# Enhancement of Ammonia Sensitivity in Swift Heavy Ion Irradiated NanocrystallineSnO2 Thin Films

Authors: Sanju Rani; Somnath C. Roy; N. K. Puri; M. C. Bhatnagar; D. Kanjilal
Journal: Journal of Nanomaterials
 (2008)

---

## Abstract

Swift heavy ion irradiation is an effective technique to induce changes in the microstructure and electronic energy levels of materials leading to significant modification of properties. Here we report enhancement of ammonia (NH3) sensitivity of SnO2 thin films subjected to high-energy Ni+ ion irradiation. Sol-gel-derived SnO2 thin films (100 nm thickness) were exposed to 75 MeV Ni+ ion irradiation, and the gas response characteristics of irradiated films were studied as a function of ion fluence. The irradiated films showed p-type conductivity with a much higher response to NH3 compared to other gases such as ethanol. The observed enhancement of NH3 sensitivity is discussed in context of ion beam generated electronic states in the SnO2 thin films.


---

## Body

## 1. Introduction

Tin oxide (SnO2)
is a material widely used for gas sensing applications because of its suitable
properties such as natural off-stoichiometry, chemical and thermal stability,
and ease of processing  [1]. Tin oxide-based gas sensors have been successfully
designed to detect a variety of toxic and hazardous gases and vapors for
applications ranging from domestic leak detections to industrial process
control. However, because of stricter environmental regulations and process
control requirements, gas sensors with higher sensitivity and selectivity are
in continuous demand; hence efforts are on to tailor the properties of sensing
materials such as tin oxide to achieve better gas response properties.Swift
heavy ion irradiation, in which an energetic ion beam is allowed to pass
through a material, is a very effective technique to induce changes in
microstructure and electronic energy levels, and has been used to tailor
properties of various metallic, semiconducting, and insulating thin films [2]. When a
high-energy ion beam passes
through a material, it loses
its energy in two distinct pathways; namely, nuclear and electronic energy
losses. The nuclear energy loss dominates at lower energies, whereas higher
energy beam results in electronic excitation of the target material. By
suitable control of the irradiation parameters such as energy and fluence of
the ion beam, it is therefore possible to tailor various properties of the
target material. Ion irradiation technique has been used to tailor the
structure-property relationships in several metal oxide systems; for tin oxide,
however, studies have so far been limited only to variation of microstructure
in powders [3, 4], and nanocrystallization in thin film [5]. An attempt to
modify the gas sensing properties of tin oxide thin films by ion irradiation
had resulted only in the baseline stabilization [6].In this work, we report on the
effect of 75 MeV Ni+ ion irradiation on gas sensing properties of
sol-gel-derived SnO2 thin films. Gas sensing experiments using
ammonia and ethanol on irradiated films revealed a much greater enhancement of
sensitivity for ammonia with respect to ethanol. Furthermore, in comparison to
unirradiated films, the irradiated films showed p-type conduction. The
observed enhancement of gas sensitivity and selectivity to ammonia has been
discussed in context of ion beam-induced changes in the material.


## 2. Experimental

SnO2 thin films were deposited
from a solution containing 10 gm of
SnCl4•5H2O   dissolved in 80 mL of 2-propanol. Thorough mixing
was ensured by refluxing the solution with magnetic stirring arrangement for 6
hours at 80°C and then the resultant solution was left for ageing
for another 6 hours. This solution was then filtered through a Whatman filter
paper to obtain a clear solution free of any particulate. The final solution
was coated onto Corning 7059 glass by dip coating system at speed of 10 cm/min2 for depositing one layer of the film. Each deposited layer was dried at 200°C
for 15 minutes before deposition of the next layer. Final sintering of the
films was carried out at 600°C after deposition of a required number
of layers to obtain a desired thickness. Thickness of the deposited films was
estimated by an Ambios surface profilometer and was approximately 1000 Å. These films were
irradiated at room temperature by using 75 MeV Ni+ ions at two different fluences, namely, 1×1011 and 1×1012 ions/cm2. During the irradiation, pressure
inside experimental chamber was at 1.5×10−6 mbar. Structural
analysis of the films was done with the help of a Philips “XPert” model
glancing angle X-ray diffractometer (GAXRD) in the 2θ range of 20°–60°.
Microstructural characterization was performed using Technai G20-Stwin (200 KeV) high-resolution transmission electron
microscope (HRTEM). For HRTEM studies the films were ultrasonically
dispersed in ethanol and taken on the carbon coated grids. Gas sensing
experiments were performed with an indigenously designed gas-sensing setup
attached with mass flow controllers for precise measurement of gas flows at ppm
level. Change in resistance of the films was measured as a function of
temperature through a computer interfaced digital multimeter (DMM).


## 3. Results and Discussion

Figure1  shows the glancing angle XRD patterns of unirradiated and irradiated SnO2 thin films. All peaks in the spectra correspond to the standard rutile phase
with polycrystalline structure. It is observed that, in comparison to
unirradiated film, the relative intensities of all peaks increase in irradiated
films. Furthermore, the intensities of XRD peaks also increase with increase in
ion fluence, implying that SHI beam irradiation enhances crystallinity of the
SnO2 thin films. A careful examination of the XRD patterns further
reveals that the relative intensity of (110) peak in the irradiated films
increases at a faster rate as compared to the intensities of the other peaks,
which implies that irradiation of Ni ions also enhances the preferred
orientation of the SnO2 thin films. Increase of crystallinity along
the (110) plane plays a crucial role in enhancing gas-sensing properties of SnO2 thin films, which is discussed later.Figure 1
Glancing angle XRD patterns of unirradiated and
irradiated SnO2  thin films.Figure2 compares the HRTEM images of unirradiated and irradiated SnO2 thin
films. In the unirradiated film (Figure 2(a)), the crystallites appear randomly
oriented, a characteristic feature of sol-gel-derived thin films. With the ion
irradiation, however, the crystallite boundaries gradually disappear and
enhancement of crystallinity is observed. At higher fluence (Figure 2(c)), the
crystallites show preferred orientation along (110) planes, which confirms XRD
observations reported in the last paragraph.High-resolution TEM images of SnO2 thin
films: (a) unirradiated, (b) irradiated with 1011 ions/cm2, and (c) irradiated with 1012 ions/cm2.
(a)(b)(c)Gas
sensing experiments performed on unirradiated SnO2  films with 1000 ppm ammonia and ethanol gases are shown in Figure 3, and Figure 4 compares the responses
of irradiated (with 1012 ions/cm2) films under similar
conditions. Here it is worth mentioning that unirradiated films show decrease
of resistance when exposed to ammonia and ethanol, both reducing gases, due to
normal n-type behavior; however, the
irradiated films show increase of resistance under similar conditions
indicating p-type conduction
resulting from ion irradiation. The sensitivity factors are calculated by the
equation:(1)s=((Rg−Ra)/Ra)%, where  Rg and
Ra represent resistance in the presence of gas and air,
respectively; the values of s for
unirradiated and irradiated films at different temperatures are tabulated in
Table 1. For the unirradiated films, the values of sensitivity “s” in ammonia and ethanol gases
at 300°C are 284 and 87, respectively, while for irradiated films they increase to 516 and 115, respectively,
for 1×1011 ions/cm2, and to 882 and 142, respectively,
for 1×1012 ions/cm2. This implies that irradiation
of SnO2 thin films by high-energy ion beam, for example with a
fluence of 1×1012 ions/cm2, results in a 213% (284 to
882) increase in its ammonia sensitivity, while only 63% (87 to 142) increase
in sensitivity for ethanol is observed under similar conditions. Such an
enhancement of ammonia sensitivity can be explained in terms of the surface
chemistry modifications due to ion beam irradiation.Table 1
Sensitivity factors of unirradiated and irradiated
SnO2 thin films for ammonia and ethanol at different temperatures.
Ni+ ion irradiation fluenceTemperature200°C (sensitivity %)Temperature250°C (sensitivity %)Temperature300°C (sensitivity %)NH3C2H5OHNH3C2H5OHNH3C2H5OHUnirradiated5259832284871×1011 ions/cm26815103535161151×1012 ions/cm21222764973882142Figure 3
Gas sensing behavior of unirradiated SnO2 thin films at 300°C, exposed to 1000 ppm ammonia (NH3)
and ethanol (C2H5OH) vapors.Figure 4
Gas sensing behavior of SnO2 thin films
irradiated with 1×1011 ions/cm2 and 1×1012 ions/cm2,
exposed to 1000 ppm ammonia (NH3) and ethanol (C2H5OH)
vapors at 300°C.The XRD data shown in Figure1 shows
an enhancement of crystallinity along the (110) plane resulting from Ni+ ion irradiation, which also indicates preferred surface orientation of the irradiated
SnO2 thin films. Such  a (110) surface of SnO2 consists
of two types of oxygen vacancies: the
bridging oxygen vacancies created by removal of oxygen ions from the top layer
of an ideal (110) surface, and the “in-plane” oxygen vacancies created by
removal of oxygen ions from the tin containing lattice plane [7]. During the
ion irradiation process, samples are kept in an ultrahigh vacuum ambience that
promotes oxygen desorption, especially from the top layer resulting in the
formation of “bridging” oxygen vacancies. Such oxygen vacancies may also result
from the thermal spikes created in the samples during ion irradiation. The formation of bridging oxygen vacancies
lowers the coordination number of the surface tin cations from an initial value
of six to four, which, in turn, reduces their charge state from Sn4+ to Sn2+. The four- coordinated Sn2+ ions are known
to be more acidic compared to Sn4+ cations, which promotes a higher
chemisorption of ammonia, a strongly basic gas, on the surface of the
irradiated films [8]. Hence the sensitivity of high-energy ion irradiated SnO2 thin films toward ammonia is significantly enhanced compared to that for any
other gas such as ethanol. Furthermore, the four coordinated Sn2+ ions create a local SnO—like environment at the surface and near surface region
of the irradiated SnO2 thin films, which results in p-type
conductivity observed in the gas sensing experiments [9].


## 4. Conclusions

The present study shows that structural and gas sensing properties of SnO2 thin films are significantly modified by 75 MeV Ni+ ion irradiation. The crystallinity of the irradiated films is enhanced along
the (110) plane that also affects the gas sensing characteristics. Gas sensing
experiments with ammonia and ethanol reveal a significant enhancement of
sensitivity for ammonia over ethanol that is attributed to acidic nature of the
irradiated SnO2 surfaces. Formation of bridging oxygen vacancies
during the ion irradiation process converts the six coordinated Sn4+ cations into four coordinated Sn2+ cations; the latter have more
acidic character than the former. This promotes a stronger chemisorption of
ammonia molecules on the surface of the ion irradiated SnO2 thin
films.



---

*Source: 395490-2008-11-25.xml*