Nanostructured Thin Films for Room Temperature Gas Sensing of

Subscriber access provided by EKU Libraries

Surfaces, Interfaces, and Applications 2

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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

ACS Applied Materials & Interfaces

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

1

SnO2 Nanostructured Thin Films for Room

2

Temperature Gas Sensing of Volatile Organic

3

Compounds

4

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

5

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

6

a

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

7

and Engineering, Washington University in St. Louis,

8

St. Louis, MO 63130, USA

9

b

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

10 11

USA c

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

12 13

Louis, St. Louis, MO 63130, USA d

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

14

63130, USA

15

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

16

Deposition; DFT calculations

ACS Paragon Plus Environment

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

Page 2 of 39

17

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

18

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

19

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

20

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

21

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

22

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

23

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

24

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

25

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

26

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

27

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

28

room temperature.


2

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


29

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

30

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

31

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

32

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

33

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

34

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

35

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

36

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

37

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

38

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

39

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

40

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

41

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

42

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

43

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

44

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

45

has emerged.9-12

46

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

47

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

48

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

49

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

50

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

51

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


3


Page 4 of 39

52

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

53

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

54

analytes at high temperatures is a potential hazard.

55

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

56

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

57

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

58

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

59

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

60

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

61

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

62

resistance.3,

63

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

64

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

65

surrounding gases at temperatures above 100 °C.14

15

The thermally activated process involves oxygen adsorption, ionization, and

66

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

67

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

68

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

69

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

70

both single crystalline

71

materials are nanoscale, including 1-D nanostructures,

72

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

73

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

74

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


4

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


75

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

76

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

77

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

78

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

79

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

80

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

81

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

82

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

83

and oxygen defects. 15, 18, 27

84

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

85

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

86

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

87

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

88

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

89

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

90

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

91

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

92

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

93

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

94

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

95

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

96

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

97

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


5


Page 6 of 39

98

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

99

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

100

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

101

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

102

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

103

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

104

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

105

2. RESULTS AND DISCUSSION.

106

2.1. Preparation and Characterization of SnO2.

The nanostructured thin films were

107

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

108

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

109

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

110

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

111

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

112

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

113

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

114

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

115

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

116

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

117

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

118

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

119

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

120

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


6

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


121

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

122

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

123

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

124

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

125

temperature appears in white in Figure 1a.

126

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

127

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

128

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

129

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

130

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

131

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

132

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

133

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

134

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

135

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

136

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

137

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

138

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

139

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

140

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

141

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

142

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


7


Page 8 of 39

143

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

144

samples as compared to the [110] facets.

145

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

146

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

147

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

148

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

149

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

150

33

151

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

152

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

153

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

154

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

155

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

156

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

157

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

158

eV.32

159

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

160

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

161

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

162

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

163

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

164

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

165

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


8

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


166

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

167

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

168

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

169

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

170

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

171

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

172

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

173

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

174

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

175

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

176

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

177

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

178

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

179

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

180

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

181

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

182

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

183

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

259

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

260

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

261

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

262

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

263

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

264

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

265

of the (101) facets.

266

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

267

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

268

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

269

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

270

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

271

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

272

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

273

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

274

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

275

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

276

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

277

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

278

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

279

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


13


Page 14 of 39

280

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

281

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

282

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

283

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

284

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

285

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

286

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

287

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

288

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

289

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

290

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

291

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

292

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

293

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

294

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

295

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

296

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

297

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

298

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

299

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

300

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

301

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

302

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

bridging

- H Ethanol), while the carbonyl group


14

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


303

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

304

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

305

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

306

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

307

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

308

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

309

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

310

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

311

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

312

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

313

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

314

still not reached saturation (Figure S7).

315

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

316

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

317

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

318

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

319

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

320

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

321

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

322

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

323

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

324

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

325

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


15


Page 16 of 39

326

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

327

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

328

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

329

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

330

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

331

sensing response, which was both slow and unsaturated.

332

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

333

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

334

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

335

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

336

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

337

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

338

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

339

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

340

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

341

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

342

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

343

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

344

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

345

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

346

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

347

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

348

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


16

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


349

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

350

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

351

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

352

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

353

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

354

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

355

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

356

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

357

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

358

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

359

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

360

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

361

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

362

understanding of these variables.45

363

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

364

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

365

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

366

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

367

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

368

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

369

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

370

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

371

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


17


Page 18 of 39

372

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

373

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

374

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

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


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

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


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


22

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


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.


24

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


479 ACS Paragon Plus Environment

25


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

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


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

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


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

(1) Taguchi, N. Gas Detecting Device. US3695848A, 1970. (2) Seiyama, T.; Kato, A.; Fujiishi, K.; Nagatani, M. A New Detector for Gaseous Components Using Semiconductive Thin Films. Analytical Chemistry 1962, 34 (11), 1502-1503, DOI: 10.1021/ac60191a001. (3) Gurlo, A. Nanosensors: Towards Morphological Control of Gas Sensing Activity. SnO2, In2O3, ZnO and WO3 Case Studies. Nanoscale 2011, 3 (1), 154-165, DOI: 10.1039/C0NR00560F. (4) Raman, B.; Meier, D. C.; Evju, J. K.; Semancik, S. Designing and Optimizing Microsensor Arrays for Recognizing Chemical Hazards in Complex Environments. Sensors and Actuators B: Chemical 2009, 137 (2), 617-629, DOI: http://dx.doi.org/10.1016/j.snb.2008.11.053.

IUSSTF/JCERDC-


29


533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577

Page 30 of 39

(5) Benkstein, K. D.; Rogers, P. H.; Montgomery, C. B.; Jin, C.; Raman, B.; Semancik, S. Analytical Capabilities of Chemiresistive Microsensor Arrays in a Simulated Martian Atmosphere. Sensors and Actuators B: Chemical 2014, 197, 280-291, DOI: http://dx.doi.org/10.1016/j.snb.2014.02.088. (6) Zhang, J.; Liu, X.; Neri, G.; Pinna, N. Nanostructured Materials for Room-Temperature Gas Sensors. Advanced Materials 2016, 28 (5), 795-831, DOI: 10.1002/adma.201503825. (7) Haddad, K.; Abokifa, A.; Kavadiya, S.; Chadha, T. S.; Shetty, P.; Wang, Y.; Fortner, J.; Biswas, P. Growth of Single Crystal, Oriented SnO2 Nanocolumn Arrays by Aerosol Chemical Vapour Deposition. CrystEngComm 2016, 18 (39), 7544-7553, DOI: 10.1039/C6CE01443G. (8) Potyrailo, R. A.; Surman, C.; Nagraj, N.; Burns, A. Materials and Transducers Toward Selective Wireless Gas Sensing. Chemical Reviews 2011, 111 (11), 7315-7354, DOI: 10.1021/cr2000477. (9) Righettoni, M.; Amann, A.; Pratsinis, S. E. Breath Analysis by Nanostructured Metal Oxides as Chemo-Resistive Gas Sensors. Materials Today 2015, 18 (3), 163-171, DOI: http://dx.doi.org/10.1016/j.mattod.2014.08.017. (10) Di Natale, C.; Paolesse, R.; Martinelli, E.; Capuano, R. Solid-state Gas Sensors for Breath Analysis: A Review. Analytica Chimica Acta 2014, 824, 1-17, DOI: http://dx.doi.org/10.1016/j.aca.2014.03.014. (11) Broza, Y. Y.; Haick, H. Nanomaterial-Based Sensors for Detection of Disease by Volatile Organic Compounds. Nanomedicine 2013, 8 (5), 785-806, DOI: 10.2217/nnm.13.64. (12) Benkstein, K. D.; Raman, B.; Montgomery, C. B.; Martinez, C. J.; Semancik, S. Microsensors in Dynamic Backgrounds: Toward Real-Time Breath Monitoring. IEEE Sensors Journal 2010, 10 (1), 137-144, DOI: 10.1109/JSEN.2009.2035738. (13) Jiménez-Cadena, G.; Riu, J.; Rius, F. X. Gas Sensors Based on Nanostructured Materials. Analyst 2007, 132 (11), 1083-1099, DOI: 10.1039/B704562J. (14) Gurlo, A. Interplay Between O2 and SnO2: Oxygen Ionosorption and Spectroscopic Evidence for Adsorbed Oxygen. ChemPhysChem 2006, 7 (10), 2041-2052. (15) Su, J.; Zou, X.-X.; Zou, Y.-C.; Li, G.-D.; Wang, P.-P.; Chen, J.-S. Porous Titania with Heavily Self-Doped Ti3+ for Specific Sensing of CO at Room Temperature. Inorganic Chemistry 2013, 52 (10), 5924-5930, DOI: 10.1021/ic400109j. (16) Wang, Y.; Jiang, X.; Xia, Y. A Solution-Phase, Precursor Route to Polycrystalline SnO2 Nanowires That Can Be Used for Gas Sensing Under Ambient Conditions. Journal of the American Chemical Society 2003, 125 (52), 16176-16177, DOI: 10.1021/ja037743f. (17) Zhao, Q.; Gao, Y.; Bai, X.; Wu, C.; Xie, Y. Facile Synthesis of SnO2 Hollow Nanospheres and Applications in Gas Sensors and Electrocatalysts. European Journal of Inorganic Chemistry 2006, 2006 (8), 1643-1648, DOI: 10.1002/ejic.200500975. (18) Chen, G.; Ji, S.; Li, H.; Kang, X.; Chang, S.; Wang, Y.; Yu, G.; Lu, J.; Claverie, J.; Sang, Y.; Liu, H. High-Energy Faceted SnO2-Coated TiO2 Nanobelt Heterostructure for Near-Ambient Temperature-Responsive Ethanol Sensor. ACS Applied Materials & Interfaces 2015, 7 (44), 24950-24956, DOI: 10.1021/acsami.5b08630. (19) Zhang, D.; Liu, Z.; Li, C.; Tang, T.; Liu, X.; Han, S.; Lei, B.; Zhou, C. Detection of NO2 Down to ppb Levels Using Individual and Multiple In2O3 Nanowire Devices. Nano Letters 2004, 4 (10), 1919-1924, DOI: 10.1021/nl0489283. (20) Li, C.; Zhang, D.; Liu, X.; Han, S.; Tang, T.; Han, J.; Zhou, C. In2O3 Nanowires as Chemical Sensors. Applied Physics Letters 2003, 82 (10), 1613.


30

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

578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623


(21) Kim, Y. S. Thermal Treatment Effects on the Material and Gas-Sensing Properties of Room-Temperature Tungsten Oxide Nanorod Sensors. Sensors and Actuators B: Chemical 2009, 137 (1), 297-304, DOI: http://dx.doi.org/10.1016/j.snb.2008.11.037. (22) Yong Shin, K.; Ha, S.-C.; Kim, K.; Yang, H.; Choi, S.-Y.; Kim, Y. T.; Park, J. T.; Lee, C. H.; Choi, J.; Paek, J.; Lee, K. Room-temperature Semiconductor Gas Sensor Based on Nonstoichiometric Tungsten Oxide Nanorod Film. Applied Physics Letters 2005, 86 (21), 213105, DOI: 10.1063/1.1929872. (23) Tshabalala, Z. P.; Motaung, D. E.; Mhlongo, G. H.; Ntwaeaborwa, O. M. Facile Synthesis of Improved Room Temperature Gas Sensing Properties of TiO2 Nanostructures: Effect Of Acid Treatment. Sensors and Actuators B: Chemical 2016, 224, 841-856, DOI: http://dx.doi.org/10.1016/j.snb.2015.10.079. (24) Liu, Z.; Yamazaki, T.; Shen, Y.; Kikuta, T.; Nakatani, N.; Kawabata, T. Room Temperature Gas Sensing of P-type TeO2 Nanowires. Applied Physics Letters 2007, 90 (17), 173119. (25) Geng, B.; Zhan, F.; Fang, C.; Yu, N. A Facile Coordination Compound Precursor Route to Controlled Synthesis Of CO3O4 Nanostructures And Their Room-Temperature Gas Sensing Properties. Journal of Materials Chemistry 2008, 18 (41), 4977-4984, DOI: 10.1039/B805378B. (26) Xu, C. H.; Lui, H. F.; Surya, C. Optical and Sensor Properties of Zno Nanostructure Grown by Thermal Oxidation in Dry or Wet Nitrogen. Journal of Electroceramics 2012, 28 (1), 27-33, DOI: 10.1007/s10832-011-9674-3. (27) Gurlo, A. Nanosensors: Does Crystal Shape Matter? Small 2010, 6 (19), 2077-2079. (28) Batzill, M.; Diebold, U. The Surface and Materials Science of Tin Oxide. Progress in Surface Science 2005, 79 (2-4), 47-154, DOI: 10.1016/j.progsurf.2005.09.002. (29) An, W. J.; Thimsen, E.; Biswas, P. Aerosol-Chemical Vapor Deposition Method For Synthesis of Nanostructured Metal Oxide Thin Films With Controlled Morphology. The Journal of Physical Chemistry Letters 2010, 1 (1), 249-253, DOI: 10.1021/jz900156d. (30) Chadha, T. S.; Tripathi, A. M.; Mitra, S.; Biswas, P. One-Dimensional, Additive-Free, Single-Crystal TiO2 Nanostructured Anodes Synthesized by a Single-Step Aerosol Process for High-Rate Lithium-Ion Batteries. Energy Technology 2014, 2 (11), 906-911, DOI: 10.1002/ente.201402054. (31) Yang, D. J.; Kamienchick, I.; Youn, D. Y.; Rothschild, A.; Kim, I.-D. Ultrasensitive and Highly Selective Gas Sensors Based on Electrospun SnO2 Nanofibers Modified by Pd Loading. Advanced Functional Materials 2010, 20 (24), 4258-4264, DOI: 10.1002/adfm.201001251. (32) Nagasawa, Y.; Choso, T.; Karasuda, T.; Shimomura, S.; Ouyang, F.; Tabata, K.; Yamaguchi, Y. Photoemission Study Of the Interaction of a Reduced Thin Film SnO2 with Oxygen. Surface Science 1999, 433–435, 226-229, DOI: http://dx.doi.org/10.1016/S00396028(99)00044-8. (33) Kolmakov, A.; Potluri, S.; Barinov, A.; Menteş, T. O.; Gregoratti, L.; Niño, M. A.; Locatelli, A.; Kiskinova, M. Spectromicroscopy for Addressing the Surface and Electron Transport Properties of Individual 1-D Nanostructures and Their Networks. ACS Nano 2008, 2 (10), 1993-2000, DOI: 10.1021/nn8003313. (34) Szuber, J.; Czempik, G.; Larciprete, R.; Koziej, D.; Adamowicz, B. XPS Study of the LCvd Deposited SnO2 Thin Films Exposed to Oxygen and Hydrogen. Thin Solid Films 2001, 391 (2), 198-203, DOI: http://dx.doi.org/10.1016/S0040-6090(01)00982-8. (35) Jarzebski, Z. M.; Marton, J. P. Physical Properties of SnO2 Materials: II . Electrical Properties. Journal of The Electrochemical Society 1976, 123 (9), 299C-310C, DOI: 10.1149/1.2133090.


31


624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650

Page 32 of 39

(36) Wang, Y.; Brezesinski, T.; Antonietti, M.; Smarsly, B. Ordered Mesoporous Sb-, Nb-, and Ta-doped SnO2 Thin Films with Adjustable Doping Levels and High Electrical Conductivity. ACS Nano 2009, 3 (6), 1373-1378. (37) Dai, Z.; Xu, L.; Duan, G.; Li, T.; Zhang, H.; Li, Y.; Wang, Y.; Wang, Y.; Cai, W. FastResponse, Sensitivitive and Low-Powered Chemosensors by Fusing Nanostructured Porous Thin Film and IDEs-Microheater Chip. Scientific Reports 2013, 3, 1669, DOI: 10.1038/srep01669. (38) Schwarzenbach, R.; Gschwend, P.; Imboden, D., Environmental Organic Chemistry. John Wiley & Sons, Inc.: Hoboken, NJ, 2003. (39) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Physical Review Letters 1996, 77 (18), 3865-3868. (40) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. The Journal of Chemical Physics 2010, 132 (15), 154104, DOI: 10.1063/1.3382344. (41) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Nanotube Molecular Wires as Chemical Sensors. Science 2000, 287 (5453), 622. (42) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Detection of Individual Gas Molecules Adsorbed on Graphene. Nature Materials 2007, 6, 652, DOI: 10.1038/nmat1967. (43) Robinson, J. T.; Perkins, F. K.; Snow, E. S.; Wei, Z.; Sheehan, P. E. Reduced Graphene Oxide Molecular Sensors. Nano Letters 2008, 8 (10), 3137-3140, DOI: 10.1021/nl8013007. (44) Robinson, J. A.; Snow, E. S.; Bǎdescu, Ş. C.; Reinecke, T. L.; Perkins, F. K. Role of Defects in Single-Walled Carbon Nanotube Chemical Sensors. Nano Letters 2006, 6 (8), 17471751, DOI: 10.1021/nl0612289. (45) Abokifa, A. A.; Haddad, K.; Fortner, J.; Lo, C. S.; Biswas, P. Sensing Mechanism of Ethanol and Acetone at Room Temperature by SnO2 Nano-Columns Synthesized by Aerosol Routes: Theoretical Calculations Compared To Experimental Results. Journal of Materials Chemistry A 2018, 6 (5), 2053-2066, DOI: 10.1039/C7TA09535J.

651 652


32

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


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

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


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)


Page 36 of 39

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


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)


Page 38 of 39

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


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


Nanostructured Thin Films for Room Temperature Gas Sensing of

Recommend Documents