cars diagnostics of molecular media under nanoporous confinement

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ISSN 1054-660X, Laser Physics, 2008, Vol. 18, No. 12, pp. 1451–1458.

© MAIK “Nauka /Interperiodica” (Russia), 2008.Original Text © Astro, Ltd., 2008.

1. INTRODUCTION

Porous structures are widely used in various physi-cal, chemical, and biological processes [1–4]. A rela-tively large internal surface, which is greater than theexternal surface by several orders of magnitude, makesit possible to use nanoporous materials in effective cat-alysts [5–7], selective biosensors [8], biomembranes [9,10], bioimplants, hydrogen accumulators [11], separa-tion devices [1, 12], and filtering and cleaning units [3].

Recent interest in the structure and dynamics ofmolecular media in nanopores has been driven mainlyby the study of the adsorption, adhesion, friction, wet-ting, and drying. The conditions for the adsorbate con-finement inside a pore and the effective molecular inter-action with the pore walls determine significant distinc-tive features of the confined substance in comparisonwith a bulk substance [13, 14]. The features of thephase behavior of a molecular fluid substantiallydepend on the pore size, topology, and morphology [15,16].

In the study of molecular fluids in porous media, asubject attracting significant interest is the behavior ofsuch fluids in the near-critical and supercritical states.An increase in the large-scale density fluctuations andan anomalously high susceptibility with respect toexternal action are observed in the vicinity of the criti-cal point [17]. Supercritical fluids (SCFs) become capa-ble of dissolving various organic substances and pene-trating in deep layers and the pores of solid substancesand materials due to a relatively high diffusivity. Theleading position in supercritical technologies belongsto the carbon dioxide, which exhibits easily reachablecritical parameters (the critical temperature and pres-sure are

T

c

= 31

°

C and

P

c

= 73.8 atm). This substance

is nontoxic, inflammable, cheap, and easily available.Multiple works are devoted to the critical effects insidepores [18–21] and the behavior of the carbon dioxideconfined in nanoporous substrates [22, 23], in particu-lar, under near-critical and supercritical conditions[24

−

26].

Nanostructured and nanocomposite materialsresulting from the pore filling with various substancescan exhibit unique optical properties and are promisingfor the creation of microelectronic [27], photonic, andnonlinear-optical devices; for the production of newlaser materials; and for the technological developmentof new laser systems [28]. Optical methods of diagnos-tics and spectroscopy are convenient for the study of thestructure and state of various optically transparent nan-oporous materials.

Among nanoporous materials, note nanoporousglasses (NGs), which exhibit a relatively high durabil-ity and chemical stability, a wide range of pore diame-ters, a high porosity (20–60%), and a high area of inter-nal pore surface (up to 300 m

2

/g). NGs are optimalobjects for optical and nonlinear-optical diagnosticsdue to their transparency [29–31].

Raman spectroscopy is the commonly accepted ana-lytical tool that is used to study molecular media in nan-opores [32, 33] and in the near-critical and supercriticalstates [34, 35].

Coherent anti-Stokes Raman spectroscopy (CARS)is one of the most effective and versatile methods forthe optical diagnostics and the study of the physico-chemical processes in rarefied and condensed media[36, 37]. Recently, CARS and its modifications wereactively used for the diagnostics and the characteriza-tion of nanostructured materials (nanoCARS) [38]. The

CARS Diagnostics of Molecular Media under Nanoporous Confinement

V. G. Arakcheev, A. A. Valeev, V. B. Morozov*, and A. N. Olenin

Faculty of Physics and International Laser Center, Moscow State University, Moscow, 119991 Russia

*e-mail: [email protected] July 29, 2008

Abstract

—The CARS spectroscopy is used for the diagnostics of carbon dioxide in a nanoporous glass at tem-peratures ranging from room temperature (20.5

°

C) to the subcritical temperature (30.5

°

C) in the pressure rangebelow the saturated-vapor pressure. The contributions of the gas-phase molecules, the molecular layer adsorbedfrom the gas phase on the pore surface, the condensed liquid-like phase, and the liquid interface in the vicinityof the pore surface can be selected using the analysis of the nonlinear spectral response. The spectral behaviorof the carbon dioxide confined in nanopores at the subcritical temperature indicates a state that is similar to thesupercritical fluid. This corresponds to a low-temperature shift of the critical point of the medium confined innanopores.

PACS numbers: 42.65.Dr, 68.43.Pg, 81.05.Rm

DOI:

10.1134/S1054660X08120128

NANOPHOTONICSAND NANOTECHNOLOGIES

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ARAKCHEEV et al.

analysis of the CARS signal intensity and spectralshape allows the diagnostics of the molecular mediumconfined in the pores. The presence of a coherent non-resonant background related to the nanoporous sub-strate leads to a decrease in the sensitivity of the con-ventional CARS and makes it possible to renormalizethe resonant signal and to estimate the concentrations[31, 38]. Porous materials with a certain pore morphol-ogy enable one to concentrate the pump energy insidethe pores rather than inside the nonresonant materialand, hence, to significantly decrease the level of thenonresonant signals and to increase the resonant signaland the measurement sensitivity [39]. This principlecan be used to construct effective gas sensors [40].

This work is devoted to the CARS analysis of thespecific characteristics of the blue

Q

band (1388 cm

–1

)of carbon dioxide confined in NG. In addition, we makean attempt at the diagnostics of the phase state of themolecular medium inside the pores.

2. NANOPOROUS SAMPLE AND EXPERIMENTAL SETUP

The sample is made of the Vycor NG with a densityof 1.5 g/cm

3

, a characteristic pore size of about 4 nm, avolume fraction of pores of about 28%, and a specificarea of the inner pore surface of about 250 m

2

/g. Thesample represents a cylinder with a diameter of 3.5 mmand a height of 5 mm. It is placed in a high-pressure cellwith a volume of about 1 cm

3

. The quartz-glass win-dows of the cell are located at a distance of 5.5 mmfrom each other. The transmission diameter of the win-dows is 10 mm, and we can probe both the sample andthe free molecular medium outside the sample. A pneu-matic press is used to exert pressure on the carbon diox-ide inside the cell, and the pressure level is controlledwith an accuracy of 0.1 atm using a digital membranemanometer. The temperature inside the cell is main-tained with an accuracy of

±

0.01

°

C using an electronictemperature stabilizer.

Under room conditions, the presence of the poresleads to the effective absorption of atmospheric waterand organic compounds, which are sedimented on thepore walls and affect the radiation absorption in poresand the interaction of the molecular fluid with the porewalls. As was demonstrated in [31], the spectra of theliquid carbon dioxide can contain a low-frequencycomponent related to the molecules from the interfacelayer at the walls. In accordance with the further analy-sis, the contribution of the interface layer increaseswhen the sample is saturated with the atmosphericimpurities, the sample becomes more yellow, and theabsorbance in the visible range increases. To clean thesample, we anneal it at a temperature of about 300

°

Cprior to each measurement series.

The experimental setup consists of a passively Q-switched Nd:YAG pulsed laser (master oscillator) witha wavelength of 1064 nm, a pulse duration of 20 ns, and

a repetition rate of 1 Hz. The second-harmonic radia-tion is used as the pump radiation

ω

1

(

λ

= 532 nm,

τ

≈

15 ns,

W

≈

0.2 mJ, and

∆ω

≈

0.5 cm

–1

), the probe radi-ation, and the pump radiation in a dye laser. The radia-tion of the tunable dye laser with a prism cavity (

λ

=570–580 nm,

τ

≈

12 ns,

W

≈

0.5 mJ, and

∆ω

≈

15 cm

–1

)is used as the second pump radiation

ω

2

in the broad-band CARS. To avoid the wing distortions in the mea-sured spectra due to the limited bandwidth of the dyelaser

∆ω

, we scan the frequency difference (

ω

1

–

ω

2

) inthe vicinity of molecular resonance

Ω

in a range ofabout 20 cm

–1

with a step of about 0.5 cm

–1

. The twobeams with identical linear polarizations are focused tothe working cell by a lens with a focal length of 17 cm.The beam diameters on the lens are about 2.5 mm.The cell windows insignificantly depolarize the radia-tion. The anti-Stokes radiation passes through adouble-grating monochromator (each grating has2400 groves/mm) with open exit slits and is detectedusing a CCD array with an image intensifier. At eachscanning step, we perform averaging over 15 lasershots. The width of the instrument function (about0.5 cm

–1

) is taken into account in the fitting procedure.In the CARS measurements in a dense gas or con-

densed phase, it is expedient to employ a noncollinearconfiguration of the beams in the interaction volume foroptimal phase matching [36]. However, in spite of acertain decrease in the signal, we prefer the collinearconfiguration, which enables one to simplify the beammatching and to more accurately determine the posi-tions of the spectral lines. The length of the inner partof the cell is greater than the length of the sample by0.5 mm. Hence, the resonant CARS signal results fromthe interference of the contributions of the moleculesinside the pores and the molecules in the space betweenthe porous sample and the cell windows. The secondcontribution corresponds to the bulk molecularmedium.

3. RESULTS

In the experiments with the nanoporous sample con-taining carbon dioxide, we measure the CARS spectra ofthe

Q

band of the

ν

1

vibrational transition (1388 cm

–1

) inthe range from room temperature (20.5

°

C) to the sub-critical temperature (30.5

°

C). The gas pressure rangesfrom 0.85

P

s

to

P

s

(

P

s

is the temperature-dependent sat-urated-vapor pressure in free volume). At each temper-ature, we control the saturated-vapor pressure based onthe appearance of meniscus in the cell.

3.1. Spectral Response of the Bulk Molecular Medium

Figure 1 demonstrates the measured pressure depen-dences of the spectral width and central-frequencyposition of the carbon dioxide

Q

band (1388 cm

–1

) in awide range of pressure variation in free volume at tem-peratures of 20.5 and 30.5

°

C. For comparison, we also

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CARS DIAGNOSTICS OF MOLECULAR MEDIA 1453

present the results obtained at the supercritical temper-ature (33

°

C). The shapes of the curves corresponding todifferent temperatures are similar. Note that the curvesexhibit a minor shift to the right-hand side when thetemperature increases due to an increase in the range ofthe possible gas densities.

The pressure-to-density recalculation based on theequation of the state for the carbon dioxide [42] showsthat, in the gas phase, the spectral width increases pro-portionally to the density, which indicates the homoge-neous collisional broadening [35, 43]. In liquid and inthe supercritical phase, the spectral width remainsalmost unchanged (about 1.6 cm

–1

) in the entire densityrange. The frequency of molecular vibrations linearlydecreases with an increasing density in both gas andliquid phase, so that the rate of decrease is higher in gas.Except for the discontinuity point in the range of thephase coexistence at the temperatures lower than thecritical point, the density dependences appear almostthe same for the room, subcritical, and supercriticaltemperatures. The dependences are used for the simula-tion of spectral responses that represent the superposi-tions of the contributions of the gas and condensedphases of the molecular medium.

3.2. Spectral Contributions of the Adsorbed and Condensed Phases

The CARS spectra measured at a temperature of20.5

°

C (far from the critical point) exhibit a developedtwo-component structure (Fig. 2) due to the fact that theanti-Stokes signal is generated by two different groupsof carbon-dioxide molecules. The main (high-fre-quency) spectral component is related to the gas-phasemolecules located in the central parts of the pores andin the spaces between the cell windows and the poroussample.

The second (low-frequency) component is related tothe contribution of the molecules in the inner parts ofthe pores. To determine the spectral characteristics ofthis contribution, we perform calculations using theformula that describes the interference of two closelylying resonant components and the nonresonant back-ground [36]:

(1)

where and determine the resonant contribu-

tions, is the amplitude of the nonresonant back-

ground,

∆

1

= and

∆

2

= are

the frequency detunings,

Γ

1

and

Γ

2

are the spectralwidths of the contributions, and

Ω

1

and

Ω

2

are the cor-responding frequencies of the vibrational transition.The values of spectral width

Γ

1

and frequency

Ω

1

,

IaχR1

3( )

–i ∆1–----------------

χR23( )

–i ∆2–---------------- χNR

3( )+ +

2

I1I2IP,∝

χR13( ) χR2

3( )

χNR3( )

ω1 ω2– Ω1–Γ1

-------------------------------ω1 ω2– Ω2–

Γ2-------------------------------

which correspond to the contribution of the gas-phasemolecules, are known from the dependences obtainedfor the free volume at the corresponding pressure

(Fig. 1). Quantities Γ2, Ω2, , , and serve asthe fitting parameters.

The analysis shows that, at a pressure of no greaterthan 0.91Ps, the spectral width of the low-frequencycontribution (greater than 3 cm–1) is almost two timesgreater than the linewidth in gas and liquid (Fig. 2a). Ata pressure of about 0.94Ps, the linewidth significantlydecreases and, starting from 0.97Ps, reaches a level ofabout 1.6 cm–1, which corresponds to the linewidth infree liquid volume (Fig. 2b). In accordance with theKelvin formula, the condensation inside the pores takes

place at pressure given by [41]

(2)

χR13( ) χR2

3( ) χNR3( )

Ps

Ps

Ps-----

µρRT-----------2σ

r------–⎝ ⎠

⎛ ⎞ ,exp=

20.5°C30.5°C33.0°C

20.5°C30.5°C33.5°C

2

1

0

1388

1387

1386

0 50 100 150 200Pressure, atm

Wav

enum

ber,

cm

–1

Lin

ewid

th, c

m–

1

Fig. 1. Plots of (a) the width and (b) central frequency of theQ band (1388 cm–1) of carbon dioxide vs. the pressure infree volume at room (20.5°C), subcritical (30.5°C), andsupercritical (33.0°C) temperatures.

(a)

(b)

1454

LASER PHYSICS Vol. 18 No. 12 2008

ARAKCHEEV et al.

where T is the temperature, µ = 0.044 kg/mol is themolar mass of carbon dioxide, ∆ρ ≈ 770 kg/m3 is thedifference between the densities of the liquid and gasphases, σ ≈ 1.2 × 10–3 J/m2 is the surface tension coef-ficient at the given temperature, and r = 2 nm is themean radius of the pores. The calculation using formula(2) yields a value of about 0.96Ps.

Thus, we can conclude that the spectral response ofthe molecules adsorbed on the pore surface is observedat a pressure of no greater than 0.91 atm. The transition

to the condensed state inside the pores is started at apressure of about 0.94Ps. In the pressure range from0.96Ps to Ps, the low-frequency component is due to themolecules condensed inside the pores and its spectralwidth corresponds to the spectral width in liquid. In theupper part of Fig. 3a, we present the pressure depen-dence of the spectral width of the low-frequency com-ponent at a temperature of 20.5°C. This dependenceillustrates the transition from the adsorption on the porewalls to the condensation inside the pores. We obtain

1

2 3

4

5

1

2

3

(a)

(b)

1380 1385 1390 1395Wavenumber, cm–1

CA

RS

inte

nsity

Fig. 2. CARS spectra of the Q band (1388 cm–1) of carbon dioxide measured in the experiments with the nanoporous sample atroom temperature (20.5°C) (a) at a pressure of 52 atm (P/Ps = 0.91): (circles) experimental data, (1) gas-phase (high-frequency)contribution, (2) best-fit data obtained using the two-component model with a variable width and position of the low-frequency con-tribution, (3) low-frequency contribution in fitting curve 2, (4) low-frequency contribution with the line width and position that cor-respond to the liquid phase and the amplitude that corresponds to fitting curve 5, and (5) best-fit data with the linewidth and positionthat correspond to liquid phase 4 and (b) at a pressure of 55.5 atm (P/Ps = 0.97): (circles) experimental data, (1) gas-phase (high-frequency) contribution, (2) best-fit data obtained using the two-component model with a variable width and the position of the low-frequency contribution, and (3) low-frequency contribution in fitting curve 2.



similar curves for temperatures of 24 and 26°C. Noteonly that a decrease in the spectral width of the low-fre-quency component under the transition from the

adsorption to the capillary condensation takes place athigher pressures owing to a decrease in the surface ten-sion coefficient with an increasing temperature [41]. It

6

4

2

0

1388

1387

1386

40 45 50 55 60 65 70Pressure, atm

Lin

ewid

th, c

m–

1W

aven

umbe

r, c

m–

1Liquid

Gas

Liquid

Gas

GasGas

LiquidLiquid

(a) (b)

Fig. 3. Plots of (upper panels) the width and (lower panels) positions of the line related to the contribution of the molecules that areadsorbed or condensed inside the pores vs. the pressure: (circles) experimental results obtained for temperatures of (a) 20.5 and(b) 30.5°C, (dashed lines) experimental data for the gas in free volume (Fig. 1), and (solid lines) experimental data for the liquidmeasured at the minimum liquid density (under meniscus) in free space. The arrows denote the pressures that correspond to thebeginning of the capillary condensation in accordance with the Kelvin equation. Triangles show the saturated-vapor pressure in freevolume.

1

2

3

1380 1385 1390 1395Wavenumber, cm–1

CA

RS

inte

nsity

Fig. 4. CARS spectra of the Q band (1388 cm–1) of carbon dioxide measured in the experiments with the nanoporous sample at thesubcritical temperature (30.5°C) and a pressure of 71 atm (P/Ps = 0.98): (circles) experimental data, (1) gas-phase (high-frequency)contribution, (2) best-fit data obtained using the two-component model with a variable width and the position of the low-frequencycontribution and (3) the low-frequency contribution in fitting curve 2.

1456


ARAKCHEEV et al.

remains unclear whether the condensation inside nan-opores represents a phase transition of the first kind.Nevertheless, the condensation pressure inside thepores calculated using the Kelvin formula (expression(2)) is in agreement with the experimental data.

3.3. Spectral Features of the Subcritical Phasein Nanopores

The CARS spectra measured at the near-criticaltemperature (30.5°C) do not exhibit the developed two-component structure. However, the asymmetrical char-acter of these spectra is due to the contribution of themolecules whose state inside the pores differs from thegas state. Figure 4 demonstrates an experimental spec-trum and its analysis. The absence of the developedtwo-component structure at the near-critical tempera-ture is related to the red spectral shift of the main (gas)

component that is caused by significantly higher gasdensities at the same normalized pressures. The calcu-lation is also performed using formula (1). At the near-critical temperature, the fitting procedure yields analmost constant spectral width of the second contribu-tion (about 1.6 cm–1) in the entire pressure range (upperpart in Fig. 4). Thus, the low-frequency contributioncannot be related to the molecules adsorbed at the poresurface. The spectral width and frequency of the low-frequency component corresponds to the spectralwidths and frequencies measured in free volume for theliquid at T < Tc and the supercritical fluid at T > Tc. Theestimation of the surface tension coefficient [41] showsthat its value at a temperature of 30.5°C is less than itsvalue at a temperature of 20.5°C by more than an orderof magnitude. With regard to the Kelvin equation, thecorresponding saturated-vapor pressure shows that thecondensation must take place at the pressure P >

1

2

12

3

×10

1380 1390Wavenumber, cm–1

(a)

(b)

CA

RS

inte

nsity

χ R13()

χ R23()

χ R33()

,,

Fig. 5. (a) CARS spectra of the Q band (1388 cm–1) of carbon dioxide measured in the experiments with the unannealed nanoporoussample at a temperature of 24.5°C and a pressure of 62.5 atm (P/Ps = 0.99): (circles) experimental data, (1) best-fit data obtainedusing the two-component model with variable linewidths and positions, and (2) best-fit data obtained using the three-componentmodel and (b) the spectral contributions related to (1) gas phase, (2) the liquid-like phase inside pores, and (3) the interface liquidlayer inside the pores.



0.99Ps. However, the pressure dependences of the spec-tral width and frequency indicate the absence of varia-tions in the phase state of the molecular medium in theentire range of measurements. Thus, the molecularmedium inside nanopores exhibits the properties of thesupercritical fluid at a temperature that is lower than thecritical point in accordance with the effect lying in theshift of the critical point in nanopores [18–20].

3.4. Contribution of the Moleculesfrom the Interface Layer

The results presented in Sections 3.2 and 3.3 areobtained for the sample that is annealed prior to eachexperimental series. The long-term contact of the sam-ple with atmospheric air leads to the saturation of thesample with water vapor and organic compounds. Thissaturation is accompanied by a yellowing and anincrease in the visible absorption. Under such condi-tions for the liquid carbon dioxide, the spectra of bothcomponents of the Fermi doublet exhibit spectral fea-tures that are red shifted to a greater extent [31]. In theexperimental series, we perform measurements at atemperature of 24.5°C using the sample that is satu-rated with atmospheric impurities. As in the case of thelower temperature, the spectrum exhibits a developedtwo-component structure (Fig. 5). Note that the widthof the low-frequency component is also greater than3 cm–1, which is in agreement with the case of adsorp-tion. When the pressure is P > 0.96Ps, the width of thelow-frequency component decreases to a level typicalof the liquid. Figure 5a shows the experimental andsimulated results for a pressure of 62.5 atm (Ps = 0.99),which is higher than the pressure of the intrapore con-densation that follows from the Kelvin equation (P ~0.98Ps). A comparison of the experimental data withthe fitting results obtained using the two-componentmodel shows that the long-wavelength wing containsan additional contribution that is similar to that result-ing from the presence of the interface liquid layer at thepore surface [31]. Such a contribution is absent underthe same conditions, but at a pressure of less than P ~0.94Ps (adsorbed molecules dominate inside pores). Totake into account the third peak, we employ the modelof the three-component resonant contribution:

(1')

where determines the amplitude of the interface-

layer contribution, ∆3 = is the frequency

detuning, Γ3 is the spectral width, and Ω3 is the corre-sponding frequency of the vibrational transition. Quan-

tities Γ3, Ω3, and represent additional variables incomparison with model (1) and serve as fitting param-

IaχR1

3( )

–i ∆1–----------------

χR23( )

–i ∆2–----------------

χR33( )

–i ∆3–---------------- χNR

3( )+ + +

2

I1I2IP,∝

χR33( )

ω1 ω2– Ω3–Γ3

-------------------------------

χR33( )

eters in model (1'). The calculated results obtainedusing the three-component model (solid line in Fig. 5a)are in good agreement with the experimental data andenable one to determine the width, frequency, andamplitude that correspond to the contribution of theinterface liquid layer (Fig. 5b).

4. CONCLUSIONSBased on the CARS results obtained for the spectral

characteristics of the blue Q band (1388 cm–1) of thecarbon dioxide confined in NG, we conclude that thenonlinear response of the medium makes it possible todiscriminate the spectral contributions of the gas phase,the molecular layer adsorbed from the gas phase on thepore surface, and the condensed liquid-like phase of theinterface liquid layer in the vicinity of the pore surface.The spectral behavior of the carbon dioxide confined inthe nanopores at the subcritical temperature indicatesthe absence of variations in the contribution of the mol-ecules adsorbed on the surface and in the phase state ofthe molecular medium at a pressure of no greater thanPs. In this case, the spectral characteristics of the con-tribution that corresponds to the molecules confined inpores correspond to the spectral characteristicsobtained for the SCF. This is in agreement with theeffect lying in the shift of the critical point under thenanopore conditions. The results show that one canemploy CARS for the diagnostics of the phase state ofthe molecular medium confined in the nanopores.

ACKNOWLEDGMENTSWe are grateful to V.N. Bagratashvili and

V.K. Popov for providing the sample and stimulatingdiscussions and A.M. Zheltikov for valuable remarks.This work was supported by the Russian Foundationfor Basic Research (project no. 07-02-01331a).

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cars diagnostics of molecular media under nanoporous confinement

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