Enhanced Ammonia Sensing By Cost-Effective ZnO Thin Films Through Yttrium Doping

Nebulizer spray coated yttrium doped ZnO thin film for ammonia vapour sensing Abstract Yttrium (Y) doped (doping concentration - 0, 1, 3 and 5 wt%) ZnO thin films were deposited using spray pyrolysis technique. The structural, surface morphological, optical and compositional properties were analysed using X-Ray diffraction (XRD), Atomic Force Microscopy (AFM), UV-vis NIR spectrophotmetry (UV), photoluminescence study (PL) and elemental composition analysis. Ammonia vapour sensing properties such as response/recovery, stability and repeatability were studied at room temperature. XRD results confirmed that the prepared samples have hexagonal wurtzite structure. ZnO:Y thin film with 5 wt% yttrium doping exhibits excellent sensing response of 99, fast response/recovery times of 29 s/ 7 s which may be due to the existence of oxygen vacancies in the case of ZnO:Y (5 wt%) film sample confirmed by photoluminescence (PL) study. These oxygen vacancies attract more electrons and thus enhance the gas sensing. In addition, increase in the number of active sites caused by the substitution of Y 3+ (trivalent) ions into the Zn 2+ (divalent) regular sites as confirmed by the observed M-B (Moss-Burstein) effect also causes an enhancement in the gas sensing. Surface roughness, another reason for the enhanced sensitivity, has been confirmed by AFM.


Introduction
Recent days, ammonia gas sensors play an important role in the industries of explosives, fertilizers, textiles and plastics [1]. The acceptable limit of ammonia for human body is 25 ppm, beyond which it can affect the various parts of the body causing damage to skin, eyes and respiratory tract [2]. Hence, researchers pay attention on making cost-effective ammonia sensors with improved '3S' parameters viz. sensitivity, stability and selectivity [3].
Moreover, literature survey shows that the gas sensing ability of ZnO can be improved through the addition of suitable dopants. Select dopants desirably alter the energy gap and surface morphology of the host material. In addition, doping causes an increase in defects and carrier concentration and thereby favouring the sensing phenomena [12]. Various rare earth metals like lanthanum, cerium, erbium, terbium, gadolinium and yttrium [13][14][15][16][17][18] have been used as dopants owing to their unique properties. Among the various rare earth elements,yttrium is used as dopant because of its peculiar properties arising from the 4f shell such as changes in energy band structure, morphology, surface to volume ratio and ability for creating more active centres at the grain boundaries [19,20]. Thus, the gas sensing response of ZnO can be improved by the addition of suitable proportion of yttrium.
Li and co-workers reported that yttrium doped ZnO nanofibers exhibit good sensing to acetic acid [21] due to the large specific surface area and small grain size of ZnO:Y nanofibers. Nithya et al. reported that yttrium doped titania nanoparticles prepared towards ethanol sensing exhibit short response/recovery times and long-term stability that may be due to the small crystal size and increase in oxygen vacancies [22]. Shruthi et al. reported that Y2O3-In2O3 nanocomposites exhibit sharp response/recovery times for methanol detection. It may be attributed to the formation of hetero junction between host and dopant materials [23]. However, to the best of our knowledge yttrium doped ZnO thin films towards ammonia sensing is scarcely available in the literature.
In this work, undoped and yttrium doped ZnO film sensors have been developed using cost effective nebulizer spray pyrolysis method. The sensing ability, response and recovery times and stability of the sensor towards the reducing vapour (NH3) at room temperature were studied and reported.

Film deposition
Zn(CH3COO)2.2H2O and Y(NO3)3·6H2O were used as host and dopant precursors, respectively and methanol (10 mL) was used as solvent. Four sets of stock solutions (0.2 M) were prepared with dopant concentrations 0, 1, 3 and 5wt%. The solution was stirred for 15 minutes at 35 o C. Substrates (glass) were cleaned well with usual cleaning procedure and then placed on the substrate holder (hot plate) maintained at 450 o C. The solution was sprayed on the hot substrates over the area of 2.5 x 7.5 cm 2 . The distance between the spray nozzle and substrate was optimized as 30 mm. Nebulizer was mounted with the help of a stand and slowly moved in horizontal direction to cover large surface area. After the deposition, the hot plate was allowed to cool to room temperature.

Characterization techniques
The structural characterization of the prepared thin films was made using X-ray diffraction (Panalytical X'Pert PRO) using Cu-Kα radiation (λ=0.1540nm). Stylus profilometer was used to measure the thickness of the prepared films. Atomic Force Microscope was used to analyse the surface topography of the samples. A UV-vis-near-IR spectrophotometer (Perkin Elmer Lamda-35) was used to study the transmittance. Fluorescence spectrophotometer (Perkin Elmer LS55) was used to record the photoluminescence spectra at room temperature (λexc= 325 nm). Keithley Meter model-2450 was used to study the room temperature (32 o C) ammonia sensing.

Sensing study
The gas sensing setup consists of a cylindrical chamber and a computer assisted Keithley electrometer (model-2450). The prepared thin film was placed inside the chamber and to attain a baseline, the sample's resistance was measured under dry air. The ammonia taken in solution form was injected into the chamber with the help of microliter syringe. The introduction of NH3 vapour produces a marked change in the resistance of the sensor. The reduction in resistance confirms the n-type behaviour of the sensor towards reducing vapours like NH3. The change in resistance was observed using Keithley meter. The chamber is opened to air ambience when the resistance attained the saturation level. The resistance values were measured with respect to base line and recorded for different ammonia concentrations. Figure 1 shows the X-Ray diffraction patternsof pure and doped ZnO thin films with yttrium doping concentrations 1, 3 and 5 wt%. From the figure, we observed that all the peaks at  intensity of this preferential orientation decreases as the yttrium doping level increases which may be due to the deterioration of crystallinity [31]. This reduction in intensity may be due to the possible movement of zinc atoms into interstitial sites from the regular zinc locations caused by the substitution of yttrium ions [32]. Scherrer's formula was applied for calculating the crystallite size (D),

XRD study
Dislocation density and lattice parameters can be calculated using the formula, The calculated values of crystallite size are presented in Table 1. We noticed that the crystallite size gradually decreases with the increase of yttrium doping level. This may be due to the incorporation of Y 3+ (0.89 Å) into Zn 2+ (0.74 Å) site which causes deformation in the lattice system. This deformation in turn can lead to the restriction of growth of crystallite boundaries called Zener pinning effect [33]. Rietveld refinement analysis was performed using Bruker Topas version 6 to study the lattice parameters. From Table 1, it can be seen that there is a slight change in the lattice parameter values after doping which may be attributed to the difference in the ionic radii of zinc and yttrium ions. The observed increase in the value of dislocation density with the increase in doping concentration of yttrium shows the formation of defects on the surface of the film [34]. Reduction in the size of the crystallites and the increase in the defects will help for adsorbing more ammonia molecules [35]. Figure 2 shows the AFM images of the pure and yttrium (1, 3 and 5 wt%) doped ZnO films. From the figure, we can observe clear edges of the grains which are generally highly reactive when exposed to target gases. This is one of the main advantages in gas sensing applications [36]. The undoped ZnO thin film exhibits relatively low surface roughness, whereas, considerably higher roughness is observed when yttrium is incorporated into ZnO lattice. The roughness value for pure ZnO increases from 26 to 46 nm due to the influence of yttrium doping. This increase in roughness results in more active centres for gas adsorption favouring better sensing as explained by Wenzel's effect [37,38]. EDX was used to identify the presence of elements in the prepared films. Figure 3a and b shows the EDX spectra which confirms the existence of zinc, oxygen and yttrium in the samples.

Photoluminescence study
In order to analyse the surface defects, photoluminescence (PL) study was used. Fig. 4 shows the PL spectra and its Lorentz fitting of pure and yttrium doped ZnO films. The emission band observed at 384 nm is related to the transition between the CB and VB [39]. This peak is found to shift to lower wavelength side due to the influence of yttrium doping. The blue-green emission peaks observed at 435 nm (2.84 eV) and 488 nm (2.53 eV) correspond to Zni (zinc interstitials) and singly ionized Vo (oxygen vacancies) as reported by Kumar et al. [40] and Radhidevi et al. [34]. Thus, the PL study confirms that the electron donor defect concentration increases when yttrium doping concentration is increased, which is favourable for gas sensing as it can be able to attract more number of gas molecules. In our case, the peaks shifted slightly towards the higher wavelength may be attributed to the bond length of Y-O (2.3 Å) is relatively higher than Zn-O (1.89 Å). Therefore, the origin of the emission defects could be varied with respect to metal doping.

Selectivity
The sensing response of the prepared sample (ZnO:Y-5wt%) tested towards six reducing vapours under room temperature is shown in Fig. 6. The selectivity study was carried out by keeping the concentration at 100 ppm and the response was calculated using the formula, = The response valuesof the films are 4, 7, 10, 16, 22 and 99 for toluene, acetone, isopropanal, ethanol, methanol and ammonia. From the results, we observed that the film shows poor response to toluene, acetone, isopropanol, ethanol, methanol compared to ammonia. The reason for the best response to ammonia may be the electron donating ability of ammonia compared to other tested vapours due to the presence of lone pair of electrons [42]. Figure 7 shows the response and recovery curves of pure and yttrium doped ZnO films exposed to NH3 vapours (100 ppm) at 32°C. When yttrium doped ZnO thin films were exposed to NH3 vapour, a rapid change in resistance was observed indicating the response at room Results of recent research works on NH3 sensing at room temperature are presented in  [45]. In the present work, yttrium doped ZnO thin films exhibit short response/recovery times of 29 s/ 7 s towards NH3 vapours.

Repeatability and stability
Repeatability, one of the important factors in sensing, was tested for 5 wt% yttrium doped ZnO film sensor for five cycles of NH3 (100 ppm). We found that the response of the sensor remains the same for all the five cycles as shown in Fig. (9a) indicating that the prepared ZnO:Y film (with 5wt% doping) is a good sensing material for sensing NH3 vapours.
The stability of the sensor is another essential parameter in vapour sensing. To determine the stability, 5 wt% yttrium doped ZnO sensor was tested after 50 days of its preparation with NH3 vapours (100 ppm) at 32 o C. Figure 9b shows that the response of the sensor is nearly the same even after 50 days.

Sensing mechanism
Gas sensing is a surface controlled reactionin which change in resistance of the sensor might be attributed to the adsorption and desorption of target vapours (Fig.10). The adsorbed oxygen molecules can form three types of ions (O2 --below 100 o C(equation 6), O --between 100 o C to 300 o C and O 2--above 300 o C). Previous reports showed the formation of O2ions on the film surface at room temperature [40]. In the present study, we believe that these oxygen ions form a depletion layer over the surface of ZnO:Y films and thus resulting in a high electrical resistance.
O2 + e -= O2 -(ads) (6) Upon exposure to NH3 vapours, desorption takes place and concomitantly electrons already trapped are released back to the film which results in reduction in resistance as per the equation    Fig.3 (a,b) EDX spectra of ZnO and ZnO:Tb (5wt%) thin films             Response and recovery curves of ZnO:Y (0, 1, 3 and 5 wt%) thin lms towards 100 ppm of NH3 at room temperature Sensing mechanism of ZnO:Y thin lm

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