multilayers in air, 0.05%, 0.5% and 1.0% H2 at RT.
increases with increasing H2 concentration. The Pd/TiO2 /SiO2 /p-Si structure exhibits high response of ∼2431% to 1.0% H2 , which are 2 orders of magnitude higher than that of nanocrystalline p-TiO2 thin film [27]. To further examine the performance of the Pd/TiO2 /SiO2 /n-Si structure H2 sensor, we investigate the time dependence of current of the Pd/TiO2 /SiO2 /n-Si multilayer in various concentrations of H2 in air, as shown in Fig. 5b. As shown in this figure, the structure shows stronger responses to 0.05%, 0.5% and 1.0% H2 at RT at the same given reverse bias voltage of −0.5 V. When the gas flow was switched from H2 to air, the current of the sample can reverted to its original value by cycling high and low concentrations of H2 in air, and its response time and recovery time are ∼3 s or ∼7 s for 1.0% H2 in air at RT, respectively. Fig. 6b shows the semi-logarithm plot of I–V curves of the Pd/TiO2 /SiO2 /n-Si (In contact on n-Si is cathode) structure in air, 0.05%, 0.5%, 1.0% H2 at RT. As shown in Fig. 6b, it is found that H2 also has a strong effect on I–V curves of the Pd/TiO2 /SiO2 /n-Si structure. When the structure is exposed to different concentrations of H2 in air, both the currents increase with increasing H2 concentration. Just like Pd/TiO2 /SiO2 /p-Si, H2 has a stronger effect on the current of the Pd/TiO2 /SiO2 /n-Si multilayer at the same given reverse bias voltage of −0.5 V. For example, the absolute values of current measured are 0.00279 (0.178), 0.00434 (0.341), 0.00720 (0.459), 0.015 (0.474) mA at bias voltage −0.5 (+0.5) V when the Pd/TiO2 /SiO2 /n-Si multilayer is exposed to air, 0.05%, 0.5%, 1.0% H2 at RT, respectively. At the same given reverse bias voltage of −0.5 V it is found that the absolute value of current increases with increasing H2 concentration. The
4. Conclusion In conclusion, our results provide an easy, effective method to produce excellent H2 gas sensor using an interfacial amplification effect of oxide/Si heterojunction. The RT Pd/TiO2 /SiO2 /Si H2 sensors were fabricated using magnetron sputtering method, which exhibited excellent sensing properties. For example, the Pd/TiO2 /SiO2 /p-Si structure exhibits great response (2431%) and fast response time (∼13 s) in 1.0% H2 in air at RT. Moreover, this sensor has many other advantages, including easy-fabrication, cheapness, operating at RT and low power requirement (0.5 V). Therefore, the sensors have significant applicability. The gas sensitive mechanism was proposed as follows, when the multilayers are exposed to H2 , the Pd film quickly reacts with H2 and forms palladium hydride which results in transferring more electrons from the Pd film to TiO2 film and the Fermi level of the TiO2 film is changed. Therefore, the I–V characteristic of the Pd/TiO2 /SiO2 /Si multilayer was greatly changed when exposed to H2 . Acknowledgments This work is supported by the National Natural Science Foundation of China (11374372), China Postdoctoral Science Foundation (2014M551983), the Fundamental Research Funds for the Central Universities (14CX02018A and 14CX02025A), and the Qingdao Science & Technology Program (14-2-4-27-jch). References [1] M. Yamaguchi, S.A. Anggraini, Y. Fujio, T. Sato, M. Breedon, N. Miura, Stabilized zirconia-based sensor utilizing SnO2 -based sensing electrode with an
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Biographies Cuicui Ling received her Ph.D. degree in materials science and engineering from China University of Petroleum in 2013. She has worked as a teacher in China University of Petroleum for nearly 8 years. Her research interest is the synthesis of film materials and their application in gas sensor. Qingzhong Xue received his Ph.D. degree in materials science and engineering from Tsinghua University in 2005. He worked at Harvard University as a visiting professor from 2007 to 2008. His research interest includes the fabrication and characterization of film materials, carbon nanotube–polymer composites, as well as to exploit their potential applications. Zhide Han received his B.Sc. degree at 1999 in Dalian University of Technology, then an M.Sc. degree at 2004 in the same college. He has worked as a teacher in China University of Petroleum for nearly 9 years. His research interest includes the fabrication and characterization of functional ceramics, amorphous materials and composites. Zhongyang Zhang received his B.Sc. degree in materials physics from China University of Petroleum in 2012. Presently he is pursuing his M.Sc. degree in the College of Science, China University of Petroleum. His areas of interest are the synthesis of film materials and their application in solar cells. Yonggang Du received his M.Sc. degree in theoretical physics from Northeast Normal University in 2005. Presently he is pursuing his Ph.D. degree in the College of Science, China University of Petroleum. His areas of interest are the synthesis of film materials and their application in gas sensor. Yanmin Liu received his B.Sc. degree in materials science from China University of Petroleum in 1998. He worked as a teacher at Materials Analysis Center in State Key Laboratory of Heavy Oil Processing, China University of Petroleum. His area of interest is X-ray diffraction. Zifeng Yan received his Ph.D. degree in physical chemistry from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. He has worked as a teacher at China University of Petroleum since 1994. His research interest includes the heavy oil processing, catalysis, adsorption and catalytic materials.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
High hydrogen response of Pd/TiO2 /SiO2 /Si multilayers at room temperature Cuicui Ling a,b , Qingzhong Xue a,b,∗ , Zhide Han b , Zhongyang Zhang b , Yonggang Du a,b , Yanmin Liu a , Zifeng Yan a,∗ a b
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, Shandong, People’s Republic of China College of Science, China University of Petroleum, Qingdao 266580, Shandong, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 1 April 2014 Received in revised form 29 July 2014 Accepted 26 August 2014 Available online 3 September 2014 Keywords: Hydrogen response Pd/TiO2 /SiO2 /Si multilayers Current–voltage curve
a b s t r a c t A series of Pd/TiO2 /SiO2 /Si multilayers were produced using magnetron sputtering method. It is found that H2 molecules have dramatic effect on the current–voltage (I–V) characteristics of the Pd/TiO2 /SiO2 /Si multilayers at room temperature (RT). When Pd/TiO2 /SiO2 /Si multilayer is exposed to H2 , the Pd film quickly reacts with H2 and forms palladium hydride which results in transferring more electrons from the Pd film to TiO2 film. Therefore, the I–V characteristic of Pd/TiO2 /SiO2 /Si multilayer was greatly changed when exposed to H2 . For example, a Pd/TiO2 /SiO2 /p-Si multilayer can show a high response (∼2431%) to 1% H2 with appreciable short response time of 13 s and recovery time of 4 s at RT. Besides, it is demonstrated that Si substrate has a great effect on the H2 response of Pd/TiO2 /SiO2 /Si multilayers. When exposed to H2 the current of Pd/TiO2 /SiO2 /p-Si multilayer at −0.5 V greatly decreases while the current of Pd/TiO2 /SiO2 /n-Si multilayer greatly increase, which can be understood by their energy band structures. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen (H2 ) as a kind of new-type and clean fuel has become one of the best choice to replace fossil energy in future. However, H2 is colorless, odorless, explosive and extremely flammable with lower explosive limit of 4% in air. Consequently, a reliable sensor is needed to detect leakage from the storage and transportation of H2 as well as to monitor its concentration over a wide range. However, the shortcomings of current H2 sensors, such as big volume, expensive price, working at high temperature, greatly limit their performance. Therefore, the development of new-type H2 sensor material possesses extremely important scientific significance [1,2]. The nanometer oxide semiconductor gas sensor with high response, light stability, corrosion resistance and simple measurement has attracted much attention [3]. For example, nano zinc oxide (ZnO) gas sensors, nano tin oxide gas sensors and nano titanium oxide (TiO2 ) etc. gas sensors show high response to polar gases [4–6] and can detect gases in harsh environments [7].
∗ Corresponding authors at: China University of Petroleum, State Key Laboratory of Heavy Oil Processing, Qingdao 266580, Shandong, People’s Republic of China. Tel.: +86 532 86981169; fax: +86 532 86981169. E-mail address: [email protected] (Q. Xue). http://dx.doi.org/10.1016/j.snb.2014.08.072 0925-4005/© 2014 Elsevier B.V. All rights reserved.
Recently, it was found that the TiO2 nanotubes and nanowires not only can detect the polar gases, but also show certain response to non-polar gases such as H2 , carbon dioxide [8,9]. TiO2 nanotubes or nanowires arrays sensors show high response (∼1000%) to 0.1% H2 [8,10], its response time and recovery time in H2 gas can reach to 13 s and 120 s, respectively [11,12]. In addition, it is found that the precious metals such as palladium (Pd) etc. can further improve the sensing performance of TiO2 nanotubes and nanowires [13–15]. Though gas sensing performance of TiO2 nanotubes and nanowires is excellent, they possess several shortcomings such as strict preparation, complex processing and high cost, which will make them difficult to realize industrialization. In addition, these gas sensors tend to obtain better gas response at high temperatures [16]. Compared with TiO2 nanotubes and nanowires, TiO2 film can be simply prepared using many low-cost methods. Moreover, the electronic structure of TiO2 thin film can be modulated by compounded with other semiconductor materials [17], doped with nonmetallic element [18,19] or modified by precious metal [20–23]. Recent studies demonstrated that TiO2 thin film is also sensitive to H2 . For example, TiO2 film doped with Pd shows fast response time (5 s) in H2 , its gas response in 0.05–0.1% H2 is more than 40% at 300 ◦ C [23,24]. It is also found that TiO2 film showed good response (270%) at 200 ◦ C in 0.1% H2 [25] and response time (1 min) in 1% H2 [26]. In addition, it is demonstrated that TiO2 film sensor shows response (∼55%) to H2 with a short response time (2 s) at 175 ◦ C in 1% H2
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with N2 as carrier gas [27]. In a word, compared with TiO2 nanotubes and nanowires the gas response of TiO2 film is very low. How to further improve the gas response of TiO2 film? Recently, it is found that the interfacial effect of the heterojunction is similar to an excellent “amplifier” and at room temperature (RT) can make the gas response of amorphous carbon film increase more than 2–5 orders of magnitude [28–32]. Based on the above discussion, we put forward an idea to improve the H2 gas response of TiO2 film. Herein, we use the interfacial amplification effect of TiO2 film/SiO2 /Si heterojunction to enhance the H2 sensing properties of TiO2 film. It is found that a Pd/TiO2 /SiO2 /p-Si multilayer can show a high response (∼2431%) to 1% H2 with appreciable short response time of 13 s and recovery time of 4 s at RT. In other words, the gas response and response time of Pd/TiO2 /SiO2 /Si heterojunction are comparable to that of TiO2 nanotubes array. 2. Materials and experiments 2.1. Synthesis and characterization of Pd/TiO2 /SiO2 /Si structure The fabrication of Pd/TiO2 /SiO2 /Si sensors can be briefly described as follows: first, a TiO2 film was grown on Si 1 0 0 substrate (10 mm × 10 mm) with native oxide layer (1.2 nm) [33] using RF magnetron sputtering method. The resistivities of p-Si or nSi substrates (the 46 Institute of Ministry of Electronics Industry, Tianjin, China) are 0.1–1 cm and 1–10 cm, respectively. The Si substrates were successively cleaned in ethanol and acetone solution using ultrasonic (Tianjin Kermel Chemical Reagent Co., Ltd., Tianjin, China) for 5 min, in the cleaning process no etching solution was used. TiO2 target is purchased at Beijing Jinyan Zhong new material Co., Ltd., Beijing, China. Before depositing TiO2 film, the sputtering chamber was pumped below 2 × 10−4 Pa and Si substrate was kept at RT. The working gas during deposition was mixed gas of argon and oxygen, two kinds of gas proportion is 1:1. The total gas pressure is 5 Pa, both oxygen and argon partial pressures are 2.5 Pa. In the deposition process, gas pressure, deposition power and time were 5 Pa, 90 W and 90 s, respectively. Second, the Pd film was deposited on the TiO2 /SiO2 /Si structure utilizing a metal mask from a Pd target (Beijing Mountain Technical Development Center, Beijing, China) using DC magnetron sputtering. In the deposition process, argon gas pressure, deposition power and time were 3 Pa, 40 W and 2 min, respectively. The substrate was still kept at RT. Thus the Pd/TiO2 /SiO2 /Si was prepared and its structure is illustrated in Fig. 1. The size of Pd film is 5.0 mm × 5.0 mm. The Pd film was used as sensitive layer. Moreover, it should be noted that the electrodes on the Pd film and Si substrate were made by In solder. Ohmic contact can be formed between In and Pd film or In
Fig. 1. The schematic illustrations of the I–V measurement of Pd/TiO2 /SiO2 /Si multilayers.
and Si substrate. The interface barriers are very little or no contact barrier even, which have little influence on I–V characteristics of the Pd/TiO2 /SiO2 /Si junction. The thicknesses of the TiO2 film and Pd film are about 15 nm and 16 nm, respectively, which are measured using Scanning Electron Microscopy. Finally, the crystal phases was observed with X-ray diffraction (XRD) on X’Pert Pro MPD XRD sys˚ It is demonstrated that the TiO2 thin film tem (CuK␣1, = 1.5406 A). is amorphous. 2.2. Hydrogen sensing measurement All the H2 sensing measurements were conducted in a chamber by exposing the multilayers to different concentrations of H2 (Qingdao Tianyuan Gas Production Limited Company, Qingdao, China) in air at RT (24 ± 1 ◦ C). The relative humidity in air is 30% and the air pressure is 1.0 × 105 Pa (normal pressure). The I–V characteristics of Pd/TiO2 /SiO2 /Si multilayers were measured using two-probe method. The two electrodes of each sensor were mounted on a probe holder and connected to Keithley 2400 source-meter (Keithley Instruments Inc., Cleveland, U.S.) controlled by a computer. 3. Results and discussions 3.1. I–V characteristics of Pd/TiO2 /SiO2 /Si multilayers Fig. 2a shows the I–V curves of the as-fabricated Pd/TiO2 / SiO2 /p-Si (In contact on p-Si is anode) and Pd/TiO2 /SiO2 /n-Si (In contact on n-Si is cathode) multilayers in air and pure H2 at RT. The I–V curve of the Pd/TiO2 /SiO2 /p-Si multilayer in air shows that Pd/TiO2 /SiO2 /p-Si is conductive when it is loaded positive or
Fig. 2. Plots of measured I–V curves of (a) the Pd/TiO2 /SiO2 /p-Si and (b) Pd/TiO2 /SiO2 /n-Si multilayers in air and after exposure to pure H2 at RT.
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Fig. 3. The sketch maps of energy-band structures of the Pd/TiO2 /SiO2 /p-Si multilayer in air (a) and in H2 (b).
Fig. 4. The sketch maps of energy-band structures of the Pd/TiO2 /SiO2 /n-Si multilayer in air (a) and in H2 (b).
negative bias voltage. However, the I–V curve of the Pd/TiO2 /SiO2 /pSi in pure H2 indicates that it shows an evident unidirectional conductive property [34,35], which indicates that the multilayer can be regarded as a diode [27]. In addition, it is found that the I–V properties of the Pd/TiO2 /SiO2 /n-Si multilayer in air and pure H2 at RT are completely contrary to that of the Pd/TiO2 /SiO2 /p-Si multilayer in air and pure H2 , as shown in Fig. 2b. In other words, the I–V curve of Pd/TiO2 /SiO2 /n-Si multilayer in air shows an evident unidirectional conductive property, while Pd/TiO2 /SiO2 /n-Si multilayer in pure H2 is conductive when it is loaded positive or negative bias voltage. In order to reveal the physical mechanism of H2 sensing properties of Pd/TiO2 /SiO2 /Si multilayers, we analyze the Fermi level of multilayers and draw the sketch map of energy-band structures of Pd/TiO2 /SiO2 /Si multilayers in air, as shown in Fig. 3a and Fig. 4a. Wherein EF , EC , EV , EVac are Fermi energy, conductanceband energy, valence-band energy and vacuum level, respectively. The subscripts 0, 1 and 2 denote Pd film, TiO2 film and Si, respectively. It is well known that the work function of Pd film is about 5.12 eV [36]. For amorphous TiO2 film, it is demonstrated that its value of energy band gap is 3.28 eV [37] which is close to 3.2 eV of anatase-structured TiO2 . According to energy band structure of anatase-structured TiO2 , EC is about located at −4.2 eV and EV is about located at −7.4 eV. Moreover, the work functions of anatasestructured TiO2 are calculated to be about 5.1 eV [38]. Therefore, we can deduce that the TiO2 film is weakly n-type in air.
Herein, the resistivities of p-Si or n-Si substrates are 0.1–1 cm and 1–10 cm, respectively. According to Physics of Semiconductor Devices [39], we can obtain the average concentration Nd (9.6 × 1014 cm3 ) and Na (2.60 × 1016 cm3 ) of n-Si or p-Si. Further, we calculated the relations among EF , EC , EV of p- or n-Si by = 1/ = 1/nqn , n = Nc exp EF − Ec /kT , EF = Si + Eg − (EF − Ec ), where , , si , and Eg are the p- or n-Si resistivity, conductivity, electron affinity and energy band gap, respectively. n , Nc , k0 and T are electron mobility, effective conduction band density of states, Boltzmann constant and absolute temperature, respectively. It is well known that band gap of Si is about 1.1 eV. Based on above discussion, the sketch maps of energy structure of p-Si or n-Si are shown in Figs. 3 and 4. As shown in Fig. 3a, due to the Fermi Levels of the TiO2 film and p-Si substrate are similar, the interfacial barrier between the TiO2 film and p-Si substrate is almost zero. Therefore, when exposed to air, the values of the current will increase with increasing absolute voltage. On one hand, When exposed to H2 , the Pd film may quickly reacts with H2 and forms palladium hydride (PdHx ) which possesses lower work function than pure Pd [40–42], resulting in transferring more electrons from the Pd film to TiO2 film. On the other hand, when H2 is absorbed on the TiO2 surface and may react with the adsorbed oxygen on the oxide surface free electrons are released. Meanwhile, H2 is dissociated on the Pd surface to H atoms [43] that get dissolved into the Pd bulk, which decrease the work
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Table 1 Response, response time and recovery time of the Pd/TiO2 /SiO2 /p-Si H2 sensor. H2 concentration in air (%)
0.05%
0.5%
1.0%
Response (S): (%) Response time (Tres ): (s) Recovery time (Trec ): (s)
821% 200.125 11.125
1541% 28.5 6.75
2431% 13 4
function of Pd [13]. These electrons trap and neutralize the holecarriers in TiO2 film [44], which lowers the hole-carrier concentration of the TiO2 film and enhances the Fermi level of the TiO2 film accordingly [27], as shown in Fig. 3b. Therefore, when exposed to H2 , the electrons will diffuse from the TiO2 film to the p-Si substrate, and finally an internal electrical field from the TiO2 film to the p-Si substrate will be built, which acts an interfacial barrier to inhibit the current form the TiO2 film to the p-Si substrate when multilayer is at the given negative bias voltage. Therefore, the Pd/TiO2 /SiO2 /p-Si multilayer can be regarded as a diode in pure H2 , as shown in Fig. 2a. The sketch map of energy-band structures of the Pd/TiO2 /SiO2 /n-Si structure is shown in Fig. 4. When exposed to air (Fig. 4a), due to the Fermi level of the TiO2 film is lower than that of the n-Si substrate, the electrons will diffuse from the n-Si substrate to the TiO2 film, and finally an internal electrical field from the n-Si substrate to the TiO2 film will be built, which acts an interfacial barrier to inhibit the current from the n-Si substrate to the TiO2 film when multilayer is at the given negative bias voltage. Therefore, the Pd/TiO2 /SiO2 /n-Si multilayer in air can be regarded as a diode, as shown in Fig. 2b. When exposed to H2 gas (Fig. 4b), the TiO2 film can obtain enough electrons from the Pd film and the Fermi level of the TiO2 film is enhanced to the vicinity of Fermi level of the n-Si substrate so that the interfacial barrier between the TiO2 film and the n-Si substrate was reduced to a smaller value. Now, the Pd/TiO2 /SiO2 /n-Si is conductive when it is loaded positive or negative bias voltage. In other words, the values of the current will increase with increasing absolute voltage.
Fig. 5. The variations of absolute value of current of (a) the Pd/TiO2 /SiO2 /p-Si or (b) n-Si multilayers with 0.05%, 0.5% and 1.0% H2 at −0.5 V and at RT.
3.2. Room-temperature hydrogen sensing performance The hydrogen sensing tests were conducted in ambient air. Usually, response time, recovery time and response are selected to evaluate and compare the device performance for H2 sensing [42]. The response time, Tres , is defined as the time required for the electrical current to reach 90% of the total change, while the recovery time, Trec , is defined as the time necessary for the electrical current to return 90% of the total change (the reference initial and final junction electrical current value are obtained from one single cycle from the sensing device). The time dependence of current of the Pd/TiO2 /SiO2 /p-Si structure at various concentrations of H2 is shown in Fig. 5a. As shown in this figure, the structure shows strong responses to 0.05% H2 , 0.5% H2 and 1.0% H2 at RT at the same given reverse bias voltage of −0.5 V. The H2 sensing parameters of the Pd/TiO2 /SiO2 /p-Si structure have been listed in Table 1. The structure exhibits good H2 sensing properties such as fast response time (∼13 s). Moreover, when the gas flow was switched from hydrogen to air, the current of the sample can reverted to its original value, and its recovery time is ∼4 s for 1.0% H2 in air at RT. Before defining the response (S), we investigate the work principle of H2 sensing device (Pd/TiO2 /SiO2 /p-Si structure) first. Fig. 6a shows the semi-logarithm plot for the I–V curves of the asfabricated Pd/TiO2 /SiO2 /p-Si structure (In contact on p-Si is anode) in air, 0.05%, 0.5%, 1.0% at RT. In view of the contacts between In and Pd or In and Si being ohmic, the I–V curves essentially reflect the characteristics of junction composted of Pd/TiO2 /SiO2 and p-Si. As observed in Fig. 6a, it is demonstrated that H2 concentration has apparent effect on the I–V curves of the Pd/TiO2 /SiO2 /p-Si
multilayer. When the Pd/TiO2 /SiO2 /p-Si structure is exposed to H2 in air, the currents decrease with increasing H2 concentration. Moreover, it is found that the response of the Pd/TiO2 /SiO2 /p-Si structure to H2 is higher under the negative bias −0.5 V. For example, the absolute values of measured current are 0.0937 (0.355), 0.0102 (0.269), 0.00571 (0.272), 0.00370 (0.273) mA at bias voltage −0.5 (+0.5) V when the Pd/TiO2 /SiO2 /p-Si structure is subjected to air, 0.05%, 0.5%, 1.0% at RT, respectively. When Pd/TiO2 /SiO2 /p-Si structure is applied negative bias −0.5 V in air or H2 , the conductivity of Pd/TiO2 /SiO2 /p-Si changes from conductive to nonconductive due to built-in electric field from TiO2 film to p-Si as Pd/TiO2 /SiO2 /pSi in H2 as shown in Fig. 3, and at this moment H2 response is much larger. Therefore, the effect of H2 molecules on the I–V characteristics of Pd/TiO2 /SiO2 /p-Si structure at bias voltage −0.5 V could serve as the work basis for the Pd/TiO2 /SiO2 /p-Si H2 sensor. As shown in Fig. 6a, the absolute value of the current of Pd/TiO2 /SiO2 /p-Si structure decrease with increasing H2 concentration at bias voltage −0.5 V. Herein, S of the sensor Pd/TiO2 /SiO2 /p-Si can be calculated according to following relationship: S=
Iair − IH2 × 100% IH2
(1)
where IH2 and Iair are the current values measured in ambient air and different concentrations of H2 in air at a given bias voltage −0.5 V at RT. Table 1 shows the S values of the sensor when exposing it to different concentrations of H2 in air at RT. As shown in Table 1, the sensor exhibits excellent H2 sensing properties. The S value
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current of Pd/TiO2 /SiO2 /n-Si structure in 1.0% H2 is 5.4 times higher than that of Pd/TiO2 /SiO2 /n-Si structure in 1.0% air. 3.3. The effects of native oxide It has been demonstrated that intrinsic SiOx layer may provide to a large number of nonradiative centers at the ZnO/Si interface and gives rise to the lattice mismatch between ZnO and Si substrate so that produce poor crystallinity in the ZnO film [45]. These lattice mismatch may cause interface state which can pin the barrier height of junction so that potential barrier between the film and Si substrate is not well responsive to the change of the Fermi level of film, resulting in a relative low gas response of film/Si junction. However, herein TiO2 film is amorphous. There is no lattice mismatch between TiO2 and Si substrate. In other words, it is thought that the very thin SiO2 has little influence on the movement of the carrier. Therefore, effect of SiO2 insulting layer in the band diagram construct was not considered and Pd/TiO2 /SiO2 /Si structure can exhibit a high H2 response as discussed above. In addition, we found that Pd/TiO2 /SiO2 /p-Si shows certain humidity response, while Pd/TiO2 /SiO2 /n-Si shows an obvious humidity response. Therefore, it needs to be calibrated before the Pd/TiO2 /SiO2 /Si structure being used to detect H2 gas. In the coming research, we will systematically study humidity response of Pd/TiO2 /SiO2 /Si multilayers and the effect of humidity on their H2 response. As is well known, H2 is very dangerous after being mixed with air over 4% (Lower Flammable Limit, LFL) concentration due to its explosive and flammable characteristics [46]. In Europe, it is state of the art that a H2 sensor should give a fast and reliable indication of danger at value of 20% of the LFL (0.8%) and a final alarm at value of 40% LFL (1.6%) [47]. Therefore, both Pd/TiO2 /SiO2 /p-Si and Pd/TiO2 /SiO2 /n-Si multilayers have a great potential in H2 detection for its high H2 response at RT. Fig. 6. The semi-logarithm plot of measured I–V curves of (a) the Pd/TiO2 /SiO2 /p-Si and (b) Pd/TiO2 /SiO2 /n-Si multilayers in air, 0.05%, 0.5% and 1.0% H2 at RT.
increases with increasing H2 concentration. The Pd/TiO2 /SiO2 /p-Si structure exhibits high response of ∼2431% to 1.0% H2 , which are 2 orders of magnitude higher than that of nanocrystalline p-TiO2 thin film [27]. To further examine the performance of the Pd/TiO2 /SiO2 /n-Si structure H2 sensor, we investigate the time dependence of current of the Pd/TiO2 /SiO2 /n-Si multilayer in various concentrations of H2 in air, as shown in Fig. 5b. As shown in this figure, the structure shows stronger responses to 0.05%, 0.5% and 1.0% H2 at RT at the same given reverse bias voltage of −0.5 V. When the gas flow was switched from H2 to air, the current of the sample can reverted to its original value by cycling high and low concentrations of H2 in air, and its response time and recovery time are ∼3 s or ∼7 s for 1.0% H2 in air at RT, respectively. Fig. 6b shows the semi-logarithm plot of I–V curves of the Pd/TiO2 /SiO2 /n-Si (In contact on n-Si is cathode) structure in air, 0.05%, 0.5%, 1.0% H2 at RT. As shown in Fig. 6b, it is found that H2 also has a strong effect on I–V curves of the Pd/TiO2 /SiO2 /n-Si structure. When the structure is exposed to different concentrations of H2 in air, both the currents increase with increasing H2 concentration. Just like Pd/TiO2 /SiO2 /p-Si, H2 has a stronger effect on the current of the Pd/TiO2 /SiO2 /n-Si multilayer at the same given reverse bias voltage of −0.5 V. For example, the absolute values of current measured are 0.00279 (0.178), 0.00434 (0.341), 0.00720 (0.459), 0.015 (0.474) mA at bias voltage −0.5 (+0.5) V when the Pd/TiO2 /SiO2 /n-Si multilayer is exposed to air, 0.05%, 0.5%, 1.0% H2 at RT, respectively. At the same given reverse bias voltage of −0.5 V it is found that the absolute value of current increases with increasing H2 concentration. The
4. Conclusion In conclusion, our results provide an easy, effective method to produce excellent H2 gas sensor using an interfacial amplification effect of oxide/Si heterojunction. The RT Pd/TiO2 /SiO2 /Si H2 sensors were fabricated using magnetron sputtering method, which exhibited excellent sensing properties. For example, the Pd/TiO2 /SiO2 /p-Si structure exhibits great response (2431%) and fast response time (∼13 s) in 1.0% H2 in air at RT. Moreover, this sensor has many other advantages, including easy-fabrication, cheapness, operating at RT and low power requirement (0.5 V). Therefore, the sensors have significant applicability. The gas sensitive mechanism was proposed as follows, when the multilayers are exposed to H2 , the Pd film quickly reacts with H2 and forms palladium hydride which results in transferring more electrons from the Pd film to TiO2 film and the Fermi level of the TiO2 film is changed. Therefore, the I–V characteristic of the Pd/TiO2 /SiO2 /Si multilayer was greatly changed when exposed to H2 . Acknowledgments This work is supported by the National Natural Science Foundation of China (11374372), China Postdoctoral Science Foundation (2014M551983), the Fundamental Research Funds for the Central Universities (14CX02018A and 14CX02025A), and the Qingdao Science & Technology Program (14-2-4-27-jch). References [1] M. Yamaguchi, S.A. Anggraini, Y. Fujio, T. Sato, M. Breedon, N. Miura, Stabilized zirconia-based sensor utilizing SnO2 -based sensing electrode with an
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Biographies Cuicui Ling received her Ph.D. degree in materials science and engineering from China University of Petroleum in 2013. She has worked as a teacher in China University of Petroleum for nearly 8 years. Her research interest is the synthesis of film materials and their application in gas sensor. Qingzhong Xue received his Ph.D. degree in materials science and engineering from Tsinghua University in 2005. He worked at Harvard University as a visiting professor from 2007 to 2008. His research interest includes the fabrication and characterization of film materials, carbon nanotube–polymer composites, as well as to exploit their potential applications. Zhide Han received his B.Sc. degree at 1999 in Dalian University of Technology, then an M.Sc. degree at 2004 in the same college. He has worked as a teacher in China University of Petroleum for nearly 9 years. His research interest includes the fabrication and characterization of functional ceramics, amorphous materials and composites. Zhongyang Zhang received his B.Sc. degree in materials physics from China University of Petroleum in 2012. Presently he is pursuing his M.Sc. degree in the College of Science, China University of Petroleum. His areas of interest are the synthesis of film materials and their application in solar cells. Yonggang Du received his M.Sc. degree in theoretical physics from Northeast Normal University in 2005. Presently he is pursuing his Ph.D. degree in the College of Science, China University of Petroleum. His areas of interest are the synthesis of film materials and their application in gas sensor. Yanmin Liu received his B.Sc. degree in materials science from China University of Petroleum in 1998. He worked as a teacher at Materials Analysis Center in State Key Laboratory of Heavy Oil Processing, China University of Petroleum. His area of interest is X-ray diffraction. Zifeng Yan received his Ph.D. degree in physical chemistry from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. He has worked as a teacher at China University of Petroleum since 1994. His research interest includes the heavy oil processing, catalysis, adsorption and catalytic materials.