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
Due to their extraordinary properties, research about strongly scattering media has become a rapidly expanding field in the last decade. This kind of media are not only involved in fundamental physical systems related to, e.g., random lasing [1,2], Anderson localization of light [3,4], and optics in heterogeneous turbid composites , but also for various practical applications such as trapping photons in solar cells , improving the sharpness of the focus , and enhancing the path length for miniature multipass gas cells . Recently, spectroscopy of embedded gas in strongly scattering media has received an enormous amount of attention. The technique - often referred to as gas in scattering media absorption spectroscopy (GASMAS)  - investigates sharp spectral features of an absorbing gas, which are typically several orders of magnitude narrower than the features of surrounding host material. This approach has already been shown to be a well-established tool for stable, selective and accurate measurements of gas concentration, and has also been successfully used for pharmaceutical analysis , porous material assessment [11,12], biomedical optics , etc. Meanwhile, the fundamental physical theory about GASMAS also becomes an interesting topic [5,14].
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 , 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 [16–18]. 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 . 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 [20,21]. It is a kind of bulk material with microstructure as urchin-like, as shown in Fig. 1(a).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 . 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 . In TDLAS, this technique is performed by scanning a sinusoidal frequency-modulated diode laser over some isolated narrow absorption lines of gas molecules . 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 :
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.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 ). 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.
Structural and scattering properties of the Al2O3 hollow-sphere xerogels with different heat treatment temperatures are presented in Table 1.
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.
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.
The pore structure has been investigated with mercury intrusion porosimetry. As summarized in Table 1, the pore sizes of samples with varying heat treatment temperatures are quite different, but all of the xerogel samples have considerably high porosity (all above 75%). The average pore size increases with the increase of the heat treatment temperature (from 600 to 1400°C), while the porosity is still around 85%. Except for the as-dried and 1600°C samples, which behave differently compared to others. Due to the residual moisture and organic ingredient, the as-dried sample has a larger pore size and smaller porosity compared with the adjacent sample after 600°C heat-treatment. Vice versa, an excessive temperature such as 1600°C, makes the spherical microstructure broken and further makes the broken spheres sintered together, which ultimately reduces the pore size and porosity. According to previous research , the pore size and porosity of samples have a strong effect on their optical scattering ability.
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, 27].
3.2 Optical path length enhancement
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 . 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
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.
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.
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, 11]. 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, 29–31]. 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.
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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