Toward the Detection of Poisonous Chemicals and Warfare Agents by

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Toward the Detection of Poisonous Chemicals and Warfare Agents by Functional Mn3O4 Nanosystems Chiara Maccato,† Lorenzo Bigiani,† Giorgio Carraro,† Alberto Gasparotto,† Cinzia Sada,‡ Elisabetta Comini,*,§ and Davide Barreca*,⊥ †

Department of Chemical Sciences and INSTM, ‡Department of Physics and Astronomy and INSTM, and ⊥CNR-ICMATE and INSTM, Department of Chemical Sciences, Padova University, 35131 Padova, Italy § Sensor Lab, Department of Information Engineering, Brescia University, 25133 Brescia, Italy S Supporting Information *

ABSTRACT: The detection of poisonous chemicals and warfare agents, such as acetonitrile and dimethyl methylphosphonate, is of utmost importance for environmental/health protection and public security. In this regard, supported Mn3O4 nanosystems were fabricated by vapor deposition on Al2O3 substrates, and their structure/morphology were characterized as a function of the used growth atmosphere (dry vs. wet O2). Thanks to the high surface and peculiar nano-organization, the target systems displayed attractive functional properties, unprecedented for similar p-type systems, in the detection of the above chemical species. Their good responses, selectivity, and sensitivity pave the way to the fabrication of low-cost and secure sensors for different harmful analytes.

KEYWORDS: Mn3O4, nanosystems, gas sensors, acetonitrile, dimethyl methyl phosphonate

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Furthermore, to the best of our knowledge, no studies on the detection of gaseous WA simulants by Mn3O4 have ever been performed. So far, various synthesis procedures have been used to tailor the chemico-physical and functional properties of Mn3O4 nanostructures.22,23,25 In the present study, supported Mn3O4 nanosystems were fabricated by chemical vapor deposition (CVD), subjected to a thorough chemico-physical characterization and used for the first time as gas sensors for acetonitrile and DMMP as toxic chemicals and WA simulants. In particular, the target materials were prepared under both dry and wet O2 atmospheres (see the Supporting Information, S-1) in order to evaluate the interplay between the adopted synthesis conditions, the resulting material characteristics and the ultimate sensing performances. Figure 1a displays representative X-ray diffraction (XRD) patterns of the obtained nanosystems. Irrespective of the preparation conditions, all the recorded Bragg reflections could be attributed to α-Mn3O4 (hausmannite) crystallographic planes,29 highlighting the formation of phase-pure systems. For specimens obtained under dry O 2 and O 2 +H 2 O atmospheres, a change in the relative peak intensities was observed, the I103/I211 intensity ratio being respectively lower, or higher, than that of the reference pattern.29 This finding

he efficient detection of poisonous chemicals and warfare agents (WAs) is a main challenge for safety, health and public security applications.1−4 Because testing of WAs at a laboratory level is indeed hazardous, experimental studies are typically carried out using less toxic chemicals,4−7 among which acetonitrile (CH3CN) and dimethyl methyl phosphonate (DMMP) are used as simulants for cyanide agents8−11 and Sarin nerve gas,5,12−17 respectively. In addition, acetonitrile is a poisonous chemical attacking the central nervous system and its early detection in the atmosphere is necessary in order to prevent health hazards.1,3,18 The development of gas sensing devices capable of an efficient detection of such analytes11 has so far been accomplished by metal oxides in a limited number of cases. In particular, acetonitrile sensing has been performed by the use of SnO2-based powders,1 films,2,3,7 and nanowires,8 as well as by WO3,8 Cr0.8Fe0.2NbO4,19 and LaCoO3-based films.18 The reports on DMMP detection have focused on the use of SnO2-based films2,7 and nanowires/nanobelts,12,14 as well as ZnO-CuO systems,17 WO3 films,8 and C nanofibers decorated with ZnO and SnO2 particles.15 Among metal oxides, manganese oxides20−24 and, in particular, Mn3O4, a p-type semiconductor (EG ≈ 2.3 eV),25,26 have received an increasing attention over the past decade, in particular for their (photo)catalytic properties.6,27 From this perspective, p-type systems like Mn3O4 are promising candidates for chemoresistors endowed with new functionalities,28 but to date only a few papers have reported on gas sensors based on Mn3O4 films/nanostructures.20−23,26 © XXXX American Chemical Society

Received: January 31, 2018 Accepted: April 4, 2018

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DOI: 10.1021/acsami.8b01835 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. (a) XRD patterns of Mn3O4 nanomaterials grown at 500 °C on Al2O3: (i) in an O2+H2O reaction atmosphere; (ii) in dry O2. Reflections pertaining to the used substrate are marked by open circles (○). Representation of α-Mn3O4 structure,29 evidencing (211) and (103) crystallographic planes. (b) SIMS depth profile for a Mn3O4 specimen obtained in O2+H2O. Plane-view (left) and cross-sectional (right) FE-SEM micrographs for Mn3O4 specimens grown: (c,d) in an O2+H2O reaction atmosphere; (e,f) in dry O2.

Regarding the latter, its presence can be related to oxygen vacancies in metal oxides, that promote water dissociative chemisorption to form − OH groups at the vacancy sites.13,19,30 It is worth noting that the presence of hydroxyl groups is inherent to the preparation of the sample grown in O2+H2O atmosphere (Figure S2c), but that, in general, the concomitant contribution of adsorbed oxygen species, especially for the sample prepared under dry O2, can indeed occur.31 The contribution of the high BE O 1s component (% of the total oxygen) was estimated to be (30 ± 2) % and (40 ± 2) % for specimens synthesized under dry and wet O2, respectively (Figure S2b, c). As shown below, these differences have a direct influence on the system sensing behavior. Additional efforts were devoted to the characterization of the system morphology by means of field emission-scanning electron microscopy (FE-SEM). The obtained data (Figures 1c−f), in accordance also with atomic force microscopy (AFM) analyses (Figure S3), revealed that the sample nanoorganization was appreciably affected by the reaction environment. For the sample obtained under a wet reaction atmosphere (Figures 1c, d), micrographs were dominated by

suggested the preferential exposure of (211) or (103) planes (see Figure 1) in the two cases. Analysis of material composition was preliminarily performed by secondary ion mass spectrometry (SIMS) (Figure 1b and Figure S1). Regardless of the preparative conditions, Mn and O displayed almost parallel ionic yields from the outermost surface down to the interface with the alumina substrate, indicating an even in-depth distribution. The inherent Al2O3 roughness was the main responsible for the apparent depositsubstrate intermixing, as testified by the penetration of Al into the overlying deposits. A further insight into the element chemical states was obtained by X-ray photoelectron spectroscopy (XPS; Figure S2a). In line with XRD results, the presence of Mn3O4 as the sole manganese-containing oxide was confirmed by the Mn 2p peak position (Figure S2, inset; binding energy (BE) = 641.9 ± 0.2 eV, spin−orbit splitting = 11.6 ± 0.2 eV) and by the multiplet splitting separation of the two Mn 3s components (5.4 eV).22−25 Two bands contributed to the O 1s line (Figures S2b, c), a main one at 530.2 ± 0.2 eV, attributed to Mn−O− Mn bonds, and a second one at BE = 531.9 ± 0.2 eV. B


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times the present sensors to CH3CN. Despite the moderate working temperatures, the calculated response and recovery times compared favorably with previous literature values reported for acetonitrile detection by oxide materials largely utilized for sensing applications, i.e. SnO2 and WO3 thin films, as well as SnO2 nanowires.3,8 As an example, for a CH3CN concentration of 2 ppm at 300 °C, response and recovery times were evaluated to be (1.0 ± 0.1) min and (8.0 ± 0.8) min, respectively. These issues, along with the obtained sensor selectivity,8,12 are important prerequisites for practical realworld applications.16,19 On the other hand, the detection of the simulant for Sarin nerve agent (DMMP) is more challenging,8 mainly due to the poisoning effects exerted by this chemical on the active material, as observed in previous studies.2,11 In this case, a careful inspection of response dynamics (Figure S4a) showed a rapid conductance drop as DMMP was introduced into the test chamber, followed by a slower change ultimately leading to an incomplete baseline recovery when the air flow was restored. The absence of a full recovery of the air conductance value after DMMP exposure prevents from a meaningful evaluation of recovery times in the present case. The observed behavior, already reported in the literature,2,4,5,7 was ascribed to the dissociative chemisorption of organophosphonate compounds over metal oxides, generating CH3−PO moieties, and methylphosphonic acid, strongly bonded to the system surface. As the gas exposure proceeds further, the accumulation of P O moieties over Mn3O4 surface leads to a progressive saturation of the available reactive sites, with a poisoning effect on the sensor surface, in line with previous reports.6,9 In spite of these phenomena, even in this case response times [(0.83 ± 0.08) min for a DMMP concentration of 0.5 ppm at 200 °C] compared favorably with those reported for DMMP sensing by SnO2-11,16 and ZnO-based systems.17 Further efforts were devoted to analyzing the sensor response dependence on the used gas concentration (Figure 3b; see eqs S1 and S2). As can be observed, in the investigated concentration range the sensor response was proportional to CH3CN concentration, and the obtained trends displayed a linear behavior in the log−log scale, at variance with previous reports,2,3,18 without exhibiting any significant saturation effect. Interestingly, a very similar behavior was also observed in experiments aimed at DMMP detection (Figure S4b), as reported for SnO2-based systems.2 The WAs toxicology is generally quantified in terms of IDLH (immediately dangerous for life and health) values, used as thresholds in security fields: a person must not be exposed to the WA IDLH concentration for more than 30 min in order to avoid permanent health issues.8,11 The best detection limits (see the Supporting Information, S-3) extrapolated in this work were estimated to be 0.6 and 0.04 ppm for CH3CN and DMMP, respectively. It is worth highlighting that the former value is appreciably lower than the IDLH of cyanide species (50 ppm9,11). On the other hand, the detection limit obtained for DMMP was better than those reported for SnO2-based sensors,16 and comparable to the corresponding IDLH value (0.05 ppm11). Nevertheless, the actual performances toward DMMP sensing are still lower than those required for the fabrication of eventual commercial devices, and acoustic sensors stand as amenable alternatives, especially for the detection of very low concentrations.32 Figures 3c, d compare the sensor responses toward fixed acetonitrile and DMMP concentrations as a function of the

low-sized particles (Ø = 80 nm; average deposit thickness = 240 nm), whose close interconnection resulted in a conformal coverage of the underlying alumina particles (dimensions 600− 800 nm). In a different way, the specimen fabricated in dry O2 was characterized by an even distribution of elongated grains (mean size = 110 nm × 270 nm), resulting in a higher overall thickness (510 nm). Overall, cross-sectional images revealed an optimal nanodeposit adhesion to the used substrates. The observed differences between the two systems could be explained taking into account that the introduction of water vapor during the system growth produces the formation of a higher nucleation sites density, resulting in an enhanced number of grains with lower dimensions per unit area. In a different way, under a dry O2 atmosphere, after the initial nucleation stages, the growth over preformed grains appears to be favored, accounting thus for the obtained morphologies. Inplane energy-dispersive X-ray spectroscopy (EDXS) mapping (Figure 2) confirmed the compositional purity of the target

Figure 2. (b, c) EDXS elemental maps, recorded on (a) the FE-SEM image, and (d) EDXS spectrum for a Mn3O4 sample grown at 500 °C in an O2+H2O environment.

specimens and revealed an even lateral distribution of O and Mn. Spectra recorded in different regions were always characterized by oxygen and manganese peaks, along with the Al signal arising from the Al2O3 substrate (see Figure 2d). Gas sensing tests were preliminarily aimed at the possible detection of various chemical compounds acting as potential interferents giving false alarms.11 In this regard, the use of CO, NO2, and NH3 with maximum concentrations of 100, 2, and 10 ppm respectively, did not result in any appreciable response (≤0.1), highlighting a good system selectivity. Figure 3a reports the specimen isothermal dynamic responses toward square concentration pulses of acetonitrile for the different Mn3O4 sensors. The curves highlighted a ptype behavior, with a conductance decrease upon reaction of the analyte with adsorbed oxygen species due to a decrease of p-type carrier concentration.20,28 The conductance showed a rapid variation as acetonitrile was introduced into the test chamber, followed by a slower change up to the steady-state value, suggesting that molecular adsorption was the limiting step of the process. In spite of an incomplete baseline recovery after the higher acetonitrile concentration pulse (Figure 3a), no significant poisoning effects could be observed exposing several C


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Figure 3. (a) Isothermal dynamic response curves (at 300 °C) to square concentration pulses of acetonitrile for Mn3O4 specimens grown: (i) in an O2+H2O reaction atmosphere; (ii) in a dry O2 environment. (b) Dependence of responses on acetonitrile concentration, at a working temperature of 300 °C. Responses as a function of working temperature toward acetonitrile (25 ppm) and DMMP (5 ppm) for Mn3O4 sensors prepared: (c) in O2+H2O, and (d) in dry O2 atmospheres.

Figure 4. Schematic representation of the gas sensing mechanisms for the target Mn3O4 nanomaterials and related HAL thickness modulation. The direction of electron flow is denoted by arrows. The different panels refer to (a) the HAL formation at the system surface after air exposure, and to the detection of (b) acetonitrile and (c) DMMP.

adopted operating temperature. Regardless of Mn3O4 preparation conditions, the optimal working temperature was between 250 and 300 °C for acetonitrile detection, and 200 °C for DMMP. In line with previous observations,1−3 the obtained responses toward CH3CN increased from 200 to 250−300 °C, indicating a progressively enhanced reaction between the analyte and adsorbed oxygen (see below).8,18 Conversely, the opposite trend was observed concerning DMMP detection,2

suggesting that for higher temperatures the decreased DMMP adsorption16 was not adequately compensated by the increased extent of surface reactions. These different temperature effects for CH3CN and DMMP indicated that the adsorption/ desorption behavior, and catalytic activity of the used materials, significantly affected their functional properties.2 In spite of the above-mentioned drawbacks, the obtained response values, unprecedented for similar p-type systems, are among the best D


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logical properties. The target materials were utilized for the first time as gas sensors toward poisonous chemicals and warfare agents, namely acenonitrile and dimethyl methyl phosphonate. The responses obtained at moderate operating temperatures were among the best ever reported up to date in the detection of the above species by chemoresistive gas sensors, demonstrating that controllable preparation of Mn3O4 nanomaterials is a key issue to tailor their performances.24 Beside yielding a valuable insight into p-type materials for sensing of the target analytes, the promising results obtained in this work pave the way to the use of the present Mn3O4 sensors for a variety of eventual applications.20 In this regard, efforts will be devoted to properly address open challenges in terms of stable sensor recovery after DMMP detection, with particular regard to reducing the poisoning effects and obtaining even lower detection limits. In addition, the target materials stand as versatile platforms for different functional end uses,23 such as hydrogen generation by photoelectrochemical water splitting or direct photocatalysis.

reported in the literature up to date for acetonitrile1,7−9 and DMMP2,8,9,15 detection by chemoresistive metal oxide sensors. The very favorable performances of the presently developed systems can be traced back to the Mn3O4 catalytic properties, as well as to the high surface-to-volume ratio and controlled nanoorganization of the obtained nanomaterials, resulting in an efficient gas chemisorption and enhanced sensor activity.9,11,12,21,22 The sensing mechanism of the target p-type systems can be explained taking into account the initial chemisorption of oxygen molecules from the surrounding atmosphere, resulting in the formation of different active oxygen species.1,16,19 In particular, O− ions are the dominant ones in the range 200− 500 °C, in which the working temperatures adopted in the present work are comprised.1,12,16 For p-type semiconductors like Mn3O4, the buildup of a low-resistance hole-accumulation layer (HAL)28 in the near surface region occurs (Figure 4 and Figure S5). After the physisorption of CH3CN or DMMP, that are both reducing agents, their chemisorption is accompanied by electron release15 and a concomitant decrease of the hole density, producing a decrease of the HAL width and, in turn, the conductance decrease observed upon analyte exposure (Figure 3a and Figure S4a).28 The relatively slow reversal to the original air situation was mainly attributed to the sluggish desorption kinetics of the resulting volatile byproducts (mainly CO2, NO2, H2O for acetonitrile,1,18,33 and CO2, methyl methyl phosphonate (MMP), H2O, for DMMP5,16,17) in the adopted temperature range. This phenomenon was particularly evident for DMMP (compare Figure 3a and Figure S4a), due to the progressive poisoning of the sensor surface occurring upon interaction with this analyte.6,11 A detailed analysis of the results displayed in Figure 3c, d showed that the best responses toward CH3CN, or DMMP, corresponded to the use of Mn3O4 sensors synthesized under dry O2, or O2+H2O reaction atmospheres, respectively, and that this difference was particularly evident in DMMP detection (compare Figure 3b and Figure S4b). This phenomenon could be related to the diverse preferentially exposed surfaces [(103) and (211) for specimens grown under O2+H2O and dry O2 atmospheres, respectively; see above and Figure 1], which promote different adsorption processes.8 In fact, the adsorption mechanism of DMMP and the subsequent redox processes are more favored by a higher availability of Lewis acid sites and of surface hydroxyl groups.6,13 As a consequence, it is reasonable to suppose that DMMP chemisorption should be favored over atomically denser Mn3O4 surfaces, like the (103) one, and further promoted by a higher concentration of available − OH groups (see above). As regards CH3CN sensing, two possible chemisorption orientations can occur. A first one, corresponding to a side-on η2(C,N) state, is slightly more favorable than the other, which requires only one chemisorption site (nitrogen end-on).34 This observation is in line with the fact that the sensor fabricated under wet O2 atmospheres, with a preferential exposure of atomically denser (103) Mn3O4 surfaces, yields a higher response to acetonitrile at 200 °C. Upon increasing the working temperature, the responses of the two sensors to acetonitrile are almost identical since an increased thermal energy supply promotes CH3CN reaction with the sensing material, regardless of the chemisorption mechanism. In conclusion, we have successfully proposed a vapor phase process for the development of high-purity p-type Mn3O4 nanosystems, endowed with tailored structural and morpho-

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b01835. Details on synthesis, XRD, XPS, SIMS, FE-SEM, EDXS, AFM, and gas sensing measurements (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +39-0303715706 (E.C.). *E-mail: [email protected]. Phone: +39-0498275170 (D.B.). ORCID

Chiara Maccato: 0000-0001-6368-5754 Alberto Gasparotto: 0000-0003-4626-651X Davide Barreca: 0000-0001-8593-0730 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has been financially supported by Padova University DOR 2016−2017, P-DiSC #03BIRD2016-UNIPD projects.

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REFERENCES

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Toward the Detection of Poisonous Chemicals and Warfare Agents by

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