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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 3 — Feb. 10, 2014
  • pp: 2584–2594
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Laser absorption spectroscopy of oxygen confined in highly porous hollow sphere xerogel

Lin Yang, Gabriel Somesfalean, and Sailing He  »View Author Affiliations


Optics Express, Vol. 22, Issue 3, pp. 2584-2594 (2014)
http://dx.doi.org/10.1364/OE.22.002584


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Abstract

An Al2O3 xerogel with a distinctive microstructure is studied for the application of laser absorption spectroscopy of oxygen. The xerogel has an exceptionally high porosity (up to 88%) and a large pore size (up to 3.6µm). Using the method of gas-in-scattering media absorption spectroscopy (GASMAS), a long optical path length (about 3.5m) and high enhancement factor (over 300 times) are achieved as the result of extremely strong multiple-scattering when the light is transmitted through the air-filled, hollow-sphere alumina xerogel. We investigate how the micro-physical feature influences the optical property. As part of the optical sensing system, the material’s gas exchange dynamics are also experimentally studied.

© 2014 Optical Society of America

1. Introduction

However, due to severe backscattering, heavy attenuation, and uncontrollable interaction lengths, GASMAS experiments suffer from a lower sensitivity than traditional approaches. Throughout the years, many efforts have been made to enhance the interaction path length and reduce the size, in order to increase sensitivity and flexibility. In this paper, we utilize a strongly scattering material, which is called hollow sphere xerogel, to significantly enhance the optical path length and investigate the interaction of light and gas in materials with many different parameters. In particular, the remarkable properties of the media, such as high porosity (over 75%), high specific surface area [15

15. T. F. Baumann, A. E. Gash, S. C. Chinn, A. M. Sawvel, R. S. Maxwell, and J. H. Stacher, “Synthesis of high-surface-area alumina aerogels without the use of alkoxide precursors,” Chem. Mater. 17(2), 395–401 (2005). [CrossRef]

], low density, etc. and their relationship with optical performance are studied.

There are three main contributions in the current work. Firstly, this is the first time this xerogel has been utilized for extreme light scattering as a miniature spectroscopic gas cell. A 3.3m path length through O2 gas for light transmitting through a 10 mm Al2O3 hollow sphere xerogel has been achieved (with enhancement factors up to ~300). Secondly, the corresponding relationships between the materials’ properties, e.g., heat treatment temperature, crystalline phase, porosity and pore size, and the light scattering capability are analyzed and discussed. Through these relationships, one can then improve the understanding of photons’ random walks in turbid media. Finally, we study the feasibility of the system using hollow sphere xerogel as multipass gas cells for sensing O2 around 760 nm, and obtain high resolution and adequate gas exchange time in the materials.

Xerogel/aerogel has also recently been a rising topic in the material field [16

16. A. E. Aliev, J. Oh, M. E. Kozlov, A. A. Kuznetsov, S. Fang, A. F. Fonseca, R. Ovalle, M. D. Lima, M. H. Haque, Y. N. Gartstein, M. Zhang, A. A. Zakhidov, and R. H. Baughman, “Giant-Stroke, Superelastic Carbon Nanotube Aerogel Muscles,” Science 323(5921), 1575–1578 (2009). [CrossRef] [PubMed]

18

18. R. T. Olsson, M. A. Azizi Samir, G. Salazar-Alvarez, L. Belova, V. Ström, L. A. Berglund, O. Ikkala, J. Nogués, and U. W. Gedde, “Making flexible magnetic aerogels and stiff magnetic nanopaper using cellulose nanofibrils as templates,” Nat. Nanotechnol. 5(8), 584–588 (2010). [CrossRef] [PubMed]

]. Compared with other materials like ceramics, xerogel/aerogel enables the light to penetrate deeper into the sample (so that a thicker xerogel/aerogel sample can be used for longer optical path length), which gives a consequently a higher sensitivity of the laser absorption spectroscopy. Thus, the method also provides an effective way of increasing the sensitivity of the system and analyzing a porous material nondestructively [19

19. M. Lewander, Z. G. Guan, K. Svanberg, S. Svanberg, and T. Svensson, “Clinical system for non-invasive in situ monitoring of gases in the human paranasal sinuses,” Opt. Express 17(13), 10849–10863 (2009). [CrossRef] [PubMed]

]. Moreover, the extraordinary scattering performance of Al2O3 xerogels will lead to the exploration of other kinds of xerogels with high refractive indices, such as ZrO2, Y2O3, and MgO, and may also be used in the future for many other gases, besides oxygen.

2. Materials and methods

2.1 Porous xerogel sample

The xerogel materials investigated in this article are hollow-sphere alumina xerogels, which are made with the epoxide-driven sol-gel method [20

20. A. E. Gash, T. M. Tillotson, J. H. Satcher, J. F. Poco, L. W. Hrubesh, and R. L. Simpson, “Use of Epoxides in the Sol- Gel Synthesis of Porous Iron (III) Oxide Monoliths from Fe (III) Salts,” Chem. Mater. 13(3), 999–1007 (2001). [CrossRef]

,21

21. A. E. Gash, T. M. Tillotson, J. H. Satcher Jr, L. W. Hrubesh, and R. L. Simpson, “New sol–gel synthetic route to transition and main-group metal oxide aerogels using inorganic salt precursors,” J. Non-Cryst. Solids 285(1–3), 22–28 (2001). [CrossRef]

]. It is a kind of bulk material with microstructure as urchin-like, as shown in Fig. 1(a).
Fig. 1 (a) Schematic diagram of the Al2O3 hollow sphere in xerogel when illumined by a 760 nm red light. (b) SEM image of an Al2O3 hollow sphere xerogel which heat-treated at 1200°C.
Although these kinds of xerogel materials have many similar features as traditional aerogels (such as low density, high porosity, enormous specific surface area, etc.), their microstructures are quite different from those of traditional aerogels (which are much more like the porous skeleton-frame structure of ceramics). Xerogel material has already been utilized in many areas due to its excellent thermal insulation.

The fabrication procedure for these xerogel materials can be described briefly as follows. The solution of Aluminum chloride hexahydrate and ethanol is prepared through several processes: adequate milling, adding polyethylene oxide as binder, and water bathing. After the sol-gel reaction, the resultant gels still need to be aged, dried, and heat-treated in different temperatures, and finally become the hollow-sphere alumina xerogels. During the whole process, the material’s microstructure is very sensitive to the temperature: the hollow-sphere structure cannot form or retain in either insufficient or excessive temperature.

The schematic illustration of the Al2O3 hollow sphere in xerogel and one SEM image of the Al2O3 hollow sphere xerogel are shown in Fig. 1. It is clear from Fig. 1 that the xerogel is an incompact aggregation/accumulation of many urchin-like hollow spheres. The spheres’ radii are between 900nm and 1500nm. The surface of the hollow sphere is not compact but mesoporous, with many crystal hairs on it. These structural properties result in the high porosity and fast gas exchange speed. In this work, it is shown that such a microstructure also prominently increases the scattering ability. Our experiments have shown that these Al2O3 xerogels have good repeatability in optical characteristics for the same temperature (and thus we will show experimental result for only one sample at each temperature).

2.2 Principles for TDLAS and WMS

The instrumentation of GASMAS utilized in this article is mainly based on tunable diode laser absorption spectroscopy (TDLAS) and wavelength modulation spectroscopy (WMS). Both TDLAS and WMS have already been widely studied in theories and implementation in recent years. Below we give a brief overview of these principles.

For sensitive, accurate and selective measurements of gas concentrations, TDLAS is a well-established optical technique and has been particularly used in assessing the gas content in solids. A review paper regarding this technique can be found in [22

22. P. Werle, “A review of recent advances in semiconductor laser based gas monitors,” Spectrochim. Acta A Mol. Biomol. Spectrosc. 54(2), 197–236 (1998). [CrossRef]

]. To improve the selectivity and sensitivity, this technique is often used together with WMS.

The wavelength of light absorption is modulated with higher frequency to perform WMS and avoid low-frequency noise from system components [23

23. S. Schilt, L. Thévenaz, and P. Robert, “Wavelength modulation spectroscopy: combined frequency and intensity laser modulation,” Appl. Opt. 42(33), 6728–6738 (2003). [CrossRef] [PubMed]

]. In TDLAS, this technique is performed by scanning a sinusoidal frequency-modulated diode laser over some isolated narrow absorption lines of gas molecules [24

24. T. Fernholz, H. Teichert, and V. Ebert, “Digital, phase-sensitive detection for in situ diode-laser spectroscopy under rapidly changing transmission conditions,” Appl. Phys. B 75(2–3), 229–236 (2002). [CrossRef]

]. In WMS, we measure the temporal evolution in terms of the amplitudes of the harmonic frequencies, and the signal can be acquired after demodulation with the lock-in amplifier technique. Following the well-known Beer–Lambert law, the WMS signal is proportional to the absorption of the light by the gas (and consequently the gas concentration), and the amplitude of its nth harmonic component, An, can be written as [22

22. P. Werle, “A review of recent advances in semiconductor laser based gas monitors,” Spectrochim. Acta A Mol. Biomol. Spectrosc. 54(2), 197–236 (1998). [CrossRef]

]:
An(ωL)=I021nNLn!δωndnσdωn|ω=ωL
(1)
where ωL is the central frequency of the modulation, I0 is the initial intensity of incident light, N is the gas concentration, L is the path length, and σ is the absorption coefficient. In general, we usually monitor only the first and second harmonic signals. The second can efficiently eliminate linear slopes of the spectra, and is thus the most useful component.

In this work we deal with the detection of gases dispersed within highly scattering samples, which involves diffused light, severe backscattering and unknown path-length distributions. The combination of TDLAS and WMS avoids the demand for a complicated system and optical accuracy, and additionally increases SNR well.

2.3 Experiment instrumentation

The experimental setup of our measurements is schematically depicted in Fig. 2.
Fig. 2 Schematic diagram of the TDLAS-WMS experimental setup, in which the rectangular dotted parts are controlled with a LabView program.
A single-mode, tunable, distributed feedback (DFB) diode laser (#LD-0760-0040-DFB-2, Toptica Photonics, Germany) with a center wavelength of 760.1 nm and an output power of 40.0 mW at 25°C is used as the spectroscopic source, and is wavelength tuned and scanned over one of the absorption lines in the R branch of molecular oxygen (R11Q12, 760.445 nm vacuum wavelength, peak absorption coefficient µa = 2.52 × 10−5 mm−1 in ambient air [25

25. L. S. Rothman, I. E. Gordon, A. Barbe, D. C. Benner, P. E. Bernath, M. Birk, V. Boudon, L. R. Brown, A. Campargue, J. P. Champion, K. Chance, L. H. Coudert, V. Dana, V. M. Devi, S. Fally, J. M. Flaud, R. R. Gamache, A. Goldman, D. Jacquemart, I. Kleiner, N. Lacome, W. J. Lafferty, J. Y. Mandin, S. T. Massie, S. N. Mikhailenko, C. E. Miller, N. Moazzen-Ahmadi, O. V. Naumenko, A. V. Nikitin, J. Orphal, V. I. Perevalov, A. Perrin, A. Predoi-Cross, C. P. Rinsland, M. Rotger, M. Simeckova, M. A. H. Smith, K. Sung, S. A. Tashkun, J. Tennyson, R. A. Toth, A. C. Vandaele, and J. Vander Auwera, “The HITRAN molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 110(9-10), 533–572 (2009). [CrossRef]

]). The operating temperature of the laser is stabilized at about 32°C by a temperature controller (TED200C, Thorlabs, NJ, USA). The operating current of the laser is modulated by a current controller (LDC205C, Thorlabs, NJ, USA), whose signal comes from a digital signal generator via an output port of the Data Acquisition Board (NI-6120, National Instruments) installed in the computer. The modulation signal consists of a triangular ramp (5 Hz) for wavelength scanning, and a superimposed sinusoidal wave (10,295Hz) for wavelength modulation. Furthermore both signals are specially chosen to optimize the 2ƒ WMS signal. The laser diode is installed in a thermoelectrically cooled mount (TCLDM9, Thorlabs, NJ, USA) with a vibration motor producing adequate dither to avoid interference fringes and thus increase SNR.

The modulated light is collimated, guided via a lens (C230TME-B, Thorlabs, NJ, USA) and then impinged onto the sample (Diameter of spot: 1.82mm). After transmitting through the sample, the divergent light signal is collected by a collimator (F240SMA-780 Thorlabs, NJ, USA) into a high quality 200 µm silica fiber bundle (NA = 0.22), and then detected by a photomultiplier tube (PMT, H10721-20, Hamamatsu Photonics, Japan). In order to collect the low intensities of transmitted light and reduce the noise, the sample and second lens are close to each other; meanwhile both ends of the fiber bundle are directly connected to the PMT and lens, respectively. The output photocurrent from the PMT is converted and amplified by a low-noise preamplifier unit (C7319, Hamamatsu Photonics, Japan) operating at 105 V/A. Then the same DAQ card with a 400 KHz sampling frequency samples the voltage signal and then transfers it to the computer for analysis. After averaged 100 times in a digital oscilloscope, which significantly improves the SNR of the detected signal, the extracted 2ƒ component is picked up by a Fourier-transform-based digital lock-in amplifier. Furthermore, both the 2ƒ and the direct signal are collected by the digital oscilloscope simultaneously and in real-time, and then are divided by each other to obtain an intensity-corrected 2ƒ WMS signal. The technique also inherently provides multiharmonic WMS detection. The minimum detectable absorption of measurement system is about 1400ppm O2 (at room temperature and atmospheric pressure).

The digital signal generator, digital lock-in amplifier and digital oscilloscope are all compiled by one LabView program, which supplies all the system control and analysis service.

3. Results

Structural and scattering properties of the Al2O3 hollow-sphere xerogels with different heat treatment temperatures are presented in Table 1.

Table 1. Comparison of structural properties and path length enhancement: material thickness (s), average pore diameter (d), porosity (ϕ), crystalline phase, detected transmission of light (T), equivalent mean path length (Leq), path length enhancement (N = L/s), and normalized path length enhancement (N* = N/s).

table-icon
View This Table

3.1 Structural properties

We have studied seven Al2O3 hollow sphere xerogel samples, which are heat-treated at different temperatures: one dried at room temperature without being heat-treated (referred to “as-dried” sample here after), and the other six treated from 600 to 1600°C. When the heat treatment temperature is below 600°C, there is a large amount of carbonized organic ingredients remaining in the samples, thus leading to the high absorption of light and cannot be utilized. All of the samples are made from the green body (i.e., the unheated xerogel after the aging process) with the same ingredients, formula ratio and procedure. Various heat treatment temperatures lead to varied porosity, pore size, crystalline phase, and microstructure.

Fig. 3 The SEM image of an Al2O3 hollow sphere xerogel which was (a) dried at room temperature, and (b) heat-treated at 1400°C.
Fig. 4 Illustration of the hollow sphere xerogel’s microstructure, and how this hollow porous media greatly increase photon path lengths in air.
As shown in Figs. 1 and 3, the special microstructures of such xerogels have significant differences from the ceramics. The most obvious difference concerns the hollow-sphere-like granules which these xerogel bulks are composed of. This key characteristic dramatically increases the porosity and thus the optical path length. Due to this feature, as shown in Fig. 4, the path length of light transmitting through the air inside and outside the sphere can both be calculated as valid “equivalent mean path length”, which is better than ceramics with the air only filling the gap. Moreover, the surface of hollow sphere is not compact like ceramics but mesoporous, on which many crystal hairs grow. The structure can diminish the capillary force on the pore walls of the gels and prevent the collapse of most of the pore volume, so urchin-like hollow spheres are able to loosely accumulate to the bulk rather than becoming powders. On the contrary, many other similar hollow-spherical structures cannot maintain a stable bulk. In addition, mesopores between crystal hairs on the sphere’s surface can enhance the gas exchange efficiency inside and outside of the sphere. On the other hand, small particles in ceramics always agglomerate together to form larger clusters, which sharply reduce the porosity.

Fig. 5 XRD patterns of alumina gels with different heat treatment temperatures. The horizontal coordinate θ refers to the Bragg angle, and Cps (counts per second) corresponds to intensity. According to the position and shape of the peaks, the crystalline phase of gels exhibits γ-AlOOH when as-dried, γ-Al2O3 from 600 to 800°C, unstable transient state (γ&θ) at 1000°C, θ-Al2O3 at 1200°C, and α-Al2O3 from 1400°C.
Another interesting aspect of the structural properties in Table 1 is the crystalline phase. XRD patterns of alumina gels in Fig. 5 show the gels’ crystalline phase.

According to the well-known Scherrer equation, the wider the linewidth broadening in diffraction peaks, the smaller the crystalline grain size. The alumina’s crystalline phase can be deduced by the shape and position of diffraction peaks in their XRD patterns. At the bottom of Fig. 5, the wide peaks of the as-dried sample with a hydrated boehmite phase (γ-AlOOH) show that this phase is obtained from the amorphous phase. Subsequently, the alumina phase moves from the γ phase (from 600 to 1000 °C) in the form of nano-crystal, to the θ phase (from 1100 to 1300 °C), and to the final phase of alumina, i.e., α phase (from 1400 °C).Various phases have different size of the crystal grain and different length of the crystal grain boundary, and both of these two features will influence the material’s refractive properties. These microscopic changes lead to variation of the intensity of photon migration in different samples, which is reflected in the variations of the refractive index, e.g., 1.70 for γ phase, 1.73 for θ phase and 1.78 for α phase [26

26. S. D. Mo and W. Y. Ching, “Electronic and optical properties of θ-Al2 O3 and comparison to α-Al2 O3,” Phys. Rev. B 57(24), 15219–15228 (1998). [CrossRef]

, 27

27. R. C. Weast, Handbook of Chemistry and Physics 82nd, (Chemical Rubber, 2001), Chapter 10.

].

3.2 Optical path length enhancement

Fig. 6 Intensity-corrected second-harmonic component of the WMS spectra for the R11Q12 line of ambient oxygen in different samples, and the reference (dashed black) is measured along a 1.5 m long path in ambient air.
In order to evaluate the scattering ability of the materials, the second-harmonic WMS signals of samples are analyzed in this work. According to Eq. (1), in the case of weak absorption, the amplitude of 2f-WMS signal is proportional to the product of the path length and gas concentration. Since in the experiment, when the samples are exposed in ambient air, the gas concentration is the same in each sample and reference. Then the difference of amplitude of 2f-WMS signals is directly decided by the mean path length through gas. As shown in Fig. 6, different amplitudes of 2f-WMS signals reflect the difference in photon-scattered intensity in a series of samples with various structural properties. Through comparing the amplitude of samples’ 2f-WMS signals with the standard 2f-WMS signals of a 1.5 m light path in ambient air under 760 nm light transmission, the equivalent mean path length through gas of every sample can be obtained. For comparison, all WMS signals have been intensity-corrected. The asymmetry of signal is caused by residual amplitude modulation (RAM) and nonlinearity of DFB Laser. For flexibility and simplicity we use time (instead of wavelength) as the unit of x-axis since each sample was measured at the same absorption line (R11Q12, 760.445 nm). Measured path length and enhancement factor are also presented in Table 1.

Various scattering performances for alumina provide abundant interest about these porous media. Although the experimental results show that different heat-treated samples present contrasting intensity of the 2f signal and their corresponding equivalent mean path length, one still cannot directly compare the enhancements between the samples because of the difference of material thickness. Other than being linearly proportional to the scattering coefficient, the equivalent mean path length of light transmitted through a turbid material is also proportional to the square of the thickness [28

28. G. Somesfalean, M. Sjöholm, J. Alnis, C. af Klinteberg, S. Andersson-Engels, and S. Svanberg, “Concentration measurement of gas embedded in scattering media by employing absorption and time-resolved laser spectroscopy,” Appl. Opt. 41(18), 3538–3544 (2002). [CrossRef] [PubMed]

]. As for the diffusive porous media, this law is also suitable for the calculation of the effective path length L through gas, in which the path length enhancement N is proportional to the thickness s. In this paper, to eliminate the influence of the thickness and ensure the accuracy and comparability of the experimental results, enhancement N is normalized to N*, which is regarded as the path length enhancement of 10 mm material thickness. Then we have

{N=Lss(Ls2)N*=Ns(irrelevanttos)
(2)

The fitting curves of the normalized path length enhancement based on heat treatment temperature and average pore diameter are shown in Fig. 7(a) and (b), respectively.
Fig. 7 (a) Plot of the normalized path length enhancement corresponding to oxygen absorption measured through alumina hollow sphere xerogel with various heat treatment temperatures, and (b) the resultant fitting multi-exponential curve of enhancement for various average pore diameters.

With the increase in average pore size, the path length enhancement also increases gradually. It is, however, important not to look only at the average pore diameter. One should note that the path length enhancement of a material is determined by its overall physical property, including refractive index, microstructure, crystalline phase, and porosity, etc., holistically. These sophisticated and interrelated parameters influence the photon migration in materials.

3.3 Gas exchange dynamics

In this paper we also pay attention to the gas exchange dynamics by measuring the change in the light signal while samples are flushed by industrial grade nitrogen (99.5%) and reinvaded by air. Here we just show the gas exchange performance of a 14.5mm xerogel that was heat-treated at 1200°C, since the performance of xerogels at other heat treatment temperatures are very similar.

The xerogel sample, sample holder, and nitrogen intake hose are covered by a small flat transparent plastic bag, and the bottom of bag is open for exhausting. There is only about 3 cm between the laser and sample; meanwhile the sample is also very close to the detector over the bag, so the additional light path outside the sample can be neglected. As shown in Fig. 8, each data point was averaged for 2 s. According to Eq. (1), in the case of weak absorption, the amplitude of 2f-WMS signal is proportional to the product of the path length and gas concentration. For the same sample the path length is always the same. Therefore, the amplitude is directly proportional to the O2 concentration in this experiment. Through comparing 2f-WMS amplitude between the sample and 1.5 m air reference, the equivalent mean path length (Leq) though O2 gas can be determined. The initial values indicated that the sample is in ambient air and the detected Leq is about 280 cm at the beginning. After about 40s, the bag was flushed with nitrogen gas N2, and the Leq of the sample is sharply reduced to about 30 cm within several seconds. Approximately 100s after the beginning, nitrogen flushing is aborted and at the same time the bag is removed. Thus air reinvaded the sample, and the Leq of the sample is rapidly restored to the initial value within several seconds. The experiments have shown good repeatability at different times.
Fig. 8 The gas exchange performance of the xerogel sample. Each data point was acquired after 10 consecutive scans (2 s average time). The sample was covered by a plastic bag. Nitrogen flushing started at 40s, and was aborted at 100s. The bag was then removed, and air reinvaded the sample at 100s. The nonzero level of Leq was mainly caused by residual O2 in the bag.

The fitting exponential curve of the acquired data indicates the velocity of gas exchange. At the falling edge the fitting equation is Leq = 28.52 + 336.1 × Exp(-t/5.19), and at the rising edge the fitting equation is Leq = 276-359.5 × Exp(-t/6.84). These results indicate the time constant τ for the exponential decay fitting. Defined as the time it takes for the system to reach e−1 of its initial value, constant τ gives a measure for the gas exchange ability of the material. The absolute value of τ is approximately 6s, which indicates a very fast gas exchange speed of alumina xerogel. Thus, these materials may be adequate for the optical in situ sensing& monitoring system.

4. Discussions and conclusion

While the samples’ properties investigated in the present paper are both optical and physical, the interrelationship between them is more worth studying. According to above experimental results, in samples with varied physical properties, the path length enhancement differs, and this implies that the scattering ability may directly correspond to the structural features, such as porosity, pore size, crystalline phase, and microstructure.

Regarding the hollow sphere xerogel samples with a lower heat treatment temperature, the relatively smaller pore size and lower porosity are obvious characteristics of their structures. In addition, their crystalline phases also have relatively small crystal grain sizes and short crystal grain boundaries, which cause insufficient refraction and macroscopically lower refractive index. Regarding the hollow sphere xerogel samples with a higher heat treatment temperature, however, their structures have relatively bigger pore sizes and higher porosity; at the same time they also have highly refractive crystalline phases, which have bigger crystal grain sizes and longer crystal grain boundaries (which cause sufficient refraction for photon migration).

It must also be particularly noted that, for the materials of the present study, the microstructure feature is the foundation of strong scattering and is more important than any other features. It is essentially the hollow-sphere structure that enables the high pore size, porosity, and stable bulk (instead of some powder of alumina aerogel) to realize the strong scattering. When the temperature is too high (1600 °C and above), the hollow spheres will collapse and the particles will agglomerate, which causes the structures of those overheated samples to be destroyed. This change leads to the decrease of porosity and pore size, and consequently a sharp decrease in equivalent mean path length, even though the alumina is still in the high refractive α phase. The hollow sphere structure results in a longer equivalent mean path length for photons than the normal solid aerogels or ceramics, due to the gas inside and outside sphere. The path length enhancements presented here have performances comparable to the enhancements reported in [8

8. T. Svensson, E. Adolfsson, M. Lewander, C. T. Xu, and S. Svanberg, “Disordered, strongly scattering porous materials as miniature multipass gas cells,” Phys. Rev. Lett. 107(14), 143901 (2011). [CrossRef] [PubMed]

, 11

11. T. Svensson, E. Alerstam, J. Johansson, and S. Andersson-Engels, “Optical porosimetry and investigations of the porosity experienced by light interacting with porous media,” Opt. Lett. 35(11), 1740–1742 (2010). [CrossRef] [PubMed]

]. For alumina, a new record is achieved. Especially when considered the longer path length along with the relative lower overall attenuation of light, xerogel materials may have great potential to be utilized in highly sensitive gas sensing system.

The special microstructure we are dealing with is a complex network of open spheres and pores with different shapes, sizes and crystalline phases, instead of isolated spherical pores of equal size. Many aspects of spectroscopy of confined gas remain to be fully understood, and the complexity of the material structure deserves further attention and efforts [5

5. T. Svensson, K. Vynck, M. Grisi, R. Savo, M. Burresi, and D. S. Wiersma, “Holey random walks: optics of heterogeneous turbid composites,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 87(2), 022120 (2013). [CrossRef] [PubMed]

, 29

29. E. Alerstam and T. Svensson, “Observation of anisotropic diffusion of light in compacted granular porous materials,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 85(4), 040301 (2012). [CrossRef] [PubMed]

31

31. T. Svensson, R. Savo, E. Alerstam, K. Vynck, M. Burresi, and D. S. Wiersma, “Exploiting breakdown of the similarity relation for diffuse light transport: simultaneous retrieval of scattering anisotropy and diffusion constant,” Opt. Lett. 38(4), 437–439 (2013). [CrossRef] [PubMed]

]. In this work, the obtained results demonstrate the strong correlation between the path length enhancement in gas and many features of the material structure, such as high porosity, large pore size, highly refractive crystalline phase, and sophisticated microstructure.

To conclude, in this paper we have utilized a strongly scattering xerogel material which has a high porosity and hollow sphere microstructure to enhance the optical path length in gas significantly. The interaction between photon and material has also been investigated. The excellent characteristics, i.e., long path length, strong scattering, fast gas exchanging, and relatively lower absorption in total transmission, may have promising sensing applications such as feasible miniature gas cells of sensor-based systems. While the present spectroscopy experiment was carried out at about 760 nm wavelength, similar results can be obtained in other spectral ranges so that the xerogels may be used for other important gases, including CO2, NH3, CH4, NO2, and H2O, etc.. Moreover, the extraordinary performance of Al2O3 xerogels may indicate the future exploration of the other kinds of xerogels with high refractive indices, such as ZrO2, Y2O3, and MgO.

Acknowledgments

The authors gratefully acknowledge the enthusiastic support and sample preparation from Li’ang Wu, Zhanglian Hong and other colleagues at Department of Materials Science and Engineering of Zhejiang University, the assistance of Meng Zeng and Runzhi Xing in optical experiments, as well as Liang Mei and Yingran He for helpful discussion. The work was partially supported by the Program of Zhejiang Leading Team (2010R50007) of Science and Technology Innovation (the Science and Technology Department of Zhejiang Province), and the Fundamental Research Funds for the Central Universities.

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O. L. Muskens, J. G. Rivas, R. E. Algra, E. P. Bakkers, and A. Lagendijk, “Design of light scattering in nanowire materials for photovoltaic applications,” Nano Lett. 8(9), 2638–2642 (2008). [CrossRef] [PubMed]

8.

T. Svensson, E. Adolfsson, M. Lewander, C. T. Xu, and S. Svanberg, “Disordered, strongly scattering porous materials as miniature multipass gas cells,” Phys. Rev. Lett. 107(14), 143901 (2011). [CrossRef] [PubMed]

9.

S. Svanberg, “Gas in Scattering Media Absorption Spectroscopy,” Proc. SPIE Vol.7142, pp. 714202 (2008). [CrossRef]

10.

T. Svensson, M. Andersson, L. Rippe, S. Svanberg, S. Andersson-Engels, J. Johansson, and S. Folestad, “VCSEL-based oxygen spectroscopy for structural analysis of pharmaceutical solids,” Appl. Phys. B 90(2), 345–354 (2008). [CrossRef]

11.

T. Svensson, E. Alerstam, J. Johansson, and S. Andersson-Engels, “Optical porosimetry and investigations of the porosity experienced by light interacting with porous media,” Opt. Lett. 35(11), 1740–1742 (2010). [CrossRef] [PubMed]

12.

T. Svensson, E. Adolfsson, M. Burresi, R. Savo, C. T. Xu, D. S. Wiersma, and S. Svanberg, “Pore size assessment based on wall collision broadening of spectral lines of confined gas: experiments on strongly scattering nanoporous ceramics with fine-tuned pore sizes,” Appl. Phys. B 110(2), 147–154 (2013). [CrossRef]

13.

S. Svanberg, “Gas in scattering media absorption spectroscopy – from basic studies to biomedical applications,” Laser Photon. Rev. 7(5), 779–796 (2013). [CrossRef]

14.

J. M. Hartmann, V. Sironneau, C. Boulet, T. Svensson, J. T. Hodges, and C. T. Xu, “Collisional broadening and spectral shapes of absorption lines of free and nanopore-confined O2 gas,” Phys. Rev. A 87(3), 032510 (2013). [CrossRef]

15.

T. F. Baumann, A. E. Gash, S. C. Chinn, A. M. Sawvel, R. S. Maxwell, and J. H. Stacher, “Synthesis of high-surface-area alumina aerogels without the use of alkoxide precursors,” Chem. Mater. 17(2), 395–401 (2005). [CrossRef]

16.

A. E. Aliev, J. Oh, M. E. Kozlov, A. A. Kuznetsov, S. Fang, A. F. Fonseca, R. Ovalle, M. D. Lima, M. H. Haque, Y. N. Gartstein, M. Zhang, A. A. Zakhidov, and R. H. Baughman, “Giant-Stroke, Superelastic Carbon Nanotube Aerogel Muscles,” Science 323(5921), 1575–1578 (2009). [CrossRef] [PubMed]

17.

J. L. Mohanan, I. U. Arachchige, and S. L. Brock, “Porous semiconductor chalcogenide aerogels,” Science 307(5708), 397–400 (2005). [PubMed]

18.

R. T. Olsson, M. A. Azizi Samir, G. Salazar-Alvarez, L. Belova, V. Ström, L. A. Berglund, O. Ikkala, J. Nogués, and U. W. Gedde, “Making flexible magnetic aerogels and stiff magnetic nanopaper using cellulose nanofibrils as templates,” Nat. Nanotechnol. 5(8), 584–588 (2010). [CrossRef] [PubMed]

19.

M. Lewander, Z. G. Guan, K. Svanberg, S. Svanberg, and T. Svensson, “Clinical system for non-invasive in situ monitoring of gases in the human paranasal sinuses,” Opt. Express 17(13), 10849–10863 (2009). [CrossRef] [PubMed]

20.

A. E. Gash, T. M. Tillotson, J. H. Satcher, J. F. Poco, L. W. Hrubesh, and R. L. Simpson, “Use of Epoxides in the Sol- Gel Synthesis of Porous Iron (III) Oxide Monoliths from Fe (III) Salts,” Chem. Mater. 13(3), 999–1007 (2001). [CrossRef]

21.

A. E. Gash, T. M. Tillotson, J. H. Satcher Jr, L. W. Hrubesh, and R. L. Simpson, “New sol–gel synthetic route to transition and main-group metal oxide aerogels using inorganic salt precursors,” J. Non-Cryst. Solids 285(1–3), 22–28 (2001). [CrossRef]

22.

P. Werle, “A review of recent advances in semiconductor laser based gas monitors,” Spectrochim. Acta A Mol. Biomol. Spectrosc. 54(2), 197–236 (1998). [CrossRef]

23.

S. Schilt, L. Thévenaz, and P. Robert, “Wavelength modulation spectroscopy: combined frequency and intensity laser modulation,” Appl. Opt. 42(33), 6728–6738 (2003). [CrossRef] [PubMed]

24.

T. Fernholz, H. Teichert, and V. Ebert, “Digital, phase-sensitive detection for in situ diode-laser spectroscopy under rapidly changing transmission conditions,” Appl. Phys. B 75(2–3), 229–236 (2002). [CrossRef]

25.

L. S. Rothman, I. E. Gordon, A. Barbe, D. C. Benner, P. E. Bernath, M. Birk, V. Boudon, L. R. Brown, A. Campargue, J. P. Champion, K. Chance, L. H. Coudert, V. Dana, V. M. Devi, S. Fally, J. M. Flaud, R. R. Gamache, A. Goldman, D. Jacquemart, I. Kleiner, N. Lacome, W. J. Lafferty, J. Y. Mandin, S. T. Massie, S. N. Mikhailenko, C. E. Miller, N. Moazzen-Ahmadi, O. V. Naumenko, A. V. Nikitin, J. Orphal, V. I. Perevalov, A. Perrin, A. Predoi-Cross, C. P. Rinsland, M. Rotger, M. Simeckova, M. A. H. Smith, K. Sung, S. A. Tashkun, J. Tennyson, R. A. Toth, A. C. Vandaele, and J. Vander Auwera, “The HITRAN molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 110(9-10), 533–572 (2009). [CrossRef]

26.

S. D. Mo and W. Y. Ching, “Electronic and optical properties of θ-Al2 O3 and comparison to α-Al2 O3,” Phys. Rev. B 57(24), 15219–15228 (1998). [CrossRef]

27.

R. C. Weast, Handbook of Chemistry and Physics 82nd, (Chemical Rubber, 2001), Chapter 10.

28.

G. Somesfalean, M. Sjöholm, J. Alnis, C. af Klinteberg, S. Andersson-Engels, and S. Svanberg, “Concentration measurement of gas embedded in scattering media by employing absorption and time-resolved laser spectroscopy,” Appl. Opt. 41(18), 3538–3544 (2002). [CrossRef] [PubMed]

29.

E. Alerstam and T. Svensson, “Observation of anisotropic diffusion of light in compacted granular porous materials,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 85(4), 040301 (2012). [CrossRef] [PubMed]

30.

C. T. Xu, M. Lewander, S. Andersson-Engels, E. Adolfsson, T. Svensson, and S. Svanberg, “Wall-collision line broadening of molecular oxygen within nanoporous materials,” Phys. Rev. A 84(4), 042705 (2011). [CrossRef]

31.

T. Svensson, R. Savo, E. Alerstam, K. Vynck, M. Burresi, and D. S. Wiersma, “Exploiting breakdown of the similarity relation for diffuse light transport: simultaneous retrieval of scattering anisotropy and diffusion constant,” Opt. Lett. 38(4), 437–439 (2013). [CrossRef] [PubMed]

OCIS Codes
(160.1890) Materials : Detector materials
(290.4210) Scattering : Multiple scattering
(290.7050) Scattering : Turbid media
(300.1030) Spectroscopy : Absorption
(300.6260) Spectroscopy : Spectroscopy, diode lasers
(300.6320) Spectroscopy : Spectroscopy, high-resolution

ToC Category:
Spectroscopy

History
Original Manuscript: December 11, 2013
Revised Manuscript: January 18, 2014
Manuscript Accepted: January 22, 2014
Published: January 29, 2014

Citation
Lin Yang, Gabriel Somesfalean, and Sailing He, "Laser absorption spectroscopy of oxygen confined in highly porous hollow sphere xerogel," Opt. Express 22, 2584-2594 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-3-2584


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References

  1. H. Cao, Y. G. Zhao, S. T. Ho, E. W. Seelig, Q. H. Wang, R. P. H. Chang, “Random laser action in semiconductor powder,” Phys. Rev. Lett. 82(11), 2278–2281 (1999). [CrossRef]
  2. D. S. Wiersma, “The physics and applications of random lasers,” Nat. Phys. 4(5), 359–367 (2008). [CrossRef]
  3. D. S. Wiersma, P. Bartolini, A. Lagendijk, R. Righini, “Localization of light in a disordered medium,” Nature 390(6661), 671–673 (1997). [CrossRef]
  4. M. Störzer, P. Gross, C. M. Aegerter, G. Maret, “Observation of the critical regime near Anderson localization of light,” Phys. Rev. Lett. 96(6), 063904 (2006). [CrossRef] [PubMed]
  5. T. Svensson, K. Vynck, M. Grisi, R. Savo, M. Burresi, D. S. Wiersma, “Holey random walks: optics of heterogeneous turbid composites,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 87(2), 022120 (2013). [CrossRef] [PubMed]
  6. I. Vellekoop, A. Lagendijk, A. P. Mosk, “Exploiting disorder for perfect focusing,” Nat. Photonics 4(5), 320–322 (2010). [CrossRef]
  7. O. L. Muskens, J. G. Rivas, R. E. Algra, E. P. Bakkers, A. Lagendijk, “Design of light scattering in nanowire materials for photovoltaic applications,” Nano Lett. 8(9), 2638–2642 (2008). [CrossRef] [PubMed]
  8. T. Svensson, E. Adolfsson, M. Lewander, C. T. Xu, S. Svanberg, “Disordered, strongly scattering porous materials as miniature multipass gas cells,” Phys. Rev. Lett. 107(14), 143901 (2011). [CrossRef] [PubMed]
  9. S. Svanberg, “Gas in Scattering Media Absorption Spectroscopy,” Proc. SPIE Vol.7142, pp. 714202 (2008). [CrossRef]
  10. T. Svensson, M. Andersson, L. Rippe, S. Svanberg, S. Andersson-Engels, J. Johansson, S. Folestad, “VCSEL-based oxygen spectroscopy for structural analysis of pharmaceutical solids,” Appl. Phys. B 90(2), 345–354 (2008). [CrossRef]
  11. T. Svensson, E. Alerstam, J. Johansson, S. Andersson-Engels, “Optical porosimetry and investigations of the porosity experienced by light interacting with porous media,” Opt. Lett. 35(11), 1740–1742 (2010). [CrossRef] [PubMed]
  12. T. Svensson, E. Adolfsson, M. Burresi, R. Savo, C. T. Xu, D. S. Wiersma, S. Svanberg, “Pore size assessment based on wall collision broadening of spectral lines of confined gas: experiments on strongly scattering nanoporous ceramics with fine-tuned pore sizes,” Appl. Phys. B 110(2), 147–154 (2013). [CrossRef]
  13. S. Svanberg, “Gas in scattering media absorption spectroscopy – from basic studies to biomedical applications,” Laser Photon. Rev. 7(5), 779–796 (2013). [CrossRef]
  14. J. M. Hartmann, V. Sironneau, C. Boulet, T. Svensson, J. T. Hodges, C. T. Xu, “Collisional broadening and spectral shapes of absorption lines of free and nanopore-confined O2 gas,” Phys. Rev. A 87(3), 032510 (2013). [CrossRef]
  15. T. F. Baumann, A. E. Gash, S. C. Chinn, A. M. Sawvel, R. S. Maxwell, J. H. Stacher, “Synthesis of high-surface-area alumina aerogels without the use of alkoxide precursors,” Chem. Mater. 17(2), 395–401 (2005). [CrossRef]
  16. A. E. Aliev, J. Oh, M. E. Kozlov, A. A. Kuznetsov, S. Fang, A. F. Fonseca, R. Ovalle, M. D. Lima, M. H. Haque, Y. N. Gartstein, M. Zhang, A. A. Zakhidov, R. H. Baughman, “Giant-Stroke, Superelastic Carbon Nanotube Aerogel Muscles,” Science 323(5921), 1575–1578 (2009). [CrossRef] [PubMed]
  17. J. L. Mohanan, I. U. Arachchige, S. L. Brock, “Porous semiconductor chalcogenide aerogels,” Science 307(5708), 397–400 (2005). [PubMed]
  18. R. T. Olsson, M. A. Azizi Samir, G. Salazar-Alvarez, L. Belova, V. Ström, L. A. Berglund, O. Ikkala, J. Nogués, U. W. Gedde, “Making flexible magnetic aerogels and stiff magnetic nanopaper using cellulose nanofibrils as templates,” Nat. Nanotechnol. 5(8), 584–588 (2010). [CrossRef] [PubMed]
  19. M. Lewander, Z. G. Guan, K. Svanberg, S. Svanberg, T. Svensson, “Clinical system for non-invasive in situ monitoring of gases in the human paranasal sinuses,” Opt. Express 17(13), 10849–10863 (2009). [CrossRef] [PubMed]
  20. A. E. Gash, T. M. Tillotson, J. H. Satcher, J. F. Poco, L. W. Hrubesh, R. L. Simpson, “Use of Epoxides in the Sol- Gel Synthesis of Porous Iron (III) Oxide Monoliths from Fe (III) Salts,” Chem. Mater. 13(3), 999–1007 (2001). [CrossRef]
  21. A. E. Gash, T. M. Tillotson, J. H. Satcher, L. W. Hrubesh, R. L. Simpson, “New sol–gel synthetic route to transition and main-group metal oxide aerogels using inorganic salt precursors,” J. Non-Cryst. Solids 285(1–3), 22–28 (2001). [CrossRef]
  22. P. Werle, “A review of recent advances in semiconductor laser based gas monitors,” Spectrochim. Acta A Mol. Biomol. Spectrosc. 54(2), 197–236 (1998). [CrossRef]
  23. S. Schilt, L. Thévenaz, P. Robert, “Wavelength modulation spectroscopy: combined frequency and intensity laser modulation,” Appl. Opt. 42(33), 6728–6738 (2003). [CrossRef] [PubMed]
  24. T. Fernholz, H. Teichert, V. Ebert, “Digital, phase-sensitive detection for in situ diode-laser spectroscopy under rapidly changing transmission conditions,” Appl. Phys. B 75(2–3), 229–236 (2002). [CrossRef]
  25. L. S. Rothman, I. E. Gordon, A. Barbe, D. C. Benner, P. E. Bernath, M. Birk, V. Boudon, L. R. Brown, A. Campargue, J. P. Champion, K. Chance, L. H. Coudert, V. Dana, V. M. Devi, S. Fally, J. M. Flaud, R. R. Gamache, A. Goldman, D. Jacquemart, I. Kleiner, N. Lacome, W. J. Lafferty, J. Y. Mandin, S. T. Massie, S. N. Mikhailenko, C. E. Miller, N. Moazzen-Ahmadi, O. V. Naumenko, A. V. Nikitin, J. Orphal, V. I. Perevalov, A. Perrin, A. Predoi-Cross, C. P. Rinsland, M. Rotger, M. Simeckova, M. A. H. Smith, K. Sung, S. A. Tashkun, J. Tennyson, R. A. Toth, A. C. Vandaele, J. Vander Auwera, “The HITRAN molecular spectroscopic database,” J. Quant. Spectrosc. Radiat. Transf. 110(9-10), 533–572 (2009). [CrossRef]
  26. S. D. Mo, W. Y. Ching, “Electronic and optical properties of θ-Al2 O3 and comparison to α-Al2 O3,” Phys. Rev. B 57(24), 15219–15228 (1998). [CrossRef]
  27. R. C. Weast, Handbook of Chemistry and Physics 82nd, (Chemical Rubber, 2001), Chapter 10.
  28. G. Somesfalean, M. Sjöholm, J. Alnis, C. af Klinteberg, S. Andersson-Engels, S. Svanberg, “Concentration measurement of gas embedded in scattering media by employing absorption and time-resolved laser spectroscopy,” Appl. Opt. 41(18), 3538–3544 (2002). [CrossRef] [PubMed]
  29. E. Alerstam, T. Svensson, “Observation of anisotropic diffusion of light in compacted granular porous materials,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 85(4), 040301 (2012). [CrossRef] [PubMed]
  30. C. T. Xu, M. Lewander, S. Andersson-Engels, E. Adolfsson, T. Svensson, S. Svanberg, “Wall-collision line broadening of molecular oxygen within nanoporous materials,” Phys. Rev. A 84(4), 042705 (2011). [CrossRef]
  31. T. Svensson, R. Savo, E. Alerstam, K. Vynck, M. Burresi, D. S. Wiersma, “Exploiting breakdown of the similarity relation for diffuse light transport: simultaneous retrieval of scattering anisotropy and diffusion constant,” Opt. Lett. 38(4), 437–439 (2013). [CrossRef] [PubMed]

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