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Nonlinear interaction of silica photonic crystals with ammonia vapor
Journal Pre-proofs Nonlinear Interaction of Silica Photonic Crystals with Ammonia Vapor A.S. Kuchyanov, P.A. Chubakov, V.P. Chubakov, S.L. Mikerin PII: DOI: Reference:
S2211-3797(19)31288-4 https://doi.org/10.1016/j.rinp.2019.102726 RINP 102726
To appear in:
Results in Physics
Received Date: Revised Date: Accepted Date:
19 April 2019 1 October 2019 1 October 2019
Please cite this article as: Kuchyanov, A.S., Chubakov, P.A., Chubakov, V.P., Mikerin, S.L., Nonlinear Interaction of Silica Photonic Crystals with Ammonia Vapor, Results in Physics (2019), doi: https://doi.org/10.1016/j.rinp. 2019.102726
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Nonlinear Interaction of Silica Photonic Crystals with Ammonia Vapor A. S. Kuchyanov*, P. A. Chubakov, V.P. Chubakov, and S.L. Mikerin Institute of Automation and Electrometry SB RAS, 1, Acad. Koptyug Ave., Novosibirsk 630090, Russian Federation *Corresponding author: E-mail address: [email protected] (A. S. Kuchyanov)
ABSTRACT Two mechanisms of gas adsorption in silica photonic crystals are shown. It is found that ammonia adsorption, in contrast to water, is nonlinear depending on its concentration. At low concentration, ammonia molecules form hydrogen bonds with silanol groups of the silica surface. At a high concentration, a contribution from dissolution of ammonia in previously adsorbed water becomes noticeable. Nonlinear dependence allows to create an optical ammonia sensor with large dynamic range (from 1 to 105 ppm) which at the same time has high sensitivity at low concentrations. A novel approach is proposed for accurate measurements of gas adsorption in photonic crystal materials. Keywords: photonic crystal; adsorption; water vapor; ammonia; Fabry-Perot interferometer; thin film devices
1. Introduction Photonic crystals (PhCs) are perspective optical materials for many industrial and scientific applications. Artificial opals are widely employed as three-dimensional photonic crystals. They can be fabricated through self assembling techniques and possess PhC properties. Adsorption of gases can affect their optical properties due to porous internal structure [1-4]. Nowadays interaction of silica PhCs with water is well-described [1, 5-6]. However, it has been found that the biggest changes of the optical properties of opal-like PhCs are caused by interaction with two gases: water and ammonia vapors. This phenomenon requires a more detailed study, since the molecules of water and ammonia have high polarity and have similar features. When a silica film is exposed to ammonia vapor, two mechanisms of interaction are possible: the first is the physical adsorption of ammonia on the silica surface with formation of hydrogen bonds and the second is the dissolution of ammonia in the previously adsorbed water. It is known that ammonia is highly soluble in water. At room temperature, approximately 800 volumes of ammonia can be dissolved in one volume of water [7]. It exceeds solubility of other gases by an order of magnitude. On the other hand, the solubility of ammonia is explained by a high polarity of its molecules, and thus it can be adsorbed on the silica surface through the formation of hydrogen bonds. Different methods have been used in this work to study it. As a result, sensitivity range of the previously proposed optical ammonia sensor is extended to higher concentrations [2]. It is shown that concentrations of ammonia up to 105 ppm do not cause sensor poisoning or degradation. Taking into account that ammonia is a toxic gas, which is widely used in industry, the development of such sensor is of a great interest [8-11]. 2. Photonic band gap method PhC opal films were prepared by a vertical deposition method [12]. Silica spheres were fabricated by Stober-Fink-Bohn synthesis with an average diameter 260 nm and a relative standard deviation less than 5% [13]. The films had an area of about 1 cm2, and a thickness of about 2.5 μm. It has been found that a just few gases have evident adsorption on the surface of porous silica PhC film [2]. Polar molecules of water can be adsorbed in the film through the hydrogen bonds formation, since silica spheres have large number of –OH groups on the surface. The amount of adsorbed molecules depends on the temperature (T) and relative humidity (RH). Water adsorbs in the three regions: inside cracks of the spheres (~ 1 nm), between the nearest spheres (~ 40 nm), forming necks, and in uniform wetting film around the spheres [1]. Inset of figure 1 shows internal structure of the sample film and cracks of the spheres. Graph on figure 1 shows results of thermogravimetric study of the silica film. At T = 25°C and RH = 50% about 4% of mass is lost when it is heated to T = 100 °C.
Fig.1. Thermogravimetric study of as-grown silica PhC film. Inset shows morphology of the investigated film.
Adsorption of gases can be measured by shift of photonic band gap (PBG) wavelength. Peak of the PBG of opal-like structure can be estimated by using a modified form of Bragg’s law:
1
B 2d (111) neff2 sin 2
(1)
where λB is the wavelength of the maximum reflected intensity, d(111)=0.86dspheres is inter-planar spacing between (111) planes of face2 2 2 centered cubic lattice, dspheres is the diameter of spheres, neff is the effective refractive index, f = 0.74 is the filling f nSiO 2 (1 f ) n Air fraction of the silica spheres, nSiO2 = 1.46, nAir = 1.0 are the refractive indexes of silica and medium (written for air) filling the void between spheres, ψ is the incident angle of light. In order to separate two mechanisms of ammonia adsorption measurements of the PBG position at various water and ammonia concentrations have been performed. Silica PhC film was placed in waterproof chamber. At room temperature T = 25°C and RH = 50% the PBG peak is at λ = 578 nm. The chamber with the sample was heated at T = 120°C and blown with dry nitrogen for complete dehydration. The PBG peak of the dry sample at room temperature T = 25°C is at λ = 570 nm (fig. 2).
Fig. 2. Reflection spectra of the silica PhC film: without water and ammonia vapors (RH = 0%, σ(NH3) = 0%), with RH = 0% and σ(NH3) = 10%, with RH = 80% and σ(NH3) = 10%.
According to eq. (1) shift of λB by 8 nm corresponds to the water filling fraction f = 0.07. It is in agreement with 4% of water mass, measured by the thermogravimetric method (fig. 1). Adding dry ammonia vapor with a concentration of σ(NH3) = 10% into the chamber shifts the PBG peak to λ = 579 nm. Subsequent nitrogen blowing returns the PBG peak to λ = 570 nm without heating (see fig. 2). It indicates absence of water inside the chamber and the silica PhC film. Removal of ammonia does not require heating of the sample above the room temperature, since it boils at T = – 33.34°C. Thus dry silica film interacts with ammonia. In absence of water, formation of hydrogen bonds is the main mechanism. Thermal annealing at temperatures above T = 150°C leads to condensation of –OH groups and to loss sensitivity to ammonia vapor in the same way as to water vapor [5]. Increase of humidity to 80% in the chamber with 10% of ammonia vapor additionally produces 13 nm shifts the PBG peak to λ = 583 nm. However such measurements are difficult to perform. It happens since bandwidth of PBG peak of the film is about 50 nm and exceeds the shift caused by gas adsorption. It determines low accuracy of such measurements. Therefore, a new optical scheme that increases accuracy has been developed. 3. Interference method A silica single crystal film 1 is formed on the surface of a glass prism 2 (Fig. 3).
Fig. 3 (a) Schematic of the experimental setup: (1) silica PhC film, (2) glass prism (3) mirror with reflectance r = 0.8, (4) lenses, (5) semiconductive laser, (α) incident angle of the laser beam, (6) image of interference fringes, (7) airproof chamber.
Use of single crystal film ensures parallel planes of its faces. As a result, the film works as a Fabry-Perot interferometer, which is illuminated by a laser 5. The laser wavelength is 650 nm and does not coincide with the wavelength of the PBG. Lens 4 focuses the laser beam 5. The glass (nglass = 1.52) prism 2 is used to introduce the laser beam into the film at angle ψ ~ 46.8°. On the boundary of the film (neff = 1.356) with air (nAir = 1) the total internal reflection occurs. A mirror 3 with R = 0.8 is deposited on the large face of the prism 2 and increases reflectivity of the boundary of the film with glass. In this case the intensity of light transmitted through the mirror to the sensor film (Transmission=20%) is sufficient for forming and recording the interference pattern The high reflectivity on the both sides of the film increases the sharpness of the interference pattern., which could be described through finesse of interferometer. Obtained finesse of interferometer is equal 30. The described optical scheme allows to place the light source 5 and the detector 6 on one side of the sensor in the airproof chamber 7 (Fig. 3). It eliminates influence of the analyte on the registration system.
2
The interference pattern is observed in the reflected light and represents a series of the dark fringes (fig. 3). Interference of light inside the film is described by the Airy’s formula [14]: 2nD cos 4 r1 r2 sin 2 , (2) R 2 2 nD cos 1 r1 r2 4 r1 r2 sin 2 where r1, r2 are the reflective coefficients of the boundaries; n is the refractive index of the film; D is the film thickness; ψ is the angle of incidence on the film. Light refraction at the boundaries of air with glass, glass with PhC and PhC with air should be also taken into account.
4. Results and discussion Figure 4 shows angular distribution of the reflected interference fringes. X-axis corresponds to the incidence angle α of light on the glass prism face (fig. 3).
Fig. 4. Angular distribution of the interference fringes reflected from the silica PhC film with neff = 1.356 (solid line) and neff = 1.386 (dash line).
If α > –5.8° the total internal reflection occurs on the boundary of the PhC film with air, the reflection coefficient is close to one. If α > 28.2° the total internal reflection occurs on the boundary of the glass prism with the PhC film, light do not enters into the film and is reflected back into the prism. Solid line corresponds to blank PhC film (neff = 1.356). Dash line corresponds to PhC film with adsorbed gas, assuming that adsorption changes only the effective refractive index to neff = 1.386. The change of Δneff = 0.03 corresponds to ΔλB = 13 nm shift of the PBG peak according to eq. (1). Choosing different interference fringes different angular shifts could be obtained. The angular shift is larger at smaller α values: at α ~ -50° the shift is about 7°, at α ~ 25° the shift is about 0.1°. Measurements have been performed at light incidence close to normal α ~ 0°. In the experiment gas mixture of ammonia with air was pumped through the chamber. A vessel with NaOH was installed at the chamber entrance. It was used as a desiccant and excluded the influence of humidity changes. Commercial sensors monitored water and ammonia concentrations at the output. In the range of 0-300 ppm, the ammonia concentration was controlled by TGS 2444 sensor. In the range of 300-105 ppm, gas cylinders with a known concentration of ammonia were used. Figure 5a shows the angular shift of the interference fringe as a function of ammonia concentration.
3
Fig. 5. Response of PhC sensor film to (a) ammonia and (b) water vapors. The inset shows the response to ammonia vapor at low concentrations.
The inset shows an enlarged region at low concentration. There are 2 regions corresponding to different ammonia concentration which is similar to the measurements of the PBG peak position. For dry silica film only initial part of the graph can be detected. We assume that the initial region is mainly responsible to formation of hydrogen bonds with the silica surface. The following part of graph with a weaker dependence corresponds to the dissolution of ammonia in water. In contrast, the angular shift of the interference fringe as a function of relative humidity is linear (Fig. 5b). Owing to the existence of two mechanisms of silica film interaction with ammonia vapor it is possible to enlarge dynamic range of the optical sensor which was proposed earlier [2]. The developed sensor provides a registration of the ammonia in the concentration range from few ppm to 105 ppm. To check stability, silica film was soaked in a 10% solution of ammonia in water for 1 month. No changes of the samples or its optical properties were registered. It has been also verified that the sensor does not respond to N2, O2, NO, NO2 and CH4, but sense ethanol. Sensitivity to ethanol is weaker by an order of magnitude than to ammonia and water, which is consistent with the weak polarity of its molecules and confirms suggested mechanism of interaction. Figure 6 shows measured time response of the sensor to ammonia vapor.
Fig. 6. Temporal response to 105 ppm of ammonia. Red line shows the rise of the signal, blue line shows the recovery to zero level.
Dry air has been pumped through the sensor. Instantly a portion of gaseous ammonia with a concentration of 105 ppm has been added (red line). The rise of the signal takes 1 second. The recovery time is 5 seconds. In fact, the recovery of the sensor can be faster, since we do not take into account the adsorption of ammonia in the input tubes and on the walls of the sensor. Response time of the sensor to water is 10 milliseconds. 5. Conclusions Study of adsorption of different gases in the highly porous structure of the silica PhC film is a complex task. At normal conditions, pores contain large amount of adsorbed water and blowing of different gases can changes properties of sample by changing concentration of water without adsorption of other gases. Therefore ammonia is an interesting object for such study since it gives response similar to water. It was verified, for the first time, that dry silica PhC film interacts with ammonia without presence of free water. Further investigation revealed existence of two mechanisms depending on ammonia concentration. At low concentrations
4
interaction of ammonia with the silica PhC film is similar to interaction with water. At high concentrations, interaction is observed only in the presence of water, and dependence is weaker. We suppose that, in the absence of water, ammonia is adsorbed on the surface through formation of hydrogen bonds with silanol groups of silica, in the presence of water an additional mechanism appears: the dissolution of ammonia in water. A new method for gas adsorption measurements has been proposed. It detects the angular shift of the fringes formed by the interference of light inside the film. In comparison with the PBG method it allows to increase ratio of band shift to its width. Nonlinear shift of the interference fringe as a function of ammonia concentration broadens dynamic range of the previously proposed optical sensor. The sensor has high sensitivity at low concentration (1 ppm) and allows to measure up 105 ppm ammonia admixture in gas. The measured response time of the sensor to ammonia vapor is about 1 second. The recovery time to zero level is about 5 seconds. Acknowledgments We would like to thank Dr. A. I. Plekhanov for helpful discussions. This work was supported by the Ministry of Education and Science of the Russian Federation [grant number AAAA A18118052390021-2].
References [1] F. Gallego-Gómez, V. Morales-Flórezc, M. Morales, A. Blanco, C. López, Colloidal crystals and water: Perspectives on liquid–solid nanoscale phenomena in wet particulate media, Adv. Colloid Interface Sci. 234 (2016) 142-160. https://doi.org/10.1016/j.cis.2016.05.004 [2] A.S. Kuchyanov, P.A. Chubakov, A.I. Plekhanov, Highly sensitive and fast response gas sensor based on a light reflection at the glass-photonic crystal interface, Opt. Commun. 351 (2015), 109-114. https://doi.org/10.1016/j.optcom.2015.04.045 [3] H. Yang, P. Jiang, B. Jiang, Vapor detection enabled by self-assembled colloidal photonic crystals, J. Colloid Interface Sci. 370 (2012) 11–18. https://doi.org/10.1016/j.jcis.2011.12.058 [4] S. N. Atutov, A. I. Plekhanov, Photo extraction of rubidium atoms from the bulk of a photonic crystal, Autom. Monit. Meas. 54 (2018) 405–410. https://doi.org/10.3103/S8756699018040131 [5] F. Gallego-Gómez, A. Blanco, V. Canalejas-Tejero, Cefe López, Water-dependent photonic bandgap in silica artificial opals, Small 7 (2011), 1838-1845. https://doi.org/10.1002/smll.201100184 [6] V.P. Chubakov, P.A. Chubakov, A.I. Plekhanov, Humidity sensor based on photonic crystal opal film, Nanotechnol. Russ. 7 (2012) 499–501. https://doi.org/10.1134/S1995078012050023 [7] International Institute of Ammonia Refrigeration, Ammonia data book, IIAR, 1993. [8] W. Thain, Monitoring toxic gases in the atmosphere for hygiene and pollution control. Pergamon Press 1980. [9] B. Timmer, W. Olthuis, A. van den Berg, Ammonia sensors and their applications—a review, Sens. Actuators B 107 (2005) 666–677. https://doi.org/10.1016/j.snb.2004.11.054 [10] P. Jiang, J. F. Bertone, K. S. Hwang, V. L. Colvin, Single-crystal colloidal multilayers of controlled thickness, Chem. Mater. 11 (1999) 2132-2140. https://doi.org/10.1021/cm990080+ [11] T. A. Ukleev, D. I. Yurasova, N. N. Shevchenko, and A. V. Sel’kin, Optical anisotropy of cubic photonic crystals under conditions of multiple-mode light propagation, J. Phys.: Conf. Ser. 769 (2016) 012051. https://doi.org/10.1134/S1063783418050335 [12] R.V. Nair , R. Vijaya, Photonic crystal sensors: An overview, Prog. Quantum Electron. 34 (2010) 89-134. https://doi.org/10.1016/j.pquantelec.2010.01.001 [13] W. Stober, A. Fink, E. Bohn, Controlled growth of monodisperse silica spheres in the micron size range, J Colloid Interface Sci. 26 (1968) 62-69. https://doi.org/10.1016/0021-9797(68)90272-5 [14] M. Born, E. Wolf, Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light, Elsevier Science Limited, 1980.
Highlights
Silica film adsorbs ammonia through hydrogen bonds formation and dissolution in water.
Applying photonic crystals as interferometers increase sensitivity of optical sensors.
Silica photonic crystal film allows ammonia detection from 1 to 105 ppm.
Alexander Kuchyanov: Conceptualization, Investigation, Writing – Original Draft, Writing – Review & Editing Pavel Chubakov: Validation , Software Vyacheslav Chubakov: Investigation, Methodology, Writing – Original Draft, Writing – Review & Editing Sergey Mikerin: Project Administration, Writing – Original Draft Fig.1. Thermogravimetric study of as-grown silica PhC film. Inset shows morphology of the investigated film.
Fig. 2. Reflection spectra of the silica PhC film: without water and ammonia vapors (RH = 0%, σ(NH3) = 0%), with RH = 0% and σ(NH3) = 10%, with RH = 80% and σ(NH3) = 10%.
5
Fig. 3 (a) Schematic of the experimental setup: (1) silica PhC film, (2) glass prism (3) mirror with reflectance r = 0.8, (4) lenses, (5) semiconductive laser, (α) incident angle of the laser beam, (6) image of interference fringes, (7) airproof chamber.
Fig. 4. Angular distribution of the interference fringes reflected from the silica PhC film with neff = 1.356 (solid line) and neff = 1.386 (dash line).
Fig. 5. Response of PhC sensor film to (a) ammonia and (b) water vapors. The inset shows the response to ammonia vapor at low concentrations.
Fig. 6. Temporal response to 105 ppm of ammonia. Red line shows the rise of the signal, blue line shows the recovery to zero level.
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Fig. 1.
Fig. 2.
Fig. 3.
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Fig. 4.
Fig. 5.
Fig. 6.
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S2211-3797(19)31288-4 https://doi.org/10.1016/j.rinp.2019.102726 RINP 102726
To appear in:
Results in Physics
Received Date: Revised Date: Accepted Date:
19 April 2019 1 October 2019 1 October 2019
Please cite this article as: Kuchyanov, A.S., Chubakov, P.A., Chubakov, V.P., Mikerin, S.L., Nonlinear Interaction of Silica Photonic Crystals with Ammonia Vapor, Results in Physics (2019), doi: https://doi.org/10.1016/j.rinp. 2019.102726
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Nonlinear Interaction of Silica Photonic Crystals with Ammonia Vapor A. S. Kuchyanov*, P. A. Chubakov, V.P. Chubakov, and S.L. Mikerin Institute of Automation and Electrometry SB RAS, 1, Acad. Koptyug Ave., Novosibirsk 630090, Russian Federation *Corresponding author: E-mail address: [email protected] (A. S. Kuchyanov)
ABSTRACT Two mechanisms of gas adsorption in silica photonic crystals are shown. It is found that ammonia adsorption, in contrast to water, is nonlinear depending on its concentration. At low concentration, ammonia molecules form hydrogen bonds with silanol groups of the silica surface. At a high concentration, a contribution from dissolution of ammonia in previously adsorbed water becomes noticeable. Nonlinear dependence allows to create an optical ammonia sensor with large dynamic range (from 1 to 105 ppm) which at the same time has high sensitivity at low concentrations. A novel approach is proposed for accurate measurements of gas adsorption in photonic crystal materials. Keywords: photonic crystal; adsorption; water vapor; ammonia; Fabry-Perot interferometer; thin film devices
1. Introduction Photonic crystals (PhCs) are perspective optical materials for many industrial and scientific applications. Artificial opals are widely employed as three-dimensional photonic crystals. They can be fabricated through self assembling techniques and possess PhC properties. Adsorption of gases can affect their optical properties due to porous internal structure [1-4]. Nowadays interaction of silica PhCs with water is well-described [1, 5-6]. However, it has been found that the biggest changes of the optical properties of opal-like PhCs are caused by interaction with two gases: water and ammonia vapors. This phenomenon requires a more detailed study, since the molecules of water and ammonia have high polarity and have similar features. When a silica film is exposed to ammonia vapor, two mechanisms of interaction are possible: the first is the physical adsorption of ammonia on the silica surface with formation of hydrogen bonds and the second is the dissolution of ammonia in the previously adsorbed water. It is known that ammonia is highly soluble in water. At room temperature, approximately 800 volumes of ammonia can be dissolved in one volume of water [7]. It exceeds solubility of other gases by an order of magnitude. On the other hand, the solubility of ammonia is explained by a high polarity of its molecules, and thus it can be adsorbed on the silica surface through the formation of hydrogen bonds. Different methods have been used in this work to study it. As a result, sensitivity range of the previously proposed optical ammonia sensor is extended to higher concentrations [2]. It is shown that concentrations of ammonia up to 105 ppm do not cause sensor poisoning or degradation. Taking into account that ammonia is a toxic gas, which is widely used in industry, the development of such sensor is of a great interest [8-11]. 2. Photonic band gap method PhC opal films were prepared by a vertical deposition method [12]. Silica spheres were fabricated by Stober-Fink-Bohn synthesis with an average diameter 260 nm and a relative standard deviation less than 5% [13]. The films had an area of about 1 cm2, and a thickness of about 2.5 μm. It has been found that a just few gases have evident adsorption on the surface of porous silica PhC film [2]. Polar molecules of water can be adsorbed in the film through the hydrogen bonds formation, since silica spheres have large number of –OH groups on the surface. The amount of adsorbed molecules depends on the temperature (T) and relative humidity (RH). Water adsorbs in the three regions: inside cracks of the spheres (~ 1 nm), between the nearest spheres (~ 40 nm), forming necks, and in uniform wetting film around the spheres [1]. Inset of figure 1 shows internal structure of the sample film and cracks of the spheres. Graph on figure 1 shows results of thermogravimetric study of the silica film. At T = 25°C and RH = 50% about 4% of mass is lost when it is heated to T = 100 °C.
Fig.1. Thermogravimetric study of as-grown silica PhC film. Inset shows morphology of the investigated film.
Adsorption of gases can be measured by shift of photonic band gap (PBG) wavelength. Peak of the PBG of opal-like structure can be estimated by using a modified form of Bragg’s law:
1
B 2d (111) neff2 sin 2
(1)
where λB is the wavelength of the maximum reflected intensity, d(111)=0.86dspheres is inter-planar spacing between (111) planes of face2 2 2 centered cubic lattice, dspheres is the diameter of spheres, neff is the effective refractive index, f = 0.74 is the filling f nSiO 2 (1 f ) n Air fraction of the silica spheres, nSiO2 = 1.46, nAir = 1.0 are the refractive indexes of silica and medium (written for air) filling the void between spheres, ψ is the incident angle of light. In order to separate two mechanisms of ammonia adsorption measurements of the PBG position at various water and ammonia concentrations have been performed. Silica PhC film was placed in waterproof chamber. At room temperature T = 25°C and RH = 50% the PBG peak is at λ = 578 nm. The chamber with the sample was heated at T = 120°C and blown with dry nitrogen for complete dehydration. The PBG peak of the dry sample at room temperature T = 25°C is at λ = 570 nm (fig. 2).
Fig. 2. Reflection spectra of the silica PhC film: without water and ammonia vapors (RH = 0%, σ(NH3) = 0%), with RH = 0% and σ(NH3) = 10%, with RH = 80% and σ(NH3) = 10%.
According to eq. (1) shift of λB by 8 nm corresponds to the water filling fraction f = 0.07. It is in agreement with 4% of water mass, measured by the thermogravimetric method (fig. 1). Adding dry ammonia vapor with a concentration of σ(NH3) = 10% into the chamber shifts the PBG peak to λ = 579 nm. Subsequent nitrogen blowing returns the PBG peak to λ = 570 nm without heating (see fig. 2). It indicates absence of water inside the chamber and the silica PhC film. Removal of ammonia does not require heating of the sample above the room temperature, since it boils at T = – 33.34°C. Thus dry silica film interacts with ammonia. In absence of water, formation of hydrogen bonds is the main mechanism. Thermal annealing at temperatures above T = 150°C leads to condensation of –OH groups and to loss sensitivity to ammonia vapor in the same way as to water vapor [5]. Increase of humidity to 80% in the chamber with 10% of ammonia vapor additionally produces 13 nm shifts the PBG peak to λ = 583 nm. However such measurements are difficult to perform. It happens since bandwidth of PBG peak of the film is about 50 nm and exceeds the shift caused by gas adsorption. It determines low accuracy of such measurements. Therefore, a new optical scheme that increases accuracy has been developed. 3. Interference method A silica single crystal film 1 is formed on the surface of a glass prism 2 (Fig. 3).
Fig. 3 (a) Schematic of the experimental setup: (1) silica PhC film, (2) glass prism (3) mirror with reflectance r = 0.8, (4) lenses, (5) semiconductive laser, (α) incident angle of the laser beam, (6) image of interference fringes, (7) airproof chamber.
Use of single crystal film ensures parallel planes of its faces. As a result, the film works as a Fabry-Perot interferometer, which is illuminated by a laser 5. The laser wavelength is 650 nm and does not coincide with the wavelength of the PBG. Lens 4 focuses the laser beam 5. The glass (nglass = 1.52) prism 2 is used to introduce the laser beam into the film at angle ψ ~ 46.8°. On the boundary of the film (neff = 1.356) with air (nAir = 1) the total internal reflection occurs. A mirror 3 with R = 0.8 is deposited on the large face of the prism 2 and increases reflectivity of the boundary of the film with glass. In this case the intensity of light transmitted through the mirror to the sensor film (Transmission=20%) is sufficient for forming and recording the interference pattern The high reflectivity on the both sides of the film increases the sharpness of the interference pattern., which could be described through finesse of interferometer. Obtained finesse of interferometer is equal 30. The described optical scheme allows to place the light source 5 and the detector 6 on one side of the sensor in the airproof chamber 7 (Fig. 3). It eliminates influence of the analyte on the registration system.
2
The interference pattern is observed in the reflected light and represents a series of the dark fringes (fig. 3). Interference of light inside the film is described by the Airy’s formula [14]: 2nD cos 4 r1 r2 sin 2 , (2) R 2 2 nD cos 1 r1 r2 4 r1 r2 sin 2 where r1, r2 are the reflective coefficients of the boundaries; n is the refractive index of the film; D is the film thickness; ψ is the angle of incidence on the film. Light refraction at the boundaries of air with glass, glass with PhC and PhC with air should be also taken into account.
4. Results and discussion Figure 4 shows angular distribution of the reflected interference fringes. X-axis corresponds to the incidence angle α of light on the glass prism face (fig. 3).
Fig. 4. Angular distribution of the interference fringes reflected from the silica PhC film with neff = 1.356 (solid line) and neff = 1.386 (dash line).
If α > –5.8° the total internal reflection occurs on the boundary of the PhC film with air, the reflection coefficient is close to one. If α > 28.2° the total internal reflection occurs on the boundary of the glass prism with the PhC film, light do not enters into the film and is reflected back into the prism. Solid line corresponds to blank PhC film (neff = 1.356). Dash line corresponds to PhC film with adsorbed gas, assuming that adsorption changes only the effective refractive index to neff = 1.386. The change of Δneff = 0.03 corresponds to ΔλB = 13 nm shift of the PBG peak according to eq. (1). Choosing different interference fringes different angular shifts could be obtained. The angular shift is larger at smaller α values: at α ~ -50° the shift is about 7°, at α ~ 25° the shift is about 0.1°. Measurements have been performed at light incidence close to normal α ~ 0°. In the experiment gas mixture of ammonia with air was pumped through the chamber. A vessel with NaOH was installed at the chamber entrance. It was used as a desiccant and excluded the influence of humidity changes. Commercial sensors monitored water and ammonia concentrations at the output. In the range of 0-300 ppm, the ammonia concentration was controlled by TGS 2444 sensor. In the range of 300-105 ppm, gas cylinders with a known concentration of ammonia were used. Figure 5a shows the angular shift of the interference fringe as a function of ammonia concentration.
3
Fig. 5. Response of PhC sensor film to (a) ammonia and (b) water vapors. The inset shows the response to ammonia vapor at low concentrations.
The inset shows an enlarged region at low concentration. There are 2 regions corresponding to different ammonia concentration which is similar to the measurements of the PBG peak position. For dry silica film only initial part of the graph can be detected. We assume that the initial region is mainly responsible to formation of hydrogen bonds with the silica surface. The following part of graph with a weaker dependence corresponds to the dissolution of ammonia in water. In contrast, the angular shift of the interference fringe as a function of relative humidity is linear (Fig. 5b). Owing to the existence of two mechanisms of silica film interaction with ammonia vapor it is possible to enlarge dynamic range of the optical sensor which was proposed earlier [2]. The developed sensor provides a registration of the ammonia in the concentration range from few ppm to 105 ppm. To check stability, silica film was soaked in a 10% solution of ammonia in water for 1 month. No changes of the samples or its optical properties were registered. It has been also verified that the sensor does not respond to N2, O2, NO, NO2 and CH4, but sense ethanol. Sensitivity to ethanol is weaker by an order of magnitude than to ammonia and water, which is consistent with the weak polarity of its molecules and confirms suggested mechanism of interaction. Figure 6 shows measured time response of the sensor to ammonia vapor.
Fig. 6. Temporal response to 105 ppm of ammonia. Red line shows the rise of the signal, blue line shows the recovery to zero level.
Dry air has been pumped through the sensor. Instantly a portion of gaseous ammonia with a concentration of 105 ppm has been added (red line). The rise of the signal takes 1 second. The recovery time is 5 seconds. In fact, the recovery of the sensor can be faster, since we do not take into account the adsorption of ammonia in the input tubes and on the walls of the sensor. Response time of the sensor to water is 10 milliseconds. 5. Conclusions Study of adsorption of different gases in the highly porous structure of the silica PhC film is a complex task. At normal conditions, pores contain large amount of adsorbed water and blowing of different gases can changes properties of sample by changing concentration of water without adsorption of other gases. Therefore ammonia is an interesting object for such study since it gives response similar to water. It was verified, for the first time, that dry silica PhC film interacts with ammonia without presence of free water. Further investigation revealed existence of two mechanisms depending on ammonia concentration. At low concentrations
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interaction of ammonia with the silica PhC film is similar to interaction with water. At high concentrations, interaction is observed only in the presence of water, and dependence is weaker. We suppose that, in the absence of water, ammonia is adsorbed on the surface through formation of hydrogen bonds with silanol groups of silica, in the presence of water an additional mechanism appears: the dissolution of ammonia in water. A new method for gas adsorption measurements has been proposed. It detects the angular shift of the fringes formed by the interference of light inside the film. In comparison with the PBG method it allows to increase ratio of band shift to its width. Nonlinear shift of the interference fringe as a function of ammonia concentration broadens dynamic range of the previously proposed optical sensor. The sensor has high sensitivity at low concentration (1 ppm) and allows to measure up 105 ppm ammonia admixture in gas. The measured response time of the sensor to ammonia vapor is about 1 second. The recovery time to zero level is about 5 seconds. Acknowledgments We would like to thank Dr. A. I. Plekhanov for helpful discussions. This work was supported by the Ministry of Education and Science of the Russian Federation [grant number AAAA A18118052390021-2].
References [1] F. Gallego-Gómez, V. Morales-Flórezc, M. Morales, A. Blanco, C. López, Colloidal crystals and water: Perspectives on liquid–solid nanoscale phenomena in wet particulate media, Adv. Colloid Interface Sci. 234 (2016) 142-160. https://doi.org/10.1016/j.cis.2016.05.004 [2] A.S. Kuchyanov, P.A. Chubakov, A.I. Plekhanov, Highly sensitive and fast response gas sensor based on a light reflection at the glass-photonic crystal interface, Opt. Commun. 351 (2015), 109-114. https://doi.org/10.1016/j.optcom.2015.04.045 [3] H. Yang, P. Jiang, B. Jiang, Vapor detection enabled by self-assembled colloidal photonic crystals, J. Colloid Interface Sci. 370 (2012) 11–18. https://doi.org/10.1016/j.jcis.2011.12.058 [4] S. N. Atutov, A. I. Plekhanov, Photo extraction of rubidium atoms from the bulk of a photonic crystal, Autom. Monit. Meas. 54 (2018) 405–410. https://doi.org/10.3103/S8756699018040131 [5] F. Gallego-Gómez, A. Blanco, V. Canalejas-Tejero, Cefe López, Water-dependent photonic bandgap in silica artificial opals, Small 7 (2011), 1838-1845. https://doi.org/10.1002/smll.201100184 [6] V.P. Chubakov, P.A. Chubakov, A.I. Plekhanov, Humidity sensor based on photonic crystal opal film, Nanotechnol. Russ. 7 (2012) 499–501. https://doi.org/10.1134/S1995078012050023 [7] International Institute of Ammonia Refrigeration, Ammonia data book, IIAR, 1993. [8] W. Thain, Monitoring toxic gases in the atmosphere for hygiene and pollution control. Pergamon Press 1980. [9] B. Timmer, W. Olthuis, A. van den Berg, Ammonia sensors and their applications—a review, Sens. Actuators B 107 (2005) 666–677. https://doi.org/10.1016/j.snb.2004.11.054 [10] P. Jiang, J. F. Bertone, K. S. Hwang, V. L. Colvin, Single-crystal colloidal multilayers of controlled thickness, Chem. Mater. 11 (1999) 2132-2140. https://doi.org/10.1021/cm990080+ [11] T. A. Ukleev, D. I. Yurasova, N. N. Shevchenko, and A. V. Sel’kin, Optical anisotropy of cubic photonic crystals under conditions of multiple-mode light propagation, J. Phys.: Conf. Ser. 769 (2016) 012051. https://doi.org/10.1134/S1063783418050335 [12] R.V. Nair , R. Vijaya, Photonic crystal sensors: An overview, Prog. Quantum Electron. 34 (2010) 89-134. https://doi.org/10.1016/j.pquantelec.2010.01.001 [13] W. Stober, A. Fink, E. Bohn, Controlled growth of monodisperse silica spheres in the micron size range, J Colloid Interface Sci. 26 (1968) 62-69. https://doi.org/10.1016/0021-9797(68)90272-5 [14] M. Born, E. Wolf, Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light, Elsevier Science Limited, 1980.
Highlights
Silica film adsorbs ammonia through hydrogen bonds formation and dissolution in water.
Applying photonic crystals as interferometers increase sensitivity of optical sensors.
Silica photonic crystal film allows ammonia detection from 1 to 105 ppm.
Alexander Kuchyanov: Conceptualization, Investigation, Writing – Original Draft, Writing – Review & Editing Pavel Chubakov: Validation , Software Vyacheslav Chubakov: Investigation, Methodology, Writing – Original Draft, Writing – Review & Editing Sergey Mikerin: Project Administration, Writing – Original Draft Fig.1. Thermogravimetric study of as-grown silica PhC film. Inset shows morphology of the investigated film.
Fig. 2. Reflection spectra of the silica PhC film: without water and ammonia vapors (RH = 0%, σ(NH3) = 0%), with RH = 0% and σ(NH3) = 10%, with RH = 80% and σ(NH3) = 10%.
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Fig. 3 (a) Schematic of the experimental setup: (1) silica PhC film, (2) glass prism (3) mirror with reflectance r = 0.8, (4) lenses, (5) semiconductive laser, (α) incident angle of the laser beam, (6) image of interference fringes, (7) airproof chamber.
Fig. 4. Angular distribution of the interference fringes reflected from the silica PhC film with neff = 1.356 (solid line) and neff = 1.386 (dash line).
Fig. 5. Response of PhC sensor film to (a) ammonia and (b) water vapors. The inset shows the response to ammonia vapor at low concentrations.
Fig. 6. Temporal response to 105 ppm of ammonia. Red line shows the rise of the signal, blue line shows the recovery to zero level.
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