SnO2 Nanostructured Thin Films for Room Temperature Gas Sensing

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SnO Nanostructured Thin Films for Room Temperature Gas Sensing of Volatile Organic Compounds Kelsey Haddad, Ahmed A. Abokifa, Shalinee Kavadiya, Byeongdu Lee, Sriya Banerjee, Baranidharan Raman, Parag Banerjee, Cynthia S Lo, John D. Fortner, and Pratim Biswas ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08397 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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ACS Applied Materials & Interfaces

Submitted to ACS Applied Materials & Interfaces May 8th, 2018

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SnO2 Nanostructured Thin Films for Room

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Temperature Gas Sensing of Volatile Organic

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Compounds

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Kelsey Haddad,a Ahmed Abokifa,a Shalinee Kavadiya,a Byeongdu Lee,b Sriya Banerjee,c

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Baranidharan Raman,d Parag Banerjee,c Cynthia Lo,a John Fortner,a and Pratim Biswasa*

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a

Department of Energy, Environmental and Chemical Engineering, Center for Aerosol Science

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and Engineering, Washington University in St. Louis,

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St. Louis, MO 63130, USA

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b

X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, IL,

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USA c

Department of Mechanical Engineering and Materials Science, Washington University in St.

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Louis, St. Louis, MO 63130, USA d

Department of Biomedical Engineering, Washington University in St. Louis, St. Louis, MO

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63130, USA

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KEYWORDS. thin films; sensors; metal-oxide nanostructures; Aerosol Chemical Vapor

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Deposition; DFT calculations

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ABSTRACT. We demonstrated room temperature gas sensing of volatile organic compounds

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(VOCs) using SnO2 nanostructured thin films grown via the Aerosol Chemical Vapor Deposition

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(ACVD) process at deposition temperatures ranging from 450-600 °C. We investigated the

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film’s sensing response to the presence of three classes of VOCs: apolar, monopolar, and

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biopolar. The synthesis process was optimized, with the most robust response observed for films

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grown at 550 °C as compared to other temperatures. The role of film morphology, exposed

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surface planes, and oxygen defects were explored using experimental techniques and theoretical

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calculations to improve the understanding of the room temperature gas sensing mechanism,

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which is proposed to be through the direct adsorption of VOCs on the sensor surface. Overall,

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the improved understanding of the material characteristics that enable room temperature sensing

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gained in this work will be beneficial for the design and application of metal oxide gas sensors at

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room temperature.


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1. INTRODUCTION. Chemiresistive gas sensors detect changes in electrical resistance of an

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active material as a function of the surrounding atmosphere.1-2 These gas sensors have found

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broad application in environmental pollutant and air quality monitoring, national security, and

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industrial processing;3-5 however, researchers are still working towards the production of cheaper

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and more stable sensors. Further, performance parameters of these devices are becoming more

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critical as their uses, and consequently the operating requirements, are expanded. The

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development and implementation of broad-based sensor networks necessitates improved sensor

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lifetime, portability, and cost. In addition, there has been a surge of interest in flexible electronics

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for portable and wearable sensing applications, where flexibility and transparency, operation at

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ambient temperature, and autonomous fast response/recovery are desired.6-7

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Beyond the requirements of the sensing hardware, the physical and chemical properties of the

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gases to be monitored, as well as their concentrations, warrant consideration. Volatile organic

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compounds (VOCs) are of particular interest, including acetone, benzene, chloroform, methanol,

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methylene chloride, and phenol which have regulated concentration levels.8 In addition, an

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increased focus on using chemiresistive gas sensors in combinational arrays for the detection of

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disease biomarkers in exhaled breath, with a complex mixture of VOCs at low concentrations,

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has emerged.9-12

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Resistive gas sensors using metal oxides as their active material have been studied extensively 1-2

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due to their low material cost, high sensitivity, ease of miniaturization, and simple operation;

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however, their high operating temperatures (200-500 °C) and poor selectivity are consistently

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cited as drawbacks.6, 13 High temperatures not only increase power consumption and the baseline

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noise of the active material but also reduce the long-term stability of the sensors by altering the

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microstructure of the material over time. Furthermore, high-temperature operation increases


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fabrication complexity and cost by requiring a heating element. Finally, high operating

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temperatures can also limit broad scale applications, because detecting flammable or explosive

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analytes at high temperatures is a potential hazard.

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Metal-oxide semiconductor gas sensing capabilities are typically explained within the

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framework of oxygen ionosorption, whereby oxygen species in the ambient air are adsorbed to

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the surface of the metal oxide. For n-type semiconductors, an electron from the conduction band

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of the metal oxide is transferred to the chemisorbed oxygen, decreasing conductivity.14 As

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mentioned, these sensors are typically operated at temperatures ranging from 200-500 °C to

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reduce the intrinsic resistance of the metal oxide semiconductor below the detection limit, and to

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ensure the kinetics of the sensor response result in a quick and measurable change in the

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resistance.3,

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charge transfer of conduction electrons.14 The atomic charged oxygen ion ( ) is considered to

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be the most reactive of the ionosorbed oxygen species, enhancing chemical reactivity with the

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surrounding gases at temperatures above 100 °C.14

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The thermally activated process involves oxygen adsorption, ionization, and

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Due to the limitations of operating pure metal-oxide chemiresistive sensors, only a small

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portion of the literature in this field focuses on the demonstration and mechanistic understanding

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of room temperature sensing. Room temperature gas sensing has been observed for several metal

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oxides, including SnO2,16-18 In2O3,19-20 WO2.72,21-22 TiO2,

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both single crystalline

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materials are nanoscale, including 1-D nanostructures,

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nanoplatelets.18 While all of these sensors showed a response at room temperature, they can

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require extensive recovery times18 or the use of UV light for response recovery,

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their application.

20, 24, 26

and polycrystalline

15, 23

16, 22

TeO2,24 Co3O4,25 and ZnO ,26 for

materials. Typically, active sensing

16, 19-22, 24, 26

nanoparticles,

17, 23

19-20

and

limiting


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For the reported studies on room temperature sensing, the sensing mechanism and origin of the

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enhanced response remain unclear. Some researchers still attribute sensing to the traditional

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interaction of analyte gases with adsorbed moieties of oxygen. 15, 23-24 Others describe the sensing

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mechanism as a function of the competitive, direct adsorption between ambient oxygen and

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analyte vapors, specifically for NO222, 24 and ethanol.26 Originally, many of the studies attributed

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the enhanced response to the intrinsically small grain size of the nanomaterial and to its high

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surface-to-volume ratio.16-17, 22, 25 However, more recent studies indicate the improved sensitivity

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cannot be a function of morphology alone, but instead also depends on the exposed crystal planes

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and oxygen defects. 15, 18, 27

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SnO2 is a wide band gap semiconductor (n-type, 3.6eV at 300K) and is commonly used for gas

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sensing applications due to its stability, high sensitivity, and fast response time at elevated

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temperatures.28 Previously, we demonstrated a single-step, template-free aerosol chemical vapor

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deposition (ACVD) method to grow well-aligned SnO2 nanocolumn arrays.7 A self-catalyzed

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vapor-solid growth mechanism was proposed and the influence of various deposition parameters

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was reported. One of the interesting features of the deposition technique was the influence of the

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substrate temperature on the morphology and crystal structure of the nanostructured films. Films

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grown at 500 °C, 550 °C, and 600 °C all had a similar columnar geometry, with strong

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diffraction peaks at the (101) and (211) planes. The major difference observed among films

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grown at these temperatures was the aspect ratio of the columnar structures, with those grown at

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higher temperatures having increased height and decreased width. Alternatively, films grown at

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450 °C showed a strong diffraction peak at the (110) plane and had a pyramidal cross-section.

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In this work, we demonstrated room temperature gas sensing of VOCs using n-type SnO2

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nanostructured thin films grown via the referenced single-step ACVD process.29-30 We


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investigated sensing response to the presence of three classes of volatile organic compounds:

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apolar, monopolar, and bipolar. For these, optimal sensor response was observed for films grown

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at 550 °C. Finally, we conducted a detailed study of film morphology, exposed surface planes,

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and oxygen defects using both advanced experimental techniques and dispersion-corrected

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density functional theory (DFT) calculations to gain a better understanding of response (sensing)

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mechanisms at room temperature. Taken together, this work advances our understanding of pure

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metal oxide enabled, chemiresistive gas sensors that can operate at room temperature.

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2. RESULTS AND DISCUSSION.

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2.1. Preparation and Characterization of SnO2.

The nanostructured thin films were

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synthesized at four different temperatures (450 °C, 500 °C, 550 °C, and 600 °C) and the column

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structure was characterized using grazing incidence small angle X-ray scattering (GISAXS). A

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more detailed understanding of the formation mechanism of the films and their morphology at

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different temperatures is described in our previous work.7 Winged GISAXS patterns of the

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columns synthesized at these four temperatures are shown in (Figure 1a). Based on the GISAXS

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scattering pattern shown in the reciprocal space, the structure of the faceted columns can be

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reconstructed in real space (white lines). The scattering directions shown by the orange lines are

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perpendicular to the surface of the column facets. From the reconstruction shown in Figure 1a,

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we see that two types of facets are present, one corresponding to the tip-facet and one to the side-

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facet. For the columns synthesized at 450 °C, the angle of the scattering pattern is small and the

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wings are narrow, implying that the column tip is flat. As the temperature increases above 450

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°C, the scattering from the tip-facets become stronger compared to the side-facets and the angle

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of the tip-wing increases. These changes correspond to the growth of a more pronounced tip and

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sharper tip facets, and are in agreement with the changes in morphology reported in our previous


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work for columns grown at 450 °C as compared to the other temperatures.7 Above 500 °C, the

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scattering from the side-facets becomes stronger with increasing temperature because of the

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vertical growth of the column, which leads to a higher aspect ratio as the cross-sectional width of

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the tip decreases and the body of the columns lengthens. The column geometry at each

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temperature appears in white in Figure 1a.

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Furthermore, grazing incidence wide angle X-ray scattering (GIWAXS), shown in Figure 1b,

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was performed to understand the exposed crystal facets of the column structures, which have

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been shown to play a role in gas sensing performance.27 The GIWAXS pattern for the

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nanocolumns synthesized at 450 °C is associated with the P42/mnm space group aligned along

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the [331] direction normal to the substrate surface, whereas the GIWAXS pattern for the

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nanocolumns synthesized at 500 °C, 550°C, and 600 °C are aligned along the [502] direction,

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normal to the substrate surface. Indexing of the GIWAXS patterns for the 450 °C sample and

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550 °C sample—taken to be representative of 500 °C, 550°C, and 600 °C substrates— are shown

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in the supplementary material (Figure S1). Although the structures are single crystals and

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oriented on the surface, the reflections are present as arches rather than discrete points due to the

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physically random orientation of the columns when averaged over the substrate surface.7

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Correlating the orientations from GIWAXS measurement to the nanocolumn geometry from the

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GISAXS measurement, we observed that for the nanocolumns synthesized at 450 °C, the tip-

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facets are oriented in the [110] direction and the side-facets are oriented in the [101] direction.

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For the nanocolumns synthesized at temperatures higher than 450 °C, the tip-facets are oriented

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in the [110] direction, and the side-facets can be oriented in the [101], [211], and [111] direction.

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Because the most exposed part of the nanocolumn is the side-facet, the [101], [211], and [111]


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surface facets are the predominant exposed crystal planes in the 500 °C, 550 °C, and 600 °C

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samples as compared to the [110] facets.

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The chemical states of the SnO2 nanostructured film surfaces were examined by X-ray

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photoelectron spectroscopy (XPS), focused on the O1s and Sn3d5/2 signals. The observed O1s

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response shown in Figure 2a has a wide, asymmetrical peak with an evident shoulder, indicating

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more than one oxidation state is present. Contributions from both SnO and SnO2 can be

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attributed to a reduced surface layer, with a SnO peak signaling the loss of a bridging oxygen.31-

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(chemisorbed oxygen or hydroxyl ions), O-Sn2+ (SnO), and O-Sn4+ (SnO2). The center of gravity

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of the O1s peaks grown at different temperatures are slightly shifted, which can be attributed to a

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shift in the relative position of the Fermi level within the band gap at the surface.34 To account

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for this shift, independent of the chemical shift of binding energy, peaks were fit relative to the

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location of the Sn-O4+ peak, while maintaining a fixed relative position of 1.0 eV and 2.5 eV for

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Sn-O2+ and Ochem, respectively.32 In addition, the Sn3d5/2 peak was also deconvoluted as Sn4+ and

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Sn2+ response, using the Sn4+ peak as a reference and maintaining a fixed relative position of 1.0

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eV.32

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reduced relative to pure SnO2.

The O1s peak was deconvoluted into three contributing states, which included Ochem

Overall, the XPS analysis shows that the surface at all four temperatures is partially

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To explore the role of deposition temperature on the electrical properties, resistivity

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measurements were performed on the SnO2 nanostructured films using the van der Pauw

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Method. Recorded values were normalized by the dense film height, and are shown in Figure 2b.

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Overall, the conduction is proposed to occur through the dense base layer. Based on the XPS

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results, the similarity between the resistivity of the nanostructured thin films grown at 500 °C,

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550 °C, and 600 °C — all on the order of 1 ohm-cm — can be attributed to the similar


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concentration of free charge carriers, assuming the major contribution of charge carriers for the

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pure material is from oxygen vacancies.35 The values of the resistivity observed in Figure 2b are

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similar to those observed for mesoporous SnO2 thin films grown at similar temperatures.36 In

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addition, the low values of the resistivity measurements further support the idea that these thin

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films are not stoichiometric but instead contain oxygen defects, as stoichiometric tin oxide films

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were shown to have a high resistivity, on the order of 108 ohm-cm, at room temperature.35

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One advantage of the ACVD deposition technique, beyond its scalability, is that both low

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resistivity and a high surface area are achieved in a single step. For techniques where

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nanostructures are grown in a separate step(s) and then later drop cast onto a sensing substrate,

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the high surface area of the materials must be balanced with the creation of a conducting

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network. By using the ACVD system, the nanostructures are directly deposited and oriented on

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the surface, and a dense base layer is created, simultaneously achieving both low resistivity and a

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high surface area. The nanocolumns create a large surface area for analyte interaction, while the

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dense base layer acts as a strong conducting network. In addition, the features of the electrodes,

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i.e., their inter-element spacing, could potentially be quite large due to the thorough coverage of

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the dense film spanning them, as shown in Figure 2b. For all sensing experiments, the SM2060

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multimeter was operated in the 2.4 kΩ range, requiring a test current of only 1 mA and a

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maximum test voltage of 2.4 V; therefore, the power consumption for the component was low (
(211) > (110), which shows that

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the highest index facets are not always the most active for gas adsorption, particularly when the

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oxidation state of the surface, the presence of surface oxygen defects, and the role of dispersive

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interactions are considered. The SnO2 structures deposited at 550 °C by ACVD are good

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candidates for the sensing of ethanol, and proximally other biopolar molecules, as these

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calculations show that the (101) reduced surface has the highest binding strength. This strong

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performance may be partially attributed to the large proportion of the column surface composed

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of the (101) facets.

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As shown in Figure 3b, the optimized adsorption configuration for ethanol on the reduced

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(101) surface is at the oxygen vacancy site, where the electronegative oxygen from ethanol binds

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to one of the two under-coordinated surface Sn cations adjacent to the oxygen vacancy (bond

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length = 2.23 Å), while a hydrogen bond is established between the hydrogen atom from ethanol

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and a surface bridging oxygen atom (bond length = 1.90 Å). A similar binding configuration is

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also observed for ethanol adsorption on the reduced (211) facet (Figure S5b). For the reduced

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(110) surface, the additional hydrogen bonding mode does not exist, contributing to the lower

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adsorption energy compared to the reduced (101) facets. Samples grown at 550 °C were tested in

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a nitrogen environment to help confirm if the response mechanism was through the direct

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binding of analyte gases to the surface rather than the traditionally cited ionosorption

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mechanism. As shown in Figure 3c, the samples had a similar response in both nitrogen and

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atmospheric conditions, which indicates, along with the theoretical calculations, that the sensing

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mechanism may be attributed to the direct binding of volatile organic compounds to the surface

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oxygen defects, even though a traditional n-type response is still observed.


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2.2.2. Gas Sensing Response Towards Monopolar Molecules. Sensing of monopolar

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molecules was explored to determine the role of electron donor interactions. While the samples

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grown at 500 °C, 550 °C, and 600 °C all showed a response, the best performance was again

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seen for the 550 °C substrates, with a correlation coefficient above 0.99 between triplicate

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measurements (Figures S2 and S4). As shown in Figure 4, sensors fabricated at 550 °C

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responded to monopolar molecules (ketones) with varying chain lengths with a rapid, step-wise

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decrease in resistance similar to that observed for the bipolar molecules. The shortest chain-

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length ketone, acetone, showed the weakest sensing response. Dispersion corrected DFT

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calculations on the adsorption of acetone on the three surface facets confirmed that the

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interaction is thermodynamically stable for all three, and follows the trend of (101)>(110)>(211)

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(Table 1). Figure 4d illustrates the most stable adsorption configuration for acetone on the

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reduced (101) surface at the electron-dense oxygen vacancy site. Similar behavior was observed

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for the other two reduced surfaces (110) and (211), where Oacetone was found to preferentially

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bind to one of the two under-coordinated surface Sn cations. The adsorption energies of acetone

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were generally less stable than ethanol, suggesting a weaker binding affinity with the notable

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exception of the (110) facet. The weaker binding affinity for acetone shown through theoretical

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calculations is consistent with the diminished sensing response seen for monopolar molecules as

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compared to bipolar molecules in this work. The stronger adsorption of bipolar molecules can be

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attributed to their higher polarity, because the polarized hydroxyl (OH-) group in ethanol binds to

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the surface via two modes (Snsurface - OEthanol) and (O

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in acetone directly interacts with surface cations. This underpins the stronger binding seen for

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ethanol compared to acetone on the (101) surfaces, where bridging oxygen atoms are more

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exposed to the adsorbed gas molecule compared to the (110) surface (Figure S5).

bridging

- H Ethanol), while the carbonyl group


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2.2.3. Gas Sensing Response Towards Apolar Molecules. Hexane, the simplest compound

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tested with regard to dipole moment, is a hydrocarbon with no electron acceptor or donor sites

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and no dipole moment. The response of the sensors grown at 550 °C is shown in Figure 5a. The

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sensors fabricated at 500 °C and 600 °C showed a similar response to hexane gas, with an

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average correlation coefficient of 0.89 between samples grown at different temperatures (Figures

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S2 and S6). Overall, the response was relatively slow, failing to reach saturation in the allotted

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five-minute time period, with no reversal in response once the stimulus was removed. The sensor

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showed an increase in resistance upon introduction of gas to the chamber, which is the opposite

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trend to that expected from a n-type semiconductor based on an ionosorbed oxygen model. To

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probe the saturation of the surface with hexane, the sensor was exposed to hexane vapors at a

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concentration of 400 ppm for an extended period of time. After 3 hours, the sensing response had

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still not reached saturation (Figure S7).

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To understand the interaction of hexane on the metal oxide surface, we studied the adsorption

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of propane on the reduced (101) SnO2 surface. Propane was used as a proxy for hexane in these

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calculations because it is a shorter aliphatic chain, which provided enough separation between

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parallel images on the p(2×2) surface slab for the required calculations. The binding mechanism

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between propane and the surface was expected to be similar to hexane because the driving force

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for adsorption should be dispersive energies (as London forces), modulated by the surface area

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of the analyte gas molecule. Consequently, the interaction between hexane (C6) and the surface

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was expected to be at least as strong as those observed theoretically with propane (C3).

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For propane’s adsorption on the reduced (101) surface, the energy of adsorption with the DFT-

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D3 correction is -0.378 eV. We found that including the correction for the non-covalent

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dispersive interactions was crucial for finding the stable adsorption configuration for propane


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(Figure 5b), where the contribution of dispersive interactions was (~96%) of the adsorption

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energy. This finding implies that weak van der Waals interactions drive the adsorption of apolar

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molecules, which was reflected in the long bonding distance (3.27 Å) between the adsorbed

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propane molecule and the surface. Consequently, the role of crystal planes is less vital and only

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the (101) surface was explored. This type of binding is supported by the experimentally observed

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sensing response, which was both slow and unsaturated.

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2.2.4. Gas Sensing Response Mechanism. The n-type sensing response of the samples

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fabricated at 550 °C for both bipolar and monopolar volatile organic compounds in air at room

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temperature is proposed to be a function of the direct adsorption on crystal surfaces. As shown

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through DFT calculations, bipolar or monopolar molecules bind preferentially to oxygen defect

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sites. Based on theoretical calculations as well as the observed response of the material, we

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propose the adsorption of these polar molecules is similar to a multisite Langmuir isotherm, with

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multiple types of binding sites with varying affinities for the analyte gas but a fixed total number

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of binding sites, resulting in the step-wise response.38 Enhanced performance of the 550 °C

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substrates compared to the 500 °C can be attributed to the large proportion of the columns

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composed of the (101) crystal facets. Unlike the polar molecules, we propose that hexane gas

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modulates charge through weak van der Waals interactions that occur across the entirety of the

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surface of the nanostructure film, in a similar fashion as observed for the Freundlich isotherm.38

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While the binding strength may be weak, the response is seen from the summation of interactions

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across the surface, with no maximum adsorption density observed within the duration of the test.

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Overall, varied binding affinities are a function of the morphology, crystal planes, and oxygen

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defects in conjunction - all of which must be considered together when designing metal oxide

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gas sensors to be applied at room temperature. The sensing mechanism proposed in this work is


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more similar to the framework used to discuss the response of carbon-based chemical sensors

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including, single-walled carbon nanotubes (SWCNT),41 graphene,42 and reduced graphene

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oxide.43 For SWCNT chemical sensors, Robinson et al. proposed molecular adsorption events on

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low-energy binding sites (i.e., defect sites) produce a rapid conductance response.44 In addition,

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the capacitance response of the sensors was observed not to saturate, which the authors’

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attributed to a different type of adsorption site. They proposed the capacitance response was the

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result of vapor condensation on the sensor surface caused by analyte-analyte interactions,

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because the adsorption energy for the anlytes on pristine SWNT was shown to be negligible. In

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this work, the calculated adsorption properties of propane were shown to be thermodynamically

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favorable (-0.378 eV), which means the vapor could directly interact with the surface. To further

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support this mechanism, detailed studies on adsorption geometries, charge transfer, and band

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structure can be performed. While beyond the scope of this report, a more detailed theoretical

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analysis of the surface and molecular dynamics was undertaken to gain an additional

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understanding of these variables.45

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3. CONCULSION. We propose a single-step, scalable deposition process to fabricate pristine

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SnO2 gas sensors for room temperature applications. The ACVD approach reduces fabrication

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cost/steps as the active material is directly deposited onto sensing substrates and requires no

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additional components for room temperature sensing. Overall, the SnO2 sensors fabricated at

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temperatures above 500 °C showed a response to apolar, monopolar, and bipolar molecules at

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room temperature. While room temperature sensing is typically explored within the context of

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the ionosorption model, the lack of atomic charged oxygen at low temperatures makes this

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explanation infeasible. Rather than an oxygen-mediated interaction, we propose that there is

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direct binding between the volatile organic compounds and the active material, resulting in


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charge modulation at these binding sites. The strong performance of the 550 °C samples can be

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partially attributed to the strong binding affinity for monopolar and bipolar molecules on the

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oxygen defective (101) crystal facets. In addition, a rapid response with a high signal to noise

375

ratio is observed for these sensors, which can be attributed to the lack of external activation such

376

as heat or UV light that typically increase the baseline noise level. Taken together, the datasets

377

presented provide fundamental and valuable insight towards the design of low cost, metal oxide

378

gas sensors for room temperature sensing. The difference in interactions observed between

379

apolar, monopolar, and bipolar compounds can be used to help distinguish between different

380

classes of volatile organic compounds, thus providing selectivity.

381

4. EXPERIMENTAL SECTION.

382

4.1. Preparation of SnO2 Thin Films. The thin films were deposited using Aerosol Chemical

383

Vapor Deposition (ACVD), with a tetramethyl tin (TMT, Sigma-Aldrich) precursor, previously

384

outlined by Haddad et al. (2016). Briefly, tetramethyl tin was delivered via a bubbler to a

385

reaction chamber whose temperature is controlled by resistive heating elements that sit below a

386

stainless steel substrate holder. As the precursor approaches the heated surface, the vapor is

387

converted to the molecular form of the oxide via thermal decomposition, resulting in

388

nanostructured thin film growth via a vapor-solid growth mechanism. Nitrogen was used as a

389

carrier gas and oxygen as a dilution gas. The distance between the feed tube and substrate was

390

fixed a 1 cm, the dilution flow was fixed at 100 ccm, the TMT feed flow was fixed at 11 ccm,

391

and the deposition time was varied to maintain similar film heights. The deposition times were

392

12 minutes, 17 minutes, 23 minutes, and 60 minutes for films grown at 600 °C, 550 °C, 500 °C,

393

and 450 °C, respectively. The electrodes were synthesized on Si (100) substrate (625 µm

394

thickness, n-type, University Wafers) heated to 1100 °C for 10 hours to form an insulating


18

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395

silicon oxide base layer. The silicon oxide substrates were patterned with interdigitated

396

electrodes (IDE, 20 fingers, 50 µm wide and 2.4 µm long, spaced 50 µm apart) comprised of a

397

70 nm thick gold electrode with a 20 nm thick chromium adhesion layer beneath, both of which

398

were deposited via thermal evaporation.

399

4.2. Characterization of SnO2 Thin Films. The morphology of the thin films was explored

400

using scanning electron microscopy (SEM, FEINova NanoSEM 230). The X-ray photoelectron

401

spectroscopy (XPS) measurements were performed on a PHI 5000 VersaProbe II equipped with

402

monochromatic Al Kα (1486.6 eV) X-ray source and the spectra were processed using the PHI

403

Multipak software. The peaks were fit following calibration by the alignment of the spectra with

404

the C 1 s peak (284.5 eV) and Shirley background subtraction. A mixed function of 90%

405

Gaussian and 10% (±10%) Lorentzian character fit was used with a full width at half maximum

406

values (FWHM) fixed at 1.3 (±0.1) eV for all peaks. Grazing incidence small angle x-ray

407

scattering (GISAXS) and grazing incidence wide angle x-ray scattering (GIWAXS) were

408

performed to investigate the crystal planes present on the facets of the nanocolumns. The

409

samples were fabricated at Washington University in St. Louis and characterized at the

410

Advanced Photon Source (APS) beamline 12-ID-B, Argonne National Laboratory using

411

monochromatic X-ray with energy of 14 keV. The scattered X-rays were detected by a Pilatus

412

2M detector for GISAXS (sample-to-detector distance of 1.923 m) and Perkin Elmer 4kx4k

413

detector for GIWAXS (sample-to-detector distance of 0.382 m). The set-up was calibrated using

414

a standard (silver behenate) with known lattice spacing. A range of X-ray incident angle (theta),

415

exposure time and in-plane rotation angle (phi) was tested. The scattering pattern was analyzed

416

using a Matlab based software provided at the beamline. Room temperature resistivity

417

measurements were performed in a commercial probe station (Janis ST500-1-2CX) using Cu–Be


19


Page 20 of 39

418

probe tips with a 250 µm tip diameter. Measurements were made using a Keithley 2400 source

419

meter and an Agilent digital multimeter (34410A). A Van der Pauw structure was constructed by

420

depositing SnO2 films on 10 mm × 10 mm silicon wafers with thermal silicon dioxide. Indium

421

dots at the four corners of the film were used as electrodes.

422

4.3. Gas-Sensing Measurements. The gas sensing measurements were performed by

423

monitoring the changes in the resistance using a NI PXI-4071 Digital Multimeter in the 1 kΩ test

424

range with a test current of 1 mA and a max test voltage of 1 V. A cyclic profile consisting of 5

425

minutes of exposure to the volatile organic compound followed by 5 minutes of exposure to

426

clean air was performed for five different concentrations: 50, 100, 400, 700, and 1000 ppm. To

427

achieve this, the gases were delivered via a bubbler filled with hexane (Sigma-aldrich), acetone

428

(Sigma-Aldrich), or ethanol (Pharmco-Aaper) using zero grade air (AI Z300, Airgas) as the

429

carrier gas controlled by a mass flow controller (MKS). The gas was diluted with dehumidified

430

and filtered (hydrocarbon trap, model HT200-4, Agilent) room air, again using a mass flow

431

controller (MKS) to achieve the desired concentration. The total gas flow rate delivered to the

432

sensor manifold was kept constant at 750 ccm for all tests. The specimens were equilibrated

433

under baseline conditions, dry and filtered room air at 750 ccm, for one hour prior to exposure to

434

test gases. To analyze the results, a low pass (3rd order, high-pass Butterworth filter) filter was

435

carried out using the Matlab function butter. Sensing measurements were carried out on three

436

substrates for each temperature and gas combination.

437

4.4. Theoretical Calculations. DFT calculations were conducted using VASP (Vienna ab

438

initio simulation package). Perdew-Burke-Ernzerhof (PBE) formulation of the generalized

439

gradient approximation (GGA) was used for the exchange correlation functional. The Projector-

440

Augmented Wave (PAW) method with a 500 eV energy cutoff is used to represent the valence


20

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441

wavefunctions near the atomic cores. To sample the Brillouin zone, k-meshes were generated

442

automatically using the Monkhorst–Pack (MP) method with (4×2×1), (3×3×1), and (2×2×1) MP

443

grids for the (110), (101), and (211) surfaces, respectively. The p(2×2) surface slabs for the

444

stoichiometric (110), (101), and (211) surfaces were all cleaved from the fully relaxed bulk

445

structure, with an imposed vacuum layer of 15 Å. The surface slabs consisted of four (Sn2O4)

446

layers, with a total of 96 atoms (Sn32O64). For the oxygen defective (reduced) surfaces, an

447

oxygen vacancy was introduced by removing one of the bridging oxygen atoms from the topmost

448

atomic layer. For all the conducted DFT calculations, the top two layers were allowed to relax

449

while the bottom two were fixed at bulk positions. For adsorption calculations, gas molecules

450

were always introduced to the top side of the relaxed slab, and hence, dipole corrections were

451

employed to obtain accurate adsorption energies. Different adsorption configurations were

452

sampled for each gas molecule, and only the most stable configurations are reported.

453 454

FIGURES


21


Page 22 of 39

455 456

Figure 1. a) 2D GISAXS patterns of tin oxide columns synthesized at four temperatures with the

457

winged pattern in reciprocal space highlighted by the orange lines and the structure of the faceted

458

columns in real space shown by the white lines (qxy and qz are the scattering wave vectors in

459

reciprocal space). b) GIWAXS image of tin oxide columns synthesized at four temperatures,

460

showing the tip-facet is oriented in [110] direction and the side-facets oriented in [101], [111]

461

and [211] direction.

462 463


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464 465

Figure 2. a) High resolution spectra at binding energy corresponding to the O1s peak and

466

Sn3d5/2 peak for films grown at 450 °C, 500 °C, 550 °C, and 600 °C. b) Resistivity of SnO2 thin

467

films, normalized by the dense film height. Triplicate measurements were performed, with the

468

standard deviation between measurements represented by the error bars.


23


Page 24 of 39

469

470 471

Figure 3. Sensing responses evoked by bipolar molecules. a) Sensor response curve of substrates

472

grown at four different temperatures towards ethanol (in room-air), with error bars representing

473

the standard deviation in response across triplicate measurements. b) Stable adsorption

474

configuration of ethanol on the (101) reduced surface (red-oxygen; purple-tin; white-hydrogen).

475

c) Representative trace showing the characteristic change in resistance of sensor substrates

476

(grown at 550 °C) towards ethanol in a nitrogen environment. Characteristic change in resistance

477

of sensor substrates (grown at 550 °C) towards d) methanol, e) ethanol, and f) propanol in room-

478

air.


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479 ACS Paragon Plus Environment

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Page 26 of 39

480

Figure 4. Sensing responses evoked by monopolar molecules. Representative trace showing the

481

characteristic change in resistance of a sensor substrates (grown at 550 °C) towards a) acetone, b)

482

2-butanone, and c) 2-pentanone in room-air. d) Stable adsorption configuration of acetone on the

483

(101) reduced surface (red-oxygen; purple-tin; white-hydrogen).

484


26

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485 486

Figure 5. a) Representative trace showing the characteristic change in resistance of sensor

487

substrates (grown at 550 °C) towards hexane and b) stable adsorption configuration on the

488

reduced (101) surface of a propane molecule (red-oxygen; purple-tin; white-hydrogen).

489

TABLES.

490


27


Page 28 of 39

Table 1. Adsorption energy of ethanol and acetone on the reduced (110), (101), and (211) surface facets. Molecule

Surface Facet Reduced (101) [eV]

Reduced (110) [eV]

Reduced (211) [eV]

Ethanol

-1.244

-1.022

-1.192

Acetone

-1.129

-1.086

-1.067

491 492

ASSOCIATED CONTENT

493

Supporting Information.

494

The following files are available free of charge.

495

Additional GIWAXS patterns at different temperatures, additional adsorption configurations,

496

additional sensing traces, and details of XPS fitting (DOC)

497

AUTHOR INFORMATION

498

Corresponding Author

499

*Pratim Biswas. E-mail: [email protected]. Phone: +1-314-935-5548

500

Author Contributions

501

The manuscript was written through contributions of all authors. All authors have given approval

502

to the final version of the manuscript.

503

Funding Sources

504

Solar Energy Research Institute for India and the U.S. (SERIIUS) funded jointly by the U.S.

505

Department of Energy subcontract DE AC36-08G028308 (Office of Science, Office of Basic


28

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506

Energy Sciences, and Energy Efficiency and Renewable Energy, Solar Energy Technology

507

Program, with support from the Office of International Affairs) and the Government of India

508

subcontract IUSSTF/JCERDC-SERIIUS/2012 dated 22nd Nov. 2012

509

ACKNOWLEDGMENT

510

This research is based upon work supported in part by the Solar Energy Research Institute for

511

India and the U.S. (SERIIUS) funded jointly by the U.S. Department of Energy subcontract DE

512

AC36-08G028308 (Office of Science, Office of Basic Energy Sciences, and Energy Efficiency

513

and Renewable Energy, Solar Energy Technology Program, with support from the Office of

514

International Affairs) and the Government of India subcontract

515

SERIIUS/2012 dated 22nd Nov. 2012. This research used resources of the Advanced Photon

516

Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the

517

DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-

518

06CH11357. The authors acknowledge financial support from Washington University in St.

519

Louis and the Institute of Materials Science and Engineering for the use of instruments and staff

520

assistance. Kelsey Haddad would like to acknowledge the McDonnell International Scholars

521

Academy for their financial support.

522

REFERENCES

523 524 525 526 527 528 529 530 531 532

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653 654

Figure 6. For Table of Contents Only


33


a) 2D GISAXS patterns of tin oxide columns synthesized at four temperatures with the winged pattern in reciprocal space highlighted by the orange lines and the structure of the faceted columns in real space shown by the white lines (qxy and qz are the scattering wave vectors in reciprocal space). b) GIWAXS image of tin oxide columns synthesized at four temperatures, showing the tip-facet is oriented in [110] direction and the side-facets oriented in [101], [111] and [211] direction. 89x46mm (300 x 300 DPI)


Page 34 of 39

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a) High resolution spectra at binding energy corresponding to the O1s peak and Sn3d5/2 peak for films grown at 450 °C, 500 °C, 550 °C, and 600 °C. b) Resistivity of SnO2 thin films, normalized by the dense film height. Triplicate measurements were performed, with the standard deviation between measurements represented by the error bars. 184x342mm (300 x 300 DPI)



Sensing responses evoked by bipolar molecules. a) Sensor response curve of substrates grown at four different temperatures towards ethanol (in room-air), with error bars representing the standard deviation in response across triplicate measurements. b) Stable adsorption configuration of ethanol on the (101) reduced surface (red-oxygen; purple-tin; white-hydrogen). c) Representative trace showing the characteristic change in resistance of sensor substrates (grown at 550 °C) towards ethanol in a nitrogen environment. Characteristic change in resistance of sensor substrates (grown at 550 °C) towards d) methanol, e) ethanol, and f) propanol in room-air. 131x91mm (300 x 300 DPI)


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Figure 4. Sensing responses evoked by monopolar molecules. Representative trace showing the characteristic response of a sensor substrates (grown at 550 °C) towards a) acetone, b) 2-butanone, and c) 2-pentanone in room-air. d) Stable adsorption configuration of acetone on the (101) reduced surface (redoxygen; purple-tin; white-hydrogen). 244x778mm (300 x 300 DPI)



Figure 5. a) Representative trace showing the characteristic response of a sensor substrates (grown at 550 °C) towards hexane and b) stable adsorption configuration on the reduced (101) surface of a propane molecule (red-oxygen; purple-tin; white-hydrogen). 163x292mm (300 x 300 DPI)


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For Table of Contents Only 87x148mm (600 x 600 DPI)


SnO2 Nanostructured Thin Films for Room Temperature Gas Sensing

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