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Phenyl-modified hybrid organic-inorganic microporous films as high efficient platforms for styrene sensing
Microporous and Mesoporous Materials xxx (xxxx) xxx
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Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso
Phenyl-modified hybrid organic-inorganic microporous films as high efficient platforms for styrene sensing Francesco Radica a, Stefania Mura b, Davide Carboni b, Luca Malfatti b, Sebastiano Garroni b, Stefano Enzo b, Giancarlo Della Ventura a, c, Giovanna Tranfo e, Augusto Marcelli c, d, Plinio Innocenzi b, * a
Department of Science, Universit� a Roma Tre, Viale Guglielmo Marconi 446, 00146, Roma, Italy Department of Chemistry and Pharmacy, Laboratory of Materials Science and Nanotechnology, CR-INSTM, Via Vienna 2, 07100, Sassari, Italy Istituto Nazionale di Fisica Nucleare, INFN-LNF, Laboratori Nazionali di Frascati, Via Enrico Fermi 40, 00044, Frascati, Italy d Rome International Centre for Materials Science Superstripes, RICMASS, Via dei Sabelli 119A, 00185, Rome, Italy e Dipartimento Medicina Epidemiologia Igiene del Lavoro e Ambientale, Istituto Nazionale Assicurazione Infortuni sul Lavoro, Monte Porzio Catone, 00040, Rome, Italy b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Microporous films Hybrid organic-inorganic materials Sensors Thin films Infrared spectroscopy VOCs
Microporous organic-inorganic hybrid silica films have been designed as active platforms to realise a styrene gas sensor. The microporous film has been synthesized by using tetraethylorthosilicate and phenyltriethoxysilane as precursors and a triblock copolymer (Pluronic F127) as a template for the pores. Multilayer films have been employed to increase the performance of the sensor and infrared spectroscopy has been used to probe the absorbed styrene molecules. The sensing response has been investigated by repeated exposures to different concentrations of styrene gas from 10 to 100 ppm. The microporous film has shown a better response compared to dense layers because of the increased surface area and the presence of the phenyl groups that enable a selective adsorption-desorption of the styrene. The diffusion coefficient of the microporous hybrid films has been calcu lated using a monodimensional plane-sheet diffusion model. This study shows that the microporous hybrid films have great potential to be used as simple and effective sensing materials for styrene and other Volatile Organic Compounds.
1. Introduction Atmospheric pollution from volatile organic compounds (VOCs), produced by human activities, is the cause of major health issues in industrialised and developing countries. The presence of VOCs triggers the atmospheric chemistry that leads to photochemical smog and climate active aerosols [1]. In urban air, they are typically connected to energy production and transport technologies as well as to the use of petrochemical-derived products. Not so much is known about VOCs pollution in indoor environments since the concentration of any pollutant indoors would depend on the balance between its sources and sinks. Moreover, to influence indoor environments, such as indoor workplaces, the reaction time scale of gas-phase pollutants must be competitive with air exchange [2]. The presence of toxic VOCs such as benzene, toluene or other aromatic molecules, as air pollutants in workplaces, has accounted for several deaths in the last decades [3].
Many epidemiological studies pointed out the need to identify and quantify the most important human-produced sources of VOC emissions to effectively mitigate air pollution and improve human health. Although several sources of VOCs have been successfully contained [4, 5] other VOC emissions are growing in relative importance [6]. How ever, emissions from the use of chemical products remain difficult to be measured in indoor ambient. Styrene, among the VOCs compounds due to its double vinyl-bond and high chemical reactivity is widely used in the manufacture of polymers, resins, and reinforced plastics. Prolonged exposure to styrene vapours is considered as one of the main hazards in the fibreglass pro cessing workplaces. The primary route of occupational exposure is via inhalation [7] and, to a lesser extent, via skin exposure [8]. Styrene is classified by the International Agency for Research on Cancer as a possible human carcinogen agent (group 2B) [9] and, according to the American Conference of Governmental Industrial Hygienists [10] the
* Corresponding author. E-mail address: [email protected] (P. Innocenzi). https://doi.org/10.1016/j.micromeso.2019.109877 Received 18 July 2019; Received in revised form 5 November 2019; Accepted 5 November 2019 Available online 6 November 2019 1387-1811/© 2019 Elsevier Inc. All rights reserved.
Please cite this article as: Francesco Radica, Microporous and Mesoporous Materials, https://doi.org/10.1016/j.micromeso.2019.109877
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threshold for a prolonged exposure is 20 ppm (Threshold Limit Value Time-Weighted Average, TLV-TWA) and 40 ppm for 15 min acute exposure (Threshold Limit Value - Short-Term Exposure Limit TLV-STEL). At present, several commercial styrene sensors designed for monitoring this VOC are available on the market. However, they barely discriminate the different volatile substances when present simulta neously in the atmosphere. Photoionization detectors (PIDs) [11], that measure the electric current generated by ionization of gases by high-energy UV light, are commonly used to detect VOCs and provide a relative fast reading of the gas concentration. They are, however, rela tively expensive and not portable. Moreover, they exhibit a non-selective broadband response, which can be used mostly in on/off situations. Thus, for detecting small concentrations of styrene in work places, a highly responsive, sensitive, selective and portable device is mandatory [8]. Most gas sensors are based on the interaction of the gas with the surface of a solid material [12]. Several systems based on microporous thin films have been tested for developing sensing devices [13,14]. Mesoporous materials, in particular, have been used for gas-sensing [15–17] because of the easy embedding of functional nanoparticles in the porous structure [18,19] and the large surface area that allows absorbing several target molecules through a proper surface design [20]. The chemical design of the sensing surface is, in fact, a fundamental tool to improve the VOCs detection as it can facilitate a selective interaction with specific functional groups of the analyte molecules. Mesoporous materials have been loaded onto microcanti levers and functionalized for linking target molecules to give large gravimetric sensing responses [21]. This type of device is, however, quite complicated, cumbersome to operate and cannot be used as a low-cost sensor. In the present work, we have developed and tested a simple device that can be potentially attached to the worker’s clothes and used to monitor the real individual exposure to styrene vapours. The system developed in the present work can be potentially used both as a sensor and as a dosimeter. Equipped with a reader suitable to evaluate the amount of styrene molecules based on the detection of its infrared bands it has the advantage to be highly selective. However, because of the small dimension, the microporous films can also be optimised as individual dosimeters. In case of an environmental alarm, the sensing layer can be read by an external calibrated instrument to determine the individual exposition to the styrene according to the existing legislation in each country. Microporous hybrid silica films have been synthesized with specific sensing groups, such as phenyls, onto the pore wall that can attract styrene vapours by physical adsorption [22]. In previous works, mesoporous hybrid materials [23,24] containing phenyl groups [25] have been synthesized to adsorb pollutants. These materials have been produced, however, as particles and powders and not specifically designed to develop a selective sensing device able to detect a specific pollutant. In the present work, hybrid silica micropo rous thin films have been deposited on silicon substrates, to obtain a platform for infrared gas sensing; this kind of systems can trap volatile molecules in a styrene-rich environment and allow monitoring the gas amount by using FTIR spectroscopy, capable of selective detection among the different VOCs at concentrations down to few ppm. A proof of concept styrene gas sensor based on microporous hybrid silica-based films is here reported; the device is simple and very effective in selec tive detection of styrene vapours.
for 7 days, at 25 � C, in a closed vial25 then TEOS-PhTES thin films were deposited by dip-coating silicon wafers in the precursor solution at 25 � C at a withdrawal rate of 15 cm min 1. A silica microporous reference film not containing the phenyl groups was prepared by mixing two solutions. In the first one 3.08 mL of EtOH were mixed with 4.26 mL of TEOS and water and, after the addition of 0.355 mL of HCl 0.768 M, the solution was stirred for 60 min. The sec ond solution was prepared by mixing 15 mL of EtOH with 1.3 g of Pluronic F127 and 1.5 mL of HCl 0.057 M. The final molar ratios were TEOS: Pluronic F127: H2O: HCl: EtOH 1: 0.005 : 5.4: 0.02 : 16.4. A dense hybrid film, also used as a reference, was prepared as previously described but without the addition of the surfactant template. The relative humidity (RH) inside the dip-coater chamber was maintained between 24 and 25% with a temperature of 25 � C, to obtain good optical quality and high structural order in the silica films. To in crease the inorganic polycondensation and stabilize the mesophase the films were thermally treated at 120 � C for 1 h. The thermal treatment is important to allow the formation of highly organized, high surface area microporous coatings. Stacked multi-layer coatings (3- and 5-layers) were tested to optimize the thickness in light of the sensing applica tion, by depositing each layer on the silicon wafer and then drying the film for 1 h in oven at 120 � C. A final calcination step was performed at 350 � C for 3 h in air, under static conditions, with a heating rate of 10 � C min 1, to remove the organic template (F127) of the stabilized coatings. The thickness of the films before and after the thermal treatments was monitored by spectroscopic ellipsometry. The hybrid silica films were transferred in a sealed chamber and exposed for about 1 h to a styrene-rich atmosphere (Sigma-Aldrich, re agent grade, >99%), temperature range 21–25 � C and RH 35–40%. Styrene concentration within the chamber (in ppm) was monitored using a Photo-Ionization Detector (PID). After adsorption the films were quickly transferred to the FTIR optical bench for infrared analysis. 3. Characterization techniques Infrared data were collected by Fourier-transform infrared (FTIR) spectroscopy using a Bruker Vertex 70v interferometer, FTIR spectra were collected every 30 s for 1 h in transmission mode, averaging 32 scans in the 4000-400 cm 1 spectral range with a resolution 4 cm 1 and scan velocity 20 kHz. A silicon wafer was used as the background; the baseline was fitted by a concave rubber band correction with OPUS™ 7.0 software and data were analyzed by ORIGIN PRO™ software. The film thickness and residual porosity were estimated by an α-SE Wollam spectroscopic ellipsometry using a Bruggeman effective me dium approximation fitting model with two components: void [26] and Cauchy film [27]. The refractive index parameters for the Cauchy film model were measured on a monolayer reference sample treated at 120 � C for 1 h and 350 � C for 3 h. Plots of Ψ and Δ as a function of incident wavelength from 370 to 700 nm were simulated using “Com pleteEASE v. 4.2” program from Wollam. The results of the fits were evaluated on the basis of the mean squared error (MSE), which was maintained below 30. Contact angle of the films was measured using a DataPhysics Contact angle System OCA 20; the measure was performed at 27 � C and 47% RH by dispensing a 4 μL droplet on the film surface. Transmission electron microscopy (TEM) images were obtained by using a FEI TECNAI 200 operating at 200 kV working with a field emission electron gun. Before analysis, the films were detached from the substrates using a scalpel blade and the resulting fragments were dispersed in ethanol by ultrasonication for 5 min. Afterwards, the ethanol solution was dropped onto silicon copper grids and the solvent was left to evaporate for 3 min before the measurements. We have used lacey reinforced grids made by silicon monoxide (Formar stabilized) on copper (Nanovision supplier). N2 sorption isotherms were collected with a Sorptomatic 1990 in strument (Fisons). Before the measure, 600 mg of sample were
2. Materials and methods 2.1. Sol preparation and film deposition The precursor sol was prepared by mixing 8.135 mL of ethanol (EtOH), 540 μL mL of phenyltriethoxysilane (PhTES), 1 mL of tetrae thoxysilane (TEOS), 138 mg of F127 Pluronic block copolymers, 300 μL of mQ water and 25 μL of HCl (2 N) in a glass vial (molar ratios PhTES: TEOS: EtOH: H2O: HCl ¼ 0.5 : 1.0: 31.2: 3.71 : 1.55). The sol was stirred 2
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introduced in a quartz tube and degassed under vacuum (1 � 10 3 bar) at 523 K for 12 h. The dead volume was evaluated through helium measurements. The surface area was calculated with BET method, the pore size was estimated by the Dollimore/Heal method and the pore volume by Dubinin Stoeckly method. Styrene concentration was monitored using a commercial PhotoIonization Detector (PID), Model TA-2100 Styrene Detector manufac tured by Mil-Ram Technology, Inc., with a detection range from 0 to 100 ppm and 1 ppm resolution, according to the manufacturer. Adsorption experiments were performed at room temperature (25 � C) in a sealed chamber by exposing films in a styrene-rich atmosphere for 1 h. For these experiments, only 3-layers films were used as they showed both mechanical stability and good signal-to-noise ratio when used as the sensing element. Experiments were also repeated multiple times in the same film to test the durability and the performances over time. The same sample was used for the first set of adsorption-desorption styrene run, left in the air to stabilize after the experiment, and then reused for a second and third adsorption-desorption run.
(red line); the 3- (blue line) and 5-layers (green line) films after calci nation at 350 � C are also reported. In the low wavelength region the most intense bands about 1076 cm 1 and the shoulder around 1200 cm 1 are assigned to Si–O–Si anti-symmetric stretching vibrations [29]. The intense band peaking at 1138 cm 1 is instead assigned to C–H stretching (Si–C in phenyl groups) from phenyltriethoxysilane [16]. The absorption bands at 810 and 480 cm 1 are also assigned to silica vibrational modes, stretching and rocking modes, respectively. The band at 960 cm 1 is ascribed to the Si–OH groups. The sharp bands at 1433 and 1595 cm 1 and the broader bands at 698 and 740 cm 1 are corre lated to the vibrations of the phenyl groups [30]. The inset in Fig. 2 shows a detail of the single-layer film FTIR spectrum before and after calcination. The FTIR spectra confirm that after thermal treatment most of OH groups (wide band peaking around 3400 cm 1) and surfactant (2981, 2989 and 2900 cm 1 peaks) have been removed. The spectra after calcinations show a new band around 3700 cm 1, which is the signature of the presence on the pore surface of isolated and twin sila nols formed upon calcination. This indicates that, as previously reported for mesoporous silica membranes [16], the thermal treatment at 350 � C, besides removing the surfactant, induces a peculiar change on the pore surface whose coverage by a continuous network of silanols is replaced by isolated and twin Si–OH groups. It is important to underline that both the microporous silica films not containing the phenyl groups and the dense hybrid films have been tested and not shown any sensing capability to styrene (See Fig. S2, Supplementary Information). The presence of porosity and of phenyl groups is thus an important feature to develop selective and effective sensing for styrene. TEM analysis (Fig. 3a) show the presence of pores in
4. Results and discussion The overall quality of the single and multilayers hybrid films has been assessed by optical microscopy and ellipsometry. Both single-layer and 3-layers films looked very homogenous with no discontinuities or surface imperfections. On the opposite, 5-layers films always presented cracks on the film surface (see Fig. S1, Supplementary Material). Refractive index, n, measurements on single and 3-layers films yield to an average value n of 1.45 � 0.02. No significant differences have been observed between single-layer and 3-layers films, while a decrease in n from 1.50 to 1.45 is observed after calcination at 350 � C. The thickness measured for the single-layer film is about 320 nm before calcination and 250 nm after calcination. As expected, the 3-layers films were about three times thicker than single-layer samples, with a thickness of 970 nm and 740 nm, before and after calcination, respectively. The observed uniaxial film shrinkage is due to the condensation of the hybrid silica walls and the partial collapse of the porous structure by thermal treat ment [28]. Ellipsometry measurements have also been used to evaluate the effect of sol ageing on the thickness and refractive index. The 3-layer films from the sols aged for two weeks have shown an average thickness increase from 740 � 11 nm to 858 � 21 nm. The thickness, after three ageing weeks of the sol, became 931 � 39 nm. The higher thickness with sol ageing is due to the proceeding of the polycondensation reactions, which causes an increase in viscosity and of the film thickness. The porosity of the films has been also evaluated by the spectroscopic ellipsometry. A single layer sample (276 � 0.2 nm), after the full cycle of thermal treatments (120 þ 350 � C), has a 9.8% � 0.43 of porosity and average surface roughness of 9.12 nm with a MSE ¼ 24.7. The hybrid microporous films have a surface which becomes hy drophobic in comparison to pure silica, due to the presence of the phenyl groups; the contact angle increases to 44.5� in comparison to 6.5� measured in silica (Fig. 1). Fig. 2 shows the FTIR absorption spectra (4000–400 cm 1) of the 1layer PhTES-TEOS film before (black line) and after thermal treatments
Fig. 2. FTIR absorption spectra of 1-layer film before (black line) and after thermal treatments (red line), 3-layers (blue line) and 5-layers films (green line) treated at 350 � C. The inset shows a detail of the high wavenumber region for 1layer film as-deposited and after calcination. (For interpretation of the refer ences to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 1. Contact angle measured in (a) silica and (b) hybrid microporous films after treatment at 350 � C. 3
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the hybrid films with a �2 nm size. The pores are not organized and likely resulting from a general collapse of larger pores. Althought the addition of a triblock-copolymer in the sol generally leads to the for mation of an ordered porous structure, the presence of PhTES as the precursor, hampers the self-assembly process, explaining the lack of long-range order observed in the films. Moreover, the thermal treat ment, which is required to densify the pore walls and remove the tem plating agent, is likely responsible of a general pore collapse, as already reported for mesoporous materials with random pore organization. Nitrogen sorption isotherms have been measured on the material obtained from the hybrid sols previously used for film deposition (Fig. 3b). The sol has been cast on Petri dishes, dried in air and then thermally treated up to 350 � C with the same thermal ramp used for preparing the films. Other analytical techniques, specifically developed for the analysis of thin films, such as the environmental ellipsometric porosimetry [31], cannot be used because the hybrid matrix does not allow measuring the porosity using water or isopropylic alchohol adsorption/desorption isotherms [32]. The adsorption isotherm shows a type I trend, typical of microporous materials, as confirmed by the sharp increase at p/p0 < 0.02 relative pressure range. On the other hand, the multilayer adsorption and the capillary condensation observed at higher pressure, for p/p0 > 0.5, suggest the existence of mesopores and mac ropores in the hybrid silica, as already reported for porous coal [33,34]. This analysis confirms the hypothesis of a partial collapse of the microporous structure as a consequence of the thermal treatment. The estimated surface area, as obtained by BET method, is 114.82 m2 g 1 while the pore radius, as calculated by the Dollimore-Heal method, in dicates a bimodal distribution with small pores with a radius of �1 nm and a larger pores with a radius of around �50 nm. Using the Dubinin-Stoeckli method we have also estimated the volume of the smaller pores as 0.24 cm3 g 1. The analysis of the N2 sorption isotherms confirms the results of TEM measurements. The films show a not completely organized porous structure with small pores in the range between 1 and 2 nm (Fig. 4). The lack of long-range pore organization has been also confirmed by low-angle XRD (the pattern can be found in Supplementary Material SI2).
interaction, however, is weak and styrene molecules are stored in the film for a limited amount of time. Fig. 5a shows the FTIR spectrum in the 950-690 cm 1 range for single and multi-layer films before (black lines) and after (red lines) exposure to a styrene concentration in the atmo sphere of 73 ppm. In each case, after the exposure, the appearance of new infrared absorption bands at 910 (out-of-plane CH2 wagging) and 780 cm 1 (deformation vibration of CH2 in the aromatic ring) is observed. These bands are characteristic of the styrene molecules and after dozens of minutes of exposure to a styrene-free environment pro gressively disappear (vide infra). The appearance of these bands is experimental evidence of the diffusion and entrapment of the styrene molecules inside the phenyl bearing hybrid matrix. The absorption band peaking at 820 cm 1 is assigned to symmetric stretching of Si–O–Si bonds [35]. The FTIR absorption spectra of single and 3-layers microporous silica films (Fig. 5b) clearly show that the styrene molecules are not adsorbed into the sensing layer if the phenyl groups do not modify the surface. They play, therefore, a fundamental role to adsorb the pollutant, no traces of adsorbed styrene molecules can be found, in fact, in the microporous silica films (The full range FTIR absorption spectra can be found in the Supplementary Material SI3). A styrene sensor based on the detection of specific infrared bands of the pollutant has the advantage to be potentially highly selective. Fig. 6 shows the FTIR absorption spectra (950–650 cm 1) of different com pounds. The 910 and 780 cm 1 vibrational modes do not overlap with any other signal from isopropanol, toluene, ethanol, acetone and xylene. It also has to be stressed that the presence of water in the environment does not affect the detection of styrene because the film is hydrophobic (Fig. 1) and in the infrared detection range, no signals of water are present. Fig. 7a and b shows the typical desorption curves obtained by monitoring the 910 and 780 cm 1 styrene absorption bands as a function of time. The details of the desorption experiments performed on the same film exposed to increasing styrene concentrations: 12, 34 and 58 ppm, are shown in Fig. 7c and d (780 cm 1 band). The maximum absorbance and the final desorption time is proportional to the styrene concentration measured at the time zero by the PID sensor, thus indi cating, that at room temperature (RT), the styrene concentration inside the PhTES/TEOS microporous film reaches the steady-state equilibrium with the environment. Additionally, most of the styrene is expelled in the first 20 min (Fig. 7a); after 1 h, the film is almost empty and ready for reuse.
4.1. Adsorption mechanism and desorption process Fig. 2 shows that the hybrid matrix still contains phenyl groups (e.g. 698 and 740 cm 1 bands) after the thermal treatment at 350 � C. The presence of these groups is important because they act as specific sensing sites for styrene molecules. As the films are exposed to a styrene-rich environment, the organic molecules diffusing inside the structure are trapped by the π-π interactions with the phenyl groups in the film. This
4.2. Calibration curve and diffusion coefficient The previous data point out that the desorption from the PhTES/ TEOS films depends on the initial styrene content. Therefore, we have modelled the data based on a mono-dimensional diffusion from a plane sheet of finite thickness [36,37]. To apply this formalism to our data we assume that: (1) the styrene concentration at any time within the film, i.e. not to be confused with the concentration in the atmosphere measured by the PID instrument (see below), can be represented by the integrated absorbance At; (2) any styrene molecule desorbed from the film surface is considered lost because the overall absorbed styrene volume inside the film is insig nificant compared to the volume of the measurement chamber; and (3) the diffusion parallel to the film surface is neglected as the analytical spot size is significantly smaller than the whole absorbing surface. The resulting equation is (Eq. (1)): � � � ∞ � X 8 Dð2n þ 1Þ2 π 2 t At ¼ A0 ⋅ ⋅exp (1) 2 2 4L2 n¼0 ð2n þ 1Þ π where At is the integrated absorbance (At) at time ¼ t, A0 is the inte grated absorbance At at time ¼ 0, D is the diffusion coefficient in m2s 1, t is the time in seconds, L is sample thickness in m, and n is the index of
Fig. 3. Nitrogen sorption isotherms of the hybrid materials, obtained from the sol used for film deposition, after treatment at 350 � C. The inset shows the pore size distribution obtained by Dollimore/Heal method. 4
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Fig. 4. Representative TEM images of the 1-layer film after thermal treatment and enlargement of the mesoporous structure (inset). The crystalline particles in C are small fragment of the Silicon substrate.
Fig. 5. a) FTIR absorption spectra in the 950-690 cm 1 range for single and multi-layer films before (black lines) and after (red lines) styrene exposure. Styrene atmospheric concentration was 73 ppm (PID reading). The absorption bands at 910 and 780 cm 1 related to styrene are highlighted by dotted vertical lines; the 740 cm 1 vibrational mode, due to phenyl group in the films, is also shown as a reference. b) FTIR absorption spectra of the single and 3-layers microporous silica films before and after exposure to styrene. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
summation. A0 and D parameters have been obtained using the fitting routine in the OriginPro™ (OriginLab, USA) software; eight exponential terms (n ¼ 8) have been used for Eq. (1) to obtain convergence. Errors on D/L2 and A0 have been calculated from the fitting error given by the used routine. Normalization of the integrated absorbances (Ai) to a
standardized 740 nm thickness (the average thickness of 3-layers sam ples) has been performed using the 780 cm 1 film absorption band as reference. Data collected after 20 min have been excluded from the fit because, as observed, most of the styrene is desorbed in the first 24 min. Moreover, for low concentrations, the extracted areas are less reliable because of the signal/noise ratio, interference fringes and/or baseline 5
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modelling. Table 1 lists the diffusion parameters calculated for both 910 cm 1 and 780 cm 1 styrene signals in the different sets of experiments. The samples 1, 2 and 3 have been used for three runs adsorption-desorption experiments (a, b and c), while the films 4, 5, 6, 7 and 8 for a single run. The R2 value for the fitted curve is usually above 95%, except the ex periments 1a and 3c (see Table 1) where low styrene concentrations at the equilibrium have given higher associated errors. Data obtained from the 780 cm 1 band have lower errors, however, A0 and D values ob tained from both 780 and 910 cm 1 styrene bands tend to overlap thus proving that both the infrared vibrational modes can be used for diffu sion and sensing purpose. The resulting average D value obtained by fitting the 780 cm 1 desorption curve is 3.8 � 0.6⋅10 16 m2s 1 (-logD ¼ 15.43 � 0.07) and for the 910 cm 1 band is 4.8 � 1.0⋅10 16 m2s 1(-logD ¼ 15.3 � 0.1) at 25 � C. 4.3. PhTES-TEOS films as sensing elements for styrene
Fig. 6. FTIR absorption spectra in the 950 - 650 cm toluene, ethanol, acetone, xylene and styrene.
1
Fig. 8 shows the correlation of the concentration of styrene at equi librium in the atmosphere (PID readings in ppm) with the absorbances at the time 0 (A0) for the 780 cm 1 styrene absorption band (Table 1) fitted by Eq. (1). The experimental points (black dots) show that, in the investigated range, the styrene concentration probed in the atmosphere is not linearly correlated to the FTIR absorbances. After steadily increasing the styrene concentration up to 40/50 ppm, the FTIR absor bance values asymptotically set to a threshold value. The following equation (Eq. (2)) well describes the styrene concentration-infrared absorbance correlation:
interval of isopropanol,
A0 ¼ a⋅ð1
bppm Þ
(2)
Fig. 7. 3D-time resolved FTIR spectra (absorbance-wavenumber-time) of the 780 and 910 cm 1 bands (a and b, respectively). Selected FTIR spectra of the 780 cm 1 styrene absorption band at different desorption times, background correction is applied (c). Desorption curves for the 780 cm 1 styrene band as a function of time. The experiments have been performed on the same film (sample 1) exposed each time to a different styrene concentration (12 ppm black line, 34 ppm red line and 58 ppm blue line) (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 6
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Table 1 PID readings and diffusion parameters, D, obtained using the monodimensional plane-sheet diffusion model for both styrene signals at 780 and 910 cm 1. The fitted integrated Absorbance (A0) is normalized to a 740 nm thick film. In brackets are reported the uncertainties (�) on the last digit. The experiments have been performed at 25 � C. Samples 1, 2 and 3 have been used for three runs adsorption-desorption experiments (a, b and c), while the films 4, 5, 6, 7 and 8 for a single run. Sample
PID reading (ppm)
Band at 780 cm A0 (cm
1a 1b 1c 2a 2b 2c 3a 3b 3c 4a 5a 6a 7a 8a
12 34 58 16 39 43 100 84 12 82 55 35 43 18
0.033 0.089 0.094 0.053 0.105 0.100 0.118 0.108 0.016 0.101 0.086 0.076 0.084 0.039
1
)
(1) (2) (2) (1) (2) (2) (2) (3) (1) (3) (2) (2) (1) (1)
1
Band at 910 cm 2
1
D (m s ) 3.0 4.7 4.6 3.5 4.2 4.3 4.1 3.9 4.2 4.1 3.1 3.1 3.8 2.8
(1)10 (2)10 (2)10 (1)10 (1)10 (1)10 (1)10 (2)10 (4)10 (2)10 (1)10 (1)10 (1)10 (1)10
16 16 16 16 16 16 16 16 16 16 16 16 16 16
1
1
-logD
A0 (cm )
D (m2s 1)
-logD
15.53 (2) 15.33 (1) 15.34 (1) 15.46 (1) 15.38 (1) 15.37 (1) 15.39 (1) 15.41 (2) 15.38 (4) 15.39 (2) 15.51 (2) 15.52 (2) 15.42 (1) 15.56 (1)
0.018 (1) 0.088 (1) 0.079 (3) 0.046 (1) 0.114 (2) 0.121 (2) 0.122 (2) 0.118 (3) 0.007 (1) 0.094 (2) 0.081 (2) 0.066 (2) 0.078 (2) 0.030 (1)
4.9 (3)10 16 5.8 (1)10 16 5.2 (2)10 16 3.7 (1)10 16 5.5 (1)10 16 3.8 (1)10 16 5.9 (1)10 16 5.5 (2)10 16 6 (1)10 16 3.0 (1)10 16 3.6 (2)10 16 4.5 (2)10 16 5.1 (2)10 16 4.3 (1)10 16
15.31 (3) 15.24 (1) 15.29 (2) 15.44 (1) 15.26 (1) 15.42 (1) 15.23 (1) 15.26 (1) 15.19 (7) 15.53 (2) 15.44 (2) 15.35 (2) 15.29 (1) 15.37 (1)
site, has a fast response and can be developed as a portable device or a dosimeter. The sensing results in this study indicate that functionalized microporous hybrid silica is also a promising material for nose-on-a-chip applications and wearable devices with many potential applications for indoor monitoring, a real emerging area of research. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work is part of the BRIC ID12 project financed by INAIL. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2019.109877.
Fig. 8. Integrated absorbance of the 780 cm 1 styrene band vs. PID reading. The experimental data (dots) are fitted using the equation: y ¼ a(1-bx) with x ¼ ppm.
References Where a ¼ 0.127 � 0.005 and b ¼ 0.975 � 0.003. The observed response is interpreted as a saturation effect of the hybrid silica film; for a given thickness, th film cannot uptake, in fact, more than a certain amount of styrene molecules.
[1] B.C. McDonald, J.A. de Gouw, J.B. Gilman, S.H. Jathar, A. Akherati, C.D. Cappa, J. L. Jimenez, J. Lee-Taylor, P.L. Hayes, S.A. McKeen, Y.Y. Cui, S.W. Kim, D. R. Gentner, G. Isaacman-VanWertz, A.H. Goldstein, R.A. Harley, G.J. Frost, J. M. Roberts, T.B. Ryerson, M. Trainer, Science 359 (2018) 760–764. [2] C.J. Weschler, N. Carslaw, Environ. Sci. Technol. 52 (2018) 2419–2428. [3] Y.H. Lanyon, G. Marrazza, I.E. Tothill, M. Mascini, Biosens. Bioelectron. 20 (2005) 2089–2096. [4] B.C. McDonald, D.R. Gentner, A.H. Goldstein, R.A. Harley, Environ. Sci. Technol. 47 (2013) 10022–10031. [5] G.J. Dollard, P. Dumitrean, S. Telling, J. Dixon, R.G. Derwent, Atmos. Environ. 41 (2007) 2559–2569. [6] B.C. McDonald, A.H. Goldstein, R.A. Harley, Environ. Sci. Technol. 49 (2015) 5178–5188. [7] R.D. Stewart, H.C. Dodd, E.D. Baretta, A.W. Schaffer, Arch. Environ. Health 16 (1968) 656–662. [8] K. Eriksson, L. Wiklund, Ann. Occup. Hyg. 48 (2004) 203–208. [9] lnternational Agency for Research on Cancer (IARC), Monographs on the Evaluation of Carcinogenic Risks to Humans, Some Industrial Chemicals, Lyon, 1994, p. 60. [10] American Conference of Governmental Industrial Hygienists, Threshold Limit Values or Chemicals and Physical Agents and Biological Exposure Indices, 2005 (Cincinnati). [11] W.R. Haag, C. Wrenn, The PID Handbook - Theory and Applications of DirectReading Photoionization Detectors (PIDs), 2nd., RAE Systems Inc, San Jose, CA, 2006. [12] T. Wagner, S. Haffer, C. Weinberger, D. Klaus, M. Tiemann, Chem. Soc. Rev. 42 (2013) 4036–4053. [13] P. Innocenzi, A. Martucci, M. Guglielmi, A. Bearzotti, E. Traversa, Sens. Actuators B 76 (2001) 299–303.
5. Conclusions Nanoporous hybrid organic-inorganic silica thin films have shown a remarkable sensing capability to detect low concentrations of styrene vapours. The combination of porosity and phenyl groups on the pore surface has allowed achieving selective adsorption of styrene. Dense or microporous films without phenyls do not uptake styrene and do not show any sensing response. The presence of styrene (<100 ppm) has been evaluated in an experimental chamber using infrared absorption spectroscopy. Threelayers microporous films have shown an optimised response to adsorption-desorption cycles. The styrene diffusion coefficient has been calculated using a mono-dimensional diffusion from a plate sheet of finite thickness model. This study shows that functionalized microporous silica film can be applied as a high-performance chemical sensing material for detection of styrene and other VOCs in the ppm range. The sensor can be used on7
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[14] A. Bearzotti, J. Mio Bertolo, P. Innocenzi, P. Falcaro, E. Traversa, Sens. Actuators B 95 (2003) 107–110. [15] K. Ariga, Y. Yamauchi, Q. Ji, Y. Yonamine, J.P. Hill, Apl. Mater. 2 (2014), 030701. [16] L. Malfatti, T. Kidchob, P. Falcaro, S. Costacurta, M. Piccinini, M. Cestelli Guidi, A. Marcelli, A. Corrias, M.F. Casula, H. Amenitsch, P. Innocenzi, Microporous Mesoporous Mater. 103 (2007) 113–122. [17] L. Malfatti, D. Carboni, M.F. Francesca Casula, S. Garroni, E. Zampetti, A. Macagnano, A. Bearzotti, P. Innocenzi, J. Mater. Chem. C 5 (2017) 11815–11823. [18] L. Malfatti, P. Innocenzi, J. Nano Res. 20 (2018) 1–13. [19] S. Costacurta, L. Malfatti, P. Innocenzi, H. Amenitsch, A. Masili, A. Corrias, M. F. Casula, Microporous Mesoporous Mater. 115 (2008) 338–344. [20] P. Xu, H. Yu, X. Li, Anal. Chem. 83 (2011) 3448–3454. [21] P. Xu, X. Li, H. Yu, T. Xu, Sensors 14 (2014) 19023–19056. [22] P. Falcaro, S. Costacurta, L. Malfatti, D. Buso, A. Patelli, P. Schiavuta, M.G. Grenci, B. Marmiroli, H. Amenitsch, P. Innocenzi, ACS Appl. Mater. Interfaces 3 (2011) 245–251. [23] J. Fan, X. Wang, W. Teng, J. Yang, X. Ran, X. Gou, N. Bai, M. LV, H. Xu, G. Li, W. Zhang, D. Zhao, J. Colloid Interface Sci. 487 (2017) 354–359. [24] Q. Hu, J.J. Li, Z.P. Hao, S.Z. Qiao, Chem. Eng. J. 149 (2017), 281-28. [25] A. Tunlert, P. Prasassarakich, S. Poompradub, Macromol. Symp. 354 (2015) 62–68. [26] For a two-component porous material, the refractive index in the Bruggemann approximation is expressed as f v[(nv 2 n2 )/(nv 2 þ 2n2 )] þ f m[(nm 2 n2
[27]
[28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
8
)/(nm 2 þ 2n2 )]) where Fv, Nv and Fm, Nm are the volume fractions and the refractive index of the pore’s void and the bulk material, respectively. The Cauchy dispersion equation, n(λ) ¼ an þ Bn/λ2 þ Cn/λ4 , allows calculating the refractive index as a function of the wavelength. An is a parameter related to the average refractive index of the material, whereas Bn and Cn are parameters that provide the shape or curvature of the n(λ) curve. G.W. Scherer, J. Sol. Gel Sci. Technol. 8 (1997) 353–363. Y. Mizuta, Y. Daiko, A. Mineshige, M. Kobune, T. Yazawa, J. Sol. Gel Sci. Technol. 58 (2011) 80–84. Z. Olejniczaka, M. Łęczkab, K. Cholewa-Kowalskab, K. Wojtachb, M. Rokitab, W. Mozgawab, J. Mol. Struct. 744 (2005) 465–471. C. Boissiere, D. Grosso, S. Lepoutre, L. Nicole, A. Brunet Bruneau, C. Sanchez, Langmuir 21 (2005) 12362–12371. D. Carboni, Y. Jiang, M. Faustini, L. Malfatti, P. Innocenzi, ACS Appl. Mater. Interfaces 8 (2016) 34098–34107. K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemeniewska, Pure Appl. Chem. 57 (1985) 603–619. Q. Lingling, T. Xu, W. Zhaofeng, P. Xinshan, Int. J. Min. Sci. Technol. 27 (2017) 371–377. P. Innocenzi, J. Non-Cryst. Solids 316 (2003) 309–319. H.S. Carslaw, J.C. Jaeger, Conduction of Heat in Solids, Oxford Univ. Press, London, 1959. J. Crank, The Mathematics of Diffusion, Oxford Univ. Press, Oxford, 1956.
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Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso
Phenyl-modified hybrid organic-inorganic microporous films as high efficient platforms for styrene sensing Francesco Radica a, Stefania Mura b, Davide Carboni b, Luca Malfatti b, Sebastiano Garroni b, Stefano Enzo b, Giancarlo Della Ventura a, c, Giovanna Tranfo e, Augusto Marcelli c, d, Plinio Innocenzi b, * a
Department of Science, Universit� a Roma Tre, Viale Guglielmo Marconi 446, 00146, Roma, Italy Department of Chemistry and Pharmacy, Laboratory of Materials Science and Nanotechnology, CR-INSTM, Via Vienna 2, 07100, Sassari, Italy Istituto Nazionale di Fisica Nucleare, INFN-LNF, Laboratori Nazionali di Frascati, Via Enrico Fermi 40, 00044, Frascati, Italy d Rome International Centre for Materials Science Superstripes, RICMASS, Via dei Sabelli 119A, 00185, Rome, Italy e Dipartimento Medicina Epidemiologia Igiene del Lavoro e Ambientale, Istituto Nazionale Assicurazione Infortuni sul Lavoro, Monte Porzio Catone, 00040, Rome, Italy b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Microporous films Hybrid organic-inorganic materials Sensors Thin films Infrared spectroscopy VOCs
Microporous organic-inorganic hybrid silica films have been designed as active platforms to realise a styrene gas sensor. The microporous film has been synthesized by using tetraethylorthosilicate and phenyltriethoxysilane as precursors and a triblock copolymer (Pluronic F127) as a template for the pores. Multilayer films have been employed to increase the performance of the sensor and infrared spectroscopy has been used to probe the absorbed styrene molecules. The sensing response has been investigated by repeated exposures to different concentrations of styrene gas from 10 to 100 ppm. The microporous film has shown a better response compared to dense layers because of the increased surface area and the presence of the phenyl groups that enable a selective adsorption-desorption of the styrene. The diffusion coefficient of the microporous hybrid films has been calcu lated using a monodimensional plane-sheet diffusion model. This study shows that the microporous hybrid films have great potential to be used as simple and effective sensing materials for styrene and other Volatile Organic Compounds.
1. Introduction Atmospheric pollution from volatile organic compounds (VOCs), produced by human activities, is the cause of major health issues in industrialised and developing countries. The presence of VOCs triggers the atmospheric chemistry that leads to photochemical smog and climate active aerosols [1]. In urban air, they are typically connected to energy production and transport technologies as well as to the use of petrochemical-derived products. Not so much is known about VOCs pollution in indoor environments since the concentration of any pollutant indoors would depend on the balance between its sources and sinks. Moreover, to influence indoor environments, such as indoor workplaces, the reaction time scale of gas-phase pollutants must be competitive with air exchange [2]. The presence of toxic VOCs such as benzene, toluene or other aromatic molecules, as air pollutants in workplaces, has accounted for several deaths in the last decades [3].
Many epidemiological studies pointed out the need to identify and quantify the most important human-produced sources of VOC emissions to effectively mitigate air pollution and improve human health. Although several sources of VOCs have been successfully contained [4, 5] other VOC emissions are growing in relative importance [6]. How ever, emissions from the use of chemical products remain difficult to be measured in indoor ambient. Styrene, among the VOCs compounds due to its double vinyl-bond and high chemical reactivity is widely used in the manufacture of polymers, resins, and reinforced plastics. Prolonged exposure to styrene vapours is considered as one of the main hazards in the fibreglass pro cessing workplaces. The primary route of occupational exposure is via inhalation [7] and, to a lesser extent, via skin exposure [8]. Styrene is classified by the International Agency for Research on Cancer as a possible human carcinogen agent (group 2B) [9] and, according to the American Conference of Governmental Industrial Hygienists [10] the
* Corresponding author. E-mail address: [email protected] (P. Innocenzi). https://doi.org/10.1016/j.micromeso.2019.109877 Received 18 July 2019; Received in revised form 5 November 2019; Accepted 5 November 2019 Available online 6 November 2019 1387-1811/© 2019 Elsevier Inc. All rights reserved.
Please cite this article as: Francesco Radica, Microporous and Mesoporous Materials, https://doi.org/10.1016/j.micromeso.2019.109877
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threshold for a prolonged exposure is 20 ppm (Threshold Limit Value Time-Weighted Average, TLV-TWA) and 40 ppm for 15 min acute exposure (Threshold Limit Value - Short-Term Exposure Limit TLV-STEL). At present, several commercial styrene sensors designed for monitoring this VOC are available on the market. However, they barely discriminate the different volatile substances when present simulta neously in the atmosphere. Photoionization detectors (PIDs) [11], that measure the electric current generated by ionization of gases by high-energy UV light, are commonly used to detect VOCs and provide a relative fast reading of the gas concentration. They are, however, rela tively expensive and not portable. Moreover, they exhibit a non-selective broadband response, which can be used mostly in on/off situations. Thus, for detecting small concentrations of styrene in work places, a highly responsive, sensitive, selective and portable device is mandatory [8]. Most gas sensors are based on the interaction of the gas with the surface of a solid material [12]. Several systems based on microporous thin films have been tested for developing sensing devices [13,14]. Mesoporous materials, in particular, have been used for gas-sensing [15–17] because of the easy embedding of functional nanoparticles in the porous structure [18,19] and the large surface area that allows absorbing several target molecules through a proper surface design [20]. The chemical design of the sensing surface is, in fact, a fundamental tool to improve the VOCs detection as it can facilitate a selective interaction with specific functional groups of the analyte molecules. Mesoporous materials have been loaded onto microcanti levers and functionalized for linking target molecules to give large gravimetric sensing responses [21]. This type of device is, however, quite complicated, cumbersome to operate and cannot be used as a low-cost sensor. In the present work, we have developed and tested a simple device that can be potentially attached to the worker’s clothes and used to monitor the real individual exposure to styrene vapours. The system developed in the present work can be potentially used both as a sensor and as a dosimeter. Equipped with a reader suitable to evaluate the amount of styrene molecules based on the detection of its infrared bands it has the advantage to be highly selective. However, because of the small dimension, the microporous films can also be optimised as individual dosimeters. In case of an environmental alarm, the sensing layer can be read by an external calibrated instrument to determine the individual exposition to the styrene according to the existing legislation in each country. Microporous hybrid silica films have been synthesized with specific sensing groups, such as phenyls, onto the pore wall that can attract styrene vapours by physical adsorption [22]. In previous works, mesoporous hybrid materials [23,24] containing phenyl groups [25] have been synthesized to adsorb pollutants. These materials have been produced, however, as particles and powders and not specifically designed to develop a selective sensing device able to detect a specific pollutant. In the present work, hybrid silica micropo rous thin films have been deposited on silicon substrates, to obtain a platform for infrared gas sensing; this kind of systems can trap volatile molecules in a styrene-rich environment and allow monitoring the gas amount by using FTIR spectroscopy, capable of selective detection among the different VOCs at concentrations down to few ppm. A proof of concept styrene gas sensor based on microporous hybrid silica-based films is here reported; the device is simple and very effective in selec tive detection of styrene vapours.
for 7 days, at 25 � C, in a closed vial25 then TEOS-PhTES thin films were deposited by dip-coating silicon wafers in the precursor solution at 25 � C at a withdrawal rate of 15 cm min 1. A silica microporous reference film not containing the phenyl groups was prepared by mixing two solutions. In the first one 3.08 mL of EtOH were mixed with 4.26 mL of TEOS and water and, after the addition of 0.355 mL of HCl 0.768 M, the solution was stirred for 60 min. The sec ond solution was prepared by mixing 15 mL of EtOH with 1.3 g of Pluronic F127 and 1.5 mL of HCl 0.057 M. The final molar ratios were TEOS: Pluronic F127: H2O: HCl: EtOH 1: 0.005 : 5.4: 0.02 : 16.4. A dense hybrid film, also used as a reference, was prepared as previously described but without the addition of the surfactant template. The relative humidity (RH) inside the dip-coater chamber was maintained between 24 and 25% with a temperature of 25 � C, to obtain good optical quality and high structural order in the silica films. To in crease the inorganic polycondensation and stabilize the mesophase the films were thermally treated at 120 � C for 1 h. The thermal treatment is important to allow the formation of highly organized, high surface area microporous coatings. Stacked multi-layer coatings (3- and 5-layers) were tested to optimize the thickness in light of the sensing applica tion, by depositing each layer on the silicon wafer and then drying the film for 1 h in oven at 120 � C. A final calcination step was performed at 350 � C for 3 h in air, under static conditions, with a heating rate of 10 � C min 1, to remove the organic template (F127) of the stabilized coatings. The thickness of the films before and after the thermal treatments was monitored by spectroscopic ellipsometry. The hybrid silica films were transferred in a sealed chamber and exposed for about 1 h to a styrene-rich atmosphere (Sigma-Aldrich, re agent grade, >99%), temperature range 21–25 � C and RH 35–40%. Styrene concentration within the chamber (in ppm) was monitored using a Photo-Ionization Detector (PID). After adsorption the films were quickly transferred to the FTIR optical bench for infrared analysis. 3. Characterization techniques Infrared data were collected by Fourier-transform infrared (FTIR) spectroscopy using a Bruker Vertex 70v interferometer, FTIR spectra were collected every 30 s for 1 h in transmission mode, averaging 32 scans in the 4000-400 cm 1 spectral range with a resolution 4 cm 1 and scan velocity 20 kHz. A silicon wafer was used as the background; the baseline was fitted by a concave rubber band correction with OPUS™ 7.0 software and data were analyzed by ORIGIN PRO™ software. The film thickness and residual porosity were estimated by an α-SE Wollam spectroscopic ellipsometry using a Bruggeman effective me dium approximation fitting model with two components: void [26] and Cauchy film [27]. The refractive index parameters for the Cauchy film model were measured on a monolayer reference sample treated at 120 � C for 1 h and 350 � C for 3 h. Plots of Ψ and Δ as a function of incident wavelength from 370 to 700 nm were simulated using “Com pleteEASE v. 4.2” program from Wollam. The results of the fits were evaluated on the basis of the mean squared error (MSE), which was maintained below 30. Contact angle of the films was measured using a DataPhysics Contact angle System OCA 20; the measure was performed at 27 � C and 47% RH by dispensing a 4 μL droplet on the film surface. Transmission electron microscopy (TEM) images were obtained by using a FEI TECNAI 200 operating at 200 kV working with a field emission electron gun. Before analysis, the films were detached from the substrates using a scalpel blade and the resulting fragments were dispersed in ethanol by ultrasonication for 5 min. Afterwards, the ethanol solution was dropped onto silicon copper grids and the solvent was left to evaporate for 3 min before the measurements. We have used lacey reinforced grids made by silicon monoxide (Formar stabilized) on copper (Nanovision supplier). N2 sorption isotherms were collected with a Sorptomatic 1990 in strument (Fisons). Before the measure, 600 mg of sample were
2. Materials and methods 2.1. Sol preparation and film deposition The precursor sol was prepared by mixing 8.135 mL of ethanol (EtOH), 540 μL mL of phenyltriethoxysilane (PhTES), 1 mL of tetrae thoxysilane (TEOS), 138 mg of F127 Pluronic block copolymers, 300 μL of mQ water and 25 μL of HCl (2 N) in a glass vial (molar ratios PhTES: TEOS: EtOH: H2O: HCl ¼ 0.5 : 1.0: 31.2: 3.71 : 1.55). The sol was stirred 2
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introduced in a quartz tube and degassed under vacuum (1 � 10 3 bar) at 523 K for 12 h. The dead volume was evaluated through helium measurements. The surface area was calculated with BET method, the pore size was estimated by the Dollimore/Heal method and the pore volume by Dubinin Stoeckly method. Styrene concentration was monitored using a commercial PhotoIonization Detector (PID), Model TA-2100 Styrene Detector manufac tured by Mil-Ram Technology, Inc., with a detection range from 0 to 100 ppm and 1 ppm resolution, according to the manufacturer. Adsorption experiments were performed at room temperature (25 � C) in a sealed chamber by exposing films in a styrene-rich atmosphere for 1 h. For these experiments, only 3-layers films were used as they showed both mechanical stability and good signal-to-noise ratio when used as the sensing element. Experiments were also repeated multiple times in the same film to test the durability and the performances over time. The same sample was used for the first set of adsorption-desorption styrene run, left in the air to stabilize after the experiment, and then reused for a second and third adsorption-desorption run.
(red line); the 3- (blue line) and 5-layers (green line) films after calci nation at 350 � C are also reported. In the low wavelength region the most intense bands about 1076 cm 1 and the shoulder around 1200 cm 1 are assigned to Si–O–Si anti-symmetric stretching vibrations [29]. The intense band peaking at 1138 cm 1 is instead assigned to C–H stretching (Si–C in phenyl groups) from phenyltriethoxysilane [16]. The absorption bands at 810 and 480 cm 1 are also assigned to silica vibrational modes, stretching and rocking modes, respectively. The band at 960 cm 1 is ascribed to the Si–OH groups. The sharp bands at 1433 and 1595 cm 1 and the broader bands at 698 and 740 cm 1 are corre lated to the vibrations of the phenyl groups [30]. The inset in Fig. 2 shows a detail of the single-layer film FTIR spectrum before and after calcination. The FTIR spectra confirm that after thermal treatment most of OH groups (wide band peaking around 3400 cm 1) and surfactant (2981, 2989 and 2900 cm 1 peaks) have been removed. The spectra after calcinations show a new band around 3700 cm 1, which is the signature of the presence on the pore surface of isolated and twin sila nols formed upon calcination. This indicates that, as previously reported for mesoporous silica membranes [16], the thermal treatment at 350 � C, besides removing the surfactant, induces a peculiar change on the pore surface whose coverage by a continuous network of silanols is replaced by isolated and twin Si–OH groups. It is important to underline that both the microporous silica films not containing the phenyl groups and the dense hybrid films have been tested and not shown any sensing capability to styrene (See Fig. S2, Supplementary Information). The presence of porosity and of phenyl groups is thus an important feature to develop selective and effective sensing for styrene. TEM analysis (Fig. 3a) show the presence of pores in
4. Results and discussion The overall quality of the single and multilayers hybrid films has been assessed by optical microscopy and ellipsometry. Both single-layer and 3-layers films looked very homogenous with no discontinuities or surface imperfections. On the opposite, 5-layers films always presented cracks on the film surface (see Fig. S1, Supplementary Material). Refractive index, n, measurements on single and 3-layers films yield to an average value n of 1.45 � 0.02. No significant differences have been observed between single-layer and 3-layers films, while a decrease in n from 1.50 to 1.45 is observed after calcination at 350 � C. The thickness measured for the single-layer film is about 320 nm before calcination and 250 nm after calcination. As expected, the 3-layers films were about three times thicker than single-layer samples, with a thickness of 970 nm and 740 nm, before and after calcination, respectively. The observed uniaxial film shrinkage is due to the condensation of the hybrid silica walls and the partial collapse of the porous structure by thermal treat ment [28]. Ellipsometry measurements have also been used to evaluate the effect of sol ageing on the thickness and refractive index. The 3-layer films from the sols aged for two weeks have shown an average thickness increase from 740 � 11 nm to 858 � 21 nm. The thickness, after three ageing weeks of the sol, became 931 � 39 nm. The higher thickness with sol ageing is due to the proceeding of the polycondensation reactions, which causes an increase in viscosity and of the film thickness. The porosity of the films has been also evaluated by the spectroscopic ellipsometry. A single layer sample (276 � 0.2 nm), after the full cycle of thermal treatments (120 þ 350 � C), has a 9.8% � 0.43 of porosity and average surface roughness of 9.12 nm with a MSE ¼ 24.7. The hybrid microporous films have a surface which becomes hy drophobic in comparison to pure silica, due to the presence of the phenyl groups; the contact angle increases to 44.5� in comparison to 6.5� measured in silica (Fig. 1). Fig. 2 shows the FTIR absorption spectra (4000–400 cm 1) of the 1layer PhTES-TEOS film before (black line) and after thermal treatments
Fig. 2. FTIR absorption spectra of 1-layer film before (black line) and after thermal treatments (red line), 3-layers (blue line) and 5-layers films (green line) treated at 350 � C. The inset shows a detail of the high wavenumber region for 1layer film as-deposited and after calcination. (For interpretation of the refer ences to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 1. Contact angle measured in (a) silica and (b) hybrid microporous films after treatment at 350 � C. 3
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the hybrid films with a �2 nm size. The pores are not organized and likely resulting from a general collapse of larger pores. Althought the addition of a triblock-copolymer in the sol generally leads to the for mation of an ordered porous structure, the presence of PhTES as the precursor, hampers the self-assembly process, explaining the lack of long-range order observed in the films. Moreover, the thermal treat ment, which is required to densify the pore walls and remove the tem plating agent, is likely responsible of a general pore collapse, as already reported for mesoporous materials with random pore organization. Nitrogen sorption isotherms have been measured on the material obtained from the hybrid sols previously used for film deposition (Fig. 3b). The sol has been cast on Petri dishes, dried in air and then thermally treated up to 350 � C with the same thermal ramp used for preparing the films. Other analytical techniques, specifically developed for the analysis of thin films, such as the environmental ellipsometric porosimetry [31], cannot be used because the hybrid matrix does not allow measuring the porosity using water or isopropylic alchohol adsorption/desorption isotherms [32]. The adsorption isotherm shows a type I trend, typical of microporous materials, as confirmed by the sharp increase at p/p0 < 0.02 relative pressure range. On the other hand, the multilayer adsorption and the capillary condensation observed at higher pressure, for p/p0 > 0.5, suggest the existence of mesopores and mac ropores in the hybrid silica, as already reported for porous coal [33,34]. This analysis confirms the hypothesis of a partial collapse of the microporous structure as a consequence of the thermal treatment. The estimated surface area, as obtained by BET method, is 114.82 m2 g 1 while the pore radius, as calculated by the Dollimore-Heal method, in dicates a bimodal distribution with small pores with a radius of �1 nm and a larger pores with a radius of around �50 nm. Using the Dubinin-Stoeckli method we have also estimated the volume of the smaller pores as 0.24 cm3 g 1. The analysis of the N2 sorption isotherms confirms the results of TEM measurements. The films show a not completely organized porous structure with small pores in the range between 1 and 2 nm (Fig. 4). The lack of long-range pore organization has been also confirmed by low-angle XRD (the pattern can be found in Supplementary Material SI2).
interaction, however, is weak and styrene molecules are stored in the film for a limited amount of time. Fig. 5a shows the FTIR spectrum in the 950-690 cm 1 range for single and multi-layer films before (black lines) and after (red lines) exposure to a styrene concentration in the atmo sphere of 73 ppm. In each case, after the exposure, the appearance of new infrared absorption bands at 910 (out-of-plane CH2 wagging) and 780 cm 1 (deformation vibration of CH2 in the aromatic ring) is observed. These bands are characteristic of the styrene molecules and after dozens of minutes of exposure to a styrene-free environment pro gressively disappear (vide infra). The appearance of these bands is experimental evidence of the diffusion and entrapment of the styrene molecules inside the phenyl bearing hybrid matrix. The absorption band peaking at 820 cm 1 is assigned to symmetric stretching of Si–O–Si bonds [35]. The FTIR absorption spectra of single and 3-layers microporous silica films (Fig. 5b) clearly show that the styrene molecules are not adsorbed into the sensing layer if the phenyl groups do not modify the surface. They play, therefore, a fundamental role to adsorb the pollutant, no traces of adsorbed styrene molecules can be found, in fact, in the microporous silica films (The full range FTIR absorption spectra can be found in the Supplementary Material SI3). A styrene sensor based on the detection of specific infrared bands of the pollutant has the advantage to be potentially highly selective. Fig. 6 shows the FTIR absorption spectra (950–650 cm 1) of different com pounds. The 910 and 780 cm 1 vibrational modes do not overlap with any other signal from isopropanol, toluene, ethanol, acetone and xylene. It also has to be stressed that the presence of water in the environment does not affect the detection of styrene because the film is hydrophobic (Fig. 1) and in the infrared detection range, no signals of water are present. Fig. 7a and b shows the typical desorption curves obtained by monitoring the 910 and 780 cm 1 styrene absorption bands as a function of time. The details of the desorption experiments performed on the same film exposed to increasing styrene concentrations: 12, 34 and 58 ppm, are shown in Fig. 7c and d (780 cm 1 band). The maximum absorbance and the final desorption time is proportional to the styrene concentration measured at the time zero by the PID sensor, thus indi cating, that at room temperature (RT), the styrene concentration inside the PhTES/TEOS microporous film reaches the steady-state equilibrium with the environment. Additionally, most of the styrene is expelled in the first 20 min (Fig. 7a); after 1 h, the film is almost empty and ready for reuse.
4.1. Adsorption mechanism and desorption process Fig. 2 shows that the hybrid matrix still contains phenyl groups (e.g. 698 and 740 cm 1 bands) after the thermal treatment at 350 � C. The presence of these groups is important because they act as specific sensing sites for styrene molecules. As the films are exposed to a styrene-rich environment, the organic molecules diffusing inside the structure are trapped by the π-π interactions with the phenyl groups in the film. This
4.2. Calibration curve and diffusion coefficient The previous data point out that the desorption from the PhTES/ TEOS films depends on the initial styrene content. Therefore, we have modelled the data based on a mono-dimensional diffusion from a plane sheet of finite thickness [36,37]. To apply this formalism to our data we assume that: (1) the styrene concentration at any time within the film, i.e. not to be confused with the concentration in the atmosphere measured by the PID instrument (see below), can be represented by the integrated absorbance At; (2) any styrene molecule desorbed from the film surface is considered lost because the overall absorbed styrene volume inside the film is insig nificant compared to the volume of the measurement chamber; and (3) the diffusion parallel to the film surface is neglected as the analytical spot size is significantly smaller than the whole absorbing surface. The resulting equation is (Eq. (1)): � � � ∞ � X 8 Dð2n þ 1Þ2 π 2 t At ¼ A0 ⋅ ⋅exp (1) 2 2 4L2 n¼0 ð2n þ 1Þ π where At is the integrated absorbance (At) at time ¼ t, A0 is the inte grated absorbance At at time ¼ 0, D is the diffusion coefficient in m2s 1, t is the time in seconds, L is sample thickness in m, and n is the index of
Fig. 3. Nitrogen sorption isotherms of the hybrid materials, obtained from the sol used for film deposition, after treatment at 350 � C. The inset shows the pore size distribution obtained by Dollimore/Heal method. 4
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Fig. 4. Representative TEM images of the 1-layer film after thermal treatment and enlargement of the mesoporous structure (inset). The crystalline particles in C are small fragment of the Silicon substrate.
Fig. 5. a) FTIR absorption spectra in the 950-690 cm 1 range for single and multi-layer films before (black lines) and after (red lines) styrene exposure. Styrene atmospheric concentration was 73 ppm (PID reading). The absorption bands at 910 and 780 cm 1 related to styrene are highlighted by dotted vertical lines; the 740 cm 1 vibrational mode, due to phenyl group in the films, is also shown as a reference. b) FTIR absorption spectra of the single and 3-layers microporous silica films before and after exposure to styrene. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
summation. A0 and D parameters have been obtained using the fitting routine in the OriginPro™ (OriginLab, USA) software; eight exponential terms (n ¼ 8) have been used for Eq. (1) to obtain convergence. Errors on D/L2 and A0 have been calculated from the fitting error given by the used routine. Normalization of the integrated absorbances (Ai) to a
standardized 740 nm thickness (the average thickness of 3-layers sam ples) has been performed using the 780 cm 1 film absorption band as reference. Data collected after 20 min have been excluded from the fit because, as observed, most of the styrene is desorbed in the first 24 min. Moreover, for low concentrations, the extracted areas are less reliable because of the signal/noise ratio, interference fringes and/or baseline 5
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modelling. Table 1 lists the diffusion parameters calculated for both 910 cm 1 and 780 cm 1 styrene signals in the different sets of experiments. The samples 1, 2 and 3 have been used for three runs adsorption-desorption experiments (a, b and c), while the films 4, 5, 6, 7 and 8 for a single run. The R2 value for the fitted curve is usually above 95%, except the ex periments 1a and 3c (see Table 1) where low styrene concentrations at the equilibrium have given higher associated errors. Data obtained from the 780 cm 1 band have lower errors, however, A0 and D values ob tained from both 780 and 910 cm 1 styrene bands tend to overlap thus proving that both the infrared vibrational modes can be used for diffu sion and sensing purpose. The resulting average D value obtained by fitting the 780 cm 1 desorption curve is 3.8 � 0.6⋅10 16 m2s 1 (-logD ¼ 15.43 � 0.07) and for the 910 cm 1 band is 4.8 � 1.0⋅10 16 m2s 1(-logD ¼ 15.3 � 0.1) at 25 � C. 4.3. PhTES-TEOS films as sensing elements for styrene
Fig. 6. FTIR absorption spectra in the 950 - 650 cm toluene, ethanol, acetone, xylene and styrene.
1
Fig. 8 shows the correlation of the concentration of styrene at equi librium in the atmosphere (PID readings in ppm) with the absorbances at the time 0 (A0) for the 780 cm 1 styrene absorption band (Table 1) fitted by Eq. (1). The experimental points (black dots) show that, in the investigated range, the styrene concentration probed in the atmosphere is not linearly correlated to the FTIR absorbances. After steadily increasing the styrene concentration up to 40/50 ppm, the FTIR absor bance values asymptotically set to a threshold value. The following equation (Eq. (2)) well describes the styrene concentration-infrared absorbance correlation:
interval of isopropanol,
A0 ¼ a⋅ð1
bppm Þ
(2)
Fig. 7. 3D-time resolved FTIR spectra (absorbance-wavenumber-time) of the 780 and 910 cm 1 bands (a and b, respectively). Selected FTIR spectra of the 780 cm 1 styrene absorption band at different desorption times, background correction is applied (c). Desorption curves for the 780 cm 1 styrene band as a function of time. The experiments have been performed on the same film (sample 1) exposed each time to a different styrene concentration (12 ppm black line, 34 ppm red line and 58 ppm blue line) (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 6
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Table 1 PID readings and diffusion parameters, D, obtained using the monodimensional plane-sheet diffusion model for both styrene signals at 780 and 910 cm 1. The fitted integrated Absorbance (A0) is normalized to a 740 nm thick film. In brackets are reported the uncertainties (�) on the last digit. The experiments have been performed at 25 � C. Samples 1, 2 and 3 have been used for three runs adsorption-desorption experiments (a, b and c), while the films 4, 5, 6, 7 and 8 for a single run. Sample
PID reading (ppm)
Band at 780 cm A0 (cm
1a 1b 1c 2a 2b 2c 3a 3b 3c 4a 5a 6a 7a 8a
12 34 58 16 39 43 100 84 12 82 55 35 43 18
0.033 0.089 0.094 0.053 0.105 0.100 0.118 0.108 0.016 0.101 0.086 0.076 0.084 0.039
1
)
(1) (2) (2) (1) (2) (2) (2) (3) (1) (3) (2) (2) (1) (1)
1
Band at 910 cm 2
1
D (m s ) 3.0 4.7 4.6 3.5 4.2 4.3 4.1 3.9 4.2 4.1 3.1 3.1 3.8 2.8
(1)10 (2)10 (2)10 (1)10 (1)10 (1)10 (1)10 (2)10 (4)10 (2)10 (1)10 (1)10 (1)10 (1)10
16 16 16 16 16 16 16 16 16 16 16 16 16 16
1
1
-logD
A0 (cm )
D (m2s 1)
-logD
15.53 (2) 15.33 (1) 15.34 (1) 15.46 (1) 15.38 (1) 15.37 (1) 15.39 (1) 15.41 (2) 15.38 (4) 15.39 (2) 15.51 (2) 15.52 (2) 15.42 (1) 15.56 (1)
0.018 (1) 0.088 (1) 0.079 (3) 0.046 (1) 0.114 (2) 0.121 (2) 0.122 (2) 0.118 (3) 0.007 (1) 0.094 (2) 0.081 (2) 0.066 (2) 0.078 (2) 0.030 (1)
4.9 (3)10 16 5.8 (1)10 16 5.2 (2)10 16 3.7 (1)10 16 5.5 (1)10 16 3.8 (1)10 16 5.9 (1)10 16 5.5 (2)10 16 6 (1)10 16 3.0 (1)10 16 3.6 (2)10 16 4.5 (2)10 16 5.1 (2)10 16 4.3 (1)10 16
15.31 (3) 15.24 (1) 15.29 (2) 15.44 (1) 15.26 (1) 15.42 (1) 15.23 (1) 15.26 (1) 15.19 (7) 15.53 (2) 15.44 (2) 15.35 (2) 15.29 (1) 15.37 (1)
site, has a fast response and can be developed as a portable device or a dosimeter. The sensing results in this study indicate that functionalized microporous hybrid silica is also a promising material for nose-on-a-chip applications and wearable devices with many potential applications for indoor monitoring, a real emerging area of research. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work is part of the BRIC ID12 project financed by INAIL. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2019.109877.
Fig. 8. Integrated absorbance of the 780 cm 1 styrene band vs. PID reading. The experimental data (dots) are fitted using the equation: y ¼ a(1-bx) with x ¼ ppm.
References Where a ¼ 0.127 � 0.005 and b ¼ 0.975 � 0.003. The observed response is interpreted as a saturation effect of the hybrid silica film; for a given thickness, th film cannot uptake, in fact, more than a certain amount of styrene molecules.
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5. Conclusions Nanoporous hybrid organic-inorganic silica thin films have shown a remarkable sensing capability to detect low concentrations of styrene vapours. The combination of porosity and phenyl groups on the pore surface has allowed achieving selective adsorption of styrene. Dense or microporous films without phenyls do not uptake styrene and do not show any sensing response. The presence of styrene (<100 ppm) has been evaluated in an experimental chamber using infrared absorption spectroscopy. Threelayers microporous films have shown an optimised response to adsorption-desorption cycles. The styrene diffusion coefficient has been calculated using a mono-dimensional diffusion from a plate sheet of finite thickness model. This study shows that functionalized microporous silica film can be applied as a high-performance chemical sensing material for detection of styrene and other VOCs in the ppm range. The sensor can be used on7
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