Monomer: A Designer of ZnO Nanostructures (Nanobush & Nanowire

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Monomer: A Designer of ZnO Nanostructures (Nanobush & Nanowire) and their Room Temperature Ethanol Vapor Sensing Signatures Prabakaran Shankar, and John Bosco Balaguru Rayappan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11561 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 10, 2017

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ACS Applied Materials & Interfaces

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Monomer: A Designer of ZnO Nanostructures (Nanobush & Nanowire) and

2

their Room Temperature Ethanol Vapor Sensing Signatures 1,2

Prabakaran Shankar and 2John Bosco Balaguru Rayappan*

1

Institute of Innovative Science and Technology, Tokai University, Hiratsuka, Kanagawa 2591292, Japan

2

Nanosensors Lab @ Centre for Nanotechnology & Advanced Biomaterials (CeNTAB),

School of Electrical & Electronics Engineering (SEEE), SASTRA University, Thanjavur – 613 401, India

Corresponding author: *Prof. John Bosco Balaguru Rayappan School of Electrical & Electronics Engineering (SEEE) & Centre for Nanotechnology & Advanced Biomaterials (CeNTAB) SASTRA University, Thanjavur – 613 401, India Email: [email protected] (J.B.B. Rayappan) [email protected]; [email protected] (P. Shankar) Tel.: +91 4362 264101-108 x2255; Fax: +91 4362 264120 Page 1 of 35 ACS Paragon Plus Environment


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Abstract

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Ethanol serves as a biomarker as well as a chemical reagent for several applications and

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has been predominantly used as an alternative fuel (E10 and E85). Development of sensors for

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the detection and monitoring of ethanol vapor at lower operating temperature has gathered

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momentum in the recent past. In this work, we reported the synthesis of self-assembled ZnO

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nanowires using electrospun technique without using any external surfactants or capping agents

9

and their room temperature ethanol sensing properties. An inherent template namely monomer of

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the polymer polyvinyl alcohol (PVA) with two different molecular weights (14,000 and 1,40,000

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g mol-1) was used along with the precursor zinc acetate dihydrate. The ZnO-PVA nanofibers has

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been tranformed to ZnO nanospheres and nanowires after calcination. Ratio of zinc precursor

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concentration to PVA polymer led to enhanced carrier concentration of the resultant ZnO

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nanowire in-turn enhanced sensing response towards ethanol vapor. The developed sensing

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elements have been systematically characterised to correlate their structural, morphological and

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electrical properties with the respective room temperature ethanol sensing characterisitics. Role

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of grain features and low activation energy of ZnO nanowires in coordination with the low

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dipole moment of ethanol resulted in the excellent response of 78 towards 100 ppm at room

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temperature with an ultra sensitive response and recovery times (9 and 12 s).

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Keywords: Electrospinning; fibrous mat; ZnO; Nanobushes; Nanowires; Ethanol sensor

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1

Introduction

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Nations across the globe emphasize on the reduction of greenhouse gas emission (GHG)

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to reduce global warming. In a bid to reduce global warming and improve air quality, majority of

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the countries have planned to reduce the carbon footprint and subsequently encourage the use of

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ethanol fuel (E85 & E15) instead of petrol and diesel. This remedial action has already been felt

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through the steep increase in the ethanol production around the globe and the global ethanol

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production (Fig. 1) advocates the impact of clean fuel.

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Fig. 1. Ethanol production in the world. (Source: Renewable Fuels Association, USA).1

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The increase in the number of production plants and consumption of ethanol definitely

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need a safety system to monitor its concentration level in both indoor and outdoor environments,

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to avoid accidents and health issues.2 The standard exposure limit of ethanol in air is 1000 ppm

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and fixed 3300 ppm as maximum by Occupational Safety and Health Administration (OSHA),3

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National Institute of Occupational Safety and Hygiene (NIOSH) and Centers for Disease Control



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and Prevention (CDC).4 Airborne ethanol causes headache, fatigue and sleepiness while,

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maximum and continuous exposure could do irreversible damage to the nerves (dementia).

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On the other hand, ethanol is one of the most important organic molecules to be detected

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in exhaled breath, and food items for health care, and food quality applications respectively.5–11

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Nair et. al.6 have reported that the presence of higher concentration of ethanol in exhaled breath

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is an indication of obesity related liver disease caused by pathogens. Similarly, various

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concentrations of ethanol were produced on the surface of food items like milk, fruits and

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vegetables due to microbial activities. 9,12,13 Thus, monitoring ethanol vapor from our breath and

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surface on food items can be used for non-invasive diagnosis and freshness level of food items.

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In this scenario, design and development of sensors with desired response characteristics to

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detect ethanol at ambient atmosphere has clear mandate.

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Nanostructured metal oxide based chemiresistive sensors play a vital role in designing handheld,

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portable and cost effective sensing system for the detection of various target vapors across

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fields.14–17 Among various metal oxides, the n-type semiconducting ZnO has been in the

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forefront owing to its inherent tunable electrical and electronic properties.18–22 It is one of the

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most exploited materials in recent times for the fabrication of gas sensors due to its flexible or

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variable low dimensional morphologies with desired degree of confinement and Density of

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States. Ethanol sensors with higher response and selectivity reported in the literature were

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opearted at elevated operating temperatures.23,24 The rapid reaction of ethanol molecule and

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stability of the sensing materials at higher operating temperatures are the major issues, which are

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motivated to develop the room temperature ethanol sensor. Further, reports describing room

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temperature ethanol sensors using ZnO nanostructures with lower and moderate figure of merits

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(Table 1)25–34 also persuaded us to finetune the properties of ZnO sensing element to achieve Page 4 of 35 ACS Paragon Plus Environment

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better performance. Amid the low dimensional materials, one-dimensional (1D) ZnO

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nanostructures have been preferred because of its selective interaction with specific target

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molecules.35,36 Hence, it has been proposed to fabricate ZnO nanowire based room temperature

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ethanol sensor with enhanced figure of merits.

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One-dimensional (1D) ZnO nanostructures like nanorods, nanowires have been

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synthesized by wide range of techniques.37 Among them, wet chemical methods are highly

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versatile for growing such nanostructures.37 Growth of highly oriented, aligned and ordered

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nanostructures requires highly controlled operating conditions, such as catalysts, surfactants or

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capping agents, seed layers and templates. Synthesizing self-assemblies of nanomaterials has

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attracted a great deal of attention for the design and development of nanostructures, wherein

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arrangement of primitive components into ordered systems or aggregates with desired

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nanostructures is the major focus. In the recent past, electrospun technique has evolved as a well-

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controlled technique for synthesizing self-assemblies of desired structure and characteristics.38–40

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In this process, polymers play a critical role in obtaining desired size and shape of specific

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nanostructures. In the same line, electrospun technique has emerged as a versatile platform for

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the synthesis of metal oxide nanostructures through polymer-metal oxide nanocomposites. Many

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synthesis parameters like type of polymer, concentration of polymer, type of solvent,

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conductivity, feeding rate of the precursor solution, applied potential and distance between

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collector and nozzle have to be controlled and optimized for obtaining the desired structure along

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with the transport properties relevant to the applications.41 Plenty of reports on the synthesis of

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1D nanowires using electrospun technique are available in the literature.41–47 But, very few

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reports are describing the formation of nanostructures with a focus on the influence of molecular

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weight of the polymer as a prominent parameter for the controlled synthesis of nanostructures. Page 5 of 35 ACS Paragon Plus Environment


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Hence, in this work, molecular weight of the polymer (poly vinyl alcohol (PVA)) was taken in to

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account for the synthesis of ZnO nanostructures and in specific nanowires (NW). Monomers

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have been considered as an inherent template for the formation of nanowires through the

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arrangement of primitive components namely nanospheres into ordered systems or aggregates as

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nanowires. In addition, the ratio of zinc acetate dihydrate to polymer PVA concentration was

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varied to synthesize ZnO nanostructures. This effectively resulted in the formation of ZnO

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nanobush (NB) and pearl-chain like nanowire with enhanced sensing performance. The

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structural, morphological, electrical and ethanol sensing characteristics of ZnO nanowire have

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been analyzed, correlated and reported.

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2

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2.1

Materials and methods Nanomaterials preparation

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ZnO nanofibers were synthesized using automated electrospinning equipment with the

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precursor source of zinc acetate dihydrate (Zn(CH3COO)2·2H2O) (Sigma Aldrich, Mw = 219.61

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g mol-1) and poly-vinyl alcohol (PVA) ([CH2CH(OH)]n). PVA with two different molecular

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weights (14,000 and 140,000 g mol-1) of 1 g each and ZnAc (1.5 g) were used as precursors to

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study the effect of molecular weight on the preparation of nanowire. In the next step, various

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concentrations of zinc acetate dihydrate (labelled as ZnAc) (1, 1.5 and 2 g) and PVA (Mw =

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1,40,000 g mol-1) were considered as precursors to study the effect of metal ion concentration on

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the preparation of nanostructures. Precursors were dissolved in 10 mL deionized water and the

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homogenous solution of 5 mL was loaded in the glass syringe equipped with a needle (24

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Gauge). The needle was connected to a high voltage power supply (15 kV) (Zeonics, Bangalore)

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and kept at distance of 12 cm from the collector and the flow rate of 0.01 mL min-1 was Page 6 of 35 ACS Paragon Plus Environment

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controlled by syringe pump (Kent Scientific, USA). The composite nanofibers were deposited on

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the aluminium foil attached with the collector. The deposited composite nanofibers were calcined

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at 873 K for 3 h to remove the organic residues to obtain ZnO nanostructures.

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2.2

Characterization

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X-ray diffractometer (D8 Focus, Bruker, Germany) with Cu Kα radiation (1.546 Å) was

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used to record the diffraction patterns of the composite fibers as well as the calcined ZnO

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nanostructures. Field Emission Scanning Electron Microscope (FE-SEM, JEOL, 6701F, Japan)

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was employed to observe the morphologies of the samples. Hall measurement system (HMS

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3000, Ecopia, South Korea) was used to measure the carrier concentration, and mobility of the

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samples. Digital pico-ammeter (SES Instruments, India) was employed to measure the activation

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energy of the samples. Grain and grain boundary resistances were measured using

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electrochemical impedance spectrometer (CHI660E, CH Instruments, USA) with gold working

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electrode, platinum counter electrode and Ag/AgCl reference electrode with 0.1 M of potassium

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hexacyanoferrate (III) as an electrolyte. Keithley electrometer (6517A, USA) was used to

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measure the change in chemiresistance of the gas sensing elements.

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2.3

Gas sensing studies

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Gas sensing measurements were carried out using a customized gas testing chamber

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reported in our previous work.48 Gas sensing characteristics like response, selectivity, stability,

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and detection limit were studied at room temperature and repeated for 3 times at different time

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pattern. Sensing response can be calculated using Ra/Rg where, Ra and Rg are the measured

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resistance of the sensing elements in ambient atmosphere and target gas respectively.49–51



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2.4

Fabrication of sensing element The prepared

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ZnO nanomaterials were mixed with appropriate amount of

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diethyleneglycol (Merck, 99%) to prepare the colloidal solution. The same was uniformly coated

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on to the glass substrate and dried at 373 K for 1 h.52 To measure the change in resistance of the

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sensing element, inter-digitated patterned silver thin film was deposited on to the glass substrate

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using DC magnetron sputtering system (12” MSPT, HHV, India). A pair of copper (Cu) wires

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were pasted on the silver inter-digitated sensing element using highly conducting silver paste.

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Fig. 2 depicts the schematic of the fabricated sensing element. Its I-V characteristics confirmed

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the ohmic contact established between Cu and ZnO.

Fig. 2: Schematic of sample coated on the inter-digitated silver thin films.

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3

Results and discussion

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3.1 Structural studies

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Fig. 3 shows the X-ray diffraction patterns of as-deposited and calcined samples prepared

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using PVA with two different molecular weights (14,000 and 140,000 g.mol-1) and ZnAc (1.5 g).

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The observed peaks at 2θ of 10 – 30° and 30 - 40° are corresponding to the PVA polymer matrix

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and small clusters of ZnO crystallites respectively. XRD pattern confirmed the polycrystalline

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nature of ZnO-PVA composite fibers and it might be due to the intermolecular hydrogen bonding Page 8 of 35 ACS Paragon Plus Environment

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of PVA monomers with ZnO. The observed peak shift from 17.7° to 16.9° might be due to the

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increase in the molecular weight of the polymer. The diffraction patterns of calcined samples

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revealed the absence of PVA polymer and confirmed the removal of organic residues from the

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samples. The well-defined peaks were observed at the diffraction planes (100), (002), (101),

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(102) and (110), which are the signatures of polycrystalline wurtzite structure of ZnO and

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matched with JCPDS (36-1451).53

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Fig. 3. X-ray diffraction patterns of as-deposited and calcined samples of ZnO with different molecular weight of PVA (14,000 and 140,000 g mol-1).

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Fig. 4 shows the X-ray diffraction patterns of as-deposited and calcined samples prepared

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using PVA (140,000 g mol-1) and various concentrations of ZnAc (1, 1.5 and 2 g). Diffraction

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patterns of the as-deposited samples exhibited the influence of metal ion concentration in the

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nanocomposite fibers and peaks at 2θ = 32, 45 and 50° have confirmed the same. The observed

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diffraction planes ((100), (002), (101), (102) and (110)) for the calcined samples revealed the

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formation of polycrystalline hexagonal wurtzite structured ZnO. The growth orientation of ZnO



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crystallites ((101) plane for NB and NW1 and (100) for NW2) varied due to the influence of

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metal ion concentration.

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Fig. 4. X-ray diffraction patterns of as-deposited and calcined samples of PVA (140,000 g mol-1) and different concentration of ZnO.

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3.1

Morphological studies

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Fig. 5 shows the FE-SEM images of as-deposited PVA-ZnO composite nanofibers and

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calcined ZnO nanospheres and nanowires. Formation of smooth, linear, uniform and beadless

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nanofibers was observed for both the as-deposited PVA-ZnO matrix. The diameter of the

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nanofibers was in the range of ~17 nm and ~32 nm for PVA with 14,000 and 140,000 g mol-1

163

respectively. The molecular weight of the polymer might have influenced the surface tension and

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electrostatic charge repulsion of the precursor solution, which in-turn resulted in the elongation

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of the fiber diameter.54,55 Calcined samples clearly exhibited the morphological transformation of

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nanofibers to nanospheres and nanowires. During the calcination, the applied thermal energy Page 10 of 35 ACS Paragon Plus Environment

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degraded the organic components which present in the nanocomposite fibers thereby aiding the

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crystalline fusion reaction to form nanostructures (nanospheres and nanowires).

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Fig. 5. FE-SEM images of as-deposited and calcined samples of ZnO with different molecular weight of PVA (14,000 and 140,000 g mol-1).

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Fig. 6 shows the FE-SEM images of as-deposited and calcined ZnO samples prepared

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using PVA (140,000 g mol-1) and various concentrations of ZnAc. The effect of metal ion

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concentration in the as-deposited ZnO-PVA nanocomposite fibers is very limited but, after

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calcination, there was a considerable change in the morphology. Prepared nanofibers with 1 g

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and 1.5 and 2 g of ZnAc transformed to nanobushes (labelled as NB) and pearl-chain like

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nanowires (labelled as NW1 and NW2) respectively. This morphological transformation has

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revealed the role of metal ion concentrations in the construction of nanostructures. The varying

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metal ion in the precursor solution influenced the crosslinking reaction. Since, the crosslinking

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process was controlled by the metal ion to polymer ratio, it resulted in the formation of Page 11 of 35 ACS Paragon Plus Environment


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uniformly distributed self-assemblies of NB and pearl-chain like NW. Fig. 7 shows the pearl-

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chain like nanowires: an oriented attachment of nanospheres forming nanowire. It might be the

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length of the polymer chain and crosslinking process, which led to the self- assembly of

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nanostructures.

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Fig. 6. FE-SEM image of as-deposited ZnO-PVA nanocomposite fibers (a-c) and calcined samples (d-f).

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Fig. 7. FE-SEM image of ZnO NW2 with different magnifications.


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3.2

Growth mechanism

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Apparent chemical reaction of polymerization between the aqueous solution of PVA and

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ZnAc leads to the formation of ZnO-PVA nanocomposite.56 The charge transfer reaction

190

between the polymer chains and dopants (inorganic components) causes the splitting of metal

191

coordination bonds and the crosslinking reaction to form ZnO-PVA composite fibers.57

192

Electrospinning technique influenced as an intermediator for the chemical reactions. The applied

193

electric field and distance between the spinneret tip and collector are the two important

194

parameters in the formation of ZnO-PVA matrix. Especially, the dehydration reaction induced by

195

the applied electric field lead to evaporation of precursor solution and formation of ZnO

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nanocomposite.58 After collecting the nanofibers mat from the aluminum foil, it was calcined at

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873 K to remove organic residues and to obtain ZnO nanostructures.26 The formation of ZnO

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nanospheres and nanowires (Sec. 3.2) could be due to the variation in the molecular weight of

199

the polymer and metal ion concentration.

200 201

Fig. 8. Schematic of (a) monomer of PVA, (b) ZnO-PVA (Mw – 14,000 g mol-1) composite and (c) ZnO-PVA (Mw – 140,000 g mol-1) composite. Page 13 of 35 ACS Paragon Plus Environment


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Scheme (Fig. 8) depicts the influence of molecular weight of the polymer on the

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formation of nanostructures. The molecular weight of polymer depends upon the number of

204

monomers (CH2CH(OH)n), which constitute the polymer chain. In the present case, the two

205

different molecular weights of 14,000 and 140,000 g mol-1 act as the inherent template with two

206

distinct polymer chains. Thus, PVA with low molecular weight and short in length (Fig. 8b)

207

resulted in the formation of ZnO nanospheres and PVA with long polymer chains and high

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molecular weight (Fig. 8c) favored the self-assembly of nanospheres to form pearl-chain like

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nanowires. Self-assembly of ligands-stabilized nanoparticles and oriented attachment of

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nanoparticles gained lattice-free energy and aided the growth of monomers into nanostructures.59

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The length of the polymer chain acted as counter ion to aggregate the particles (oriented

212

attachment or crystalline fusion particles) to form as pearl-chain like nanowires. Further, metal

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ion concentration in the precursor might have favored the formation of crystalline fusion reaction

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to construct the oriented attachment of crystallites in the periodical format. The fusion reaction

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and oriented attachment reduced the surface free energy to form pearl-chain like nanowires. At

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the same time, insufficient metal ions in the precursor (1 g of ZnAc) has not supported the fusion

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reaction and oriented attachment to construct nanowire and therefore ended like a nanobushes.

+ ∙ 2 + → ↓ + ↑

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(1)

The possible chemical reaction is as follows:

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3.3

Electrical properties

Sample

Carrier Concentration (cm-2)

Mobility (cm2 V-1s-1)

Grain Grain resistance boundary (Ω) resistance (Ω)

Activation energy (eV)

Response

NS

1.79×1011

20

198

294

0.6777

48

NB

1.72×1011

58

136

192

0.7091

39

NW1

2.67×1011

146

125

176

0.5707

72

NW2

8.42×1011

140

121

164

0.5202

78

221

Carrier concentration, mobility, grain and grain boundary resistances and activation

222

energy were measured to investigate the electron transport mechanism of the ZnO nanostructures

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like nanospheres, nanobushes and pearl-chain nanowires (Table 1).

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Table 1. Electrical properties of the sample nanospheres (NS), nanobushes (NB), nanowires 1 (NW1) and nanowires 2 (NW2).

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The observed higher carrier concentration for NW2 has strongly confirmed the influence of high

227

metal ion concentration in the precursor, which acted as donor ions in the ZnO nanowires.

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Similarly, the observed mobility for both the nanowires (NW1 and NW2) was higher and it

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might be due to the impact of self-assembly like (connected-nanospheres) morphology.

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Generally, mobility of the crystalline materials is directly influenced by carrier density60 and

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morphology.61 Though the carrier concentration of NW2 is more than that of NW1, the slight

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decrease in the mobility of NW2 might be due to the enhanced scattering of charge carriers as

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the effect of increased carrier concentration.60 The native donor ions in NW2 might have

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increased the point defects in lattice, which are responsible for the scattering of carriers. This has

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reflected in grain and grain boundary resistances and potential barrier. Grain and grain boundary

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resistances were calculated from Nyquist impedance plot (Fig. 9a).



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Fig. 9: (a) Nyquist plot and (b) Arrhenius plot of nanostructured ZnO samples.

238

The scanning from high to low frequencies exhibited a semi-circle arc, which attributed

239

to grain resistance, grain boundary resistance and capacitance of nanomaterials. The equivalent

240

circuit model was fitted on the obtained signal to depict the grain and grain boundary resistances.

241

The existence of high grain and grain boundary resistances for ZnO nanospheres and nanobushes

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might be due to the defects, trap states, disoriented lattice at the boundaries and random

243

boundaries. In general, grain boundaries have been classified as coincidence or random

244

boundaries with high or low symmetry respectively.62 It has a strong influence on deciding the

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conductivity of the nanostructures. In contrast, lengthy polymer chains reduced the surface free

246

energy to form oriented attachment of lattice, which favored the formation of grains in pearl-

247

chain like nanowires with reduced grain and grain boundary resistances. This trend has been

248

confirmed by the observed low activation energy (Fig. 9b) and it might be due to the self-

249

assembled wire like ZnO morphology. So, the combined effect of high carrier concentration,


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mobility, low grain and grain boundary resistances and low activation energy reflected in the

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sensing characteristics by swift transfer of electrons in the ZnO nanowires.63

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3.4

253

3.4.1

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Sensing studies Selectivity

Fig. 10: Sensing response of the ZnO nanostructures towards 100 ppm of ethanol, methanol, acetone and acetaldehyde at ambient temperature.

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Selectivity of the sensor plays a vital role in deciding its overall sensing performance. To

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confirm the selectivity, the fabricated sensing element was exposed to 100 ppm of volatile

258

organic compounds (VOCs) such as ethanol, methanol, acetone and acetaldehyde and the

259

corresponding change in resistance was recorded. The observed response confirmed the

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selectivity of ZnO nanostructures towards ethanol (Fig. 10). Particularly pearl-chain like ZnO

261

nanowires showed better room temperature response. Selectivity towards ethanol might be due to

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the low dipole moment of ethanol48,64 and enhanced surface to volume ratio, low activation

263

energy, surface confined electronic states of ZnO nanowires and matching of the lowest



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unoccupied molecular orbital energy level of ethanol molecule with the activation energy of the

265

sensing surface.30,65,66

266

3.4.2

Response

267

Response is based on the change in resistance of the sensing element influenced by

268

adsorption and desorption of gas molecules on the sensor surface. The transient responses of

269

ZnO samples (NS, NB, NW1 and NW2) towards 100 ppm of ethanol vapor are shown in Fig 11.

270

Response curve can be fragmented as base-line resistance (Ra), stabilized-response resistance

271

(Rg) and recovery base-line resistance. While exposing the sensing element made up of ZnO

272

nanostructures in the ambient air, oxygen molecules in the atmosphere adsorbed on its surface by

273

trapping the electrons from the conduction band of ZnO, which resulted in the formation of space

274

charge region (Fig. 15). This in-turn has resulted in the increased surface resistance and the same

275

has been set as the base line resistance for the sensing studies. In this background, when the

276

target vapor (ethanol) was introduced, a sudden drop (transient) in the base resistance followed

277

by the chemical interactions (Eqs. 2 and 3)

278

The transient resistance reached the steady state in the ethanol mixed atmosphere, which

279

indicated that all the available target gas molecules have interacted with the active sites available

280

on the surface of the sensing element (Fig. 11), which is called as stabilized-response resistance

281

(Rg). Decrease in resistance confirmed the reducing nature of ethanol and n-type semiconducting

282

behaviour of ZnO. During the gas – solid interaction, the number of electrons increased in the

283

conduction band of the sensing element, which resulted in the reduction of the surface resistance.

284

Hence, the change in resistance with reference to the base line resistance depends on the carrier

285

concentration of the sensing element and concentration of the target vapor.


Page 19 of 36

1000

400

Ethanol 100 ppm 900

Ra = 8.64 ×

109

Ω

Ethanol 100 ppm Ra = 3.46 × 109 Ω

350 300

700

Resistance (×107 Ω)


800

600 500 400 300

250 200 150 100

200

Rg = 1.79 ×

108

Ω

50

NS

100 0

Rg = 8.77 × 108 Ω

NB

0 0

10

20

30

40

50

60

70

80

0

10

20

30

Time (s)

Ra = 1.19 × 108 Ω

60

70

80

Ethanol 100 ppm Ra = 1.04 ×

100

108

Ω


100 80 60 40 20

Rg = 1.65 ×

NW1

106

Ω

80

60

40

Rg = 1.33 × 106 Ω

20

NW2 0

0 0

10

20

30

40

50

60

70

80

0

10

20

30

40

50

60

70

80

90

Time (s)

Time (s)

286 287

50

120

Ethanol 100 ppm

120

40

Time (s)

140


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


Fig. 11. Transient resistance response of ZnO samples (NS, NB, NW1 and NW2) for 100 ppm of ethanol.

288

At the same time, mobility, grain and grain boundary resistances have been acting as

289

strong influencing factors on the sensing response through the conduction process. Once the

290

steady state is reached, the sensing element was again exposed to the ambient atmosphere.

291

During this process, the resistance of the sensing element again increased to the base line

292

resistance due to the desorption of target molecules. The observed response and recovery times

293

were found to be 9 and 12 s respectively. The switch like transient response might be due to the

294

confinement of electrons in one-dimensional structure with desired density of states.

295

Response of ZnO nanowires was observed for the various concentrations of ethanol

296

ranging from 1 to 1000 ppm (Fig. 12). Sensor exhibited a linear response till 100 ppm and after Page 19 of 35 ACS Paragon Plus Environment


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Page 20 of 36

297

that rate of response was reduced. This trend has been explained based on the availability of

298

active sites for the adsorption/desorption processes.

Fig. 12. Response trend of NW2 towards ethanol.

299 300

3.4.3

Effect of humidity

301

Humidity is one of the most influencing parameters in determining the overall sensing

302

response of the sensing system. With reference to the chemical sensor technology, humidity has

303

been considered as an obstacles which always results in different response value. Hence,

304

chemiresistive sensing element should be stable against the variation in the humidity level of the

305

environment. In this context, the sensing response of the synthesized NW2 nanowire was tested

306

in the different RH conditions developed using saturated aqueous solutions.26,67 The response of

307

ZnO NW2 towards 100 ppm of ethanol at different RH levels of 24%, 54% and 76% is 81, 78,

308

and 74.6 respectively. The observed response revealed that the variation in the response of the

309

sensing element in different RH levels were below ±5%. At lower relative humidity level (24%),

310

influence of chemisorbed hydroxyl ions and subsequent charge transport on the surface of

311

sensing element follows hopping transport mechanism22,68. Both the adsorbed hydroxyl ions and

312

oxygen molecules on the surface involve in the sorption process with the target ethanol vapour. Page 20 of 35 ACS Paragon Plus Environment

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313

At higher RH (76%) level, physisorption of water molecules on the chemisorbed hydroxyl layer

314

based on the surface charge densities69 lead to Grotthus transport mechanism on the sensing

315

surface14. At higher humidity levels, presence of more number of hydroxyl molecules causes

316

multiple physisorbed layers. These layers increase the H+ concentration on the surface in-turn

317

influence the charge transfer process and conductivity22,68. The interaction of ethanol molecules

318

on this surface follow the hydrogenation and dehydrogenation process to increase the carrier

319

concentration in the conduction band by desorbing the adsorbed hydroxyl molecules from the

320

surface25.

321

3.4.4

Stability

322

Fig. 13 shows the transient resistance response of the NW2 towards 100 ppm of ethanol

323

observed for 60 days. It specified the occurrence of base line resistance with very minimum drift

324

corresponding to the duration. Thus, the fabricated sensing element using ZnO nanowire (NW2)

325

can be used for the real-time monitoring of ethanol in different applications.

326 327

Fig. 13. Cyclic transient resistance response of NW2 sample towards 100 ppm of ethanol observed for 60 days. Page 21 of 35 ACS Paragon Plus Environment


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328

329 330

3.5

Page 22 of 36

Sensing mechanism

Fig. 14. Adsorption and desorption process on nanoparticle and nanowire surface a) in ambient and b) in ethanol atmosphere at room temperature.

331

Fig. 14 depicts the physical and potential barrier models representing the sensing

332

mechanisms of nanoparticle and nanowire towards ethanol vapor at room temperature. The

333

formation of depletion layer or space charge region due to the chemisorbed oxygen molecule on

334

the surface act as a barrier for electron transport. At the intergranular contacts of ZnO

335

nanoparticles, the presence of double space charge layers forms a formidable barrier against the Page 22 of 35 ACS Paragon Plus Environment

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336

electron transport, which in-turn influences the percolation path. Since, width of the space charge

337

layer depends on the carrier concentration or surface charge density, it decides the adsorption-

338

desorption rate and hence, the response of the sensing element towards the target vapor or

339

oxygen molecules in the ambient.70

340

Formation of space charge layer due to the receptor function (oxygen adsorption) and

341

transduction function (intergranular contacts) of the nanoparticle tend to increase the height of

342

potential barrier. While exposing the sensing element to ethanol vapor, reaction with

343

chemisorbed oxygen decrease the width of space charge region and height of the potential

344

barrier, which enhanced the electron transport. But, reduction in the intergranular contact

345

resistance of nanowire than nanoparticles and nanobushes resulted in the decreased height of

346

potential barrier, which resulted in the switch like transient response in the absence and presence

347

of target gas (Fig. 8)

348

Fig. 15. Schematic of oxygen adsorption on nanowire surface.



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Page 24 of 36

349

The enhanced sensing response may also be due to the unidirectional percolation path

350

through the pearl-chain like nanowire. The observed sensing mechanism and response confirmed

351

the grain and neck boundary models proposed by Yamazoe.70

352

Surface coverage and adsorption of oxygen molecules on the surface of sensing element

353

depends upon the number of actives sites or carrier concentration, which mainly depends on the

354

structural and morphological properties of the sensing element. In this case, NW1 and NW2

355

showed similar morphology of pearl-chain like nanostructure with varying carrier density. Fig.

356

15 depicts the increased number of active sites or carrier concentration on the nanowire surface

357

which in-turn enhanced the sensing response through elevated oxygen sorption process.

358

Similarly, surface adsorption or coverage by the atmospheric oxygen on the sensing surface is

359

influenced by temperature and surface dopants. The atmospheric oxygen chemisorbed as

360

molecular ( ) or atomic ( , ) forms wherein, molecular form dominates below 423 K.

361

Further, the interactions of vapor (methanol, ethanol, acetone and acetaldehyde) on the oxygen

362

adsorbed surface is as follows (Eqs. 2-6):

!"#$%$

+ & ' !(%)*$ ⟶ & '

Ethanol14

2 , + 5 .!

Methanol71

+ .!

Acetone72

+ 4 .!

Acetaldehyde14

2 + 5 .!

(2)

%/$.

%/$.

⟶ 4 ↑ +4 ↑ +5

⟶ ↑ + ↑ + %/$.

%/$.

⟶ 3 ↑ +3 ↑ +4

⟶ 4 ↑ +4 ↑ +5

(3) (4) (5) (6)

363

The response trend confirmed the reducing nature of VOCs and n-type semiconducting

364

behavior of nanostructured ZnO sensing elements (Fig. 4 and 5). The observed response

365

established the selectivity of the sensing element towards ethanol. This might be due to the Page 24 of 35 ACS Paragon Plus Environment

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366

matching of the activation energy of the pearl-chain like nanowire (NW2) and the bond-

367

dissociation energy26 (436 kJ mol-1) and dipole moment (1.69)64 of ethanol molecule. During the

368

ethanol vapor and ZnO NW2 interaction the conduction band electrons increased in the sensing

369

element following the reaction mechanism given in Eqs. 2 and 3 (Fig. 14 and 15). ZnO NW2

370

sensing element with less activation energy, grain resistance and grain boundary resistance

371

(Table 2) exhibited better sensing performance towards ethanol at room temperature and

372

compared to the available room temperature ZnO ethanol sensors (Table 2).

373

Table 2. Ethanol sensing performances of ZnO at room temperature.

S.No.

ZnO morphology

Response

Concentration

Ref.

1.

Nanowire

78

100 ppm

This work

2.

Nanosphere

61.90

100 ppm

26

3.

Nanosphere

~39

100 ppm

27

4.

Nanorod

23

100 ppm

25

5.

Nanosphere

~19

250 ppm

28

6.

Nanorod

13.62

100 ppm

26

7.

Nanosphere

~7.50

50 ppm

29

8.

Nanorod

~6

100 ppm

30

9.

Nanosphere

03.73

50 ppm

31

10.

Nanosphere

03.20

100 ppm

32

11.

Nanosphere

2.80

100 ppm

33

12.

Nanoflakes

1.02

200 ppm

34

374 375

4

Conclusion

376

Novel and facile growth process for the synthesis of ZnO nanowires through template and

377

surfactant free electropsun technique has been successfully standardized. From this investigation, Page 25 of 35 ACS Paragon Plus Environment


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Page 26 of 36

378

it can be concluded that the monomers of the polymer can be used as a template to construct the

379

self-assembled nanowires. The observed carrier concentration, mobility, grain and grain

380

boundary resistances and activation energy of nanospheres and nanowires have revealed the

381

influence of varying weight ratio of ZnAc and PVA (different molecular weight). Pearl-chain

382

like nanowire (NW2) with high carrier concentration, unidirectional flow of carriers without

383

scattering at the boundaries and uniform contraction and relaxation of potential barrier exhibited

384

a better sensing performance towards ethanol at room temperature. The fabricated ethanol

385

sensing element using ZnO nanowires can be used for applications such as ethanol breath

386

analyser to avoid accdients (concnentraion limit of 300 ppm (30 mg dL-1) for India and 800 ppm

387

(0.08%) for USA), indoor air quality monitoring and petrochemical industries (1000 ppm,

388

exposure limit – OSHA, NIOSH and CDC). Since ethanol is one of the biomarkers of obesity

389

related liver diseases, freshness level of milk, fruits and vegetables, the same sensor can be used

390

for the design and development of non-invasive diagnostic device (0.5 – 10 ppm) as well as

391

electronic nose for non-destructive quality discrimination of food items. Hence, the proposed

392

novel processes can be attempted for exploring various self-assembled nanostructures for tuning

393

the electron conduction mechanism thereby the sensing response towards the desired target gas

394

molecule.

395

Acknowledgements

396

The authors wish to express their sincere thanks to Department of Biotechnology

397

(BT/PR10437/PFN/20/779/2013) and Department of Science & Technology

(SR/FST/ETI-

398

284/2011(C))), Government of India for their financial support. We also wish to acknowledge

399

SASTRA University, Thanjavur for extending infrastructure support to carry out this work.


Page 27 of 36

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400

Conflict of interest

401

The authors declare no competing financial interest and conflict of interest.

402



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List of tables

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Table 1. Electrical properties of the sample nanospheres (NS), nanobushes (NB), nanowires 1

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(NW1) and nanowires 2 (NW2).

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Table 2. Ethanol sensing performances of ZnO at room temperature.

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List of figures

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Fig. 1. Ethanol production in the world. (Source: Renewable Fuels Association, USA).1

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Fig. 2: Schematic of sample coated on the inter-digitated silver thin films.

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Fig. 3. X-ray diffraction patterns of as-deposited and calcined samples of ZnO with different

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molecular weight of PVA (14,000 and 140,000 g mol-1).

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Fig. 4. X-ray diffraction patterns of as-deposited and calcined samples of PVA (140,000 g mol-1)

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and different concentration of ZnO.

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Fig. 5. FE-SEM images of as-deposited and calcined samples of ZnO with different molecular

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weight of PVA (14,000 and 140,000 g mol-1).

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Fig. 6. FE-SEM image of as-deposited ZnO-PVA nanocomposite fibers (a-c) and calcined

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samples (d-f).

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Fig. 7. FE-SEM image of ZnO NW2 with different magnifications.

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Fig. 8. Schematic of (a) monomer of PVA, (b) ZnO-PVA (Mw – 14,000 g mol-1) composite and

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(c) ZnO-PVA (Mw – 140,000 g mol-1) composite.

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Fig. 9: (a) Nyquist plot and (b) Arrhenius plot of nanostructured ZnO samples.

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Fig. 10: Sensing response of the ZnO nanostructures towards 100 ppm of ethanol, methanol,

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acetone and acetaldehyde at ambient temperature.

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Fig. 11. Transient resistance response of ZnO samples (NS, NB, NW1 and NW2) for 100 ppm

611

of ethanol.

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Fig. 12. Response of NW2 towards various concentrations of ethanol.

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Fig. 13. Cyclic transient resistance response of NW2 sample towards 100 ppm of ethanol

614

observed for 60 days.

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Fig. 14. Adsorption and desorption process on nanoparticle and nanowire surface a) in ambient

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and b) in ethanol atmosphere at room temperature.

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Fig. 15. Schematic of oxygen adsorption on nanowire surface. Page 35 of 35 ACS Paragon Plus Environment


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Graphical abstract

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Page 36 of 36

Monomer: A Designer of ZnO Nanostructures (Nanobush & Nanowire

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