Growth mechanism of Pt modified TiO2 thick film

Sensors and Actuators B 176 (2013) 723–728

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Growth mechanism of Pt modified TiO2 thick film Maolin Zhang a,∗ , Zhanheng Yuan b , Tao Ning c , Jianping Song b , Cheng Zheng b a

School of Technical Physics, Xidian University, No.2, South TaiBai Road, Xi’an, Shaanxi 710071, PR China School of Electronic & Information Engineering, Xi’an Jiaotong University, No.28, Xianning West Road, Xi’an, Shaanxi 710049, PR China c Construction Engineering Research Institute of the Department of Logistics, PLA, No.16, North Jinghua Road, Xi’an, Shaanxi 710032, PR China b

a r t i c l e

i n f o

Article history: Received 3 February 2012 Received in revised form 24 August 2012 Accepted 17 September 2012 Available online 25 September 2012 Keywords: Pt/TiO2 Surface state Defect Growth mechanism

a b s t r a c t Platinum is widely used to modify TiO2 based gas sensing materials, and the promotion mechanism accompanied by praiseworthy results are discussed deeply. In this work, the growth mechanism of Pt dispersed on TiO2 thick film surface is discussed. The surface state of Pt is studied by XRD, XPS, SEM and Kröger-Vink defect theory in depth. Results indicate that Pt is not only dispersed on TiO2 thick film surface in metallic state but also occupy the lattice of Ti vacancy and form the oxidation state Ti1−x Ptx O2 . A simple model is proposed to explain the growth process of Pt and Ti1−x Ptx O2 . © 2012 Elsevier B.V. All rights reserved.

1. Introduction Noble metals are widely used to improve gas sensing properties of semiconductive metal oxides [1–9]. Not only the sensitivity and selectivity but also the response speed of metal oxide based gas sensors has been shown to be improved considerably by noble metals according to recent studies. In general, there are two types of sensitization mechanisms for noble metals, electronic and chemical [10,11]. The chemical sensitization, represented by Pt, takes place via a “spill-over” effect. The promoting effect of Pt arises from its ability to activate testing gases by enhancing their spillover, so that they can react with oxygen adsorbents more easily. This process does not affect the work function of metal oxide. On the other hand, the promoting effect of electronic sensitization which represented by Ag and Pd, arises mainly from the change in oxidation state of noble metal. Rational utilization of noble metal is an important technical problem on gas sensing materials. In field effect gas sensors, noble metals are usually deposited over an oxide layer, used as catalytic metal [12,13]. Also, based on its high catalytic activities and good thermal/mechanical stability, noble metal has received much attention on electrode material [1,8,14]. The most common using method of noble metal is doping, by sol–gel, pyrolysis and nanotechnology. These methods are used to get noble metal doped sensing material [4,6,9] or to get nanoparticles of noble metal which dispersed on sensing film surface [5,7,15]. The gas sensing

∗ Corresponding author. Tel.: +86 29 88202554. E-mail address: [email protected] (M. Zhang). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.09.054

properties of metal oxides have been further enhanced by these methods. However, these technology methods are complicated, expensive and need long periods of preparation. In our previous work [16], a simple method is successfully proposed to prepare a Pt modified TiO2 gas sensor. It is found that sensors with lower activation energy exhibited a faster rate of response when the magnitude of response was approximately uniform. The response properties, on the other hand, were promoted by increasing the amount of platinum particles dispersed on the sensor surface via a spill-over effect. TiO2 films were modified by platinum using chloroplatonic acid. After different treatment conditions, nano sized Pt particles were achieved. However, the growth mechanism of Pt dispersed on TiO2 thick film surface is not discussed. In this paper, the surface state of Pt modified TiO2 is studied in depth by XRD, XPS, SEM and KrögerVink defect theory. The growth process of Pt dispersed on TiO2 thick film surface is considered to include three main processes. Results indicate that Pt is not only dispersed on the TiO2 thick film surface but also occupy the lattice of Ti vacancy and form oxidation state. These results may bring a new light in preparation method on noble metals modified metal oxide sensing materials. 2. Experiment The experiment process was the same as our previous work [16]. TiO2 thick film was prepared on an alumina substrate by thick-film technique and calcined at 1280 ◦ C to form a gas sensing layer about 30 ␮m in thickness and TiO2 particle about 2–3 ␮m in diameter. The substrate equipped with comb-type platinum microelectrodes. Platinum electrodes were fabricated on alumina

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Fig. 1. XRD patterns of H2 PtCl6 -modified TiO2 thick films.

Fig. 3. Pt characteristic peaks of Pt-modified TiO2 thick film which treated at different temperatures (a) and preservation time (b).

Fig. 2. The whole XPS pattern of T1100 thick film.

substrate by screen printing method using a commercial platinum paste (Northwest Institute for Non-ferrous Metal Research, China), and were calcined at 1300 ◦ C. And platinum wires were used to contact the sensor device in the test system. TiO2 films were modified by platinum using chloroplatonic acid by dipping films into an aqueous solution of H2 PtCl6 . The response properties have been shown in our previous work [16]. Based on this result, the growth process of Pt dispersed on TiO2 thick film surface is discussed in this work. Therefore, treatment condition of sensing film is the same as our previous work. To obtain different surface state, one group of thick films was treated at 800–1100 ◦ C for 2 h and labeled as T800, T900, T1000, and T1100, respectively.

Table 1 Treatment condition and label of different film. Temperature (◦ C)

Time (h)

800 0 2 4 6

900 T0 T900(T2) T4 T6

T800

1000

1100

T1000

T1100

Then, another group was treated for different time at 900 ◦ C. The samples were labeled as T0, T4, and T6 and were treated for 0, 4, and 6 h, as shown in Table 1.

Table 2 The binding energy of Pt 4f7/2 characteristic peak with different samples. Sample

T800

T900(T2)

T1000

T1100

T0

T2

T4

T6

Pt [13]

Pt 4f7/2/ eV

72.02

71.37

71.28

70.97

71.63

71.37

70.95

71.05

70.90

Table 3 The relative abundance of lattice oxygen (OI) and chemisorbed oxygen (OII). Sample

T800

T900(T2)

T1000

T1100

T0

T2

T4

T6

OII/(OI + OII)

0.51

0.53

0.48

0.34

0.53

0.53

0.41

0.37

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Fig. 4. Oxygen 1s photoemission spectra for four Pt/TiO2 thick films. Plotted on the figures are the probable individual peak contributions corresponding to lattice oxygen (OI) and chemisorbed oxygen (OII).

X-ray diffraction (XRD, D/MAX-2400, Rigaku, Japan) was used to characterize the crystalline structure of modified TiO2 thick film at room temperature using Cu K␣ radiation. XRD studies were carried out over a 2 range from 20◦ to 70◦ at a step size of 0.02◦ (2) and a speed of 0.12◦ s−1 . Scanning electron microscope (SEM, JSM6700F, Jeol, Japan) was used to analysis the surface morphology of Pt. Surface elements analysis was carried out by X-ray photoelectron spectroscopy (XPS, VG-MK-II, China). X-ray energy was 1253.60 eV and the scanning range was 0–1000 eV at a step size of 1.00 eV. 3. Results and discussion 3.1. Surface state Fig. 1 shows the X-ray diffraction (XRD) spectra of Pt modified TiO2 treated at different conditions. All TiO2 samples are in rutile phase, but there are three peaks at 39.6◦ , 46.1◦ and 67.5◦ (2), which are typical of platinum. There is no phase transformation as treatment temperature differs. The presence of platinum is firmly demonstrated by EDS [16] and XPS. The whole XPS pattern of T1100, which provides high spatially resolved insight about the present of Pt, Ti and O, is shown in Fig. 2. The characteristic peaks of Pt and O are picked out for further study. The characteristic peaks of Pt dispersed on different films are shown in Fig. 3. It is found that there are some distinguishing features of Pt when the films treated at different temperatures, as compared with pure platinum. It shows that the intensity of Pt 4f7/2 is much lower than Pt 4f5/2 . However, the intensity of Pt 4f7/2 is much higher than Pt 4f5/2 in pure platinum XPS pattern [17]. On the other hand, as treatment temperature arising, the binding energy

of Pt 4f7/2 characteristic peak is skewed toward lower energy values, approaching the binding energy of pure Pt at 70.9 eV [17]. The binding energy of Pt 4f7/2 peak with different samples is shown in Table 2 in detail. In general, when an atom has gained one or more extra electrons, it present negative valence and the binding energy skewed toward low energy values in XPS pattern. In contrast, when an atom has lost one or more electrons, it present positive valence and the binding energy skewed toward high energy values in XPS pattern. It is indicated that some Pt has formed oxidation state by losing some electrons. And this oxidation state is closely related to treatment temperature. Similarly, as treatment time increasing, the binding energy of Pt 4f7/2 peak is also skewed toward lower energy values, approaching pure Pt at 70.9 eV [17], as shown in Fig. 3(b). From Fig. 2, it can be seen that O 1s XPS is asymmetric. This indicates that there are some different types of oxygen existed on the film surface. To investigate further detail of surface oxygen, O 1s spectra are fitted, as indicated in Fig. 4. O 1s spectra are fitted roughly with two peaks, OI and OII, representing two different kinds of surface species. OI with BE from 528.3 to 530.8 eV is the characteristic of lattice oxide, while OII with BE over 532 eV could be ascribed to chemisorbed oxygen [18–22]. The relative abundance of these two kinds of oxygen species are listed in Table 3. Decomposition of O 1s spectrum on samples T0–T6 is the same as Fig. 4. According to the rules of qualitative and quantitative analysis of XPS, XPS data of O 1s on T0–T6 is also listed in Table 3. It is shown that the percentage of chemisorbed oxygen (OII) to total oxygen decreases as treat temperature arises and preservation time increases. For samples T800, T900 and T0, the relative surface concentration of adsorbed oxygen (OII) is greater than 51%, while the OII is

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Fig. 5. Surface SEM micrographs and distribution histograms of all samples.

only 34% on T1100 and 37% on T6 surface. Generally speaking, the chemisorbed oxygen which has higher mobility than lattice oxygen, needs much active center to adsorb on film surface. This strongly suggests that the OII species is associated with the number of Pt dispersed on TiO2 surface. SEM is performed to examine the grain size and dispersion of Pt metal clusters [16], shown in Fig. 5. As seen in SEM pictures, platinum agglomerated on TiO2 thick film surface is clearly visible. However, changes in sintering temperature and preservation time have great influence on average diameter. Platinum particle diameters of T800, T900 and T0 are distributed in 0–200 nm ranges, while T1100 and T6 are mainly in the range from 200 to 600 nm. This matches well with Table 3. Samples with smaller Pt particle diameter can provide more active center which helps accept chemisorbed oxygen. Thus, T800, T900 and T0 with Pt particle diameter distributed in 0–200 nm ranges have more chemisorbed oxygen.

metal vacancies at film surface. At high oxygen partial pressure, Ti vacancy (VTi ) ionizes as follows:

3.2. Defect constant

p=

The resistance characteristics of Pt/TiO2 film are determined as a function of the content of oxygen at 600 ◦ C, plotted in Fig. 6. Resistance of Pt/TiO2 film is measured in terms of stabilization in oxygen concentration (0.1–40%). Kröger-Vink defect theory [23] is used to explain this result. When TiO2 thick film sintered at atmospheric air, oxygen is superfluous. The main phase of TiO2 produces a lot of

When Pt is dispersed on TiO2 film surface, some Pt may substitute for the site of VTi , and ionize as follows:

1 • n O2 (g) → O× O + VTi + nh 2

(1)

where O× O is an oxygen ion on an anionic site of the crystal and VTi is a Ti vacancy, n is the number of charge after VTi ionization, and h represents hole. Using the law of mass action, we find the following equilibrium:





n VTi

• 



1/2

× OO pn = k1 PO

(2)

2

where p is the hole concentration, k1 is the equilibrium constant and PO2 is the oxygen partial pressure. Using the charge neutral request,







n n VTi =p

(3)

and introducing Eq. (3) into Eq. (2), we can get:

1 n

1/2

k1 PO

1/n+1

2



1/2(n+1)

∝ PO

2

•

PtTi → PtlTi + lh

.

(4)

(5)

where l is the number of charge after PtTi ionization. Similarly, using the law of mass action and charge neutral request, we can get the

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to Eq. (4), the resistance on oxygen partial pressure of pure TiO2 thick film can be given as follows: R = A exp

E kT

−1/2(n+1)

PO

2

, m = 2(n + 1).

(9)

However, when Pt dispersed on TiO2 surface, some Pt substitutes the site of VTi . According to Eq. (6), the resistance on oxygen partial pressure is: R = A exp

E kT

−1/2(n+l+1)

PO

2

, m = 2(n + l + 1).

(10)

Therefore, the relation between the resistance (R) and oxygen pressure is: ln R = ln A +

1 E − ln PO2 kT 2(n + l + 1)

(11)

From Eqs. (9–11), it can be proved that the defect constant (m) is increase as Pt dispersed on TiO2 surface. As shown in Fig. 6(a), the defect constant (m) decrease from 7.63 to 5.66 when the treat temperature increase from 800 to 1100 ◦ C. As treatment temperature increases, the defect constant (m) gradually approach to pure TiO2 film (m = 6), in agreement with Gao and Xu’s work [26,27]. This is because when the treat temperature is low, more Pt can substitute the site of VTi . As the temperature increase, Pt agglomerated, grown and the Pt substituted at VTi decrease. Thus, the resistance characteristic of Pt/TiO2 approaches pure TiO2 . The defect constant (m) of another group which treated for different preservation time decrease from 8.55 to 6.64 when the preservation time increase from 0 to 6 h, shown in Fig. 6(b). The defect constant (m) of T6 is 6.64. Similarly, this means that as the preservation time increases, the defect constant of Pt/TiO2 gradually approach to pure TiO2 film (m = 6). 3.3. Growth mechanism

Fig. 6. Resistance response to different oxygen concentrations of films treated at different temperatures (a) and preservation time (b). These samples are tested at 600 ◦ C.

relation ship between p and PO2 as follows when Pt dispersed on TiO2 film: p=

k k 1 2 n

1/2

PO

1/n+l+1

2

1/2(n+l+1)

∝ PO

2

.

(6)

where k2 is the equilibrium constant when Pt dispersed on the film. At high oxygen partial pressure, resistance of the film is inversely proportional to hole concentration. In general, for TiO2 the dependence of the resistance on temperature and oxygen partial pressure can be given by such a formula [24,25]: R = A exp

E kT

−1/m

PO

2

(7)

where A is a constant, E is the activation energy, k is the Boltzmann constant, and m is a constant dependent on the nature of the defects. At high oxygen partial pressure, it is possible that the conduction process is mainly controlled by the transport of holes. The relationship between R and p can be written as: R=

1 1 1 1 = + ∝  ne pe p

(8)

where R is the resistance of material,  is the conductance,  is the mobility, n is the electron concentration, p is the hole concentration, e is the electric charge. At high oxygen partial pressure, according

Based on the foregoing discussions on surface state and defect state, the growth process of Pt modified TiO2 thick film can be proposed in three steps. Firstly, when TiO2 thick films are calcined at 1280 ◦ C, stable rutile phase are formed. As the sensing films sintered at atmospheric air, oxygen is superfluous and lots of metal vacancies (VTi ) formed at the surface of TiO2 , as shown in Eq. (1). Secondly, the sensing film is dipped into H2 PtCl6 contained solution. After heat treatment, Pt decomposes from H2 PtCl6 . One part of Pt dispersed directly on the film surface in metallic state which confirmed by XRD, as shown in Fig. 1. On the other hand, the other part of Pt may substitute on the site of VTi at film surface forming the oxidation state. It can be seen from XPS that the intensity of Pt 4f7/2 is much higher than Pt 4f5/2 in pure platinum XPS pattern. Thirdly, as the sintering temperature and preservation time increasing, Pt which dispersed on the surface in metallic state agglomerated and grown. Changes in treatment condition considerably influenced the average diameter, which was significantly enlarged from 20 to 500 nm when the temperature increased from 800 to 1100 ◦ C, as seen in Fig. 5. This directly affected the concentration of chemisorbed oxygen on TiO2 surface, shown in Table 3. It is shown that the percentage of chemisorbed oxygen (OII) to total oxygen (OI + OII) decreases as treat temperature arises and preservation time increases. On the other hand, as treatment temperature arising and treatment time increasing, the binding energy of Pt 4f7/2 characteristic peak is skewed toward lower energy values, shown in Table 2. This indicates that the proportion of Pt in oxidation state is decreased. This can be assumed that Pt which substituted on the site of VTi in oxidation state can also partly enter into the metallic Pt particles on TiO2 surface. The grain size of Pt particles increases as the treatment temperature and time increases. However, some

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Pt still occupied VTi and the defect constant (m) is changed. As the treatment temperature and preservation time increases, the defect constant of Pt/TiO2 gradually approach to pure TiO2 film (m = 6), as shown in Fig. 6. That means Pt can partly substitute on the site of VTi , affecting the oxidation state and defect constant, besides chemical sensitization. 4. Conclusions Pt/TiO2 thick-films are prepared by thick film technique and surface-modified in an H2 PtCl6 -containing solution by dipping method. Then these films are treated under different conditions. The growth process of this Pt/TiO2 thick film can be proposed in the following steps. Firstly, metal vacancies VTi formed at the surface of TiO2 when the film sintered at atmospheric air with oxygen superfluous. Then, Pt decomposes from H2 PtCl6 , partly dispersed directly on film surface in metallic state and another portion substitutes on the site of VTi , forming oxidation state Ti1−x Ptx O2 . Even though these Pt/TiO2 films are then treated under different conditions, some Pt still occupied VTi as Ti1−x Ptx O2 . This means that there are two type of Pt forms on Pt/TiO2 surface, metallic state (Pt) and oxidation state (Ti1−x Ptx O2 ). Acknowledgments This work was supported by National Natural Science Foundation of China (no. 61078020). References [1] T. Hyodo, H. Shibata, Y. Shimizu, M. Egashira, H2 sensing properties of diodetype gas sensors fabricated with Ti- and/or Nb-based materials, Sensors and Actuators B 142 (2009) 97–104. [2] S. Zachary, J.C. Young, S. Bose, Citrate-nitrate synthesis of nano-structured titanium dioxide ceramics for gas sensors, Sensors and Actuators B 140 (2009) 98–103. [3] S. Zhuiykov, Y.J. Choi, Z. Seeley, A. Bandyopadhyay, S. Bose, S.A. Akbar, Aluminum doped TiO2 nano-powders for gas sensors, Sensors and Actuators B 124 (2007) 111–117. [4] J.X. Sheng, N. Yoshida, J. Karasawa, T. Fukami, Investigation of platinum-doped TiO2 film lambda-sensor, Sensors and Materials 9 (1997) 97–106. [5] J. Trimboli, P.K. Dutta, Oxidation chemistry and electrical activity of Pt on titania: development of a novel zeolite-filter hydrocarbon sensor, Sensors and Actuators B 102 (2004) 132–141. [6] L. Francioso, D.S. Presicce, P. Siciliano, A. Ficarella, Combustion conditions discrimination properties of Pt-doped TiO2 thin film oxygen sensor, Sensors and Actuators B 123 (2007) 516–521. [7] C.H. Han, D.W. Hong, I.J. Kim, J. Gwak, S.D. Han, K.C. Singh, Synthesis of Pd or Pt/titanate nanotube and its application to catalytic type hydrogen gas sensor, Sensors and Actuators B 128 (2007) 320–325. [8] Y. Shimizu, N. Kuwano, T. Hyodo, M. Egashira, High H2 sensing performance of anodically oxidized TiO2 film contacted with Pd, Sensors and Actuators B 83 (2002) 195–201. ˜ A. Maldonado, M. de la, L. Olvera, Sensing properties of chemically [9] L. Castaneda, sprayed TiO2 thin films using Ni, Ir, and Rh as catalysts, Sensors and Actuators B 133 (2008) 687–693. [10] Y. Shimizu, M. Egashira, Basic aspects and challenges of semiconductor gas sensors, MRS Bulletin 24 (1999) 18–24. [11] N. Yamazoe, New approaches for improving semiconductor gas sensors, Sensors and Actuators B 5 (1991) 7–19.

[12] A. Trinchi, S. Kandasamy, W. Wlodarski, High temperature field effect hydrogen and hydrocarbon gas sensors based on SiC MOS devices, Sensors and Actuators B 133 (2008) 705–716. [13] C. Hsu, H. Chen, C. Chang, T. Chen, C. Huang, P. Chou, W. Liu, On the hydrogen sensing characteristics of a Pd/AlGaN/GaN heterostructure field-effect transistor (HFET), Sensors and Actuators B 165 (2012) 19–23. [14] J. Yang, P.K. Dutta, High temperature potentiometric NO2 sensor with asymmetric sensing and reference Pt electrodes, Sensors and Actuators B 143 (2010) 459–463. [15] L.D. Luca, A. Donato, S. Santangelo, G. Faggio, G. Messina, N. Donato, G. Neri, Hydrogen sensing characteristics of Pt/TiO2 /MWCNTs composites, International Journal of Hydrogen Energy 37 (2012) 1842–1851. [16] M.L. Zhang, Z.H. Yuan, J.P. Song, C. Zheng, Improvement and mechanism for the fast response of a Pt/TiO2 gas sensor, Sensors and Actuators B 148 (2010) 87–92. [17] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Mulienberg, Handbook of X-ray Photoeelectron Spectroscopy, Perkin-Elmer Corporation, Minnesota, 1979. [18] P.M. Kumar, S. Badrinarayanan, M. Sastry, Nanocrystalline TiO2 studied by optical, FTIR and X-ray photoelectron spectroscopy: correlation to presence of surface states, Thin Solid Films 358 (2000) 122–130. [19] F. Larachi, A. Adnot, A. Bernis, Ce 3d XPS study of composite Cex Mn1−x O2−y wet oxidation catalysts, Applied Surface Science 195 (2002) 236–250. [20] H. Chen, A. Sayari, A. Adnot, F. Larachi, Composition-activity effects of Mn-Ce-O composites on phenol catalytic wet oxidation, Applied Catalysis B: Environmental 32 (2001) 195–204. [21] S.X. Yang, Y.J. Feng, J.F. Wan, W.P. Zhu, Z.P. Jiang, Effect of CeO2 addition on the structure and activity of RuO2 /g-Al2 O3 catalyst, Applied Surface Science 246 (2005) 222–228. [22] L.Q. Jing, Z.L. Xu, X.J. Sun, J. Shang, W.M. Cai, The surface properties and photocatalytic activities of ZnO ultrafine particles, Applied Surface Science 180 (2001) 308–314. [23] D.M. Smyth, The Defect Chemistry of Metal Oxides, Oxford University Press, Inc, Oxford, 2000. [24] R.C. Bnchanan, Ceramic Materials for Electronics, Marcel Dekker, New York, 1986. [25] P.T. Moselay, Material selection for semiconductor gas sensors, Sensors and Actuators B 6 (1992) 149–156. [26] L. Gao, Q. Li, Z. Song, J. Wang, Preparation of nano-scale titania thick film and its oxygen sensitivity, Sensors and Actuators B 71 (2000) 179–183. [27] Y.L. Xu, X.H. Zhou, O.T. Sorensen, Oxygen sensor based on semiconducting metal oxides: an overview, Sensors and Actuators B 65 (2000) 2–4.

Biographies Maolin Zhang received his PhD degree from the Xi’an JiaoTong University in the Department of Electronic Science and Technology. He is currently a Lecturer of the Xidian University working in the area of sensing materials and devices. Zhanheng Yuan graduated from the Xi’an JiaoTong University in 1976. He is currently a Professor of the Xi’an JiaoTong University. His work focuses on the functional composite materials for gas-sensitive and chemical sensitive sensors. Tao Ning received his MS degree from the Xi’an JiaoTong University in 2008. Now, he works in the Construction Engineering Research Institute of the Department of Logistics, P. L. A. His current fields of interest focus on the thick film materials. Jianping Song received his PhD degree from the Xi’an JiaoTong University in 1990 and then worked in the Technical University of Denmark. He is currently a Professor of the Xi’an JiaoTong University and the Vice-chair of the Institute of Physical Electronics & Optoelectronics. His research interests are in the area of multifunctional sensing materials, nonlinear optics and the scanning tunneling microscopy. Cheng Zheng received his BS degree from the Xi’an JiaoTong University in 1998. Since 2002 he has been working at the Speciality of Microelectronic & Solidelectronic Engineering of Xi’an JiaoTong University. His current fields of interest focus on the applications of titania and stannic oxide to sensors.