May 1, 2004 / Vol. 29, No. 9 / OPTICS LETTERS 1031

Terahertz time-domain spectroscopy of submonolayer wateradsorption in hydrophilic silica aerogel

Jiangquan Zhang and Daniel Grischkowsky

School of Electrical and Computer Engineering, Oklahoma State University, Stillwater, Oklahoma 74078

Received October 24, 2003

We report a terahertz time-domain spectroscopy study of the adsorption of water in hydrophilic silica aerogel.The adsorbed water is in submonolayer form and shows properties of index of refraction similar to those ofbulk water but different absorption properties. © 2004 Optical Society of America

OCIS codes: 300.6490, 320.7150.

Silica aerogels are remarkable artificial materialscombining low density, low thermal conductivity, lowindex of refraction, high transparency, and extremelyhigh surface area.1 They consist of a fractal struc-ture of relatively uniform silica strands typically 3 nmin diameter. This structure results in their openultrasmall interconnected pores, providing access tothe entire surface area for the reaction or depositionof materials. Consequently, silica aerogels are anexcellent substrate for use in studying the propertiesof submonolayer adsorbed molecules.

In this Letter we report a terahertz time-domainspectroscopy (THz-TDS) study of the adsorption ofwater in hydrophilic silica aerogel.2 – 4 The adsorbedwater is in submonolayer form and shows propertiesof index of refraction similar to those of bulk waterbut different absorption properties. Figure 1 showsthe experimental setup of the broadband THz-TDSsystem, which consists of a THz transmitter andreceiver, along with beam-shaping and steering com-ponents.5 The THz pulse is generated and detectedby photoconductively switching the transmitter and re-ceiver by 40-fs optical pulses from a Ti:sapphire laser.Between the paraboloidal mirrors is a cylindrical vac-uum chamber of 188 mm length 3 80 mm diameter,with two 50 mm diameter 3 10 mm thickness sili-con windows for the THz beam to pass through. Theentire setup is placed in an airtight enclosure purgedwith dry air. The hydrophilic silica aerogel sample,with dimensions of 50.0 mm 3 49.3 mm 3 12.1 mmand weight of 2.65 g, is centered perpendicular tothe THz beam path in the middle of the vacuumchamber. Obtained from MarkeTech International,this lightweight transparent solid has an enormoussurface area per weight of 800 m2g, corresponding toa total surface area of 2120 m2.

As in other THz-TDS studies,5 a reference pulse anda signal pulse were measured in the time domain, andthe complex spectra were obtained through digital fastFourier transformation of the THz electrical pulses.In this experiment the reference was the measuredpulse transmitted through the sample when the cham-ber was pumped out and kept below 10 mTorr. Afterthis scan the vacuum chamber was filled with 18 Torrof water vapor at room temperature, and the aerogelwas allowed to equilibrate for approximately 1 h. Thesignal pulse was then measured. The measured refer-

0146-9592/04/091031-03$15.00/0

ence and signal pulses are presented in Fig. 2(a), wherethe reference pulse has been shifted for clarity. Therespective spectra are shown in Fig. 2(b). The sharpabsorption lines in the signal spectrum are the char-acteristic lines of water vapor; the broadband signaldrop is due to the adsorbed water inside the aerogel.The inset of Fig. 2(a) is the enlargement of the mainpeaks of the time-domain pulses, which shows that thesignal pulse is delayed by approximately 0.16 ps (withan estimated error of 60.02 ps) relative to the refer-ence pulse, caused by both the adsorbed water insidethe aerogel and the water vapor outside the aerogel.

To estimate the delay caused by the adsorbed waterand to reduce the effect of the sharp absorption linesof water vapor in the analysis of the measurement,we conducted the same experiment with the aerogelsample removed from the vacuum chamber. The ref-erence pulse was obtained with the vacuum chamberpumped out, and the signal scan was measured withthe chamber f illed with 18 Torr of water vapor at roomtemperature. The measured pulses showed that thesignal scan is delayed by approximately 0.08 6 0.02 psbecause of the water vapor, while their correspondingspectra differed only by the strong absorption of thewater vapor lines.

With this result the delay caused by the adsorbedwater in the aerogel experiment is then obtained asDt 0.08 6 0.03 ps. We assume that the index of re-fraction of the adsorbed water is the same as that of thebulk water. The equivalent water layer thickness canthen be calculated as L cDtnw 2 1 19 6 8 mm,where c is the speed of light in vacuum and nw 2.27is the index of refraction of bulk water at 0.5 THz,6 – 8

the frequency of the peak spectral amplitude. Given

Fig. 1. Experimental setup.

© 2004 Optical Society of America

1032 OPTICS LETTERS / Vol. 29, No. 9 / May 1, 2004

Fig. 2. (a) Output pulses of reference (top trace) and sig-nal scans and (b) their respective spectra. The referenceand signal scans have been shifted in time and separatedvertically for clarity. Inset, enlargement of the pulses inthe vicinity of the peaks to show the relative signal pulsedelay.

this equivalent water layer thickness, and the to-tal surface area of 2120 m2 inside the aerogel, thesurface water layer thickness can be calculated asDl 0.022 6 0.008 nm, corresponding to 0.07 6 0.03monolayer thickness of water. This simple calculationshows that, on the silica aerogel surfaces, the adsorbedwater is in submonolayer form. As an independentcheck on this value of the equivalent water layerthickness, we relocated our sample onto a battery-powered electronic balance with 2-mg sensitivity thatwas in a larger vacuum chamber. The addition of18 Torr of water vapor to the chamber caused theweight of the sample to increase monotonically by70 6 15 mg within 1 h. This additional weight wasdue to the adsorbed water, which corresponds tothe equivalent water layer thickness of 28 6 6 mm,somewhat higher than the previous measurement.

The complex spectrum of the signal pulse of aerogelsample, with both the adsorbed water inside and thewater vapor outside, can be calculated as

Aa1aw1vv A0vHa1awvHvv . (1)

For the aerogel sample with the vacuum chamberpumped out this relation is Aav A0vHav,where v is the angular frequency, A0v is thecomplex spectrum of the vacuum reference pulsewithout aerogel sample, and H v represents thecomplex frequency transfer functions for differentconditions. The subscripts a, aw, and v stand foraerogel, adsorbed water, and water vapor, respectively,so Ha1awv is the complex frequency transfer function

of the aerogel with adsorbed submonolayer water,and so on. Correspondingly, the complex spectrum ofwater vapor in the chamber without silica aerogel canbe written as Avv A0vHvv. Here we assumethat Hvv is the same with and without the aerogelin the chamber. From the equations above it is easyto extract the effect caused by the adsorbed water inthe aerogel sample as

Hawv Ha1awvHav

Aa1aw1vw

AavA0vAvv

. (2)

Figure 3(a) shows the amplitude ratio of Aa1aw1vvAvv compared with Hawv based on Eq. (2); the re-spective phases are shown in Fig. 3(b). It can be seenthat most of the effect of water vapor is removed afterthis treatment.

One of the advantages of the coherent THz-TDS sys-tem is that the time-domain scans contain both thephase and the amplitude information on the pulses,so one can simultaneously retrieve both the index ofrefraction and the power absorption coeff icient of asample of known thickness. In our case, however, itis impractical to measure the thickness of the adsorbedwater sample inside the aerogel, as the aerogel itself isused as the reference. To obtain the absorption coeffi-cient, it is necessary to get a more precise estimation ofthe equivalent sample thickness of bulk water for theadsorbed water. By use of the transfer function ob-tained from Eq. (2), the index of refraction nawv andthe power absorption coeff icient aawv of the equiva-lent water layer of thickness d are defined by therelation

Fig. 3. (a) Amplitudes and (b) phases of Aa1aw1vvAav compared with Hawv based on Eq. (2). Theamplitude of Hawv has been raised by 0.2 for clarity.

May 1, 2004 / Vol. 29, No. 9 / OPTICS LETTERS 1033

Fig. 4. (a) Index of refraction and (b) power absorption co-eff icient aawv due to the adsorbed water inside the silicaaerogel. The dashed curves are those of the bulk water.

Haw v expΩi2pdl0

nawv 2 1æexp2aawvd2 ,

(3)

where l0 is the free-space wavelength. In the calcu-lation we adjust the layer thickness d to match theindex of refraction with a precision of 60.05 to thatof bulk water in the range 0.75–0.99 THz betweenthe absorption peaks of water vapor. By this proce-dure the equivalent water layer thickness is obtainedas d 21 6 1 mm, in reasonable agreement with thelower-precision earlier estimates of 19 6 8 mm based onrelative pulse delay, and that of 28 6 6 mm determinedby the weight increase of the aerogel when exposed towater vapor.

The change in the power absorption of the aerogelsample caused by the adsorbed water is then calcu-lated as presented in Fig. 4(b), together with the in-dex of refraction in Fig. 4(a), as well as those for bulkwater.8 It is worth pointing out that, in a usual ab-sorption calculation, the Fresnel transmission coeff i-cient has to be obtained for a real sample with abruptinterfaces and a definite thickness. However, in ourcase this correction is not made, since our sample ofadsorbed water does not present abrupt interfaces. InFig. 4(a), the obtained index of refraction of adsorbedwater shows the remnants of the water vapor absorp-tion lines, which could not be completely removed byEq. (2). However, the overall results indicate that the

adsorbed submonolayer water has an index of refrac-tion similar to that of bulk water.

The change in the transmitted power through theaerogel appears to show two effects; f irst an increase inabsorption due to the adsorbed water itself and second,starting at 1 THz and increasing with frequency, adecrease in absorption as a result of the adsorbed waterpassivating the hydrophilic Si–OH groups.9 Our un-published THz-TDS comparison (in vacuum) betweenthe hydrophilic aerogel sample above and a hydropho-bic sample shows essentially the same absorption up to1 THz. Thereafter, the absorption of the hydrophilicsample increases more rapidly with frequency; at 1.5and 2.0 THz the hydrophilic absorption coeff icient is1.29 and 1.56 times larger, respectively, than that ofthe hydrophobic sample.

As shown in Fig. 2(b), in the frequency range from0.25 to 1.0 THz, a significant amplitude drop as aresult of absorption is observed for the signal scan.However, at 1.25 THz, the signal amplitude is almostthe same as the reference signal, indicating littleabsorption as a result of the adsorbed water. InFig. 4(b) the resulting aawv is completely differentfrom that of the bulk water, for which the power ab-sorption increases monotonically with frequency. Theapparent negative absorption seen above 1.25 THz isconsidered to be due to the adsorbed water passivatingthe hydrophilic Si–OH group. These remarkablechanges in the far-infrared absorption of silica aerogelcaused by the submonolayer of adsorbed water awaittheoretical explanation.

This investigation shows the feasibility of using sil-ica aerogel as the adsorbing medium in studies of thefar-infrared properties caused by the adsorbed mate-rial. The large surface area greatly increases mea-surement sensitivity.

This work was partially supported by the Na-tional Science Foundation, the U.S. Army Re-search Off ice, and the U.S. Department of Energy.D. Grischkowsky’s e-mail address is grischd@ceat.okstate.edu.

References

1. S. S. Kistler, J. Phys. Chem. 36, 52 (1932). Anoverview of the properties of aerogels is available athttp://eande.lbl.gov/ecs/aerogels/.

2. G. J. Young, J. Colloid Sci. 13, 67 (1958).3. A. da Silva, P. Donoso, and M. A. Aegerter, J. Non-Cryst.

Solids 145, 168 (1992).4. M. A. Henderson, Surf. Sci. Rep. 46, 1 (2002).5. D. Grischkowsky, S. Keiding, M. van Exter, and

Ch. Fattinger, J. Opt. Soc. Am. B 7, 2006 (1990).6. L. Thrane, R. H. Jacobsen, P. Uhd Jepsen, and S. R.

Keiding, Chem. Phys. Lett. 240, 330 (1995).7. J. T. Kindt and C. A. Schmuttenmaer, J. Phys. Chem.

100, 10373 (1996).8. J. Dai and D. Grischkowsky, unpublished measurement

of bulk water in our laboratory.9. P. B. Wagh and S. V. Ingale, Ceramics Int. 28, 43 (2002).