polymer - embedded stannic oxide nanoparticles as humidity sensors

Synthesizing porous nanoparticulate thin films using soft chemistry approach or incor-poration of metal-oxide nanoparticles in polymer matrix are two different fascinating approaches, whi

Polymer-embedded stannic oxide nanoparticles as humidity sensors

Shadie Hatamiea, Vivek Dhasb, B.B Kalec, I.S Mullab, S.N Kalea,⁎

a

Department of Electronic-Science, Fergusson College, Pune 411 004, India

b

Physical and Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India

c Center for Materials for Electronics Technology (C-MET), Panchawati, Pashan Road, Pune 411 008, India

a b s t r a c t

a r t i c l e i n f o

Article history:

Received 22 April 2008

Received in revised form 21 June 2008

Accepted 29 July 2008

Available online 8 August 2008

Keywords:

Stannic oxide

Nanoparticles

Polymer

Humidity sensor

Stannic oxide (SnO2) nanoparticles have been suspended in polyvinyl alcohol (PVA) matrix in different PVA: SnO2molar ratios ranging from 1:1 to 1:5 using simple chemical route This suspension was deposited on ceramic substrate and upon drying was carefully detached from the substrate SnO2-embedded self-standing, transparent andflexible thin films were hence synthesized Transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques show the rutile tetragonal structure of SnO2with particle size ~ 5 nm UV– Visible spectroscopy demonstrates the band gap of 3.9 eV, which does not alter when embedded in polymer Fourier transform infrared spectroscopy (FTIR) reveals that the properties of SnO2do not modify due to incorporation in the PVA matrix The structures work as excellent humidity sensors at room temperature For

a critical PVA:SnO2molar ratio of 1:3, the resistance changes tofive times of magnitude in 92% humidity within fraction of second when compared with resistance at 11% humidity The sample regains its original resistance almost instantaneously after being removed from humid chamber Nanodimensions of SnO2 particles and percolation mechanism related to transport through polymer matrix and water molecule as a carrier has been used to understand the mechanism

© 2008 Elsevier B.V All rights reserved

1 Introduction

Relationship between nanostructures and their implications on the

electrical, optical and thermal properties of materials is an extremely

interesting area of material science Metal-oxides form attractive

domain therein due to their wide range of properties like

ferroelec-tricity, superconductivity and piezoelectricity Synthesizing porous

nanoparticulate thin films using soft chemistry approach or

incor-poration of metal-oxide nanoparticles in polymer matrix are two

different fascinating approaches, which have been recently adopted as

routes to explore interesting physics of self-assembly and study the

range of properties exhibited by these oxide structures [1–8] In

polymer-embedded metal-oxide thinfilms, polymer controls viscosity

and binds the metal-oxide ions, resulting in their homogeneous

distribution in thefilm These uniform, flexible and crack-free

metal-oxides— polymer films can be synthesized on much larger scale, in

bigger dimensions and for variety of applications Some interesting

attempts have been made in recent past by Q.X Jia et al and N.V

Kolytcheva et al [1,5]on metal-oxide nanoparticles embedded in

polymer matrix According to Jia et al titanium dioxide thinfilms can

be synthesized in epitaxial manner using simple polymer-assisted

deposition technique, and the route promises good sensing devices In

an attempt to synthesize and explore nanoparticulate thinfilms in

porous configurations, Brousse et al.[6]and Horillo et al.[9] have explored nanomaterials of tin oxide and compared them with bulk systems, for their gas sensing properties It has been argued that since nanoparticles have higher surface-to-volume ratio, surface states are more, which increase the gas molecules adsorbed on nanoparticles, as compared to bulk systems; thereby improving the sensing ability in their nanoforms Further, as indicated by Mizsei[10] the faceted or non-faceted grains, and hence the surface morphology has its impact

on surface characteristics, which further controls sensing properties For such reasons, porous SnO2films are projected to be superior by M Honore et al.[11]and their transduction to conductivity changes have been studied Y Shimizu et al.[12]have studied porous ZnOfilms as varistors and using similar arguments have studied the non-linear response of these materials as a function of particle size It is hence important to address the issue of polymer-embedded sensors, under-stand the interesting science therein and explore its technological importance Finding out the role of polymer and exact ratio of

maximum response to the incident gas/humid ambience, is the key

to apply these materials to technology andfinally establish a base to yield extremely good, selective room temperature sensors

In this communication, we report synthesis of stannic oxide (SnO2) nanoparticles embedded in polyvinyl alcohol (PVA) matrix Nearly mono-dispersed nanoparticles of SnO2having size of ~ 5 nm and band gap of 3.9 eV have been formed and when embedded in PVA, yielded self-supporting thinfilms which were highly flexible, transparent and non-degradable in ambient atmosphere Thesefilms when subjected

⁎ Corresponding author Department of Electronic-Science, Fergusson College, F.C.

Road, Pune 411004, India Tel.: +91 20 2565 5119.

E-mail address: sangeetakale2004@gmail.com (S.N Kale).

0928-4931/$ – see front matter © 2008 Elsevier B.V All rights reserved.

Contents lists available atScienceDirect Materials Science and Engineering C

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to humid environment showed change in resistance The jump in

resistance in given humidity is a function of PVA:SnO2ratio, which has

been varied from molar ratios of 1:1 to 1:5 It has been found that

maximum change in resistance occurs forfilm with 1:3 molar ratio

and that the sensitivity response decreases on either side The possible

reasons have been related to the adsorption sites offered by SnO2,

their interconnectivity and active polymer medium as a tunneling

percolation track

2 Experimental

2.1 SnO2nanoparticle synthesis

SnO2 nanoparticles were prepared by a simple co-precipitation

method [13] Stannic chloride (SnCl4.5H2O (A.R), 0.01 M) was

dissolved in deionized water and stirred for 30 min at room

temperature 8 ml ammonia solution was added drop wise to the

above solution to attain pH ~10 The resultant gel wasfiltered and

dried for 24 h in ambient temperature and then later for 2 h at 100 °C

to ensure that the powder was totally devoid of water The powder

was ground for 10 min, and heated in oven at 400 °C for 3 h and cooled

to room temperature

2.2 PVA/SnO2nanocompositefilm

PVA was dissolved in deionized water (1 M) and the solution was

heated in a water bath at 90 °C for 1 h Then the SnO2nanoparticles

suspended in water (with molarity varying from 1 to 5 M) were added

to the PVA solution and well dispersed using ultrasonication for

20 min The homogeneous solution was then spin-coated on ceramic

substrate and dried in air at ambient temperature After drying, this

compositefilm was easily removed from the ceramic substrate The

similar procedure was obtained and multiplefilms were synthesized

with different PVA:SnO2molar ratios viz varying from 1:1 to 1:5

2.3 Humidity measurements

Different films were subjected to supersaturated solutions for

humidity measurements Two different solutions, namely, 50 ml of

lithium chloride (LiCl) and potassium nitrate (KNO) were put in the

air tight 250 ml plastic chambers, which provide different constant relative humidity (% RH) at 30 °C of 11%, 92% after 24 h, respectively Two electrical contacts were made on the compositefilms using silver paste and copper wires for contacts to meters The probes were long wires, which were connected to voltage source, and current meter, which were placed outside the humidity chamber The chamber cover had sealed microholes that allowed the wires to come out of the chamber without offering any leak during measurements Thefilms werefirst subjected to 11% humidity chamber and current (and hence, resistance (R11)) was noted down This was done to ensure that all samples had uniform reference for comparison Then the sample was shifted to 92% humidity chamber and current (and hence, resistance (R92)) was noted down For all samples, the voltage was kept constant

at 15 V The change in current, which was converted to resistance, was studied as a function of PVA:SnO2ratio All samples had dimensions of

1 cm × 1 cm × 0.03 cm The time analysis was also done tofind out the amount of time thefilm takes to regain its original value of resistance (basically R11) after the sample has been removed from 92% humidity chamber and re-subjecting it to 11% humidity chamber This gave us information of recovery time and reusability

characterized for structural, compositional properties using Fourier transform infrared spectroscopy (FTIR, Shimadzu 8400S Spectro-meter), X-ray Diffraction Technique (XRD, Philips PW 1830 40 kV,

Spectroscopy (Jasco V570UV–VIS–NIR) Keithley meters were used for transport measurements

3 Results and discussion

Fig 1(a) shows the XRD pattern of the sample synthesized using co-precipitation method described above The spectrum has been compared with standard commercial bulk powder (Fig 1(b)) Typical tetragonal rutile structure can be clearly seen in the nanopowder and peak broadening confirms the smaller particle size The Miller indices gave lattice constants as a=b=4.738 and c=3.187, which matched well with bulk SnO2(JCPDS File No 41-1445) No impurity peaks were observed,

Fig.1 XRD pattern of SnO 2 nanoparticles (a) and compared with SnO 2 bulk (b) As can be seen

rutile structure was formed with the broad spectrum indicating formation of nanoparticles.

The inset shows the graph of (αE) 1/2

versus (E) showing the band gap of 3.9 eV.

Fig 2 FTIR spectrum of SnO 2 nanoparticles (a), PVA (b) and PVA: SnO 2 composite (c) All signatures in PVA and SnO 2 are seen in the composite with no modifications in the positions, indicating the formation of a composite The inset shows photographs of the films detached from ceramic substrates, which are highly flexible, self-supporting and transparent.

indicating the high purity of thefinal products The average crystal

size of SnO2calculated from the Scherrer's formula (D = Kλ / β cosθ,

where D is the average diameter of the crystalline particles (nm),λ is

maximum intensity (in radians) for a certain powder peak andθ is the

corresponding angle) was found to be 5.32 nm The UV–Vis

spectro-scopy data was used and from the absorption coefficient (α) and

energy values, band gap was determined by extrapolating the linear

portion of the plot of (αE)1/2versus (E), which indicated band gap to

be 3.9 eV, as is shown in the inset ofFig 1, which was blue-shifted

from the bulk value (3.6 eV) confirming the nanoparticle formation

Fig 2 shows FTIR spectroscopy data for SnO2 nanoparticles

(Fig 2(a)), Polyvinyl alcohol (Fig 2(b)) and the PVA: SnO2

standard FTIR reference book [14] The typical signatures in PVA

which were due to O–H stretching (3333 cm− 1), C–H stretching

(2912–2945 cm− 1), C–H bending (1416–1331 cm− 1), O–H bending

(1416–1333 and 650 cm− 1) and C–O (1090 cm− 1) were seen in PVA

signature of Sn–O at 610 cm− 1in the composite were also observed,

which were also seen in pure SnO2 As can be seen from thisfigure,

there was no modification of bonds of PVA after the composite had

been formed; nor there was any shift or intensity modification after

formation of the PVA:SnO2sample Owing to the procedure that was

being used and using the well-established fact that SnO2is a highly

stable oxide, the spectrum was well anticipated Thus we can conclude

nanoparticles in the PVA matrix, homogeneously The insets ofFig 2

transparency andflexibility However, as the percentage of SnO2was

changed in the composite, though FTIR did not show any changed

signature, one could expect that sample becomes denser and that can

affect the transport property of the sample

Fig 3confirmed the results of FTIR using TEM pictures As seen in

Fig 3, we observed nearly mono-dispersed SnO2nanoparticles with

particle size of ~5 nm (this was also confirmed using particle size

analysis) Lower inset shows a detailed TEM image at 10 nm length-scale and upper left inset shows selected area electron diffraction (SAED) pattern, which exhibited characteristics of polycrystalline particles and the rings could be easily indexed with reference to the rutile tetragonal SnO2structure, which was highly consistent with the XRD results

The most telling results are shown inFig 4 Humidity measure-ments were done as described in the experimental section Since we were not very sure of the dimensional differences of different samples, the comparison was done of the resistance ratios (R11/R92) The inset

ofFig 4(a) shows the change in absolute values of resistance R11and

performed for films with different molar ratios, namely 1:1, 1:2, 1:2.5, 1:3, 1:3.5, 1:4 and 1:5 To get fair comparison, results were also

interestingly it was found that the change in current with the change

in SnO2proportion in the sample was not monotonic, as is shown in

Fig 4(a) It was seen that the ratio R11/R92increased initially with SnO2, reached maximum (at 1:3 ratio) and decreased again The maximum change in resistance (current) was almost 5 times in the 1:3 sample When compared to bulk SnO2, it was seen that the change

Fig 3 TEM viewgraph of PVA: SnO 2 film, showing particle size ~5 nm The inset below

shows the viewgraph on the scale of 10 nm The inset on the top shows the SAED pattern

of the sample.

Fig 4 (a) Plot of sensitivity (R 11 /R 92 ) as a function of PVA:SnO 2 ratio The inset shows the change in resistance in 11% humidity and 92% humidity versus PVA:SnO 2 ratio (b) Plot of change in resistance of the 1:3 molar ratio film as a function of humidity The inset shows schematic with lower PVA: SnO 2 ratio (i) and critical threshold ratio (b) Schematic exhibits a percolation threshold in (ii) which shows a conductivity between SnO 2 nanoparticles (orange balls) via tunneling through the PVA matrix and water molecule (blue ball) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

in resistance with the change in humidity from 11% to 92% was only by

few ohms, which suggested that SnO2in nanoparticles form and PVA

matrix was important in the sensing phenomenon However, when

the similar experiment was done using only PVA polymer (1 M), we

did observe some change in current (R11/R92~ 1.89) indicating that

PVA was itself contributing to the humidity sensing However, the

change was of the order of few ohms, which was quite insignificant as

compared to the SnO2embeddedfilm It is important to state here that

the response time was of the order of few msec Also, similar rate of

recovery was observed as we removed the sample from 92% humidity

chamber and put it back in chamber at 11% humidity The readings of

allfilms were taken at least two times to confirm these observations,

and same experiments were done on different batches of synthesized

samples Thesefindings gave us two different hints: i) role of PVA was

quite important in the sensing phenomenon; probably it offered

ap-preciable conducting tracks in between SnO2 nanoparticles, and

ii) nanoparticles improvised the ability to sense humidity After

getting the optimized molar ratio for maximum sensitivity, studies

were done using the 1:3 sample for practical applicability The

samples were subjected to different humidity values ranging from 11%

to 92%.Fig 4(b) shows the corresponding behavior, which depicts that

as the humidity increases, the resistance of the sample decreases

With proper fitting of this data the sample can be calibrated for

outdoor applications For further checking the reusability of the

synthesized samples, every sample was measured twice, using

dif-ferent contact positions The samples were preserved in natural

ambience and after about three months, the samples exhibited the

readings within an error ofb5%

Looking carefully into the literature, it can be envisaged that any

sensing device needs more adsorbing surface area, to adsorb moisture

(in our case) and yield some property changes Since nanoparticles are

well known to have more surface-to-volume ratio, the increased jump

with nanoparticles as compared to their bulk value can be anticipated

[6,9] Further, the polymer PVA helps this activity in two ways:firstly,

it works as a weak sensor and secondly, it is a hydrophilic polymer,

which hold the adsorbed water in the matrix This helps the

system-as-a-whole to connect via the water molecules, to yield large current

values Similar results have been observed by Andreev et al.[15], on

their system of PVA-calcium chloride Their system offers relevant

conductivity variation by 4.5–5 times the magnitude with relative

hydrophilic polymers doped with metal salts, which increase the

sensor sensitivity due to the appearance of ionic conductivity during

water sorption Similarly, Ogura et al.[16]have studied a

humidity-sensitive compositefilm that consists of conducting polyaniline and

water-loving PVA Polyaniline gave a percolation threshold at a

particular volume fraction The results have been interpreted on the

basis of doping level, which was affected by the concentration of water

molecules surrounding the conducting polymer

In our case, on similar lines, we propose an explanation on

weaker humidity sensitivity of their own The high sensitivity

occurred in a percolation regime where SnO2grains would almost

begin to touch each other building chain like configurations and

eventually leading to full percolation Such tunneling phenomenon

has been reported in various systems, in recent past[17,18] From our

standpoint the high humidity sensitivity at intermediate ratio of

nanocomponents can be explained based on the nature of the two

components of the system at their electronic proximity SnO2is an n-type conductor with high electron concentration at room tempera-ture, hence although humidity may affect this concentration by adsorption, the corresponding percentage change is very small On the other hand PVA has hopping conduction, which leads to low sensitivity because of limited mobility At the optimum intermediate molar concentration (1:3) the layer of PVA polymer would just be in the tunneling regime with the humidity adsorption (as shown in the schematic in the inset ofFig 4(b) by introducing electronic states which aid tunneling, effectively weakening the barrier Hence we envisage here that this is a novel system, which can be explored to yield extremely sensitive devices The film is highly flexible and reusable The synthesis mechanism promises useful applications of this system as micro-sensors

4 Conclusion

In conclusion, we have synthesized polycrystalline rutile structures

of SnO2nanoparticles of ~5 nm size using co-precipitation method Using simple chemical route SnO2has been embedded in PVA matrix The molarity ratio of PVA:SnO2was varied from 1:1 to 1:5 It was seen that allfilms worked as humidity sensors At a characteristic ratio of 1:3, the response offilm was maximum and it decreased on either sides of the optimum ratio The results have been understood by considering the critical tunneling regime, increased surface area of the nanocomposites and the active role of PVA in the system

Acknowledgements Authors sincerely thank Dr S.B Ogale from National Chemical Laboratory, Pune for his valuable guidance and suggestions S.N Kale acknowledges International Centre for Theoretical Physics (ICTP), Italy for her Associate affiliation and for the rich library access used for this work

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