New Insights into the SnO2 Sensing Mechanism Based on the

Aug 15, 2013 - James S. McManus , Patrick D. Cunningham , Laura B. Regan , Alison Smith , Dermot W. McGrath , and Peter W. Dunne. Crystal Growth ...
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New Insights into the SnO2 Sensing Mechanism Based on the Properties of Shape Controlled Tin Oxide Nanoparticles Massimiliano D’Arienzo,*,† Davide Cristofori,‡ Roberto Scotti,† and Franca Morazzoni† †

INSTM, Department of Materials Science, University of Milano-Bicocca, Via R. Cozzi 53, I-20125 Milano, Italy Department of Molecular Sciences and Nanosystems, University Ca’ Foscari of Venezia, Via Torino 155/b, I-30172 Venezia, Italy

‡

S Supporting Information *

ABSTRACT: We report on the sensing behavior of SnO2 shape controlled nanocrystals in order to evaluate the role of their exposed crystal surfaces in the sensing mechanism. Octahedral (OCT), elongated dodecahedral (DOD), and nanobar shaped (NBA) nanocrystals were synthesized by previously reported procedures and their performances were evaluated in the sensing toward CO. Singly ionized oxygen vacancies (VO•) were detected by electron spin resonance (ESR), and their abundance and reactivity were associated to the exposed crystal faces and, in turn, to the sensing responses of the nanocrystals. Results indicated that the electrical properties and the formation/ reactivity of the VO• centers are interconnected and are relatable to the nanoparticle specific surfaces. Two different temperaturedependent sensing mechanisms were proposed, depending on the prevalence of the surface structure or of the specific surface area on the sensing ability of shape controlled SnO2 nanoparticles. KEYWORDS: SnO2 gas sensors, shape controlled nanocrystals, ESR, crystal surfaces

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INTRODUCTION The sensing properties of SnO2, the most used semiconductor base material for toxic gas sensors, have been measured for a long time and the sensing mechanism has been extensively discussed.1−6 This was done also taking advantage of the different methods to obtain the material, such as the conventional hydroxide precipitation,7 the sol−gel precipitation,8,17 the hydrothermal crystallization,9 and other several physical procedures.10,11 In the past few years, the synthesis techniques have allowed us to obtain SnO2 with a homogeneous nanometric size,12 which due to the high surface area, displays high electrical response when exposed to reducing gases (e.g., CO, NO, H2, CH3OH).13−17 Tin oxide was also obtained as meso and macroporous material, where the compact network of the nanoparticles facilitates the charge transport and the high porosity significantly improves the gas diffusion throughout the layer, upgrading the sensing properties.18−28 Recently, several studies demonstrated that the electrical response and the selectivity of SnO2 chemical sensors are highly affected by the shape of nanocrystals and specifically by their exposed surfaces.29−32 In this perspective, the addition of capping molecules to the reaction medium as well as the control of the pH and temperature led to the production of shape controlled nanocrystals with specific exposed facets.30−34 As the chemisorption properties and the oxo-reductive ability of these facets depend on their surface structure, and the facets with identical structure should display identical reactivity, a greater electrical response is expected in the case of shape controlled nanoparticles.29,30 © 2013 American Chemical Society

Nevertheless, for these kinds of materials, the sensing mechanism and the species responsible for the conductivity variations, which were discussed in our previous studies on the basis of the spectromagnetic properties,17,27,35−39 can be more precisely depicted. Previous studies, beginning from the first ones of Yamazoe,22,40,41 suggested that the chemisorbed oxygen species (O2−, O−, O−• 2 ) were responsible for the resistance variations when interacting with the reducing gases. We proposed instead that the electrical modifications would be originated by the alternate formation and filling of oxygen vacancies during the gas pulse; this is based on comprehensive electron spin resonance investigations on the paramagnetic species present in the semiconductor oxide.37 In particular, we observed an increase of the amount of paramagnetic singly ionized oxygen vacancies, Vo•, in connection with the reducing gas treatment and a decrease in the presence of oxidation treatments. This mechanism was validated on nanosized sol−gel obtained SnO2.35−39 Recently, Kuang et al. reported on the sensing properties of octahedral, elongated dodecahedral and lance shaped SnO2 nanocrystals with different percentages of {221}, {111} and {110} surfaces.31,32 They found that octahedral nanocrystals that highly expose {221} or {111} surfaces exhibit far better gas-sensing performance toward ethanol than particles with predominant {110} facets. Received: June 11, 2013 Revised: August 7, 2013 Published: August 15, 2013 3675

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autoclave (25 mL) and kept at 200 °C for 12 h. The products were collected by centrifugation, washed several times with distilled water, ethanol and acetone, and finally dried in air at room temperature.32 Tin oxide nanobars (NBA) with predominant {110} surfaces were prepared by dissolving 0.250 g of SnCl4•5H2O in 100 mL of distilled water. To this solution, 2.5 g of CO(NH2)2, and 5 mL of HCl were successively added under mild stirring. The colorless transparent solution obtained was transferred into a Teflon-lined stainless steel autoclave of 125 mL of capacity. The autoclave was sealed and maintained at 90 °C for 24 h. The resulting white product was retrieved by centrifugation and washed several times with water, absolute ethanol, and acetone, and finally dried in air at room temperature.34 Hereafter, SnO2 nanoparticles with octahedral, dodecahedral and nanobar shapes will be labeled as OCT, DOD and NBA, respectively. Structural and Morphological Characterization. X-ray diffraction (XRD) patterns of tin oxide nanocrystals were collected with a Bruker D8 Advance (Cu Kα radiation) in the range 10−70° 2θ (2θ step 0.025°, count time of 2 s per step). Calculation of unit cell parameters and polymorph composition for OCT sample was performed by the Rietveld method using the ICSD database as reference. The crystallite size, LXRD, of SnO2 NBA nanocrystals was estimated from the broadening of the XRD peaks by means of the Scherrer equation, after correction for instrumental broadening, and assuming negligible the microstrain broadening. The peak used for the calculation was {110}. Glancing incidence X-ray diffraction (GIXRD) patterns of OCT and DOD films were recorded by a Bruker D8 Advance diffractometer equipped with a Göbel mirror and a Cu Kα source (40 kV, 40 mA), at a fixed incidence angle of 0.5°. Specific surface area (SSABET) by the BET method44 was measured by nitrogen physisorption using Quantachrome Autosorb-1 apparatus. The nanopowders were evacuated at 200 °C for 16 h before the analysis. Scanning electron microscopy (SEM) measurements of the films were performed by a Vega TS5136 XM Tescan microscope in a highvacuum configuration. The electron beam excitation was 30 kV at a beam current of 25 pA, and the working distance was 12 mm. In this configuration, the beam spot was 38 nm. Prior to SEM analysis, samples were gold-sputtered. High-resolution transmission electron microscopy (HRTEM) and electron diffraction (ED) were performed using a Jeol 3010 apparatus operated at 300 kV with a high-resolution pole piece (0.17 nm pointto-point resolution) and equipped with a Gatan slow-scan 794 CCD camera. Samples were obtained by removing a film portion from the substrates in order to obtain a fine powder sample, then suspended in 2-propanol. A 5 μL drop of this suspension was deposited on a holey carbon film supported on a 3 mm copper grid for TEM investigation. According to the model proposed by Kuang et al.,31 octahedral SnO2 nanoparticles (OCT) entirely expose the {221} surfaces. To determine the percentage of high energy {111} and lower energy {110} exposed surfaces in dodecahedral nanocrystals (DOD), we utilized the following equation:

The differences in the sensing ability were attributed both to the higher surface energy of the {221} and {111} compared to the {110} faces and to the presence on these surfaces of undercoordinated cations, i.e., 5- and 4-fold coordinated Sn4+, which could favor the chemisorption of ionized oxygen species, enhancing the reaction with the reducing gas.30 This hypothesis is still based on the Yamazoe mechanism, which attributes the conductivity variations to the surface chemisorption and removal of oxygen ions.22,41,42 Nevertheless, no direct evidence was supplied of the reduced oxygen species implied in the mentioned oxo-reductive processes. In contrast with the previous results, other investigations recently revealed that SnO2 nanocrystals with predominant {110} surfaces exhibit enhanced sensing properties.43,44 However, there has not been convincing evidence on their sensing mechanism and on the relative involvement of the crystal faces yet. To investigate the effect of the crystal surfaces on the sensing mechanism, SnO2 nanocrystals with tailored morphology and specific exposed crystal faces were synthesized by previously reported hydrothermal reactions and used to realize new thin films.31,32,34 The structural and morphological features of the films were investigated in detail and revealed the presence of well-crystallized particles with homogeneous shape and definite surfaces. Their sensing behavior toward CO was examined and the obtained performances were discussed in connection with the paramagnetic oxygen defects detected by electron spin resonance (ESR), in order to suggest a rationale for the relative importance of the exposed crystal facets in the sensing properties modulation. It turned out that the sensing ability of shape controlled SnO2 nanoparticles is essentially related to the formation and the variation in the amount of oxygen vacancies. In particular, the differences in the sensing properties appear related to the different capability of the specific exposed faces to form Vo• species and to refill them with oxygen. These results support our previous suggestions about the role of the oxygen vacancies in the sensing mechanism and better detail the location of the sensing action.17,35−39

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EXPERIMENTAL SECTION

Chemicals. SnCl4•5H2O, Hydrochloric acid (37.5%), polyvinyl pyrrolidone (PVP, M.W. 55000), aqueous tetramethylammonium hydroxide (TMAH; 1.1 M), and urea (CO(NH2)2) were all purchased from Aldrich and used without further purification. Synthesis of Shape Controlled SnO2 Nanocrystals. The preparation of morphology controlled SnO2 nanoparticles was performed according to previously reported procedures.31,32,34 Tin oxide nanoparticles with octahedral, dodecahedral, and nanobar shapes were prepared and used to fabricate thin films for gas sensing measurements. In a typical synthesis of octahedral nanocrystals (OCT) with highly exposed {221} surfaces, 0.350 g of SnCl4•5H2O were dissolved in a solution of ethanol/distilled water (6.00 mL, 1/1 v/v) under stirring. Upon addition of 0.60 mL of HCl, the reaction mixture was heated at 40 °C, and under vigorous stirring, 0.315 g of PVP was added. The resulting pale-pink solution was cooled down to room temperature, transferred to a 25 mL Teflon-lined stainless steel autoclave and kept at 200 °C for 12 h. The solid was recovered by centrifugation, washed twice with ethanol and acetone and finally dried in air at room temperature.31 Elongated dodecahedral SnO2 nanocrystal (DOD) with {111} and {110} exposed crystal facets were synthesized by dissolving 0.350 g of SnCl4•5H2O in a solution of ethanol/distilled water (6.00 mL, 1:1 v/v); 9.2 mL of 1.1 M aqueous TMAH were successively added under stirring. The resulting solution was transferred to a Teflon-lined stainless steel

%S{111}exp = S{111}/(S{111} + S{110} = [2b2 × tag(45°)]/{[2b2 × tag(45°)] + (4a × b)} × 100

(1)

which is based on the geometrical model of an elongated dodecahedron reported in Figure 1A constituted by eight {111} and four {110} facets. a and b correspond to the sides of the rectangular {110} surfaces, while 45° is the angle between the {110} and the {111} facets. The NBA shape was approximately that of a rectangular parallelepiped (Figure 1B), and the percentage of exposed {110} faces was simply estimated by the following equation:

%S{110}exp = S{110}/(S{110} + S{001}) = 4l × w/[4l × w + 2w 2] × 100 3676

(2)

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between the film resistance under flowing air, RAIR, and under flowing CO/air mixture, RMIX, respectively (S = RAIR/ RMIX). In order to carry out the electrical measurements at each operating temperature, the samples were maintained in the sensing apparatus for about 3 weeks. The sensing behavior was initially checked at the lowest working temperature (200 °C). Then the sensors were heated up to 375° (the highest working T) and the resistance was recorded. After that, the measurements were performed at progressively lower temperatures (from 350 to 225 °C) and again at 200 °C to check the reproducibility and the stability of the electrical response. The results suggested that, in this time frame, the electrical response (S) of the SnO2 sensors remain more less constant with an uncertainty of ±10%. The reliability as well as the reproducibility of the devices is also satisfactory; in fact, all the produced sensors (at least two for each material) were working, and the response reproducibility (obtained by at least six pulses) was calculated to be ±13%. ESR Spectroscopy. The electron spin resonance (ESR) investigation was performed by a Bruker EMX spectrometer operating at the X-band frequency and equipped with an Oxford cryostat working in the temperature range of 4−298 K. The EPR spectra were recorded on samples of shape controlled SnO2 nanocrystals after the following successive treatments:17 (a) As prepared OCT, DOD and NBA nanocrystals. (b) Treatment under air stream (80 cm3 min−1) for 1 h at 225 or 350 °C. (c) Treatment under CO (580 ppm)/Ar mixture stream (80 cm3 min−1) for 30 min at 225 or 350 °C. (d) Interaction with air stream (80 cm3 min−1) at room temperature (RT) after treatment b, for 10 min at 298 K. Spectra were recorded at 130 K in vacuum conditions (p < 10−2 mbar) to avoid line broadening due to the magnetic interaction of O2 with the surface paramagnetic species. Spectra were recorded with a modulation frequency of 100 kHz, modulation amplitudes of 2−5 gauss, and microwave powers of 10 mW. The g values were calculated by standardization with α,α′-diphenyl-β-picryl hydrazyl (DPPH). The spin concentration was obtained by double integration of the resonance lines, referring to the area of the standard Bruker weak pitch (9.7 × 1012 ± 5% spins cm−1). Accuracy on double integration was ±15%. Care was taken to always keep the most sensitive part of the ESR cavity (1 cm length) filled.

Figure 1. (A) Geometrical model of elongated dodecahedral tin oxide nanocrystals (DOD) exposing eight {111} and four {110} facets. (B) Geometrical model of NBA nanocrystals with {110} and {001} exposed surfaces and shape resembling that of a rectangular parallelepiped with a square basis.

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RESULTS AND DISCUSSION Structural and Morphological Characterization. The XRD pattern of the as-obtained OCT nanopowders was refined by Rietveld method in the tetragonal space group P42/mnm. The observed, calculated, and difference profiles together with the allowed Bragg reflections are displayed in Figure 2A. The OCT sample displays a single phase with lattice parameters a = b = 4.7444(3) Å and c = 3.1834(8) Å, which are in good agreement with the values reported for the rutile phase of bulk tin oxide (JCPDS No. 41−1445). Similarly the diffraction peaks of the DOD and NBA samples have been indexed based on the same tetragonal crystal system (Figure 2B). No other crystalline phases were detected in the synthesized products. The sharpness of the diffraction peaks for OCT and DOD indicates the presence of nanoparticles with large average dimensions and good crystallinity. Conversely, the broadening of the pattern indicates that the NBA sample is composed of nanocrystals with a very small size. This deduction has been confirmed by SEM and TEM images (see Figure 4). The average crystallite size (LXRD) of NBA calculated from the broadening of the (110) peak by the Scherrer equation was 2.9 ± 0.4 nm, in good agreement with the crystal width estimated from the TEM images. To better describe the materials used for the gas sensing measurements, GIXRD analysis was performed on drop-casted

where w and l correspond to the sides of the {001} and of the {110} faces, respectively. The average values of a, b, l, and w were evaluated by measuring the sizes of ∼100 particles in TEM images. Electrical Measurements. The films for the sensing measurements were prepared by simply depositing by drop-casting a few drops of SnO2 paste (consisting of SnO2 nanoparticles mixed with ethanol) onto Suprasil quartz slides (20 × 20 mm, 0.25 mm thickness). Suprasil quartz slides were equipped, before film deposition, with two gold current collectors (20 mm) deposited at a distance of about 2 mm from each other by the dc sputtering technique. Then the samples were placed in a quartz chamber inserted in an oven, and the measurements were performed at different temperatures, ranging from 200 to 375 °C. The electrical resistance was measured by a programmable electrometer controlled by a PC. To dynamically reproduce environmental conditions in a controlled and reproducible way, a system based on volumetric gas mixing through mass flow controllers and certified bottles was used. The sensing element was initially equilibrated in air flow (100 mL min−1) at the selected temperature, then CO (range 72.5−585 ppm)/air mixture was introduced (100 mL min−1) up to equilibrium conditions. The different gas mixtures were obtained by dilution of the starting CO (585 ppm)/air mixture using mass flow controllers. The initial resistance conditions of the film were restored by air equilibration, before again introducing the CO/air mixture. The electrical response (S) is defined as the ratio 3677

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Figure 3. GIXRD patterns of thick films obtained by drop-casting of octahedral (OCT) and dodecahedral (DOD) SnO2 nanocrystals.

about 1.6 and an apex angle between two side surfaces of about 88.7°. These morphological features agree well with the model suggested of octahedral SnO2 nanoparticles entirely exposing {221} facets. These surfaces have been described as a combination of (111) terraces and (110) steps (HRTEM images, Figure 4d, f). Therefore the {221} surfaces are very similar to the {111}. As reported by Wang et al.,32 when TMAH is added to the tin oxide precursor solution, compact aggregates of nanocrystals with elongated dodecahedral shape are obtained (Figure 5a−c). It is quite evident that the size distribution of DOD nanoparticles is broader than that observed for the OCT sample and ranges from 50 to 100 nm in length and from 20 to 50 nm in width. The HRTEM image demonstrates that the two tips of these elongated dodecahedra are bound by high-energy {111} facets and the middle section by low-energy {110} facets (Figure 5c, d). Moreover, it is possible to note that the DOD nanocrystals grow along the [110] direction. Finally, Figure 5e−h shows the TEM and HRTEM images of the NBA nanocrystals obtained in the presence of urea. As described by Ye et al.,34 the product consists of very compact aggregates of nanobar-shaped particles (Figure 5e). High magnification images (Figure 5f, g) reveal that these small nanoparticles are almost 2.0 nm in width and 12 nm in length. HRTEM investigation (Figure 5h), highlights the singlecrystal nature of the SnO2 nanobars with lattice spacing of 0.335 nm between adjacent crystal planes, which perfectly matches the interplanar distance between two {110} crystal planes. It can also be observed that the preferred growth direction for the nanocrystals is the [001]. On the basis of the TEM and HRTEM analysis, a, b, l and w average values were calculated, which correspond to the to the sides of the rectangular {110} surfaces in DOD, and to the sides of the {001} and of the {110} faces of NBA. From these values and according to the eq 1 and 2, the percentages of {111} and {110} exposed crystal faces for DOD nanocrystals were estimated as 34.2 and 65.8%, respectively (see Table 1). For NBA nanocrystals, the fraction of the {110} surfaces results 88.0%, while the remnant percentage can be easily associated to the {001} exposed crystal faces. According to these calculations, we can infer that NBA particles present a higher percentage of {110} faces than OCT and DOD nanocrystals (Table 1). Nitrogen physisorption experiments were performed on SnO2 powdered samples, as the characterization on film is difficult to

Figure 2. (A) Observed (red circle) and calculated (solid line) XRD patterns of tetragonal tin oxide OCT nanocrystals after Rietveld refinement in the tetragonal space group P42/mnm with lattice parameters a = b = 4.7444(3) Å and c = 3.1834(8) Å. The difference profile is shown at the bottom. (B) XRD patterns of (a) octahedral (OCT), (b) dodecahedral (DOD), and (c) nanobar (NBA) SnO2 nanocrystals. The vertical marks below the patterns give the positions of the allowed Bragg reflections.

OCT and DOD nanocrystals (see Electrical Measurement section). According to the XRD results, the patterns obtained (Figure 3) were indexed again based on the tetragonal crystal system of the rutile phase of bulk tin oxide. The sharp peaks detected indicate the high crystallinity of the films. Figure 4 reports a summary of the SEM, TEM and HRTEM images collected for OCT nanocrystals. According to the results of Kuang et al.,31 when the reaction is carried out in the presence of PVP and under high acidic pH, octahedral nanocrystals (OCT) with regular size and shape (length ∼200 nm and width ∼150 nm) are obtained (Figure 4a). The octahedral shape is better visible from TEM images (Figure 4b−f). In particular, Figure 4e shows a low-magnification TEM image of an individual octahedral particle viewed along the [111] direction. The crystal presents an aspect ratio α (α = length/width) of 3678

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Figure 4. Representative (a) SEM, (b−d) TEM and (e, f) HRTEM of octahedral SnO2 nanocrystals (OCT). In particular, (e) reports a lowmagnification image of an octahedral particle viewed along the [111] direction with the schematic model of an ideal SnO2 octahedron enclosed within {221} facets. The inset in (f) is the FFT pattern of the octahedral nanoparticle.

t-plot, no micropores were detected. The specific surface areas (SSABET) of differently shaped SnO2 nanocrystals are reported in Table 1. Using the percentage of the mainly exposed {221}, {111} and {110} crystal faces, their relative SSABET were also calculated. Electrical Measurements. Aiming to use the obtained materials for environmental monitoring applications, the tin dioxide nanocrystals were mixed with EtOH to give a dense mixture. Then, in order to obtain thin films, a few drops of this

be performed with high accuracy (see Experimental Section). In particular, the adsorption−desorption isotherms of OCT and DOD samples are shown in Figure 6. Octahedral nanoparticles show a pseudotype II Brunauer isotherm, typical of nonporous materials (Figure 6a). Conversely, the DOD sample appears mesoporous, displaying a type IV isotherm (Figure 6b). A similar behavior was observed for the small NBA nanocrystals (not shown). According to the 3679

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Figure 5. TEM and HRTEM images of (a−d) DOD and (e−h) NBA tin oxide nanocrystals. The inset in (a) corresponds to the SAED pattern of the DOD nanocrystals.

a quartz slide is showed in Figure 7a. Although the agglomeration increased, the morphological features of the nanocrystals do not change.

paste were simply deposited by drop-casting onto Suprasil quartz slides equipped with gold current collectors. As an example, the SEM image of octahedral (OCT) nanocrystals deposited on 3680

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although the increased agglomeration, the nanocrystals retain their initial morphology. Figure 8 reports the electrical responses of the obtained shape controlled SnO2 nanocrystals as a function of the operating temperatures under CO (580 ppm)/air, the highest used CO concentration. Two different regions can be observed. At working temperature between 300 and 375 °C (Region I), the best sensing performances (maximum S value of ∼12 at 350 °C) are displayed by the large octahedral nanocrystals (OCT) with very low surface area (see Table 1). In spite of their relatively low SSABET and DCPV (see Table 1), DOD nanocrystals show also relevant response, with a S maximum of 6.6 at 375 °C. Above this temperature, their electrical response decreases (S ≈ 4.5 at 400 °C, not shown), indicating that the best operating temperature for CO detection for DOD nanocrystals is 375 °C. Finally, though the very high specific surface area (227 m2 g−1), in the same temperature range, small nanobars (NBA) exhibit the worst sensing behavior, which appears almost independent from the temperature. These results suggest that the differences in the total surface area and porosity among SnO2 nanocrystals are not representative of their sensing properties in this temperature range, while the shape (see Morphological Characterization), the exposed crystal faces and their relative specific surface area seem to play a major role. In detail, the total exposition of {221} surfaces and the largest SSABET of the exposed {111} faces (Table 1) appear crucial in determining the remarkable electrical response of OCT and DOD nanocrystals, respectively. This is in agreement with previous studies31,32 reporting the excellent gas-sensing performance toward EtOH of octahedral and elongated dodecahedral SnO2 nanocrystals predominantly exposing high energy {221} and {111} faces. Conversely, the relatively low specific surface area of exposed {111} crystal faces and the high area of low-energy {110} surfaces probably limit the response toward CO of the small NBA nanocrystals. In the Region II (ranging between 200 and 275 °C), the trend of the sensing performances of the shape controlled SnO2 nanocrystals appears inverted, and the effect of the total surface area and porosity of the nanocrystals seem to determine their functional behavior. In fact, the NBA sample with the highest SSABET (see Table 1) shows the best electrical response at each operating temperature with a maximum S value of ∼5 at 225 °C. Conversely, the large octahedral and dodecahedral nanocrystals display much lower responses, which slightly increase with the increase of the working temperature. In order to better describe the electrical sensitivity toward carbon monoxide, we also report the electrical response of all layers as a function of CO concentration at two representative temperatures (Figure 9). At each temperature, layers generally discriminate well the different concentration of the gas (see also Figure 7b). At 350 °C, the sensing performances of the large OCT nanocrystals are significantly better than that of the DOD and the small NBA nanocrystals, and the electrical response follows a linear trend increasing the CO concentration (Figure 9a). This supports the idea that at a high temperature the specific exposed crystal surfaces play a key role in determining the sensing properties. At 225 °C, the electrical response of NBA nanocrystals is the highest and almost linearly increases with the CO concentration (Figure 9b). Moreover, OCT and DOD show very similar sensing performance, which appear unaffected by the

Table 1. Specific Surface Area (SSABET) of Shape Controlled SnO2 Nanocrystals and Relative SSABET of Their Exposed Surfaces

a

sample

SSABET (m2 g−1)

OCTa DOD NBA

5.19 ± 0.8 56.4 ± 8.5 227.0 ± 2.8

exposed {111} crystal facets (%)

exposed {110} crystal facets (%)

SSABET of exposed {111} crystal facets (m2 g−1)

SSABET of exposed {110} crystal facets (m2 g−1)

36.8

63.2 88.0

20.7

35.6 199.8

OCT nanocrystals entirely expose {221} surfaces (100%).31

Figure 6. Adsorption/desorption isotherm at liquid nitrogen temperature for (a) OCT and (b) DOD SnO2 nanocrystals.

Films were tested with a CO/Air gas mixture (72.5−580 ppm), and the working temperature was varied in a selected range (200−375 °C). Figure 7b reports the resistance variation for films of OCT SnO2 nanocrystals when pulses of the target gas (CO/Air) with known CO concentration (72.5−580 ppm) were introduced into the measuring chamber, alternating with air pulses at 250 °C. The resistance in the sensing layers regularly decreases under the reducing gases and increases under air, thus confirming n-type semiconductor behavior of the oxide. The sensor response was evaluated as S = RAIR/RMIX, where RAIR is the resistance under air and RMIX is the electrical resistance under CO/Air mixed gas. To demonstrate that at the operating temperatures of the sensors SnO2 nanocrystals maintain the same morphological features, we have performed also SEM investigation of OCT and DOD films after thermal annealing in air at 400 °C for 3 h. The images (Figure S2, Supporting Information) revealed that, 3681

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Figure 7. (a) SEM image of OCT nanocrystals deposited by drop-casting onto a Suprasil quartz slide. (b) Resistance variation for OCT SnO2 films after exposure to pulses of CO (72.5−580 ppm)/Air and Air at 250 °C.

Figure 8. Comparison among the electrical responses of the films of (○) OCT, (△) DOD and (□) NBA tin oxide nanocrystals at different temperatures recorded for 580 ppm [CO] in dry air.

increase of the gas concentration. These results confirm that the sensing behavior at low temperatures is mainly determined by the specific surface area of the nanocrystals, which allows in the case of high surface area NBAs a better interaction of the target gas with the surface sites. An accurate comparison between the electrical responses of the films obtained from shape controlled SnO2 and those of other systems reported in literature is difficult to be achieved due to differences in the sensing layer composition, loading and kind of metal-dopant, and in the operating conditions (e.g., sensing apparatus, gas concentration and temperature). However, in order to highlight the effect of the exposed crystal facets on the sensing properties, we compared the sensitivity of OCT, DOD and NBA with the behavior of nanostructured SnO2 films without specific prominent surfaces obtained by our group via a sol−gel conventional method.36 These layers display spherical cassiterite nanoparticles with average size of 4−6 nm, high surface area (SSABET = 150 m2 g−1), irregularly organized in large aggregates (see Figure S1, Supporting Information). In this case, under the same conditions

Figure 9. Comparison among the electrical responses of (○) OCT, (△) DOD and (□) NBA films as a function of CO concentration in dry air at (a) 350 °C and (b) 225 °C. Insets in (a) and (b): representative HRTEM images of OCT and NBA SnO2 nanocrystals which show the best electrical response at the corresponding temperatures.

where octahedral and elongated dodecahedral nanocrystals showed the highest electrical sensitivity (OCT: T = 350° and 3682

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Figure 10. Experimental ESR spectra of shape controlled SnO2 nanocrystals: (A) As prepared samples; (B) after treatment under air stream (80 cm3 min−1) for 1 h at 350 °C; and (C) after treatment under CO (580 ppm)/Ar mixture stream (80 cm3 min−1) for 30 min at 350 °C. Insets in B show the corresponding shapes.

DOD: T = 375 °C for [CO] = 580 ppm), the response of the conventional sol−gel material (S = 2 ± 0.3) was remarkably lower than the value found for the OCT (S = 12 ± 1.5) and DOD (S = 6.6 ± 1) films. This confirms that the morphological control of the nanocrystals and specifically the presence of selected surfaces (i.e., high energy surfaces), induces a more effective interaction of the gas with the sensing layer and enhances the electrical sensitivity of the shape controlled SnO2 nanocrystals.30 Further investigation on the electronic properties of these nanoparticles is mandatory to support the electrical results, and in particular to evaluate the role played by these surfaces at the electronic level in the sensing mechanism (see ESR Investigation). ESR Investigation. In order to study how the exposition of specific crystal faces affects the formation and the reactivity of the species (e.g., singly ionized oxygen vacancies Vo•; reduced oxygen centers O−, O2‑, O2−•) involved in the SnO2 sensing mechanism and, in turn, the electrical response, a comprehensive ESR investigation of OCT, DOD and NBA nanocrystals was performed (Figure 10). The spectra were recorded under vacuum (p < 10−2 mbar) at 130 K before and after subjecting the samples to Air, and to CO/Air mixture (see Experimental Section) at 350 or 225 °C, in order to reproduce the conditions where a different trend of the sensing performances was observed (see Region I and region II in Figure 8, Electrical Measurements). The spectra of as-prepared OCT and DOD samples, show the presence of resonance lines at g ≈ 1.890 (Figure 10A). Such resonances are identical to those generally observed after reduction treatment in polycrystalline SnO2 and they are attributable to unpaired electrons trapped in singly ionized oxygen vacancies VO•.17,27,35−39 At variance, in the case of NBA nanoparticles, no ESR signals are detectable. After treatment under air stream (80 cm3 min−1) for 1 h at 350 °C, the shape and the intensity of the ESR signals remain almost the same (Figure 10B). The treatment under CO/Ar atmosphere at the same temperature, leads to a significant increase of the VO• amount only for OCT and DOD nanocrystals (Figure 10C). This indicates that new oxygen vacancies have been created according to the following reactions:17,37 CO + OO ⇆ CO2 + VO

VO ⇆ V •O + e−

(4)

Again, the spectrum of NBA does not show any resonance after CO/Ar treatment. In the ESR spectra acquired on samples subjected to Air and then CO/Air streams at 225 °C (not reported), no new signals arise and the amount of VO• species remains almost constant. As already proposed in our previous studies,17,27,35−39 the amount of the paramagnetic oxygen vacancies detected by ESR is relatable to that of the electrons entering the SnO2 conduction band and therefore parallels the increase of the semiconductor sensing performance. In order to associate the paramagnetic defects with the sensing performance, the amount of VO• species calculated by using the area of integrated signals after CO/Ar treatments at 350 °C and at 225 °C was plotted against the electrical response S of shape controlled SnO2 nanocrystals at the same temperatures (Figure 11A). It turned out that at 350 °C the concentration of VO• centers increases with increasing S, suggesting a direct correlation between the electrical response of faceted nanocrystals and the amount of oxygen vacancies. Instead, at lower temperature (225 °C), the amount of VO• centers follows almost an opposite trend, decreasing with the increase of the electrical response (Figure 11A) and the VO• centers do not seem implied in the sensing mechanism. To gain deeper insight into the role that the different crystal surfaces play in determining the electrical response, the amounts of VO• centers detected after CO/Ar treatment at 350 °C and at 225 °C in OCT, DOD and NBA SnO2 nanocrystals were related to the structure of the different exposed surfaces and specifically plotted vs the area of the exposed {110} crystal face (Figure 11B). At a higher temperature (350 °C), the concentration of oxygen vacancies decreases by increasing the surface area of {110} faces and reaches the minimum value (no paramagnetic species) for NBA nanocrystals, which show the worst electrical response at 350 °C (see Region I, Figure 8). From these observations, it appears that at a high temperature the {110} surfaces do not promote the generation of oxygen vacancies and have a detrimental effect on the sensing performance. According to this idea, the very sensitive OCT nanoparticles which entirely expose

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At a temperature above 300 °C, the sensing reaction appears to be more influenced by the production of oxygen vacancies; thus, octahedral nanoparticles (OCT) with entirely exposed {221} surfaces display the best performance. Conversely, at a lower temperature (below 275 °C), the effect of the particle size and specific surface area of the nanocrystals seems to prevail over the role played by the crystal surfaces, and small NBAs with highly exposed {110} facets show the highest electrical response.34 To check the presence of surface reduced oxygen species involved in the sensing mechanism, the reactivity of the VO• species was studied, by exposing the nanocrystals to air stream at RT after the treatment under CO/Ar atmosphere at 350 or 225 °C. This procedure is suitable to locate the paramagnetic VO• centers37 and, in the present case, allows us to further elucidate the influence of the crystal facets on the sensing performance. Unexpectedly, no effect on the resonance lines was noticeable after the air contact. This suggests that, at variance with the conventional sol−gel obtained SnO2 powders,17 the singly ionized oxygen defects in OCT and DOD SnO2 nanoparticles do not undergo the surface reaction: Sn 4 + + V •O + O2 ⇆ Sn 4 + − O−• 2

(5)

That is, the VO• centers are unable to transfer their electrons to O2 at room temperature and to produce the paramagnetic superoxide anions (O−• 2 ).This indicates that in octahedral and elongated dodecahedral nanocrystals the defects are located in a subsurface region. More significantly, the lack of the surface reactivity toward molecular oxygen also suggests that in SnO2 nanocrystals with specific exposed crystal facets the surface damage, induced by the interaction with the reducing gas, undergoes a much faster self-repair, followed by the reconstruction of the {221} and {111} surfaces, compared to the almost spherical sol−gel obtained nanoparticles.17 This pushes the oxygen defects toward the bulk, where they can react only if thermally activated. The subsurface location of the oxygen vacancies further justifies the improved sensing properties of OCT and DOD nanocrystals. In fact, the bulk oxygen defects, unable to interact with molecular oxygen, are fully active for injecting electrons into the conduction band of the oxide.

Figure 11. (A) Trends of the relative amounts of VO• species at 350 °C (●) and at 225 °C (○) calculated for differently shaped SnO2 nanocrystals vs the electrical response (S = RAIR/RMIX) at the same temperatures. (B) Trends of the relative abundance of VO• species for OCT, DOD and NBA nanocrystals as a function of the specific surface area (SSABET) of their {110} exposed crystal faces.

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the {221} surfaces and do not possess any {110} face, show the highest amount of VO• centers. A lower quantity of paramagnetic defects is observable for DOD nanocrystals, which exhibit, besides the {111} surfaces, a relatively high surface area of {110} faces and an electrical response much lower than the OCT sample. These results can be associated to the arrangement and coordination of the surface atoms on the different crystal facets exposed by the nanocrystals.31,32,34 In fact, the high energy {221} and {111} faces, mainly occurring in OCT and DOD, present a higher density of undercoordinated Sn centers compared to the low-energy {110}, which are instead predominant in NBA. Thus, the {221} and {111} surfaces seem to have a higher tendency than the {110} ones to stabilize the defective sites. This can account for the presence of more VO• species in OCT than in DOD and NBA, even in the as-prepared powders. At 225 °C, the amount of VO• centers follows exactly the same trend and again displays the lowest value for the NBA sample, which at variance with the higher T conditions shows the best sensing behavior (see Region II, Figure 8). This suggests the occurrence of two different temperature-dependent sensing mechanisms.

CONCLUSIONS In the present study, we comprehensively investigated the sensing behaviors of SnO2 shape controlled nanocrystals in connection with the generation and the reactivity of their oxygen defects, in order to obtain deeper insight into the influence of their exposed crystal facets on the sensing mechanism. The electrical response and the formation of the VO• centers resulted greatly affected by the specific exposed surfaces of the nanocrystals. In particular, at temperatures above 300 °C, the concentration of oxygen vacancies increases by decreasing the area of the low-energy {110} surface, and displays the maximum value for the highly sensitive OCT nanocrystals with entirely exposed high-energy {221} faces and very low surface area. At lower temperatures (below 275 °C), although the VO• species show a very similar trend, the best sensing performances were observed for small NBA nanocrystals with predominant {110} surfaces and very high SSABET. 3684

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These outcomes suggest the occurrence of two different temperature-dependent sensing mechanisms. At T > 300 °C, the sensing reactions appear to be mainly influenced by the presence of the {221} and {111} surfaces, which enhance the generation of oxygen vacancies and boost the performance of low surface area OCT and DOD nanocrystals. Conversely, at lower temperatures (below 275 °C), the effect of the particle size and of the specific surface area of the nanocrystals seems to prevail over the role played by the structure of the crystal surfaces, and small NBAs show the best electrical response. Unexpectedly, the interaction with air at RT after CO/Air treatment does not evidence the formation of O−• 2 species, usually occurring in polycrystalline samples. This indicates that in SnO2 nanocrystals with specific exposed crystal facets, the surface damage, induced by the interaction with the reducing gas, undergoes a very fast selfrepair followed by the reconstruction of the crystal surfaces. Thus, the oxygen defects are pushed toward the bulk where they can react only if thermally activated. This further justifies the improved sensing properties of OCT and DOD nanocrystals, since bulk oxygen defects are the only ones active to inject electrons to the conduction band of the oxide. Therefore, by quantitatively monitoring the formation and the interfacial reactivity of VO• centers in shape controlled SnO2 nanocrystals, we provided an effective tool to shed light on the role of the exposed crystal faces in the sensing mechanism.

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ASSOCIATED CONTENT

* Supporting Information S

Details on the preparation of conventional SnO2 sol−gel films and their morphological characterization by TEM. SEM images of OCT and DOD films after calcination in air at 400 °C. Several examples of row resistance data for each SnO2 sample. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Milano group gratefully acknowledges the financial support of the COST-HINT action MP1202. The authors also personally thank Alessandro Ciappei for his support in the experimental work, Dr. Ilya Pinus for XRD analysis, Dr. Lidia Armealo for GIXRD measurements and Dr. Paolo Gentile for his assistance with SEM investigation.

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