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Chemical Response of Tungsten Trioxide Sensing Layer Deposited on

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									                             NANOSTRUCTURED ANODIC TUNGSTEN OXIDE FILMS AS
                             A PROMISING MATERIAL FOR GAS SENSOR APPLICATION
                      Khatko*     Guirado,    Llobet,
                   V. Khatko*, F. Guirado, E. Llobet, X. Correig, Departament d'Enginyeria
                   Electronica,             Automatica,                       Virgili, Avd.
                   Electronica, Electrica i Automatica, Universitat Rovira i Virgili, Avd.
                                                                E-
                   Paisos Catalans 26, 43007 Tarragona, Spain, E-mail: vkhatko@urv.cat
                       Gorokh,        Mozalev,        Solovei,
                   G. Gorokh, A. Mozalev, D. Solovei, Nanotechnology Research                                                                           UNIVERSITAT                BELARUSIAN STATE UNIVERSITY OF
                   Laboratory, Belarusian State University of Informatics and                                                                          ROVIRA I VIRGILI          INFORMATICS AND RADIOELECTRONIC
                   Radioelectronic,         Str.
                   Radioelectronic, Brovka Str. 6, 220013 Minsk, Belarus
            Introduction In the last few years the new ways for preparing gas sensors with improved performance have appeared. Most of these
          ways are connected with the attempt of finding methods to increase the surface area (or the surface to volume ratio) of any active layer
          used for chemical sensing. One of these methods consists in employing thin porous layers with enlarged surface areas as support
          material for semiconductor, metal-oxide gas sensitive films. Particularly, nanoporous alumina resulting from the anodisation of
          aluminium can be used as a nanotemplate providing a connection between silicon microsystem technology and nano-chemistry.
          Different methods including sputter-deposition, CVD and electrochemistry can be considered to fill the pores with gas sensitive metal
          oxides or mesoporous materials. For example, the tungsten oxide gas sensing structures supported by nanoporous alumina templates
          showed high responsiveness to toxic gases, especially to NO2 [1]. It is known that the porous tungsten oxide layers can be formed by
          direct anodization of W in NaF electrolytes [2]. The morphology of the layers depends strongly on the NaF concentration as well as the
          formation potential.

             •The main aims of this work were to develop a new technology for the formation of nanostructured tungsten oxide films
             and to study the properties of the anodic films formed.
  Preparation of nanostructured tungsten oxide thin films                                                                               Schematic sketches of nanostructured film formation
  Nanostructured tungsten oxide thin films were formed in the several steps.
  First,
  First, thin tungsten films, up to 300 nm thick, were deposited onto alumina
             (Rubalite
substrates (Rubalite 710) by rf magnetron sputtering of a tungsten target in an
Ar atmosphere using an ESM100 Edwards sputtering system. A tungsten
                                            r.f.
target of 99.95% purity was used. The r.f. sputtering power was 100 W.
Subsequently, thin aluminum layers, up to 1.2 μm thick, were sputter-   sputter-
                                             W-
deposited onto the tungsten layers to form W-Al bilayer stacks.
  Second,
  Second, the overlying aluminum layer was transformed into its porous
anodic oxide in 0.2 M H2C2O4 stirred electrolyte at a constant current density
of 10 mA/cm2 at 296 K. During the galvanostatic period, the forming voltage
rises almost linearly up to about 55 V, before reaching a maximum of 60 V, and
                                   steady-
then decreases gradually until a steady-state value of 53 V is naturally taken
                                                                 rate
up. Over this period, a porous alumina grows with a constant rate up to the
underlying tungsten. According to these current density and film thickness,
after 5 min of galvanostatic anodizing the alumina barrier layer reaches the
                                                                0.1-
tungsten metal and the voltage begins to increase. Practically, 0.1-0.2 V rise is
                                                                  switched
enough to distinguish this moment and then the process is switched into
potentiostatic mode and anodizing is carried out until the current lowered to
                                                           underlying
its leakage value. During this period, oxidation of the underlying tungsten                                                               Cross-sections and surface of nanostructured WO3 film
                                                                                                                                          Cross-
occurred through the pores in the alumina film, and an array of the nanosized
                                            W-
tungsten oxide hillocks was formed at the W-Al interface.
   Third, after electrochemical treatments, the samples were rinsed with
deionized water and dried with N2 stream.
                  Investigation of sensor structures
                                                                             oxide
       The physical properties and chemical response of the tungsten oxide films to
                                                                          temperature
       nitrogen dioxide were investigated. The films were kept in a temperature and
       moisture controlled test chamber. The resistance of the films in the presence
       of either pure air or the different pollutants at the different concentrations was
       monitored and stored in a PC.

                                                                               films
                  Temperature (a) and chemical (b) responses of tungsten oxide films                                                    X-ray diffractogram of tungsten oxide film annealed in N2
                   430                                                                           500                                                   2500
                                               o
                                                                                                                o
                                                                                                           T=130 C                      off                                                                      RT
                                            T=70 C                                                                                                                                                                  o
                   420                                                                           480        NO2                                                                                                  400 C
                                                                                                                                                                                                                    o
                                                                                                                                                       2000                                                      800 C
                                                                                                 460
                                                                               Resistance, Ohm
Resistance, Ohm




                   410                                                                                                                                                                              Pt
                                                                                                 440
                   400                                                                                                                                 1500        WO3
                                                                                                                                              Counts




                                                                                                 420                                                             (020)
                   390                                                                                          2 ppm
                                                                                                 400
                                                                                                       1 ppm                                           1000
                   380                                                   o                                                                                                                Al2O3          Al2O3
                                                                   T=130 C                       380
                                                                                                                                                              WO3
                   370                                                                           360                                                    500   (002)      Al2O3

                         0   20   40   60      80      100   120   140       160                               200         400    600
                                             Time, s                                                                    Time, s
                                                a)                                                                                                        0
                                                                                                                           b)                              20 22      24 26 28 30   32 34 36 38   40 42 44 46 48 50 52
                                                                               oxidized
     Conclusiones After electrochemical processing the tungsten film was fully oxidized through a matrix                                                                          2Theta (degrees)
                                                                oxide
   of alumina. In the result of volumetric growth, the tungsten oxide has filled in the bottom parts of
                                                                                          low-
   alumina pores and has got the form of hillocks. The tungsten oxide films were low-resistance
   semiconductors with n-type of the conductivity and had the chemical response to NO2.
                        n-
     References
                         Khatko,    Gorokh,    Mozalev,      Solovei,    Llobet,   Vilanova,
                  [1] V. Khatko, G. Gorokh, A. Mozalev, D. Solovei, E. Llobet, X. Vilanova, X. Correig; Sensors & Actuators, B.
                                              255-
                    Chemical, 118 (2006), pp. 255-262.
                                         Macak, Sieber, Taveira,          Ghicov,    Sirotna,    Schmuki;
                  [2] H. Tsuchiya, J. M. Macak, I. Sieber, L. Taveira, A. Ghicov, K. Sirotna, P. Schmuki; Electrochemistry
                                                     295- 298
                    Communications, 7 (2005), pp. 295-298.

								
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