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Tailoring vapor phase fabrication of Mn3O4 nanosystems: from synthesis to gas sensing applications Lorenzo Bigiani, Chiara Maccato, Giorgio Carraro, Alberto Gasparotto, Cinzia Sada, Davide Barreca, and Elisabetta Comini ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00584 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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ACS Applied Nano Materials

Tailoring Vapor Phase Fabrication of Mn3O4 Nanosystems: From Synthesis to Gas Sensing Applications Lorenzo Bigiani,a Chiara Maccato,a,*, Giorgio Carraro,a Alberto Gasparotto,a Cinzia Sada,b Elisabetta Comini,c,* and Davide Barrecad

a

Department of Chemical Sciences, Padova University and INSTM, Via Marzolo 1, 35131

Padova, Italy b

Department of Physics and Astronomy, Padova University and INSTM, Via Marzolo 8,

35131 Padova, Italy c

Sensor Lab, Department of Information Engineering, Brescia University, Via Valotti 9,

25133 Brescia, Italy d

CNR-ICMATE and INSTM, Department of Chemical Sciences, Padova University, Via

Marzolo 1, 35131 Padova, Italy

*

Corresponding authors; phone: +39-0498275234; e-mail: [email protected] (C.M.); phone: +39-0303715706; e-mail: [email protected] (E.C.). 1 Environment ACS Paragon Plus

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Supported p-type α-Mn3O4 nanosystems were fabricated by means of chemical vapor deposition (CVD) on polycrystalline alumina substrates at temperatures of 400 and 500°C, using Mn(hfa)2•TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N’,N’-tetramethylethylenediamine) as precursor compound. The structure, chemical composition and morphology of the obtained deposits were characterized in detail, devoting particular attention to the influence of the used reaction atmosphere (dry O2 vs. O2+H2O) on the system characteristics. For the first time, the gas sensing performances of the obtained CVD Mn3O4 nanomaterials were investigated towards ethanol and acetone vapors, with concentrations ranging from 10 to 50 and from 25 to 100 ppm, respectively. The developed systems showed the best activity ever reported in the literature for Mn3O4 chemoresistive sensors in the detection of the target gases, a result that, along with their low detection limits and good selectivity, is an appealing starting point for eventual technological applications.

KEYWORDS: Mn3O4 nanosystems, chemical vapor deposition, gas sensors, ethanol, acetone.

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INTRODUCTION Manganese oxides, including Mn2O3, Mn3O4 and MnO2, have been extensively investigated thanks to their multitude of accessible oxidation states and diversified electronic, structural and chemical properties.1-10 Among these systems, α-Mn3O4, a p-type semiconductor (EG ≈ 2.3 eV)11-13 with low cost, large abundance and environmental compatibility,14-16 has attracted an increasing interest for various eventual applications, encompassing electronic devices,2,6 electrochromics,13,17,

batteries,18-19

electrochemical

capacitors,4,13,15,20-21

heterogeneous

(photo)catalysts,3,7,12,14,18,22-23 and sensors. Concerning the latter field, Mn3O4-based systems have been utilized for the amperometric determination of hydrogen peroxide,24-25 bisphenol26 and nimorazole drug,19 non-enzymatic glucose sensing,27 electrochemical detection of Na+ ions,28 2-butanone,3 hydrazine29 and biosensors for fish freshness monitoring,30 detection of heavy metals in aqueous solutions,16 potentiometric O2 sensing,31 and cataluminescence sensing of acetone5 and H2S,14 as well as impedimetric sensors for humidity and mechanical pressure.32 Additional studies have concerned their use in the chemoresistive sensing of ethanol,1-2,4,33 acetone,33-34 CO,33 H2,12 and humidity.35 In general, p-type oxides like α-Mn3O4 have been less investigated than n-type ones36 since their response is typically lower than that of n-type oxides with comparable morphology,37-40 making the design of highly efficient ptype gas sensors an open challenge. Nevertheless, p-type oxide semiconductors have an important potential as gas sensors, taking into account their appreciable catalytic activity in the selective oxidation of organic compounds37-38 and the possibility to enhance their responses by tailoring their chemico-physical properties.4,11 In fact, the design and control of their structure and morphology is of key importance to obtain smaller internal resistances and reversible charge transfer,21,41 enhancing the ultimate functional behavior as chemoresistive gas sensors. Up to date, various studies have focused on the preparation of Mn3O4 systems in the form of 3 Environment ACS Paragon Plus


powders,3,5,14,18,23,28,31,33-35,42 whereas modern frontiers concern the tailored fabrication of supported nanosystems, that enable to overcome the disadvantages associated with powdered materials.7,43 Nevertheless, so far only a few works have reported on supported Mn3O4 for gas sensors.1-2,4,44 Among the possible preparation routes for Mn3O4 materials, CVD processes offer significant advantages due to the possibility of achieving in-situ, large area growth of thin films and nanostructured materials with controlled properties, as well as to the compatibility with current processing standards.11 In the framework of a comprehensive research project focused on the preparation and characterization of multi-functional manganese oxide-based materials, we have developed and optimized a CVD route to Mn3O4 deposits starting from a βdiketonate diamine Mn(II) precursor, Mn(hfa)2•TMEDA.11,45 Despite this compound had been preliminarily proposed as CVD precursor for Mn3O4 films,46 a detailed investigation of the interplay between processing conditions and chemico-physical characteristics of the resulting Mn3O4 nanomaterials has not been carried out to date. Yet, the elucidation of such issues is of key relevance from both a fundamental and an applicative point of view, and undoubtedly deserves further attention. In the present work, CVD Mn3O4 nanosystems are fabricated from Mn(hfa)2•TMEDA on alumina substrates at growth temperatures of 400 and 500°C under different reaction atmospheres, i.e. dry O2 and O2 containing water vapor (O2+H2O). The prepared materials were thoroughly characterized as a function of the adopted experimental conditions, with particular regard to their structure and morphology. For the first time, the sensing behavior of the obtained CVD Mn3O4 nanomaterials is investigated in the detection of two organic analytes, ethanol and acetone, of importance for biomedical industries, breath analyzers and safety testing of food packaging, especially in the monitoring of wine quality and of fermentation processes.1-2,41,43,47 The functional properties of the developed CVD Mn3O4


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materials are discussed in terms of their morphological organization, proposing for the first time a possible mechanism for their detection of the above gases and highlighting the most appealing features in view of technological end-uses.

EXPERIMENTAL SECTION Synthesis. Mn3O4 nanodeposits were fabricated using a custom-built cold-wall CVD reactor under electronic grade O2-based reaction atmospheres. In each experiment, the Mn(hfa)2•TMEDA precursor, synthesized and characterized as recently reported,11,48 was placed in an external vaporization vessel (0.3 g for each deposition), heated at 60°C by means of an oil bath, and transported into the reaction chamber by an electronic grade O2 flow (rate = 100 standard cubic centimeters per minute (sccm)). In all cases, connection gas lines were maintained at 100°C to prevent undesired condensation phenomena, and the total operating pressure and deposition time were set at 10.0 mbar and 60 min, respectively. Polycrystalline Al2O3 slides (3×3 mm2, thickness = 254 µm) were used as deposition substrates and cleaned prior to each deposition by sonication in dichloromethane, rinsing in 2-propanol and drying in air. For experiments in dry O2 atmospheres, an additional O2 flow (rate = 100 sccm) was separately introduced into the reaction chamber. For growth processes in wet (O2+H2O) atmospheres, the same configuration and O2 flow rates were used, but a water reservoir kept at 35°C was introduced in the auxiliary gas line. Basing on preliminary optimization experiments aimed at ensuring the reproducibility of material characteristics, two different growth temperatures (T), namely 400 and 500°C, were used. At the end of each deposition, samples were cooled down to room temperature under flowing O2. In the following, samples are labelled as TA, where T is the adopted growth temperature and A refers to the used reaction atmosphere (D: dry, pure O2; W: wet, O2+H2O). Characterization. Glancing incidence X-ray diffraction (XRD) patterns were recorded at a



fixed incidence angle of 1° on a Bruker D8 Advance X-ray diffractometer, equipped with a Göbel mirror and a CuKα X-ray source powered at 40 kV and 40 mA. The Scherrer equation was used to estimate the average crystallite dimensions.29,40,44 X-ray photoelectron spectroscopy (XPS) characterization was performed by means of a Perkin–Elmer Φ 5600ci instrument, using a standard AlKα source (1486.6 eV) and working pressures lower than 10-8 mbar. BE values (standard deviation = ±0.2 eV) were corrected for charging by assigning to the C1s signal of adventitious carbon a BE of 284.8 eV.49 The analysis involved Shirley-type background subtraction50 and peak area integration, with subsequent determination of atomic percentages using Φ V5.4A sensitivity factors. The experimental uncertainty on the reported atomic composition values does not exceed ±2%. Peak fitting was carried out by a least-squares procedure, using Gaussian–Lorentzian peak shapes. Secondary ion mass spectrometry (SIMS) analysis was carried out by a Cameca IMS 4f spectrometer at pressures lower than 10−10 mbar, using a Cs+ primary ion beam (14.5 keV, 20 nA) and negative secondary ion detection. Depth profile acquisition was performed rastering over a 150×150 µm2 area and sampling secondary ions from a sub-region close to 8×8 µm2, to avoid crater effects. Charging phenomena were compensated by means of an electron gun. Beam blanking mode and high mass resolution configuration were used to improve in-depth resolution and avoid possible mass interference artifacts. Field emission-scanning electron microscopy (FE-SEM) images were acquired using a Zeiss SUPRA 40VP instrument, at a primary beam acceleration voltage of 10.0 kV. Specimen thickness and aggregate size values were obtained by the statistical analysis of cross-sectional and plane-view micrographs using the ImageJ® software (http://imagej.nih.gov/ij/, accessed September 2017), by averaging over various independent measurements. A NT-MDT SPM Solver P47H-PRO instrument operated in tapping mode and in air was used


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for atomic force microscopy (AFM) measurements. After background subtraction and plane fitting, Root Mean Square (RMS) roughness values were calculated from 2×2 µm2 micrographs. Gas sensing tests were carried out using a standard configuration for resistive sensor measurements, with Pt interdigitated contacts51 and a Pt heater deposited by sputtering with shadow masking on the top of the sensing material and on the back side of Al2O3 substrates, respectively. Analyses were carried out in a sealed chamber (relative humidity = 40% at 20°C) in the 100-300°C temperature range. Before each test, samples were stabilized at the used working temperature for 8 h. A constant synthetic air flow (rate = 300 sccm) at atmospheric pressure was used as carrier for dispersion of the analytes at the desired concentrations. Resistance values were obtained by applying a constant bias voltage (1 V) to the sensing materials, measuring the flowing current through a picoammeter. For a p-type semiconductor like Mn3O4 in the presence of reducing analytes (acetone and ethanol in the present case), the sensor response is defined as: Response = (Rf − R0)×100 / R0 = ∆R / R0 ×100

(1)

with R0 corresponding to air baseline resistance, whereas Rf is the corresponding steady state value after equilibration with the gaseous analyte. Repeated measurements under the same conditions on up to 8 identical sensors yielded stable responses, with a maximum uncertainty of 10%. A similar variation was also estimated as the sensor response value drift upon their cyclability over 4 months of tests, an important prerequisite for practical end uses.41,47 The lifetime of the present Mn3O4 sensors can be estimated to be > 1 year, over which the samples are still working without any evident deterioration. Experimental response data were fitted by the relation:12 Response = A×CB

(2)



where A is a constant, C is the analyte concentration and B is an exponent depending on the stoichiometry of the involved reactions.52

RESULTS AND DISCUSSION A sketch of the synthetic approach adopted in the present work for the fabrication of manganese oxide nanostructures is reported in Scheme 1. Particular attention was devoted to the analysis of material chemico-phsyical properties as a function of the adopted growth temperature and reaction atmosphere.

Scheme 1. Graphical representation of the synthetic approach developed for the CVD growth of Mn3O4 nanosystems on alumina substrates. Material characterization. Figure 1 reports the GIXRD patterns for samples obtained both under dry and wet reaction atmospheres at growth temperatures of 400 and 500°C. Irrespective of the preparation conditions, beside diffraction peaks related to the alumina substrate, the recorded patterns were characterized by the sole signals of tetragonal α-Mn3O4 (hausmannite; space group I41/amd; a = 5.762 Å, c = 9.470 Å) with Mn(III) and Mn(II) respectively in the octahedral and tetrahedral sites of the spinel structure.7-8,15,53-55


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Interestingly, the obtainment of crystalline hausmannite was achieved already at a growth temperature of 400°C, lower than those utilized in previous CVD experiments.46 No other reflections from different Mn oxides could be detected, indicating the selective formation of α-Mn3O4, a challenging issue as reported in various studies.5,17,20-21 For each of the adopted reaction atmospheres (dry O2 and O2+H2O), the calculated nanocrystal diameters increased with the growth temperature (≈35 and 45 nm, for samples 400W and 500W; ≈32 and 50 nm, for specimens 400D and 500D), suggesting a concomitant enhancement of material crystallinity. The latter was confirmed by the parallel increase of the overall diffracted

20

25

30

35

2ϑ (°)

40

(220)

(004)

(101)

(112) (200) (103)

(211)

intensity, attributable also to higher film thickness values at higher temperatures (see below).8

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500D 500W 400D 400W

45

50

Figure 1. GIXRD patterns of the target Mn3O4 samples synthesized under different conditions. Miller indexes refer to the tetragonal Mn3O4 reflections,53 whereas diffraction peaks pertaining to the Al2O3 substrate are marked by (). In addition, the relative peak intensities were directly affected by the presence of water vapor in the reaction environment. In fact, for samples 400D and 500D, obtained under dry atmospheres, (211) signals were more intense than (103) ones, and the I103/I211 intensity ratio was lower than that of the powder spectrum (Figure S1). Conversely, for specimens 400W and 500W, fabricated in the presence of water vapor, the I103/I211 intensity ratio was reversed



and higher than the reference value. Strong reflections from (103) planes have also been observed in the electron diffraction characterization of Mn3O4 nanosystems prepared by hydrolytic routes.33 The above phenomena suggest: a) the preferential exposure of (211) or

Mn2p3/2

Mn2p

Mn2p1/2

Intensity (a.u.)

500D 500W 400D 400W

660

655

650 645 BE (eV)

Intensity (a.u.)

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640

635

Mn3s

500D 500W 400D 400W

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85 BE (eV)

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O1s 400D

Intensity (a.u.)

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I

II

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534

532 530 BE (eV)

528


526

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Figure 2. Surface Mn2p, Mn3s and O1s photopeaks for Mn3O4 nanosystems.

(103) planes for specimens obtained under dry or wet atmospheres, respectively, and/or b) the occurrence of an anisotropic growth.51 For wet samples, the latter effect could be reasonably ruled out, whereas it was definitely not negligible for specimens fabricated using a dry O2 atmosphere (see below for FE-SEM characterization data). The system surface composition was first investigated by XPS, which revealed no significant presence of impurities (Figure S2), except for a minor contribution due to carbon contamination arising from air exposure.8,47,56 For all the investigated systems, the Mn2p photoelectron peak position (Figure 2; BE(Mn2p3/2) = 642.0 eV; BE(Mn2p1/2) = 653.5 eV) was in good agreement with previously reported values for Mn3O4.3-5,8-9,11,23,27,29,32,42,57 To obtain a specific finger-print for Mn3O4 presence, attention was devoted to the analysis of the Mn3s signal (Figure 2), and, in particular, to the BE separation of the two components arising from multiplet splitting phenomena.49 The obtained value (5.4 eV) confirmed the selective formation of Mn3O4 as the only manganese oxide,8,11,15,17,22 regardless of the adopted synthesis parameters. In all cases, the O/Mn atomic ratio was 1.5, higher than the expected value for stoichiometric Mn3O4 (1.3). In fact, the O1s surface XPS could be resolved in two different components (Figure 2). The most intense (I), at 530.3 eV (≈65% of the total oxygen) was referred to lattice oxygen, whereas the higher energy one (II) at BE = 531.9 eV could be attributed to surface adsorbed oxygen species.7,11,17,20,43,56,58-59 The latter are very important in reacting with the target gases, and their presence is beneficial to achieve improved sensing performances.34 To gain complementary information on the in-depth material composition, SIMS analyses were also carried out. The recorded data highlighted an even distribution of manganese and oxygen throughout the investigated depth (Figure 3), with no segregation in the outermost regions and/or at the interface with the Al2O3 substrate, indicating a homogeneous in-depth 11 Environment ACS Paragon Plus


elemental distribution. In particular, the similarity of O and Mn ionic yields throughout the deposit thickness was compatible with the presence of Mn3O4 as the sole Mn-containing oxide, in line with the above results. The relatively broad interface between the deposit and alumina substrate was mainly ascribed to the high roughness of the latter.41

-1

SIMS yield (counts x s )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2 4

10

Al 4

O

2 3

10

4

Mn

2 2

10

200 400 600 Sputtering time (s)

Figure 3. Representative SIMS depth profile for the 400D specimen.

A deeper insight into the system morphology, one of the most important factors affecting sensing performances,40 was attained by FE-SEM analyses (Figure 4). For samples obtained under wet O2 atmosphere (Figures 4a,b and 4e,f), plane-view micrographs showed the formation of densely packed α-Mn3O4 particles, yielding a conformal coverage of the underlying alumina substrate and suggesting an isotropic 3D growth mode.41 The formation of similar faceted grains has been reported for hausmannite-type Mn3O4 prepared by a solution route.28 Upon going from sample 400W to 500W, the enhanced thermal energy supply produced an increase of both the mean aggregate size and deposit thickness [for sample 400W: (80±20) nm and (240±25) nm, respectively; for specimen 500W: (120±40) nm and (270±30) nm, respectively].8 For systems fabricated under dry O2 (400D and 500D, Figures 4c,d and 4g,h), more porous structures arising from the assembly of anisotropic grains, with one dimension (D) much longer than the other (d), could be observed. In line


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with the above results, both particle size and deposit thickness increased with the substrate temperature from 400°C [sample 400D: d = (40±10) nm, D = (200±40) nm, thickness = (460±40) nm] to 500°C [sample 500D: d = (110±30) nm, D = (270±50) nm, thickness = (510±30) nm]. These results suggested the



Figure 4. Plane-view (left) and cross-sectional (right) FE-SEM micrographs for samples 400W (a,b), 400D (c,d), 500W (e,f), and 500D (g,h).


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occurrence of a kinetically controlled growth regime, the rate-limiting step being the surface reactions.58 Taken together, the above observations highlight that the effect of the reaction atmosphere (wet vs. dry O2) on the system morphology was more pronounced than that of deposition temperature. This phenomenon could be explained taking into account that water vapor produces a high density of nucleation sites (–OH groups) on the growth surface, enhancing thus the precursor decomposition by favoring ligand removal.58 As a result, 3D deposits characterized by a lower grain size were obtained in an O2+H2O environment. Under dry oxygen atmospheres the growth of elongated grains took place (specimens 400D and 500D). A similar phenomenon, already observed for hausmannite Mn3O4 nanosystems, was traced back to soft aggregation processes of the system building blocks.33 Gas sensing properties. As already discussed, specimens obtained under O2+H2O atmospheres (400W and 500W) displayed an apparently more compact morphology (see Figure 4), likely resulting in a lower area available for the interaction with surrounding gases. Hence, in the following the attention is focused in detail on the sensing behavior towards CH3CH2OH and CH3COCH3 of systems obtained under dry O2 atmospheres (400D and 500D), whose nano-organization appeared more promising in view of the ultimate applications. Gas sensing performances were preliminary investigated between 100 and 400°C. The identification of the optimal working temperature plays in fact an important role in the sensing process and is directly dependent both on the gas to be detected and on the active material properties.1,36,40 The above tests revealed that 200°C was the minimum temperature requested for the obtainment of appreciable responses (≥ 0.1), and that for T ≥ 400°C sensing performances were degraded due to detrimental Mn3O4 thermal alteration.23 As a consequence, the optimal operating conditions corresponded to working temperatures



between 200 and 300°C. In this range, preliminary screening experiments aimed at the detection of potentially interfering gases such as NO2, CO, and NH3 (maximum concentrations of 2, 100 and 10 ppm, respectively) yielded negligible responses (≤ 0.1). Overall, these findings suggested a good selectivity of the present sensors, a great concern in view of their eventual practical utilization.14,43,56 In a different way, previous Mn3O4 sensors have shown a poor selectivity to gaseous acetone.34 10-5

80

Conductance (S)

400D 500D

ethanol 60

10-6

40

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10-7

Concentration (ppm)

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Time (min) 150

10-5

acetone

Conductance (S)

400D 500D

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0 30

70

110

150

Time (min)

Figure 5. Dynamic responses of selected Mn3O4 nanodeposits towards square concentration pulses of ethanol and acetone, at a working temperature of 300°C. As a matter of fact, the explanation of the reasons underlying the observed selectivity is not a


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straightforward task, since the sensor selectivity is influenced in a complex way by the type of the active material, as well as by its morphology and surface active sites nature and distribution. In fact, an in-depth elucidation of these issues would require additional experimental studies specifically dedicated to the analysis of chemical interactions between the involved gases and the active sensing material. In the present case, the lack of appreciable responses to nitrogen dioxide, could be related to the fact that p-type materials like Mn3O4 are oxidation catalysts, making thus unfeasible the detection of oxidizing gases.37 Conversely, the insensitivity to CO and NH3, in accordance with previous results reported for other Mn3O4 sensors,33-34 could be related to a limited carbon monoxide chemisorption in the absence of noble metal particles,10 and to a scarce ammonia adsorption and activation under the adopted temperature conditions.60 Isothermal dynamic response curves (Figure 5) showed that the sensor conductance (resistance) underwent a decrease (increase) upon contact with the target analytes. This behavior is in line with that expected for p-type sensors exposed to reducing gases,1,34 due to a decrease of the majority p-type carrier density upon analyte reaction with adsorbed oxygen species (see § 3.3 for more details).41,43,47,61 As a general rule, the conductance variation was proportional to gas concentration, and the almost complete recovery of air state upon switching off the analyte pulses suggested a reversible interaction of the used gases with the sensing materials.1,41,61 A careful inspection of response dynamics revealed a net conductance variation as the chemical species were introduced into the test chamber, followed by a slower change up to the end of the gas pulse. A similar behavior, already reported for Mn3O4 thin films,1-2,4 indicated that for both ethanol and acetone detection the rate-limiting step of the overall process was the analyte adsorption on the sensor surface61-62 (see § 3.3). Figure 6 compares the response data as a function of analyte concentration collected at 300°C,



the optimal working temperature (see also below and Figure 7). In accordance with previous studies, Mn3O4 responses increased proportionally to the analyte concentration in agreement with equation (2),1,33 confirming the absence of saturation phenomena.

1000

ethanol

Response

400D 500D

100

10 1

10

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Concentration (ppm) 1000

acetone

400D 500D

Response

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100

10 10

100

1000

Concentration (ppm)

Figure 6. Responses of selected Mn3O4 sensors as a function of ethanol and acetone concentrations (working temperature = 300°C).

The best fitting parameters for the target analytes are reported in the Supporting Information (§ S-4). By assuming the validity of equation (2) even for low analyte concentrations, limits of detection (LODs)1 were estimated for a fixed response value of 30, yielding 1.0 ppm (ethanol) and 0.01 ppm (acetone) for the best performing sample (500D). It is worth noticing that the present systems are much more sensitive than commercial breath analyzers (LOD:


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200 ppm).4,47,51,61 In addition, it is important to highlight that the obtained LODs are inferior than those of p-type Co3O4 sensors47 and appreciably lower than those of MnO2 nanowires/nanorods63 and of CuO-TiO2-Au nanomaterials at the same operating temperature.61 Experimental results displayed in Figs. 5 and 6 indicate that, irrespective of the test analyte, the obtained sensing responses were systematically higher for sample 500D in comparison to the 400D one. Since both specimens were fabricated using the same reaction atmosphere, the higher activity of the former could be first attributed to a larger active area. To clarify this issue, AFM analyses were carried out on the above specimens, and pertaining images are displayed in Figure S3. In line with FE-SEM results, a crack-free granular topography, characterized by interconnected nanoparticles giving rise to porous deposits, was observed. The obtained RMS roughness values were 20.0 nm and 26.0 nm for samples 400D and 500D, respectively. Since the effective surface area is typically larger for a rough surface, higher RMS roughness values imply a larger area available for the chemisorption of surrounding gases,1,41,43-44,47 supporting thus the higher activity of the 500D sample.64 Additional concurring contributions could be traced back to the enhanced crystallinity (see above) and larger grain size of the 500D specimen (see above),1 beneficially affecting the corresponding sensing performances due to the reduction of defects and grain boundaries content. It is generally accepted that materials with smaller grain sizes could have a larger sensor response,38,44 although the type of metal oxides and the involved mechanism should also be comprehensively considered. Grain boundaries can in fact generate potential barriers that act as traps or scatter charge carriers,38 and the increase in grain connectivity for the present 500D specimen is likely to have a positive influence on its functional behavior.37,47,65 The actual sensing properties result thus from the counterbalance of different concurring factors.47 To attain a deeper insight into the system performances, the responses to selected CH3CH2OH



and CH3COCH3 concentrations are plotted in Figure 7. For both ethanol and acetone, the obtained values increased monotonically with the working temperature, a feature that, along with previous data, suggested similar reaction mechanisms.41 The obtained trend, already reported for Mn3O4 in the same temperature interval,33 could be explained taking into account that the contact between nano-aggregates evidenced by FE-SEM analysis (Figure 4) ensures percolation and crossing of barriers between adjacent grains. Thus, the system conduction properties are exponentially dependent on the height of potential barriers,65 related, in turn, to the activation energy of conduction upon changing the temperature. Under the adopted conditions, the response is governed by the rate of chemical reactions,33,56 which are promoted by a temperature increase.62

Figure 7. Responses of sample 500D to selected concentrations of ethanol and acetone at various working temperatures.

The highest responses obtained herein compare favorably with those reported under analogous conditions for various p-type oxide gas sensors, encompassing NiO,37,65 NiCr2O3 nanofibers,37 F-doped and Au-functionalized Co3O4,43,47 CuxO (x = 1,2),41 and CuO-TiO2 nanoarchitectures.61 To the best of our knowledge, the responses at a working temperature of 300°C were also higher than those previously obtained for all Mn3O4 sensors reported in the literature up to date, even for those doped with Zr or Li, in the detection of both ethanol120 Environment ACS Paragon Plus

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2,4,33,37,42,44

and acetone,33-34 and also better than those pertaining to Mn2O366 and MnO259,63,67

in the sensing of the same analytes. These findings, along with the relatively low operating temperatures,1 highlight the great potential of the present systems for further implementation of highly efficient sensing devices.

Figure 8. Representation of the mechanism involved in ethanol sensing by the target Mn3O4 materials and related HAL thickness modulation. The direction of electron flow is denoted by green arrows. Upon exposure to air, the formation of a HAL at the system surface occurs. After ethanol chemisorption (i), electron injection reduces the HAL thickness, which is further lowered as the analyte oxidation proceeds (ii,iii). Finally, byproducts desorption (iv) at the end of the target gas exposure restores the pristine HAL in air.

The above discussed performances can be examined in more detail in relation to a detailed gas sensing mechanism, which has never been proposed for CVD Mn3O4 sensors. Upon Mn3O4 exposure to air, O2 chemisorption generates active oxygen species by capturing electrons from surface states:39-42,61 O2 (g)

2O− (ads) + 2h+

(3)



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As a consequence, a low resistance hole-accumulation layer (HAL) is generated in the material near-surface region (Figure 8),2,4,33-34,37,40 whose formation results in conduction patterns and mechanisms different from n-type semiconductors.4,56 The subsequent analyte chemisorption:1,3 CH3CH2OH + 6O− (ads)

2CO2 (g) + 3H2O (g) + 6e−

(4)

CH3COCH3 + 8O− (ads)

3CO2 (g) + 3H2O (g) + 8e−

(5)

results in an electron release into the material conduction band,34,44 leading to a lower hole concentration and in a parallel reduction of the HAL width, accounting for the observed conductance drop-off37,39 (compare Figure 5). The HAL width decrease is further enhanced with the progress of the analyte oxidation, whereas the final desorption of oxidation products from the sensor surface restores the original condition in air (Figure 8). The relatively slow reversal to the air conductance value upon switching off gas pulses (Figure 5) could be traced back to the sluggish recovery processes involving the out-diffusion, as well as to the slow desorption kinetics of oxidized species in the adopted temperature range.52 Basing on the above observations, a very similar reaction mechanism can be envisaged in the case of acetone41 (see Figure S4), and the higher number of released electrons in Eqn. (5) with respect to (4) is in good agreement with the higher conductance variations (and higher responses) obtained in acetone detection (see Figs. 5-7).

CONCLUSIONS In summary, the present work has been dedicated to the CVD preparation of supported Mn3O4 nanosystems starting from Mn(hfa)2•TMEDA, to their chemico-physical characterization and to the analyses of their functional performances in chemoresistive gas sensing applications. To the best of our knowledge, no analogous studies have ever been reported in the literature


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up to date. The obtained materials materials were characterized by the presence of high purity hausmannite Mn3O4, with structural and morphological features dependent both on the adopted reaction atmosphere (dry O2 vs. O2+H2O) and growth temperature. For the first time, Mn3O4 systems obtained by CVD were utilized as gas sensors for the detection of ethanol and acetone, and a possible sensing mechanism involving the target materials was proposed and discussed. The response values obtained in the present work under optimized conditions were the highest ever reported in the literature up to date for manganese oxide-based gas sensors. These issues, along with the amenable preparation route, good selectivity, low detection limits and working temperatures for the proposed sensors (≤300°C), disclose interesting perspectives for future advances and developments of the present research activities. In particular, the latter will concern the design of Mn3O4-based nanocomposites either modified by suitable doping, or functionalized with noble metal or oxide additives yielding tailored heterojunctions,1,34,37 in order to enhance the sensor functional performances and further improve selectivity. In this regard, interesting perspectives for future research developments will be also focused on the development and implementation of suitable sensor arrays, with the ultimate aim of discriminating the considered analytes, which still remains an open challenge. The attention will also be dedicated to the fabrication of nanosystems based on other manganese oxides, like MnO2, and the analysis of their gas sensing performances,59 in an attempt to open the way to a novel class of sensing architectures.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publication website at DOI: 10.1021/acsanm.XXXDetails on XRD, XPS, AFM and gas sensing data and mechanism.



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.

Conflict of interests The authors declare no conflict of interests.

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from Padova University DOR 2016– 2017 and P-DiSC #03BIRD2016-UNIPD projects.


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