Enhancement of vanadium oxide doped Eu + 3 for gas sensor application

Thin films of vanadium oxide nanoparticles doped with different concentrations of europium oxide (2, 4, 6, and 8) wt % are deposited on glass and Si substrates with orientation (111) utilizing by pulsed laser deposition technique using Nd:YAG laser that has a wavelength of 1064 nm, average frequency of 6 Hz and pulse duration of 10 ns. The films were annealed in air at 300 °C for two hours, then the structural, morphological and optical properties are characterized using x-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM) and UV-Vis spectroscopy respectively. The X-ray diffraction results of V2O5:Eu2O3 exhibit that the film has apolycrystalline monoclinic V2O5 and triclinic V4O7 phases. The FESEM image shows a homogeneous pattern and confirms the formation of uniform nanostructures on the glass substrate. The type of the particle found nanoparticles with different doping concentrations of Eu2O3. The optical energy gap increases with the increase of doping concentration and it varies from 2.67 eV to 2.71 eV. The prepared thin films are used to fabricate sensor against nitrogen dioxide gas. The dependence of sensitivity and response time on doping ratio and operation temperature of gas sensors has been studied, the maximum sensitivity was about 100%, the response time is equal to 24s and recovery time 16s for V2O5 doped 2% Eu2O3 at 50 °C.


Introduction
Controlling the free-surface electrostatic potential of semiconducting metal oxides offers possibilities for improving the performance of sensors and catalysts and photocatalysts. However, methods to exert such control have typically proven to be inexact and unreliable. The present work demonstrates an approach based on semiconductor heterojunctions, wherein an oxide substrate with controlled carrier concentration supports a much thinner layer. The layer is too thin to absorb all the charge that would normally transfer, so some of the excess charge propagates to the free surface and changes the surface potential. A combination of standard heterojunction analysis via Poisson's equation and surface potential measurements verifies the workability of this concept for thin polycrystalline V 2 O 5 grown on polycrystalline anatase TiO 2 [1]. Vanadium forms various morphologies with different coordination arrangements. The most common coordination arrangements are: tetrahedral (VO 4 ), trigonal bipyramids or square bipyramids (VO 5 ) distorted and regular octahedrons (VO 6 ) [2,3]. There are two bulk structures for vanadium pentoxide: α-V 2 O 5 and γ-V 2 O 5 .The structure of α V 2 O 5 has an orthorhombic layered structure. Pyramid structural arrangement builds with five oxygen atoms surrounding one vanadium atom. Vanadium forms various morphologies with different coordination arrangements [4]. Tetrahedral coordination is a preferred arrangement for +5 oxidation state [5]. The layered structure of γ-V 2 O 5 resembles closely that of α-V 2 O 5 . As a result the γ-V 2 O 5 unit cell may be obtained from that of α-V 2 O 5 by a few rearrangements and twists. These twists make the γ-V 2 O 5 layers more flexible but also render the structure metastable [6,7]. The structure of vanadium pentoxide (V 2 O 5 ) exhibits intercalation layered structure. As a result, it offers a possibility of reversible intercalations of different atoms, molecules or ions [8]. The interlayer separation of V 2 O 5 changes depending the size and shape of the intercalated particles [9,10].  (111) utilizing pulsed laser deposition technique the laser energy is a setup of 200 mJ with constant shoot of 600 are carried out. The distance between the targetand the substrate is 2 cm, while the distance between the target and the laser source is 12 cm. The thickness about of 100 nm, furthermore the films are annealed in air at 400 °C for two hours.

Experimental
Two types of substrate were used in this study. Glass slides which were used to study the optical properties of V 2 O 5 : Eu 2 O 3 films, and p-type Si wafer substrate with crystal orientation (111) for gas sensing measurments. Substrates cleaned using detergent with water to remove any oil or dust that might be attached to the surface of substrates, then placed under tap water and rubbing gently for 15 minutes. Then placed in a clean beaker containing distilled water, then rinsed in an ultrasonic unit for 15 minutes also.
Field emission scanning electron microscopy (FESEM) allows sample images to be quickly collected in the magnification range of 10-250,000X. In FESEM, a finely focused electron beam 5-30 keV is directed onto the examined area in a high vacuum environment. The interaction of the electron beam with the sample can yield backscattered electrons, secondary electrons, X-rays with characteristic energies, or photons. The information restored as images form or held on the area for static analysis.

Results and discussion 1. X-RAY diffraction spectra
The X-ray diffraction test is widely used as a characterization technique because it gives a lot of crystalline, lattice parameters, size of the crystallites and any other phase information about the structure of films. It is useful to determine texture and it is a non-destructive technique and requires minimum sample preparation. The interplaner distanced d hkl for different planes are measured using Eq. (1) nλ = 2dsinθ (1) Fig. 1 shows the x-ray diffraction patterns of deposit V 2  Also it can be seen that the full width at half maximum (FWHM) increases, i.e the grain size (G.S) decrease, with increasing doping ratio. This is good result for using samples in gas sensors.

Field emission scanning electron microscope (FESEM)
Field emission scanning electron microscope (FESEM) can be used to obtain three dimensions like topographical images of a wide variety of samples. FESEM images of undoped and doped films with different concentration of Eu 2 O 3 were shown in Fig. 2. The FESEM image shows a homogeneous pattern and confirms the formation of uniform structures on the glass. The size of the particles of pure and doped with 2%, 4%, 6%, 8% wt of Eu 2 O 3 are in the range of nanostructure. The Nano particle size decrease from 200 nm in pure samples to less than 50 nm at 8% sample.  Fig.3  There was stability in transmittance about 90% from 500 nm to 1100 nm. Spectrum is a significantly associated with the structure of energy levels which are in turn connected with chemical and crystalline structure of the material and therefore general characteristics of that material. Fig. 3 shows the variation of transmittance for deposited V 2 O 5 thin films on glass substrates with wavelength for different doping ratio with E 2 O 3 . It clear from this figure that the transmission for all films about 90% at the range 500 to 1100 nm and increase with increasing doping ratio, which is useful for used as optically transparent films. Fig.3: Transmittance as a function of wavelength for V 2 O 5 films doped with (2, 4, 6, and  8) Fig. 6 shows the variation of resistance as a function of the time with on/off gas valve. The resistance of gas sensor decreases exponentially when open the gas till reach the minimum value and the action reverse with gas closing, because it is p-type. Gas sensor measurement of pure and doped V 2 O 5 show high sensitivity to NO 2 gas, and the sensitivity was increases with the increase of operation temperature. The maximum sensitivity was found about 100% and the best response time is equal to 24s, while, recovery time 16s for 2% Eu 2 O 3 sample at 50 °C. All gas sensors parameters were shown in Table 2.   )  50°C  100°C  200°C  50°C  100°C  200°C  50°C  100°C  200°C  96  96  95  18  8  21  27  7  23  2%  100  98  60  24  28  19  16  53  18  4%  90  97  96  24  33  30  28  26  31  6%  90  98  96  24  27  27  26  29  28  8%  99  95  97  24  28  18  14  27  18 Conclusions Polycrystalline V 2 O 5 -doped Eu 2 O 3 structures were successfully prepared by PLD technique. The x-ray diffraction reveals that there was decrease in intensities up to 4% and some peaks of V 2 O 5 disappear. A new phase of vanadium oxide (V 4 O 7 ) cubic structure was appeared at 6% and 8% with peaks ( ̅ 20), (1 ̅ 2) and (104) at diffraction angle of 26.7660°, 30.1340° and 36.2210° respectively. The FESEM image shows a homogeneous pattern and confirms the formation of uniform structures on the glass. The maximum sensitivity is 100% and the best response time is equal to 24s, while, recovery time 16s for 2% Eu 2 O 3 sample at 50 °C.