Crystalline Microporous Organo-Silicates with Reversed

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Crystalline Microporous Organo-Silicates with Reversed Functionalities of the Organic and Inorganic Components for Room-Temperature Gas Sensing Barbara Fabbri, Lucia Bonoldi, Vincenzo Guidi, Giuseppe Cruciani, Davide Casotti, Cesare Malagu, Giuseppe Bellussi, Roberto Millini, Luciano Montanari, Angela Carati, Caterina Rizzo, Erica Montanari, and Stefano Zanardi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b02122 • Publication Date (Web): 28 Jun 2017 Downloaded from http://pubs.acs.org on July 4, 2017

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ACS Applied Materials & Interfaces

Crystalline Microporous Organo-Silicates with Reversed Functionalities of the Organic and Inorganic Components for Room-Temperature Gas Sensing Barbara Fabbri,† * Lucia Bonoldi, ‡ Vincenzo Guidi, † Giuseppe Cruciani, † Davide Casotti, † Cesare Malagù, † Giuseppe Bellussi, ‡ Roberto Millini, ‡ Luciano Montanari, ‡ Angela Carati, ‡ Caterina Rizzo, ‡ Erica Montanari, ‡ and Stefano Zanardi ⸸ AUTHOR ADDRESS. † University of Ferrara, Department of Physics and Earth Sciences, via G. Saragat 1, 44122, Ferrara, Italy, ‡ Eni Spa, San Donato Milanese Research Center, via F. Maritano 26, 20097, San Donato Milanese, Italy, ⸸Eni Spa, Renewable Energy and Environmental Laboratories, via G. Fauser 4, 28100, Novara, Italy * Address correspondence to [email protected] KEYWORDS. Eni Carbon Silicates; microporous hybrids; humidity sensor; room temperature; reversed functionalities.

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ABSTRACT. A deepened investigation on an innovative organic-inorganic hybrid material, referred to as ECS-14 (Eni Carbon Silicates), revealed the possibility to use them as gas sensors. Indeed, among ECS phases, the crystalline state and the hexagonal microplatelet-like morphology characteristic of ECS-14 seemed favorable properties to obtain continuous and uniform films. ECS-14 phase was used as functional material in screen-printable compositions, thus deposited by drop coating for morphological, structural, thermal, and electrical characterizations. Possible operation at room temperature was investigated as technological progress, offering intrinsic safety in sensors working in harsh or industrial environment, and avoiding high power consumption of most common sensors based on metal-oxides semiconductors. Electrical characterization of the sensors based on ECS-14 vs. concentrations of gaseous analytes gave significant results at room temperature in presence of humidity, thereby demonstrating fundamental properties for a good quality sensor, i.e. speed, reversibility and selectivity that make them competitive with respect to systems currently in use. Remarkably, we observed functionality reversal of the organic and inorganic components, i.e., in contrast to other hybrids, for ECS-14 the functional site has been ascribed to the inorganic phase whilst the organic component provided structural stability to the material. The sensing mechanism of humidity was also investigated.

1. INTRODUCTION The great expectations of gas-sensor market have fostered the research in material science and technology towards a constant development. In particular, room-temperature operation is a keyfeature due to intrinsic safety requirements for the sensors working in harsh or industrial environment.1,2 Such an operation mode may also be attractive to reduce the power consumption


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of the sensing unit, which negatively characterizes the overwhelming majority of currently used materials for sensing, e.g. Metal-OXides semiconductors (MOX), and ultimately scales down the size of associated electronics. In fact, for metal oxides, the operational temperature, typically ranging within 250-650 °C, promotes the reversibility of chemical reactions between gaseous molecules and semiconductor nanostructures inducing bulk/surface charge transfer. At room/low temperature, MOX sensors lack in sensitivity, stability and ultimately in reversibility. In recent years, photo-activation of MOX sensors has represented a valuable method for gas sensing at room temperature to recover reversibility, showing as rapid a kinetics of reaction as for heated devices, through their sensitivity still keeps significantly lower than for the thermo-activation mode. Thus, the search for more suitable materials is under continuous evolution.3 With this aim, hybrid organic-inorganic nanostructures constitute an effective alternative, because they may combine the advantages of both components. Ideally, the mechanical, structural, and hydrothermal stabilities of the inorganic counterpart can be merged with the flexibility and functionality of the organic part, which is also able to induce highly specific chemical reactions.4,5 Among the wide assortment of functional hybrids, silica-based mesoporous materials are attractive because of their very high specific surface area, an ideal situation for the applications where functionality is magnified by the surface exposed for interaction with the environment, such as gas detection or storage, heterogeneous catalysis, and solar cells. Basically, there are three routes to bond an organic component to a silica matrix, i.e., by grafting, cocondensation, and the use of single-source precursor.6,7 The advent of ECSs (Eni Carbon Silicates) has revolutionized the synthesis of organic–silica materials.8,9 Indeed, ECSs are the result of a highly refined development of hybrids synthesis. At variance to the hybrids obtained by post-synthetic functionalization of silica, organic and inorganic components are perfectly


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merged each other in ECSs, due to the synthesis by condensation of bis-silylated organic precursor, yielding a complex structure where each silicon atom is involved in one Si-C covalent bond. Several organo-silica sources were employed as precursors, without surfactant agent, and with addition of NaAlO2 as an aluminum source, yielding crystalline phases. The presence of aluminum plays relevant role by favoring the crystallization of ECSs.10,11 The novelty of the crystalline ECSs concerns their structures, i.e. aluminosilicate scaffoldings with long-range 3D order, which distinguish them from the previously reported amorphous or at most “crystal-like” silica-based PMOs (Periodic Mesoporous Organosilicas).12,13 Such materials feature functional active sites with tunable geometrical and chemical properties. In particular, their structure allows facile access of gaseous molecules into the framework, as reported by Yuliarto et al. (2009) for PMOs.14 ̶ 16 A recent trend in gas sensing revealed remarkable dependence of the sensing performance on sample morphology. In particular, quasi one- or two-dimensional (Q1D, Q2D) nanostructures resulted in long-term stability with respect to their nanograined counterparts, which are traditionally employed for gas sensing.17 Thereby, ECSs would be well suited because their highly ordered arrangement is extended on 3D and the majority of crystalline phases possesses plate-like morphology.10,18,19 It is the purpose of this work to identify a suitable ECS phase, which could be easily printed as films, and to prove its possible operation as a functional material for gas sensing at room temperature. Among ECS phases, it was thought that the crystalline state and the hexagonal microplatelet-like morphology characteristic of ECS-14 were favorable properties to obtain continuous and uniform films. ECS-14 consists of organic–inorganic layers bonded together by


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sodium ions and crossed by linear channels, 7 Å in diameter including benzene rings — an ideal condition for diffusion of gaseous analytes. 10,18,19 By applying a multiscale characterization approach (Scanning Electron Microscopy, X-Ray Diffraction, Thermo-Gravimetric analysis), we demonstrated the stability and functionality of films prepared with ECS-14 powder by simple deposition technique. Furthermore, we studied possible electrical activity of these films for their employement as functional layers in gas sensors exposing them to different gaseous compounds, i.e., alkanes, aldehyde, nitrogen compounds, ketones, alcohols, aromatic compounds, and humidity. In particular, the sensing performance of the films were studied at room-temperature operation mode in order to highlight possible technological advantages for such completely novel application of ECSs materials.

2. MATERIALS AND METHODS 2.1 Materials and Devices Preparation. The ECS-14 powder (named ECS-14) was used as functional material in a process refined for the production of printing pastes for thick films. The study on the interaction between ECS-14 phase and screen-printing paste components led to an optimal formulation based on ECS-14 powder as functional material and an organic vehicle as dispersing medium. The suspensions were treated with two cycles of ultrasounds (each of one hour, 300 W power at 50 °C) limiting the temperature in order to avoid possible material degradations (Figure S1, Supporting Information). After these treatments, the suspensions were still too dense to be deposited by drop-coating, then 100 µl of absolute ethanol were added to the samples, which were subsequently treated by magnetic stirring for almost 12 hours. This formulation hereinafter is referred to as ECS-14P. Preliminary analysis showed a reduction of the agglomerates by means of ethanol. The introduction of magnetic agitation as disaggregating


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treatment conferred greater uniformity to the films considerably reducing the quantity and the size of agglomerates (Figure S2, Supporting Information). By means of drop-casting the samples were deposited onto silicon (for SEM, XRD, thermal analysis) and alumina substrates (for electrical characterization). 2.2 Nanomorphology and Nanostructure Investigation. SEM images were recorded by Zeiss SEM EVO 40 (Source LaB6, maximum acceleration voltage 30 kV, variable pressure chamber non-metallic samples). The structural investigation by XRD w performed with Bruker D8 Advance diffractometer (tube: Cu Kα1, α2; 40kV; 40mA; detector: SiLi (SOLX), geometry θ-θ, scan-step 0.02 ° 2θ, 10s/step), and XRD data were processed using the program Topas® Bruker AXS. The XRD patterns collected on film depositions were performed on sample holder in zero back-ground silicon. N2 adsorption/desorption isotherms for ECS-14 as-synthesized and ECS-14P film were obtained at – 196 °C with Micromeritics ASAP 2020 surface area and porosity analyzer. The isotherms acquisition was carried out after out-gassing the samples overnight at increasing temperatures under vacuum. The specific surface area were obtained applying the Brunauer–Emmett–Teller (BET) theory. 2.3 Thermal and Optical Characterization. Thermal analyses were carried out by a thermogravimetric balance Netzsch STA 409 PC Luxx. In this analysis, three samples were considered: pure ECS-14 powder, ECS-14P, and a mixture of organic vehicle and alumina in order to achieve depositions that contained the component in suspension with the characteristic of being inert to a thermogravimetric analysis. The acquisitions were performed in airflow (20 ml/min) in the ramp (10 °C /min), from 25 °C up to a maximum temperature of 800 °C since the organic component was completely burned out at this a temperature.


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Reflectance and fluorescence spectra were recorded on powder samples in quartz cells with 1 mm optical path, provided with a stopcock for dehydration treatment. Reflectance spectra were recorded with a Perkin Elmer Λ-19 spectrophotometer equipped with a 60 mm diameter integrating sphere. Fluorescence spectra were measured with the spectrofluorometer Perkin Elmer LS50-B in front face configuration, at 30°-60° excitation-emission geometry with a 1% transmission neutral filter between the sample and the detector in order to avoid detector saturation due to the strong intensity of the emitted light. Reported emission spectra were corrected for the detector sensitivity with the correction file provided with the instrument. 2.4 Electrical Characterization. The electrical characterizations were carried out at room temperature by a dedicate apparatus, which consists of a flow meters system for gas mixing, a test chamber, and a data acquisition system (Figure S3, Supporting Information). The test chamber (volume of 500 cm3) is regulated by a circuit whose main component is constituted by an operational amplifier (OA). The input signal is connected at the negative entrance of the amplifier whereas the positive one is grounded. The voltage values Vin and Vout are connected at the ends of sensor resistor RS and applied load resistor Rf, respectively. Then, the gain is given by Vout / Vin= - Rf / RS. Fixing Vin equal to -5V, because of the virtual short circuit to the inputs of the OA, the inverting input terminal results at the same potential that the non-inverting input terminal. In this way, the sensor is subjected to a constant potential difference of 5 V until the OA works far from the saturation. Being the values of Vin and Rf known and constant, the output voltage Vout is then proportional to the conductance. The expression for sensor conductance GS results to be:

GS =

1 V = − OUT RS R f ⋅ VIN

(1)


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Then, the expression of response R assumes the simple shape of a normalized conductance variation, independent from the circuit parameters:

Re sponse =

∆G G gas − G air VOUT ( gas ) = = −1 G air G air V IN (air )

(2)

The acquisition system must manage all phases regarding the sensor electronics and it is composed of a Multimeter K2000 (Keitheley), to convert all output voltages from analog to digital, and a management/acquisition software consisting of a program (TestPoint) capable of acquiring, at regular intervals, the input channel of the multimeter (gas sensor, a temperature sensor and a humidity sensor). 3. RESULTS AND DISCUSSION An extensive study about the interaction of the ECS-14 phase (Figure 1) with each component commonly employed in the screen-printing pastes has identified an optimal formulation through which the ECS powder was added to a suitable organic vehicle.20


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Figure 1. ECS-14 structure (top left), and projection along [001] (top right): [SiO3C] tetrahedral in blue, [AlO4] in cyan, phenylene rings in brown, oxygen atoms in red; sodium in yellow takes up three sites which have different structural role. H2O molecules can be distinguished by H atoms in white. Linear channels of 7 Å in diameter are along [001] direction. This formulation (ECS-14P) combined with the use of selected solvents (ethanol), ultrasounds and magnetic stirring, proved successful to obtain well adherent, continuous and uniform films. The last were deposited by drop-casting onto alumina substrates with interdigitated gold electrodes (Figure 2A), and their thickness was in the order of 30ൊ40 µm. We chose a deposition method for which no thermal treatment was needed because a preliminary study on ECS-14 powder heated at 200 °C for 1 hour resulted in material degradation (Figure S1, Supporting Information).


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Figure 2. A) Device based on ECS-14 phase. SEM image of B) ECS-14 powder, C) ECS-14P film. 3.1 Morphological, Structural and Thermal Properties of the Films. SEM investigations on ECS-14 powder (Figure 2B) and on ECS-14P film (Figure 2C) showed hexagonal micrometer-sized platelets in agreement with previoulsy reported observations.10 Both samples showed a narrow distribution of particle size. ECS-14 powder exhibited little aggregation of crystals, while in the ECS-14P film crystals were loosely aggregated with large intergrain porosity. At the same time, XRD patterns revealed that the structural properties of the ECS-14 powder are maintained, i.e., the diffraction pattern of ECS-14P film perfectly overlaps that of pristine powder. Moreover, the XRD analysis highlights the almost perfect iso-orientation of the crystallites lying with the 001 lattice plane parallel to the sample surface as revealed by the major occurrence of diffraction lines with indexes 00l (l = 2,4,6, etc.), i.e., the 2nd, 4th, 6th, etc. order reflections of the 001 plane (Figure 3).


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Figure 3. Diffraction patterns of ECS-14 pure powder (blue, below), and ECS-14P film (black, above). Morphological and structural characterizations confirmed the expectations for ECS-14 as a printable material: the iso-orientation of its crystallites promotes the continuity and the adhesion of the film, while the open diffused porosity of channels increases the sensing area (Table S1 and S2, Supporting Information).a TG/DTG analyses were performed on ECS-14 pure powder, organic vehicle on alumina, and ECS-14P. It came clear that weight losses in ECS-14P were mainly ascribable to evaporation of the organic-vehicle (Figure S6, Supporting Information). For both ECS-14 powder and the ECS-14P film, significant weight losses occurred at temperatures close to that previously identified as degrading for the material (200 °C). Thereby, it was chosen to operate at room temperature, not only during the processing but also during the electrical characterization. 3.2 Electrical Activity of the Films. In order to assess any possible employment of this material as a sensing device, we exposed the film to several analytes, i.e., methane, acetaldehyde, ammonia, acetone, toluene, ethanol, benzene, and humidity. The real-time electrical characterization was carried out at room temperature in a dedicated test chamber under controlled atmosphere (Figure S3, Supporting Information). The sensing performance of ECS-

a

Unfortunately, it was not possible to measure directly the specific surface area of ECS-14, because of structural modifications occurring during the vacuum pretreatments prior to N2 adsorption in the BET measurements (Table S1 and S2, Supporting Information). Briefly, after degassing/dehydration XRD measurements show that the structure is significantly modified. Therefore, the specific surface area calculated by BET is not representative of the pristine crystalline material one. See the detailed discussion in the Supporting Information, Section 4, Study of the porosity.


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14P film was evaluated vs. the standard approach of “3S-Rule”b, i.e., Sensitivity, Selectivity and Stability.21-25 Measurements showed high Sensitivity to humidity, resulting in significant conductance change, whereas the response for the other gases resulted negligible (Figure S5 and S6, Supporting Information). Therefore, it was clear that ECS-14P film was Selective to humidity with its calibration shown in Figure 4A. Here, one can observe that ECS-14P sensing films detect humidity even at RH = 20% (inset), exhibiting a fully reversible process.

b

The Sensitivity is the slope of the calibration curve, i.e., how large is the change in the sensor signal upon a certain change in the analyte concentration. The Selectivity is the ability of a sensor to respond primarily to only one chemical species (specific sensor response) in the presence of other species (interferants). The Stability is the ability of a sensor to provide reproducible results for a certain period of time and it depends on the drift of the baseline signal, the drift of response, and the regeneration after the interaction with analyte. In particular, longterm stability is related to ageing of the sensor materials.25


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Figure 4. A) Relative humidity (at room temperature) calibration of sensor based on ECS-14P, as dynamic response. Inset: zoom of the range 0-8000 s. B) Humidity calibration curves: resistance values of ECS-14P (red) and Na-Y-P (black) sensing films vs. humidity concentration. Inset shows normalized percentage responses of ECS-14P sensor vs. humidity concentration. Numerical values are reported in Table S3, Supporting Information.


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The response and recovery times of these films, calculated as the time necessary to attain 90% of steady-state sensor response and as the e-folding response, respectively, ranged from 500 s to 1000 s. These are four times lower than the response and recovery times exhibited by commercial HIH-4000 Honeywell capacitive humidity sensor used to control the relative humidity in the test chamber. Therefore, the characteristic times of ECS-14P sensor are perfectly comparable to those of other humidity sensors.26-28 Figure 4B shows resistance variation values of ECS-14P film vs. the humidity concentration (red curve). These values are in the same order of those measured in the presence of humidity with sensing film based both on pure inorganic and pure organic materials, such as MOX semiconductors (e.g. MnO) and polymers (e.g. polyaniline (PANI)).26-28 It has been proved that the employment of organic-inorganic hybrid materials enhances humidity sensing properties. In particular, porous hybrid structures exhibit higher humidity sensitivity than the nonporous counterparts. In this sense, ECS-14P film satisfactorily inserts itself in the state of art. 26-28 The inset in Figure 4B shows the percentage normalized response of ECS-14P sensor as function of humidity concentration. The values increase gradually vs. RH up to about 40%, beyond this value the increase proceeds at a faster rate. Since the response is nonlinear, one can observe that the Sensitivity of the ECS-14P sensor, the slope of the calibration curve, is not a constant value. This is in line with the systems currently in use.26-28 The Stability of the sensing film and the repeatability of its response vs. humidity were not affected by any memory effects in the material during the dry-wet cycles. 3.3 Study on the Functionalities of the Organic and Inorganic Components. The chemoresistive behavior showed by ECS-14P film required further investigation to understand how ECS-14 components, organic and inorganic, play a role in the sensing mechanism of


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humidity. Indeed, it was necessary to identify which component acts as a receptor and/or transducer for gas sensing.29 To evaluate separately the role of organic and inorganic components of ECS-14, we tested one sample of Na-Y zeolite (HSZ 320 NAA, TOSOH)30 and 1,4-bis(triethoxysilyl)-benzene (BTEB),10 as inorganic and organic reference material, respectively.c Considering the above-mentioned components, the interaction with water molecules is well documented only for zeolites. Indeed, these microporous aluminosilicates are commonly used as adsorbents and catalysts because their porous structure can accommodate a wide variety of cations and select molecules primarily based on a size-exclusion process. It was also demonstrated that Y zeolites containing sodium adsorb more water than the same zeolite without sodium.31,32 Water molecules adsorbed in zeolite frameworks tend to form 3D hydrogen bonded ring structure, and water desorption is promoted only by heating the zeolite.31,32 Hence, in order to investigate the interaction of the two ECS-14 components with moisture, we prepared two devices following the same preparation employed for ECS-14P and using active layers obtained by deposition of the inorganic Na-Y zeolite (Figure S7, and Figure S8, Supporting Information), hereinafter referred to as Na-Y-P, and the bis-silylated organic precursor. Unfortunately, in the last case the quality of the film did not allow the electrical tests. Figure 4B shows the comparison of resistance values of ECS-14P and Na-Y-P Paste films vs. RH%. One can observe that the resistance value decreases with an increase in RH for ECS-14P

c

The structural elucidation of ECS-14 showed that its inorganic part is similar to the ALPO-5 (IZA code AFI) type zeolite. However, to the best of our knowledge, an AFI type zeolite with Si/Al molar ratio (and then amount of cations) similar to that obtained for ECS-14 is still unreported. Therefore, for the compared investigation we choose zeolite Y as reference materials for the inorganic component. Among the different, commercially available, zeolite Y the HSZ 320 NAA by Tosoh Corporation was selected. Concerning the organic reference, the choice was driven considering that 1,4-bis-(triethoxysilyl)-benzene (BTEB) was the silicon precursor in the ECS-14 synthesis.


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as well as for Na-Y-P film, i.e., the change of resistance ranges from 0.7 MΩ at 5% RH for both devices to 0.03 and 0.01 MΩ at 94.4% RH for ECS-14P and Na-Y-P, respectively. The decrease trend of ECS-14P film resistance is very smooth with an increase in %RH, in line with the results obtained with other hybrid sensing materials.33 On the contrary, the Na-Y-P film shows rather an unstable behavior, i.e. it suddenly turns from one saturation state to another, which it is not a good feature for sensing. The humidity calibration carried out with Na-Y-P film resulted in neither stable nor repeatable and reliable signal characterized by a lack of sensitivity up to a concentration lower than 50% in Relative Humidity (%RH) (Figure S9, Supporting Information). The electrical characterization demonstrated that the inorganic component most likely acts as receptor in humidity detection of ECS-14P film at room temperature. Indeed, the structure of ECS-14, with its open channels, can host a large amount of water molecules, presumably coordinated to the sodium atoms nestled in the framework, in agreement with its highly hydrophilic behavior (the variation in weight by dehydration is 15%). After dehydration, XRD measurements show that the structure is shrunk and partly collapsed (Figure S4, Supporting Information), disclosing that the interaction of ECS-14 with water con not be represented as absorption at the surface of a rigid scaffold, but changes the material in bulk. XRD measurements after dehydration-hydration cycles show that this change is reversible, making the sensitive mechanism possible (Figure S5, Supporting Information). Water molecules may interact with the film framework under two modes, by 1.

either forming hydrogen bonds with the scaffold or arranging themselves as clusters, as it occurs for zeolites;34

2.

or creating a physisorbed layer, due to the interaction with hydroxyl sites, which promotes the proton mobility, as it occurs in mesostructured silica films.35


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However, it was also proved that Na-Y zeolite by itself was not sufficient to reproduce the performance obtained with sensor based on ECS-14, i.e., it came clear that the organic component should play some role in the sensing mechanism. Then further investigation was needed. 3.4 Optical Properties: Effect of Hydration and Dehydration. In order to understand the physico-chemical reaction mechanism for sensing, the effects of hydration and dehydration were studied by spectroscopic analysis, i.e. electronic absorption and emission (Figure 5A and 5B, respectively), and NIR absorption (Figure 5C).


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Figure 5. Left. A) Absorption, B) emission spectra of ECS-14 as-synthesized (blue), after dehydration overnight under vacuum at 70 °C (red), and after re-equilibration under ambient humidity (purple), and of the BTEB precursor (dots; A: pure BTEB; B:cyclohexane solution, 105

M). C) Top: NIR spectra of ECS-14 as-synthesized (blue), after dehydration overnight under

vacuum at 70 °C (red), and after requilibration under ambient conditions (purple). Bottom: NIR spectra of the BTEB precursor (lines) and Na−Y zeolite (lines and dots). Right. Top: bis-silylated organic precursor representation. Bottom: spatial configuration of aromatic rings in ECS-14.

Absorption and emission properties of ECS-14 in the UV-vis range are ascribed to the organic component through the optically active aromatic rings, as shown by comparison with the BTEB precursor (Figure 5A and 5B). The absorption spectrum of pristine ECS-14 (atmospheric conditions) is in fact quite similar to that of BTEB, with the absorption maximum at 270 nm, with only 1.5 nm red-shift while maintaining the vibrational structure. Upon overnight dehydration at 70 °C (10-3 mbar) only a slight reversible decrease in intensity and loss of vibrational structure were observed. No further shift in the position of band maxima occurred, at variance with what observed in other ECS materials.36 Concerning the fluorescence properties, despite the high spatial packing, which could favor radiation quenching, ECS-14 is highly emissive.10 This was observed also in other members of the ECS family36 and hybrids obtained from bis-silylated precursors37 and indicates a lack of electronic interactions among the rings. Confirming the isolated state of the aromatics, the emission spectrum looks very similar to that of the BTEB in dilute solution (Figure 5B), with a minimal red shift of the maximum position (289.1 nm). Just a little additional shoulder is present


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at about 330 nm, an indication of few interacting rings in excimeric configuration, possibly due to defective sites. Upon dehydration, a reversible quench of the monomeric emission was observed, with persistence of the excimeric component. This shows that the monomeric emission is favored by the presence of water molecules. Indeed, in hydrophobic-like amorphous materials (generally prepared in the absence of an aluminum source), only the excimeric species are observed. The fluorescence quench (increase in non-radiative decay) of the monomeric emission in dehydrated conditions could be due to facilitated ring rotation in absence of sterical hindrance of water molecules. This is confirmed by the reduced vibrational structure in the phenyl absorption spectrum of the dehydrated material.38 At variance, the excimeric species are not modified by the hydration conditions, in agreement with their assignment to inaccessible hydrophobic defective sites. In summary, both absorption and emission spectroscopies of ECS-14 mainly point to rings in non-interacting state, with optical properties changing reversibly upon dehydration/rehydration due to a varied environment of the rings. Both organic and inorganic functional groups contribute to vibrational absorptions of ECS-14 within the NIR range. In Figure 5C the spectra recorded for the hydrated/dehydrated material are compared with the spectra of pure BTEB precursor and a Na-Y zeolite. The vibrational absorption bands arise from: -

silanols bonding water molecules, giving rise to a broad band (first overtone of OH groups) with two resolved maxima at 6961 and 6759 cm-1 and one shoulder at 7214 cm-1 similarly to silanols in Y zeolite (7350-6650 cm-1),

-

physisorbed water with a strong band centered at 5177 cm-1,


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-

C-H groups (first overtone) of organic compounds giving a weak band at 5909 cm-1,

-

organic (C-H and C-C combination) components in the complex 4200 ̶ 4700 cm-1 region,

with peaks at 4517 and 4416 cm-1, similarly to the bands detected in BTEB. Overnight treatment under vacuum (10-3 mbar) at 70 °C effectively dehydrated the sample, vanishing the signal at 5177 cm-1 due to physisorbed water. The OH signals sharpened and shifted to higher energy (peaking at 7193 cm-1) in the position attributed to isolated silanols in inorganic matrixes (e.g. the band with maximum at 7182 cm-1 in Na-Y zeolite). However, it is significant to highlight that organic component of ECS-14 (peaks at 5909 and in the range 4700-4200 cm−1) is stable during the dehydration/rehydration process. This last evidence suggests that the benzene rings constitute a scaffolding bridge between the inorganic layers during the dehydration/rehydration process, providing the robustness to the ECS structure, similarly to ECS.36 3.5 Charge Transport Properties of the Films: a Possible Sensing Mechanism. On the basis of the above characterization, a model for the reaction mechanism can be proposed: the ECS-14 channels allow the access of water molecules which, due to their high polarity, are coordinated by sodium atoms (receptors), belonging to inorganic layers (Figure 6).


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Figure 6. Representation of the protonic conduction (H+ ions) along the [010] in ECS-14 phase. Si atoms in blue, Al atoms in cyan, benzene rings in brown, oxygen atoms in red, Na atoms in yellow, H atoms in white. The interaction of adsorbed water molecules with hydroxyl groups, by hydrogen bridges, gives rise to a protonic transport (Grotthuss mechanism) along the ECS-14 phase, similar to that in inorganic mesostructured silica films.39 ̶ 41 The benzene rings actively participate in the conduction mechanism due to the electrophilic aromatic substitution of the H+ ions. This mechanism is allowed by the close proximity of benzenes and water molecules, as evidenced by the effects of water presence on spectroscopic features of benzene rings.


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The number of phenylene rings in the precursor used for the ECS phase synthesis may affect the protonic transport. Indeed, the single benzene ring in ECS-14 phase makes available its hydrogens to maintain the proton mobility; a larger number of phenylene rings in the scaffold could inhibit the formation of hydrogen bridges and then the flux of positive charge carriers. Therefore, the benzene rings actively participate as transduction elements in sensing mechanism of humidity. Concerning the reversibility of the sensing response, it was clear that the whole ECS-14 structure is involved. On the basis of the above model for gas sensing, the ECS-14 shows unexpected role reversal in the sensing mechanism with respect to traditional hybrid organic-inorganic materials. Usually inorganic compounds show high chemical and thermal stability and they can be synthetized by simple and low-cost processes and then deposited by different standard techniques. By side, organic compounds show novelty and versatility from the synthesis point of view that, together with an effective functional reactivity, allowing to control the molecular structure to enhance the interaction of the sensing material with specific analytes.33 On the contrary for ECS-14 the reception of water molecules appears to belong to the inorganic phase, while the organic component provides structural stability to the material. 4. CONCLUSIONS In summary, for the first time we experimentally proved the gas sensing properties of ECS-14 phase and proposed a model for its exploration. It was also demonstrated the capability to prepare robust and electrically active ECS-14 based films, in which the optical and structural properties of the pristine phase are preserved. The possibility of screen-printing the films, based on the selection of suitable paste components as dispersing medium, represents an important advantage with respect to other deposition techniques because of the large-scale production of


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the method. The devices obtained with the ECS-14 hybrid showed a chemoresistive behaviour, exhibiting properties of speed, reversibility and selectivity fundamental for a good quality electrical response that makes them competitive with respect to the materials currently in use. The technological advantage of room temperature operation and the selectivity are added to the novelty of the application of ECSs as active materials in gas sensing. We finally proposed a possible explanation for conductance variation, measured during the humidity calibration of ECS-14P sensors, in terms of protonic conduction. This sensing mechanism finds analogy with the behavior of chemoresistive gas sensors based on p-type semiconductor in which the charge transport owes to positive carriers. However, the ECS-14P sensor accomplishes sensing performance of traditional MOX sensors without employing any activation energy. As it occurs for solid-state devices, the functionalization of ECS phases with other materials, e.g. noble metals (Au, Pd, Pt, etc.), might enhance their sensing properties. Indeed the engineering of the ECS-14 material, e.g. the dispersion of nanoparticles inside the ECS-14 matrix or the deposition of the ECS-14 film on a nanoparticles sub-monolayer, should potentiate its receptivity. Supporting Information (PDF). Effect of heat treatment on ECS-14; Film deposition: ethanol and magnetic stirring effect; Gas measurements: experimental setup; Study of the porosity; Thermal characterization; Humidity sensing at room temperature; Zeolite-based sensor; Humidity tests in dry/nitrogen condition. AUTHOR INFORMATION Corresponding Author * Dr. Barbara Fabbri, Department of Physics and Earth Sciences, University of Ferrara, via G. Saragat 1/C, 44122, Ferrara (Italy), tel. +390532974213, [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by Spinner 2013, Regione Emilia-Romagna, project “Nanoscience: materials and emerging strategies for sustainable technologies”. ACKNOWLEDGMENT The authors thank Eni Spa for the financial support, Prof. A. Quaranta, A. Gaiardo and M. Valt for enlightening discussions on the sensing mechanism of humidity. REFERENCES (1) Liu, L.; Li, X.; Dutta, P.K.; Wang, J.; Room Temperature Impedance Spectroscopy-based Sensing of Formaldehyde with Porous TiO2 under UV Illumination. Sens. Actuators B 2013, 185, 1–9. (2) Zhang, C.; Boudiba, A.; De Marco, P.; Snyders, R.; Olivier, M.-G.; Debliquy, M.; Room Temperature Responses of Visible-light Illuminated WO3 Sensors to NO2 in sub-ppm Range. Sens. Actuators B 2013, 181, 395–401. (3) Fabbri, B.; Gaiardo, A.; Giberti, A.; Guidi, V.; Malagù, C.; Martucci, A.; Sturaro, M.; Zonta, G.; Gherardi, S.; Bernardoni, P.; Chemoresistive Properties of Photo-activated Thin and Thick ZnO Films. Sens. Actuators B 2016, 222, 1251–1256. (4) Kickelbick, G. Hybrid Materials. Synthesis, Characterization, and Applications, WileyVCH Verlag GmbH & Co. KGaA, Weinheim, USA 2007. (5) Gómez-Romero, P.; Sanchez, C.; Functional Hybrid Materials, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, USA 2004.


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(6) Hoffmann, F.; Cornelius, M.; Morell, J.; Fröba, M.; Silica-based Mesoporous OrganicInorganic Hybrid Materials. Angew. Chem. Int. Ed. 2006, 45, 3216–3251. (7) Rurack, K.; Martínez-Máňez, R.; The Supramolecular Chemistry of Organic–Inorganic Hybrid Materials, John Wiley & Sons, Inc., USA 2010, Ch. 3. (8) Bellussi, G.; Carati, A.; Di Paola, E.; Millini, R.; Parker Jr., W. O.; Rizzo, C.; Zanardi, S.; Crystalline Hybrid Organic-Inorganic Alumino-Silicates. Microporous Mesoporous Mater. 2008, 113, 252–260. (9) Bellussi, G.; Carati, A.; Rizzo, C.; Diaz Morales, U.; Zanardi, S.; Parker Jr., W. O.; Millini R.; ENI Spa, WO 2008/017513 A2, 2008. (10) Bellussi, G.; Millini, R.; Montanari, E.; Carati, A.; Rizzo, C.; Parker Jr., W. O.; Cruciani, G.; De Angelis, A.; Bonoldi, L.; Zanardi, S.; A Highly Crystalline Microporous Hybrid Organic-Inorganic Aluminosilicate Resembling the AFI-type Zeolite. Chem. Commun. 2012, 48, 7356–7358. (11) Bellussi, G.; Carati, A.; Millini, R.; Rizzo, C.; Zanardi S.; ENI Spa, WO 2013/098261 A1, 2013. (12) Inagaki, S.;Guan, S.; Ohsuna, T.; Terasaki, O.; An Ordered Mesoporous Organosilica Hybrid Material with a Crystal-like Wall Structure. Nature 2002, 416, 304–307. (13) Hoffmann, F.; Fröba, M.; Vitalising Porous Inorganic Silica Networks with Organic Functions--PMOs and Related Hybrid Materials. Chem. Soc. Rev. 2011, 40, 608–620. (14) Goto, Y.; Mizoshita, N.; Ohtani, O.; Okada, T.; Shimada, T.; Tani, T.; Inagaki, S.; Synthesis of Mesoporous Aromatic Silica Thin Films and Their Optical Properties. Chem.Mater. 2008, 20, 4495–4498. (15) Mizoshita, N.; Ikai, M.; Tani, T.; Inagaki, S.; Hole-Transporting Periodic Mesostructured Organosilica. J. Am. Chem. Soc. 2009, 131, 14225–14227.


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(16) Yuliarto, B.; Kumai, Y.; Inagaki, S.; Zhou, H.; Enhanced Benzene Selectivity of Mesoporous Silica SPV Sensors by Incorporating Phenylene Groups in the Silica Framework. Sens. Actuators B 2009, 138, 417–421. (17) Ponzoni, A.; Russo, V.; Bailini, A.; Casari, C.S.; Ferroni, M.; Li Bassi, A.; Migliori, A.; Morandi, V.; Ortolani, L.; Sberveglieri, G.; Bottani, C.E.; Structural and Gas-Sensing Characterization of Tungsten Oxide Nanorods and Nanoparticles. Sens. Actuators B 2011, 153, 340–346. (18) Zanardi, S.; Bellussi, G.; Parker Jr., W. O.; Montanari, E.; Bellettato, M.; Cruciani, G.; Carati, A.; Guidetti, S.; Rizzo, C.; Millini, R.; The Role of Boric Acid in the Synthesis of Eni Carbon Silicates. Dalton Trans. 2014, 43, 10617–10627. (19) Fabbri, B.; Eni Carbon Silicates as Crystalline and Mesoporous Hybrids for Gas Sensing. PhD Thesis, University of Ferrara, Italy, 2015. (20) Guidi, V.; Malagu', C.; Carotta, M.C.; Vendemiati, B.; Printed films: Materials Science and Applications in Sensors, Electronics and Photonics. In WOODHEAD PUBLISHING SERIES IN ELECTRONIC AND OPTICAL MATERIALS, 2012, Ch. 11. (21) Barsan, N.; Koziej, D.; Weimar, U.; Metal oxide-based gas sensor research: How to?. Sens. Actuators B 2007, 121, 18–35. (22) Korotcenkov, G.; Metal oxides for solid-state gas sensors: What determines our choice?. Mater. Sci. Eng., B 2007, 139, 1–23. (23) Yamazoe, N.; Toward innovations of gas sensor technology. Sens. Actuators B 2005, 108, 2–14. (24) Gaiardo, A.; Fabbri, B.; Guidi, V.; Bellutti, P.; Giberti, A.; Gherardi, S.; Vanzetti, L.; Malagù, C.; Zonta, G.; Metal Sulfides as Sensing Materials for Chemoresistive Gas Sensors. Sensors 2016, 16, 296–314. (25) Bochenkov, V. E.; Sergeev, G. B. In Metal Oxide Nanostructures and Their Applications; Umar A., Hahn Y.-B., Eds.; American Scientific Publishers, 2010; Chapter 2, pp 31–52.


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BRIEFS. Structural and electrical properties of ECS-14 hybrid are demonstrated to be enabling it as functional sensing material for room temperature detection of humidity SYNOPSIS .


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Figure 1. ECS-14 structure (top left), and projection along [001] (top right): [SiO3C] tetrahedral in blue, [AlO4] in cyan, phenylene rings in brown, oxygen atoms in red; sodium in yellow takes up three sites which have different structural role. H2O molecules can be distinguished by H atoms in white. Linear channels of 7 Å in diameter are along [001] direction. 2405x1259mm (80 x 80 DPI)



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Figure 2. A) Device based on ECS-14 phase. SEM image of B) ECS-14 powder, C) ECS-14P film. 280x235mm (96 x 96 DPI)


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Figure 3. Diffraction patterns of ECS-14 pure powder (blue, below), and ECS-14P film (black, above). 238x121mm (96 x 96 DPI)



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Figure 4. A) Relative humidity (at room temperature) calibration of sensor based on ECS-14P, as dynamic response. Inset: zoom of the range 0-8000 s. B) Humidity calibration curves: resistance values of ECS-14P (red) and Na-Y-P (black) sensing films vs. humidity concentration. Inset shows normalized percentage responses of ECS-14P sensor vs. humidity concentration. Numerical values are reported in Table S3, Supporting Information. 375x548mm (96 x 96 DPI)


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Figure 5. Left. A) Absorption, B) emission spectra of ECS-14 as-synthesized (blue), after dehydration overnight under vacuum at 70 °C (red), and after re-equilibration under ambient humidity (purple), and of the BTEB precursor (dots; A: pure BTEB; B:cyclohexane solution, 10-5 M). C) Top: NIR spectra of ECS-14 as-synthesized (blue), after dehydration overnight under vacuum at 70 °C (red), and after requilibration under ambient conditions (purple). Bottom: NIR spectra of the BTEB precursor (lines) and Na−Y zeolite (lines and dots). Right. Top: bis-silylated organic precursor representation. Bottom: spatial configuration of aromatic rings in ECS-14. 213x226mm (96 x 96 DPI)



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Figure 6. Representation of the protonic conduction (H+ ions) along the [010] in ECS-14 phase. Si atoms in blue, Al atoms in cyan, benzene rings in brown, oxygen atoms in red, Na atoms in yellow, H atoms in white. 178x236mm (600 x 600 DPI)


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Structural and electrical properties of ECS-14 hybrid are demonstrated to be enabling it as functional sensing material for room temperature detection of humidity 35x15mm (600 x 600 DPI)


Crystalline Microporous Organo-Silicates with Reversed

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