Precursor Dependent Morphologies of Microwave

Available online at www.sciencedirect.com

ScienceDirect Materials Today: Proceedings 5 (2018) 9831–9838

www.materialstoday.com/proceedings

IC-FNM 2016

Preccursor Dependent Morphologies of Microwave Assisted ZnO Nanostructures and their VOC Detection Properties Anupam Nandi, Pratanu Nag, Hiranmay Saha and Sanhita Majumdar* Centre of Excellence for Green Energy and Sensor Systems (CEGESS), Indian Institute of Engineering Science and Technology (IIEST), Shibpur, Howrah 711103, West Bengal, India

______________________________________________________________________________ Abstract In the present work, Zinc Oxide (ZnO) nanostructures have been prepared by microwave assisted synthesis method. ZnO powders from four different precursor solutions were synthesized followed by calcination at 600oC to form polycrystalline ZnO nanostructures. The wurzite crystal structure of ZnO powders were determined by X-ray diffraction (XRD). Surface morphology of the prepared four ZnO powders were studied using Field Emission Scanning Electron Microscopy (FESEM) and showed four different types of surface alignments. Gas sensing characteristics for four Volatile Organic Compounds (VOCs), namely, acetone, formaldehyde, isopropanol and toluene in their vapor forms, were studied to investigate the response behavior of the prepared ZnO powders. ZnO synthesized from the zinc nitrate precursor showed the highest sensitivity compared with the rest three powders synthesized from acetate, chloride and sulphate precursors of zinc. The sensitivity of the ZnO (nitrate) sensor was found to exhibit tthe highest response [%S = 50.54% & 66.21%] for 10 and 20 ppm formaldehyde, respectivvely at room temperature, in presence of the other three VOCs and thus, it can be inferred that the synthesizeed ZnO is selective to formaldehyde. The mechanism of sensing has been explained according to the ionosorption model. The influence of the precursor on the morphology of derived ZnO samples and the effect of the morphologies on the gas sensing activities has been discussed. © 2017 Ellsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Functional Nano-Materials, 2016. Keywords: ZnO; Microwave assisted wet chemical synthesis; VOC; Sensor.

___________________________________________________________________________________________________

* Corresponding author. +91-33-6455-1644; fax: +91-33-2668-4564. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Functional Nano-Materials, 2016.

9832

A. Nandi et al. / Materials Today: Proceedings 5 (2018) 9831–9838

1. Introduction Zinc oxide (ZnO), a direct wide bandgap compound, is extensively used in various applications such as sensors, piezoelectric devices, varistors, and so on due to its unique properties [1-2]. ZnO consists of alternating planes in which each atom is tetrahedrally coordinated, with the O2- and Zn2+ ions stacked alternately along the c-axis, and crystallizes in two major forms, hexagonal wurtzite and cubic zinc blend. The wurtzite structure is most stable at ambient conditions with exciton binding energy of 60 meV [3]. ZnO is known as an n-type semiconductor for its dominant electrons contributed by the point defects like oxygen vacancies (VO) and/or Zn interstitials (Zni) [4]. On the other hand, Volatile Organic Compounds (VOCs) comprise of a large group of organic chemicals of low boiling point that find wide applications as solvents and dry cleaners and in many household products like paint, varnishes, adhesives, composite wood products and others; home & personal care products like air fresheners, cosmetics, and many more. Acetone, isopropyl alcohol, ethylene glycol, formaldehyde and toluene are among the most common of VOCs of great importance. VOCs, however, have tremendous pollution potential for the environment and adverse human health implications. Several studies suggest that exposure to VOCs may cause symptoms worse than for asthma patients. Each VOC has its own toxicity and potential for adverse health effects. VOCs’ short-term exposures (hours to days) may cause eye, nose & throat irritation, headaches, nausea, vomiting, etc.; and chronic exposures (years to a lifetime) may cause cancer, liver & kidney damages, central nervous system damage etc. These VOCs are generally colorless, tasteless and may be odorless but their inhalation is hazardous to health and needs instantaneous, accurate detection and treatment. In this work, ZnO nano-particles with a range of different morphologies have been synthesized (without using any expensive noble metal catalyst) via microwave assisted synthesis method from four different precursors viz. zinc acetate, zinc chloride, zinc nitrate and zinc sulphate. Microwave irradiation helps to form de-agglomerated nanoparticles with enhanced surface area which is favorable for sensing phenomena. Capabilities of these nano-particles towards detection of VOCs have been demonstrated for acetone, formaldehyde, isopropanol and toluene vapors. It was observed that ZnO derived from zinc nitrate precursor is efficient and selective for formaldehyde sensing at room temperature.

2. Experimental 2.1. Powder preparation 100 mL of 0.001 (M) solution of 4 different zinc salts having different anionic groups viz., acetate [Zn(CH3COO)2.2H2O], chloride (ZnCl2), nitrate [Zn(NO3)2.6H2O] and sulphate [ZnSO4.7H2O] were taken in four separate 250 mL beakers for obtaining Zn precursor solutions wh i ch were subjected to identical synthesis procedure. All chemicals were of analytical grade and were used in the as-received condition. The beakers were placed in microwave oven (Samsung, MW73AD-B/XTL) for 10 min. period running with full power (microwave frequency: 2450 MHz); at 2 min. intervals, the beakers were taken out and placed in cold water baths with stirring (to arrest temperature rise, if any), and simultaneous addition of 2-3 drops of ~0.1 (N) ammonium hydroxide (NH4OH) solution. The slow addition of dil. NH4OH was found to be the controlling step in getting a homogeneous precipitate. The process of microwave irradiation followed by dropwise addition of ammonium hydroxide in small doses was continued until the pH of the solution reached 9 [5]. By shortening the time of precipitation through molecular in situ heating, microwave irradiation helps formation of de-agglomerated nanoparticles with enhanced surface area which is desirable for sensing materials. Following this procedure milky white homogeneous suspensions were obtained in all the four beakers. They were sonicated in an ultrasonic bath (Wensar WUC Series-12L) for another 60 min. at room temperature and were then centrifuged (Labman; LMCF6.20) at 3000 rpm to separate out the solids from the liquid medium. The precipitates were washed with de-ionized water and acetone alternately and finally dried in an air oven (maintained at ~90 °C) for ~12 h. The dried and crushed (mortar-pestle) as-synthesized powder materials were calcined at 600 oC for 2 h to get the desired phase pure oxide powder materials (ZnO).


9833

2.2. Vapor preparation Two different concentrations (10 ppm and 20 ppm) of four different VOCs viz. formaldehyde, acetone, isopropyl alcohol, and toluene vapor were made within desiccators by gradual dilution method in our laboratory. First, a small known volume of a VOC (acetone etc.) was allowed to fill the space of an evacuated desiccator with VOC vapor. The desiccator was carefully equilibrated with atmospheric pressure and then closed again. The concentration of VOC vapor in the desiccator was calculated from the amount of the VOC used and the volume of the desiccator. Lower VOC vapor environment was created in a second desiccator by drawing a known volume of vapor-air mix from the first desiccaator and injecting it into a second desiccator. These steps were repeated and allowed to homogenize for ~1h at each step to get the VOC of desired dilution. However, there was no carrier gas involved in this process and from that point of view, it was not a dynamic process. 2.3. Vapoor sensing procedure In order to investigate the behavior of the prepared ZnO powders towards sensing of VOC vapors, the ZnO nanostructures were spin coated as thin films on indium-tin-oxide (ITO) substrates, having surface resistivity in the range of 24-28 Ω/cm2, and were placed in a close sensing chamber at room temperature, connected to VOC vapors in the desiccator of appropriate concentration. All the four VOCs viz., acetone, formaldehyde, isopropyl alcohol and toluene were allowed through the inlet of the test chamber one by one. The sensor response was determined by measuring the change in resistance when exposed to those vapors. Sensor response was quantified as percent sensitivity S(%) defined for a given test vapor by equation (1) shown below [6]: % =

∆

=

100

………………………………………. (1)

where ΔR=RA-RG and RA=Ro=initial base resistance in aerial medium and RG= base resistance in presence of target analyte gas or vapor. 3. Results and Discussions 3.1. XRD studies The XRD analysis was carried out by an X-ray diffractometer (XRD; RIGAKU Ultima IV) using Cu Kα1 radiation source. The scanning range was from 2θ = 20o to 70o with λ=1.5418 Å wavelength. Fig. 1. shows the four X-ray diffraction spectra of ZnO powders prepared from four different precursors and calcined at 600 oC. It is clearly shown that all the ZnO powders had polycrystalline structures but ZnO derived from the nitrate precursor has the lowest crystallite size with the broadest peak width over the entire range of diffraction angles. Three peaks corresponding to the (100), (002) and (101) planes were detected in the range of 2θ = 30o - 40o, which confirmed the presence of single-phase ZnO with the wurtzite crystal structure for all the powders. Among these four XRD patterns, ZnO from acetate has the lowest intensity. On the other hand, ZnO prepared from chloride and sulphate exhibit sharp XRD peaks with similar high intensity indicating high degree of crystallinity of those powers.

Fig. 1. XRD of ZnO nanostructures synthesized from zinc acetate, zinc chloride, zinc nitrate and zinc sulphate, respectively (from lower to upper) by microwave assisted wet chemical synthesis.

9834


3.2. FESEM studies ZnO has many different structural forms and shapes grown under different conditions. The crystallographic morphology of ZnO can be influenced by many experimental parameters such as, pH of the reaction medium, precursor, solvent, time and temperature [7]. Among these parameters, the precursor composition is found to have determinative effects on different morphologies of ZnO in the investigated aqqueous mediuum. Therefore, the effect of the precursor composition on the morphology of ZnO has been investigated in this work by keeping the other parameters same, through FESEM (SIGMA ZEISS) surface studies.

Fig. 2. FESEM microgrraphs of ZnO nanostructures prepared from zinc (a) acetate, (b) chloride, (c) nitrate, and (d) sulphate prrecursors.

Fig. 2 shows the FESEM images of the ZnO nanocrystals (calcined at 6600 oC) prepared from four different precursors (viz., zinc - acetate, chloride, nitrate, and sulphate). ZnO generally presents various types of one dimensional (1D) nanostructures including nanowires (NWs), nanorods (NRs), nanobelts (NBs), and nanotubes, which can be synthesized under specific growth conditions, as has been recently reviewed by Wang [8] and Niederberger [9]. The four FESEM images in Fig. 2 also shows that the crystalline ZnO has different growth habits for the different precursors: ZnO prepared from zinc acetate exhibits disordered nanoflake-like morphology [Fig. 2(a)], ZnO prepared from zinc citrate exhibits truncated hexagonal tubular shaped-like morphology [Fig. 2(b)], ZnO prepared from zinc nitrate exhibits nanorod-like morphology [Fig. 2(c)] and ZnO prepared from zinc sulphate exhibits spherical or pseudo-spherical morphology [Fig. 2(d)]. In Fig. 2(a), heterogeneous nucleation is probably responsible at the beginning of the processes followed by particle growth in different crystalline planes forming random nano-flake secondary particles. However, for the next two precursors viz. zinc citrate and zinc nitrate, the particle growth become more c-axis oriented with the calcination (600 oC temperature). This phenomenon could be due to the surface grain growth along with the surface which has the lowest free energy. This results in growth of highly oriented ZnO truncated hexagonal tubular [Fig. 2(b)] and nanorod-like structures perpendicular to substrate surface [Fig. 2(c)], respectively. For zinc sulphate derived ZnO, it had spherical or pseudo-spherical shapes in the nanometric range. These morphologies suggest that the crystal orientation and growth of synthesized ZnO is dependent on precursor composition. All the morphologies are found without any significant structural deformation.


9835

3.3. Vapor sensing results Combined dynamic response towards 10 and 20 ppm concentrations of acetone, formaldehyde, isopropyl alcohol and toluene for two consecutive cycles for ZnO synthesized from zinc acetate, zinc chloride, zinc nitrate and zinc sulphate has been examined at room temperature and it has been found that amongst the four synthesized ZnO nanoparticles, those from zinc nitrate (ZnOnitrate) displays the highest sensitivity for both the concentrations of VOCs and the corresponding combined dynamic response curves has been presented in Fig. 3. Here, the relative resistance variation for the zinc nitrate derived ZnO has been shown towards 10 and 20 ppm concentrations of acetone, formaldehyde, isopropyl alcohol and toluene for two consecutive cycles, respectively. ZnOnitrate nano-particles having the highest surface area and widest distribution (evident from the FESEM micrographs) displays the highest sensing performances probably because sensitivity is manifest form surface phenomenon. In Fig. 3, the shadow indicates a full sensing cycle, i.e., the pulse starting from resistance drop due to ‘vapor on’ to complete recovery of the sensor signal to come back to its original base resistance when the vapor was off. The material (ZnOnitrate) exhibited the highest response towards formaldehyde and lowest towards toluene. The sensitivity order is toluene