polymer-matrix nanocomposite gas-sensing materials

ISSN 0020�1685, Inorganic Materials, 2014, Vol. 50, No. 3, pp. 296–305. © Pleiades Publishing, Ltd., 2014.Original Russian Text © D.A. Pomogailo, S. Singh, M. Singhc, B.C. Yadav, P. Tandon, S.I. Pomogailo, G.I. Dzhardimalieva, K.A. Kydralieva, 2014, published in NeorganicheskieMaterialy, 2014, Vol. 50, No. 3, pp. 320–330.

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INTRODUCTION

To determine the content of readily flammable,potentially hazardous burning gases that are employedin industrial and household applications, use is typi�cally made of sensors that rely on the semiconductingproperties of transition metal oxides [1]. The first ofsuch sensors was reported in 1962 [2]: a zinc�oxide�based thin�film sensor operating at 400°C was shownto ensure determination of the toluene, benzene, ethylether, and propane concentrations in the vapor phasedown to 1–50 ppm.

Known gas�sensing materials offer acceptable sen�sitivity to changes in the composition of the gas phase,but have low selectivity to various gases and their mix�tures and high working temperatures.

At the same time, recent years have seen the use ofvarious polymers as basic components or elements ofcomposite materials for chemical gas sensors in com�bination with transition metals, for example, conduc�tive polymers: polyaniline, polypyrrole, poly�thiophene, and their derivatives [3, 4].

The purpose of this work was to test various semi�conducting (oxide and sulfide types) metallopolymersystems as gas�sensing materials for analysis of lique�fied petroleum gas (LPG) at room temperature.

EXPERIMENTAL

Starting materials. The starting chemicals used wereZn(NO3)2 ⋅ 6H2O (pure grade), Pb(NO3)2 (pure grade),Co(NO3)2 ⋅ 6H2O (Aldrich, 99.0%), Cd(NO3)2 ⋅ 6H2O(Aldrich, 99.0%), acrylamide (Fluka, 99.0%), and thio�

urea (as purchased). As solvents, we used benzene(pure grade) and ethyl ether (pure grade), which werepurified to remove stabilizers, distilled, and dried. Toproduce metal–polymer gas�sensing composites, weutilized monomeric and polymeric zinc, lead, cad�mium, and cobalt acrylamide (AAm) complexes withthe general formula M(NO3)2 ⋅ (AAm)4 ⋅ 2H2O (M =Zn (I), Pb (II), Cd (III), and Co (IV)). The complexeswere synthesized by a technique reported previously[5]. The metal salt : acrylamide molar ratio was 1 : 5.Excess AAm was removed by washing with ethyl etherand dried in vacuum to constant weight. The compo�sitions of the complexes obtained are listed in Table 1.

Preparation of metal�oxide nanocomposites. CoO�and CdO�containing nanocomposites were producedthrough frontal polymerization (FP) of monomericcomplexes III and IV, followed by controlled thermol�ysis of the forming metallopolymers. To conduct fron�tal polymerization, we used solid monomer samples inthe form of pressed cylindrical pellets (diameter d =0.8 cm, height h = 1.2–1.5 cm). The samples wereplaced in glass ampules, and polymerization was initi�ated through thermal perturbation produced in thebottom part of the ampule by immersing it in a bathcontaining a heating agent (for no more than 15 s) to adepth of 0.2 cm at a temperature of 180°C. Nanocom�posites were produced by controlled thermolysis of theresultant metallopolymers under isothermal condi�tions as described elsewhere [6]. Thermolysis was rununder stationary isothermal conditions at a tempera�ture of 723 K in a self�generated atmosphere (prior tothermolysis, the samples were held in vacuum at roomtemperature for 30 min).

Polymer�Matrix Nanocomposite Gas�Sensing MaterialsD. A. Pomogailoa,b, S. Singhc, M. Singhc, B. C. Yadavc, P. Tandonc,

S. I. Pomogailoa,b, G. I. Dzhardimalievaa,b, and K. A. Kydralievab

a Institute of Problems of Chemical Physics, Russian Academy of Sciences, pr. Akademika Semenova 1, Chernogolovka, Moscow oblast, 142432 Russia

b Moscow Institute of Aviation (National Research University), Volokolamskoe sh. 4, Moscow, 125080 Russia

c Department of Physics, University of Lucknow, 226007 UP, Indiae�mail: [email protected]

Received June 3, 2013

Abstract—A new approach has been proposed for producing nanocomposite gas�sensing materials: in situpreparation of a polymer matrix and metal sulfide or oxide nanoparticles through the frontal polymerizationof Co(II), Cd(II), Zn(II) and Pb(II) acrylamide complexes. The composition and structure of the nanocom�posites thus obtained have been determined using X�ray diffraction, scanning and transmission electronmicroscopy, and Raman spectroscopy. The nanocomposites have been tested as room�temperature liquefiedpetroleum gas sensors.

DOI: 10.1134/S0020168514030108

INORGANIC MATERIALS Vol. 50 No. 3 2014

POLYMER�MATRIX NANOCOMPOSITE GAS�SENSING MATERIALS 297

Preparation of metal�sulfide nanocomposites. Toprepare metal�sulfide nanoparticles in a polyacryla�mide matrix, the synthesis and frontal polymerizationof the acrylamide complexes I–III were carried out inthe presence of thiourea (TU) as a sulfiding agent. Ina typical experiment, to a metal nitrate AAm complexprepared as described above was added TU (1 : 1 molarratio), and frontal polymerization of a sample pressedinto a cylinder (h = 30 mm, d = 0.5 mm) was carriedout by the above procedure. During polymerization,the color of the reaction medium changed from whiteto yellow or black, characteristic of cadmium and zinc(lead) sulfides, respectively. Elemental analysis data for

the nanocomposites thus prepared are presented inTable 2.

Gas�sensing measurements. To perform gas�sensingmeasurements, films of gas�sensing materials wereproduced on the surface of an aluminum substrate byscreen printing followed by heat treatment at a tem�perature of 80°C for 6 h and annealing at 450°C for anadditional 2 h. Next, the substrate with the film andsilver contacts were secured in a purpose�designedchamber having gas inlet and outlet holes (Fig. 1).

Electrical resistances at varied LPG content(vol %) were measured with a Keithley 6514 electrom�

Table 1. Elemental analysis data for metal nitrate acrylamide complexes

Compound

Found/calculated, %

C H N M

Zn(CH2=CHCONH2)4 · (NO3)2 · 2H2O (I) 27.8/28.1 4.6/4.7 15.4/16.4 11.5/12.8

Pb(CH2=CHCONH2)4 · (NO3)2 · 2H2O (II) 22.8/23.4 3.7/3.2 12.1/13.6 34.4/33.6

Cd(CH2=CHCONH2)4 · (NO3)2 · 2H2O (III) 27.8/25.9 4.8/4.3 14.9/15.1 18.9/20.14

Co(CH2=CHCONH2)4 · (NO3)2 · 2H2O(IV) 29.6/28.6 5.0/4.4 16.7/16.7 11.7/11.6

Table 2. Elemental analysis data for metal–polymer nanocomposites

Compound

Found/calculated, %

C H N S M (metal�con�taining residue)

Zn(CH2=CHCONH2)4 (NO3)22H2O (I) + TU (ZnS/PAAm) 29.39/31.7 5.5/5.8 15.5/14.8 7.2/8.5 17.2/17.19

Pb(CH2=CHCONH2)4 (NO3)22H2O (II) + TU (PbS/PAAm) 21.2/21.3 4.16/4.18 17.7/17.8 3.9/4.1 27.7/26.3

Cd(CH2=CHCONH2)4 (NO3)22H2O (III) + TU (CdS/PAAm) 30.0/30.9 6.0/6.0 12.1/12.0 4.5/4.5 16.5/16.5

Cd(CH2=CHCONH2)4 (NO3)22H2O (III) + TU* 20.0 5.9 – 6.8 71.7 (residue)

Cd(CH2=CHCONH2)4 (NO3)22H2O (III)** 1.4 0.24 – – 95.2 (residue)

Co(CH2=CHCONH2)4 (NO3)22H2O (IV)*** 37.0 2.6 8.7 – 48.8 (residue) (38.4% Co)

* Product of thermolysis at 450°C for 2 h in air.** Product of thermolysis at 450°C for 2 h in air.

*** Product of thermolysis at 370°C for 1 h in a self�generated atmosphere.

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1

2

345

6

7

8 9

10

Fig. 1. Schematic of the chamber for gas�sensing measurements: (1) gas inlet, (2) gas outlet, (3) to a measurement system,(4) disks, (5) pinholes in the disk, (6) silver electrode, (7) glass tube, (8) insulating cylinder, (9) supporting rod, (10) disk spring.

eter. The sensitivity of gas�sensing materials (S) wasevaluated as:

where Ra is the electrical resistance of the gas�sensingmaterial in air and Rg is that in the presence of gaseousimpurities.

IR absorption spectra were measured in the range400–4000 cm–1 on a Specord 75 IR spectrophotome�ter. X�ray diffraction studies of powder samples wereperformed on a DRON UM�2 and a Philips PW 1050diffractometer with CuK

α radiation (λ = 1.5418 Å).

The metal–polymer nanocomposites were character�ized by electron microscopy on a Hitachi 3500 scan�ning electron microscope (15 keV), JEOL transmis�sion electron microscope operated at an acceleratingvoltage of 100 kV, and JEM 3010 high�resolution elec�tron microscope (300 kV).

RESULTS AND DISCUSSION

Preparation and characterization of metal–polymernanocomposites. AAm complexes of Co(II), Cd(II),Zn(II), and Pb(II) nitrates were obtained by substitut�ing AAm molecules for water of crystallization in crys�talline metal nitrate hydrates. The composition of theresultant complexes (M(NO3)2 ⋅ 4AAm ⋅ 2H2O, M =Co2+, Zn2+, Cd2+, Pb2+) was checked by elementalanalysis (Table 1). IR spectroscopy data demonstratesthat the metal atoms are coordinated by the oxygens ofthe carbonyl group of the AAm ligand: the ν(CO)band (1672 cm–1) is shifted to longer wavelengths,similarly to what was found for analogous transitionmetal complexes [5]. The spectra of the complexesalso show the stretching band of the nitrate anion,ν(NO3), at 1384 cm–1; the symmetric and asymmetricstretching bands ν(NH) at 3200 and 3350 cm–1, andthe ν(OH) band of the water of crystallization.

Metal–polymer composites were produced by self�propagating FP of metal�containing monomers in

S ΔRRa

��Ra Rg–

Ra

��,= =

condensed phase [5, 7]. The optimal temperaturerange for reaction initiation was 423–443 K. Belowthese temperatures, no polymerization wave wasformed. At higher temperatures, a deep oxidationfront was produced, leading to the formation of metaloxides and carbides. For the Co–AAm and Cd–AAmcomplexes, the Tmax of the front was 488 and 483 K,respectively, and the velocities of the front differedmarkedly: 1.4 × 10–2 and 7.0 × 10–2 cm/s.

Metal�oxide nanocomposites. Two approaches wereused to produce nanocomposites. In one of them,matrix�stabilized metallic nanoparticles were synthe�sized in situ in matched processes of thermal solid�state polymerization of metal acrylamide complexesand controlled thermolysis under isothermal condi�tions. The kinetics and mechanisms of the main ther�mal transformations of metal�containing monomerswere considered previously [8]. In the microstructureof the composites obtained, metallic nanoparticles areuniformly distributed over a pyrolyzed polymer matrix(Fig. 2). The particles are predominantly spherical inshape, with an average diameter of 7 nm.

According to X�ray diffraction data, the metal�containing phase in the product of the thermolysis ofcomplex III in a self�generated atmosphere at Т =723 K is Co nanoparticles in a pyrolyzed polymermatrix. The products of thermal transformations ofcomplexes III and IV in air are the correspondingmetal oxides (Fig. 3).

Important characteristics of gas�sensing materialsare their specific surface area and porosity. Table 3 pre�sents data on the specific surface area and pore size ofthe metal�oxide nanocomposites under consideration.The nanocomposites have a microporous structurewith a uniform distribution of spherical nanoparticlesover the matrix of the composite. This is favorable foreffective adsorption of gaseous substances to be ana�lyzed.

Metal�sulfide nanocomposites. Metal chalcogenidenanoparticles in polymer matrices are often rather dif�ficult to produce by conventional processes because of



(b)

(а) 100 nm

300 nm

Fig. 2. Microstructure and morphology of a material obtained by the thermolysis (T = 773 K) of the Co(II) AAm complex:(a) transmission and (b) scanning electron microscopy images.

Table 3. Specific surface area and pore size of metal�oxide nanocomposites

Sample S, m2/g Vpore, cm3/g Å

Product of Co(CH2=CHCONH2)4 (NO3)22H2O (IV) thermolysis* 5.6 0.069 245.7

Product of Cd(CH2=CHCONH2)4 (NO3)22H2O (III) thermolysis** 4.0 0.036 178.3

* Product of thermolysis at 370°C for 1 h in a self�generated atmosphere.** Product of thermolysis at 450°C for 2 h in air.

rpore,

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POMOGAILO et al.

problems related to compatibility of components,phase homogeneity, process duration, etc., which mayinfluence the photostability of the resultant nanocom�posites, lead to a reduction of quenching of their lumi�nescence, etc. [9–11]. Here, we propose a newapproach for fabricating nanocomposites of the typeunder consideration: in situ formation of a polymermatrix and metal sulfide nanoparticles through the FPof the AAm complexes I–III. In preparing a starting

monomer mixture, an appropriate amount of TU as asulfiding agent was added to the system. Next, thereaction mixture was subjected to frontal polymeriza�tion. The phase composition of the forming nanopar�ticles was determined by concentration relationships.In other words, the metallopolymer chains formingduring the self�propagating polymerization processare considered in this approach as chemical nanoreac�tors. The main transformation paths can be repre�sented as follows:

(1)

М = Cd2+, Zn2+, Pb2+

(2)

(3)

(4)

The formation of a composite in the course of FP isevidenced by a characteristic change in the color of thesolid reaction mixture: from white to lemon yellow inthe case of the CdS/PAAm system (cadmium sulfidein a polyacrylamide matrix), to beige in the case ofZnS/PAAm, and to black for PbS/PAAm. X�ray dif�

M CH2=CHCONH2( )4 NO3( )2 2H2O⋅

M CH2=CHCONH2( )4 NO3( )2 2H2O,+

H2N C NH2

S

+ 2H2O 2NH3 + CO2 + H2S,

M CH2=CHCONH2( )4 NO3( )2∂FP CH2 CH

C

O

NH2

M(NO3)2

n

4

,

Mn+

H2S M2Sn 2H+

.+ +

1100

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10090807060100

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Inte

nsi

ty,

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ty,

arb.

un

its

2θ, deg

33.0

538

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55.3

65.9

569

.25

82

(а) (b)

Fig. 3. X�ray diffraction patterns of the products of thermolysis (723 K) of AAm complexes in (a) a self�generated atmosphere(complex IV) and (b) air (complex III).

Table 4. Characteristic frequencies in the Raman spectrumof the PAAm/CdS nanocomposite

Characteristic frequency, cm–1 Assignment

PAAm/ CdS

3330 νa NH2

3200 νs NH2

2836 νs CH2

1663 ν CO

1603 δ NH2

1426 ν C–N

1315 δ C–H

1215 ω NH2

1118 ν C–C

1042 ν C–C

795 ω C–H

Note: νa and νs are asymmetrical and symmetrical stretching vi�brations; δ and ω are twisting and rocking vibrations.



fraction data confirms the presence of crystalline CdS,ZnS, and PbS in the corresponding nanocomposites(Fig. 4). The large width of the diffraction peaks andline broadening attest to the formation of nanoparti�cles. The nanocrystallite size evaluated using the

Scherrer formula is 5–6 nm for CdS and 4.5 nm forPbS. An important point is that additional annealingof the nanocomposites at 450°C produced no changesin the phase composition or size of the metal sulfidenanoparticles, as illustrated by the data for the CdSsystem in Fig. 5. Simultaneous formation of a poly�acrylamide matrix and metal sulfide nanoparticles isconfirmed by Raman spectroscopy data (Table 4).Figure 6 shows low�frequency Raman spectra of theCdS/PAAm nanocomposite. The peaks at 354 and560 cm–1 are due to optical vibrational modes of CdSnanoparticles in a polymer�matrix composite. Com�parison with the spectrum of individual CdS nanopar�

500

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100

100806040200

2000

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un

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1000900

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nsi

ty,

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un

its

2θ, deg

800

500400300200

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5

43.8

5

52.0

5

15.5

5

28.8

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2θ, deg

26.0

530

.2

45.1

5

51.1

53.5

62.6

68.9

571

7984

.5

(а)

(b)

(c)

Inte

nsi

ty,

arb.

un

its

Fig. 4. X�ray diffraction patterns of the nanocompositesproduced by frontal polymerization in the (a) III + TU,(b) I + TU, and (c) II + TU systems.

300

200

100

100806040200

50 nm

Inte

nsi

ty,

arb.

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its

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29.8

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70.1

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5

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(а)

(b)

Fig. 5. (a) X�ray diffraction pattern and (b) SEM micro�graph of the III + TU nanocomposite annealed at 450°Cin air.

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POMOGAILO et al.

ticles indicates that the longitudinal optical phononmode of the CdS nanocrystals at 595 cm–1 is shifted to560 cm–1 in the PAAm�matrix composite, which isprobably caused by the smaller size of the formingnanoparticles. This behavior is consistent with thephonon confinement model [12] and with the fact thatthe polymer matrix may have a significant effect on theoptical and gas�sensing properties of CdS nanoparti�cles [13].

Gas�sensing properties of the metal–polymer nano�composites. As gases to be detected, we used liquefiedoil gases, which consist primarily of propane andbutane. The operating principle of semiconductorchemical gas sensors is that the electrical properties ofa sensing semiconductor layer changes in response tochanges in the composition of the gas phase. In semi�conductor crystals, conduction electrons are gener�ated through the ionization of oxygen vacancies(donors) containing localized electrons relativelyweakly bound to the crystal lattice of the oxide. Theelectronic levels of such donors are close to the con�duction band bottom. Oxygen molecules adsorbed onthe surface of the sensitive layer of a gas�sensing mate�rial become ionized by capturing electrons from thesemiconductor film:

O2(gas) O2(ads),

O2(ads) + e– .

Because of this, the near�surface region becomes defi�cient in charge carriers, which reduces the electricalconductance of the sensor. The reducing�gas sensing

O2–

mechanism is a change in electrical resistance inresponse to reaction between gas molecules andadsorbed oxygen on the surface of the gas�sensingmaterial. When a reducing gas appears in the gasphase, it reacts with oxygen ions, which is followed bythe desorption of the reaction products:

2CnH2n + 2 + 2 2CnH2n + 2 + 2H2O + 2e–.

The state and amount of adsorbed oxygen stronglydepend on the microstructure of the material, namely,on its specific surface area, particle size, and surfacemorphology [14, 15]. The performance of a chemicalsensor improves with decreasing particle size [16].

Metal�oxide nanocomposites. Figures 7a and 7bshow the electrical resistance as a function of timeand LPG concentration for a gas�sensing materialbased on Co(CH2=CHCONH2)4 · (NO3)2 ⋅ 2H2Oand its analog, nanocomposite IV, respectively. It isseen that the sensing element responds immediatelyafter LPG is introduced into the measurementchamber. The gas sensitivity of nanocomposite IV,prepared through Co(CH2=CHCONH2)4 ⋅

(NO3)2 ⋅ 2H2O thermolysis in a self�generatedatmosphere, is almost one order of magnitudehigher than the LPG sensitivity of the materialobtained by annealing Co(CH2=CHCONH2)4 ⋅(NO3)2 ⋅ 2H2O at 450°C in air. This difference inproperties can, probably, be understood in terms ofthe morphology, microstructure, and particle size ofnanocomposite IV, as discussed above. At the sametime, the highest sensitivity is offered by the system

O2–

800700600500400300 900Wavenumber, cm–1

354 49

1

560

750600450300 900Wavenumber, cm–1

301

595

(а) (b)

Fig. 6. Raman spectra of (a) the CdS/PAAm nanocomposite and (b) CdS nanoparticles.



based on Cd(CH2=CHCONH2)4 ⋅ (NO3)2 ⋅ 2H2O(Fig. 7c), even though the specific surface area andpore volume are even smaller in this system(Table 3), which may be related to the particularcharacter of chemisorption processes and changesin the density of surface states. Further detailedinvestigation is needed to shed light on the exactorigin of this behavior.

One of the most important parameters of gassensors is their speed, which is quantified by theirresponse time, over which the measurand changesto 0.9 of its maximum value, and their recoverytime. The response and recovery times of sample IVwere determined to be 2 and 9 min, respectively;and those of nanocomposite III are 2 and 8 min.The highest sensitivity of the gas�sensing materialsbased on Co(CH2=CHCONH2)4 ⋅ (NO3)2 ⋅ 2H2Oand nanocomposite IV is 2.9 and 23.6 MΩ/s,respectively. It is important to note that the gas�sensing materials under investigation offer stableoperation. After one month of storage, the repro�ducibility of the gas�sensing characteristics of nano�composites IV and III was 96 and 97%, respectively(Fig. 8).

Metal�sulfide nanocomposites. The gas�sensingproperties of chalcogenide semiconductor materialshave been studied in less detail than those of oxidematerials even though their working range may extenddown to room temperature [17]. We assessed the gas�sensing properties of PAAm matrix composites con�taining ZnS, PbS, and CdS (ZnS/PAAm,PbS/PAAm, and CdS/PAAm). All of the samplesstudied had a fast sensing response to LPG at roomtemperature (Fig. 9, Table 5). Exposure to LPG led toa rapid increase in their resistance, followed by satura�tion. After removal of the gas, the resistance returnedto its original level.

CONCLUSIONS

The key gas�sensing characteristics of the nano�composite materials studied here—their fast room�temperature response, good sensitivity, and operationstability—suggest that they can be used as sensing ele�ments for LPG detection in industry and environmen�tal monitoring.

ACKNOWLEDGMENTS

We are grateful to S.P. Gubin for his assistance withthis study.

This work was supported by the Russian Founda�tion for Basic Research (project nos. 11�03�00769 and13�03�92693) and the Presidium of the Russian Acad�emy of Sciences (basic research program no. 24: Prin�ciples of Basic Research on Nanotechnologies andNanomaterials).

0.05

0.450.400.350.300.250.200.150.10

25002000150010005000

Res

ista

nce

, GΩ 1 vol % LPG

Time, s

2 vol %3 vol %4 vol %5 vol %

0.8

4.03.63.22.82.42.01.61.2

25002000150010005000

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ista

nce

, GΩ 1 vol % LPG

Time, s


14000

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4000

2000

25002000150010005000

Res

ista

nce

, GΩ

1 vol % LPG

Time, s


0

0.40

(а)

(b)

(c)

Fig. 7. Room�temperature electrical resistance as afunction of time at varied LPG concentration for (a) agas�sensing material based onCo(CH2=CHCONH2)4 ⋅ (NO3)2 ⋅ 2H2O, (b) nano�composite IV, and (c) the product ofCd(CH2=CHCONH2)4 ⋅ (NO3)2 ⋅ 2H2O (III) ther�molysis.

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Time, s

5 vol %Repr. (5 vol %)

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0.5

0

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siti

vity

, MΩ

/s

(а) (b)

(c) (d)

2 5431vol % LPG

2 5431vol % LPG

Fig. 8. Sensitivity as a function of LPG concentration for sensing elements made from nanocomposites (a) IV and (c) III; sensi�tivity reproducibility after one month of storage for (b) IV and (d) III.

Table 5. Gas�sensing characteristics of polymer�matrix metal sulfide nanocomposites

Nanocomposite Maximum sensitivity, MΩ/min Response time, min Recovery time, min Reproducibility, %

ZnS/PAAm 2 2 8 83*

PbS/PAAm 12 3 5 91*

CdS/PAAm 1300 3 16 92**

* Reproducibility after three months of storage.** Reproducibility after one month of storage.



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Translated by O. Tsarev

656055504540

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2500200015001000500–5

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Fig. 9. Sensing response as a function of time and LPGconcentration for the chalcogenide nanocomposites(a) ZnS/PAAm, (b) PbS/PAAm, and (c) CdS/PAAm.

polymer-matrix nanocomposite gas-sensing materials

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