A simple preparation of ZnO nanocones and exposure to formaldehyde

Shahid Hussain a,b,n, Tianmo Liu a,b,n, M. Kashif c, Shixiu Cao a,b, Wen Zeng a,b, Sibo Xu a,b,Khalid Naseer d, Uda Hashim c

a College of Materials Science and Engineering, Chongqing University, Chongqing 400044, Chinab National Engineering Research Centre for Magnesium Alloys, Chongqing 400030, Chinac Nano Biochip Research Group, Institute of Nano Electronic Engineering (INEE), Universiti Malaysia Perlis (UniMAP), 01000 Kangar, Perlis, Malaysiad Department of Physics, University of Sargodha, Sargodha 40100, Pakistan

a r t i c l e i n f o

Article history:Received 9 April 2014Accepted 21 April 2014Available online 30 April 2014

Keywords:Crystal growthCrystal structureZnO nanoconesSensor

a b s t r a c t

Hexagonal ZnO nanocones were successfully prepared using a controlled hydrothermal method with theassistance of double anionic surfactants. The products were characterized using X-ray powder diffrac-tion. The morphologies of ZnO nanostructures were characterized using field-emission scanning electronmicroscopy and high-resolution transmission electron microscopy. The effects of the addition of oneanionic to another anionic reagent in the crystal structure formation were discussed in detail. The gassensing properties were measured for formaldehyde.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

The anisotropic fabrication of novel new nanostructures alwaysattracted researcher attention because of importance in tailoringthe different chemical and physical properties along various facetsor directions. The surface-controlled designing of nanostructuresis employed in many applications and is still challenging and veryimportant. Among metal oxides semiconductors, the most fash-ioned n-type ZnO has a wide range of applications because of itsversatile structure, wide range of interesting morphologies andsurface chemistry [1]. ZnO materials are very promising as sensors,piezoelectric devices and photoinduced piezooptics material [2–4].Numerous ZnO nanostructures have been grown using a hydro-thermal method such as nanoflowers, twinned nanodisks, nano-cones, porous sphere and chrysanthemum-like composites [5–8].In the present study, a simple and unique route was followed toprepare nanocone structures. The effect of anionic surfactants inthe formation of nanostructure was also discussed. The gas sensingproperties were measured for the as-prepared ZnO nanostruc-tures. The effect of reagents on crystal formation was alsodiscussed, and a growth mechanism was proposed.

2. Experimental

The preliminary materials used were zinc nitrate hexahydrate,Polyvinyl pyrrolidone (PVP) and sodium dodecyl sulfate (SDS) (98%purity, analytical grade) purchased from Keshi Chendu DevelopmentCo. Ltd., China, and used without further purification. At roomtemperature, PVP (0.1 g) and SDS (0.1 g) were dissolved in 20 mL ofdistilled water under vigorous stirring for 1 h. Then, Zn(NO3)2 �6H2O(0.8 g) and NaOH (0.08 g) were dissolved in 20 mL of distilled waterand added to the above solution under continuous stirring following1 h of vigorous stirring. The resulting solution was transferred to aTeflon-lined stainless steel autoclave of 50mL capacity, and heated at180 1C for 24 h. The obtained white solid product was separated fromthe solution by centrifugation, washed with distilled water andabsolute ethanol for four times, and dried in air at 80 1C. Structuralanalysis was accomplished using X-ray diffraction (XRD) with a RigakuD/Max-1200X diffractometer and Cu Kα radiation (λ¼1.5406 Å)operated at 30 kV and 100 mA. Surface morphologies were observedusing a Hitachi S-4300 field emission scanning electron microscope(FESEM) and a transmission electron microscope (TEM, ZEISS,LIBRA200) at an accelerating voltage of 200 kV. Gas sensing propertieswere measured using a computer-controlled static system (HW-30A,Hanwei Electronics Co. Ltd.).

3. Results and discussions

The XRD pattern shown in Fig. 1(a) is sharp and exhibits highcrystallinity. The peaks were indexed to the wurtzite hexagonal

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Materials Letters

http://dx.doi.org/10.1016/j.matlet.2014.04.1150167-577X/& 2014 Elsevier B.V. All rights reserved.

n Corresponding authors at: College of Materials Science and Engineering,Chongqing University, Chongqing 400044, China. Tel.: þ86 22 217 5933;fax: þ86 22 217 5930.

E-mail addresses: shahidsargodha@gmail.com (S. Hussain),tmliu@cqu.edu.cn (T. Liu).

Materials Letters 128 (2014) 35–38

structure of ZnO, which was consistent with standard ZnO. Fig. 1(b) shows the SEM images of ZnO nanocones with an average lengthand base-diameter of 1.3 and 1.2 μm respectively. The tip-end TEM,and insets of SAED and HRTEM results with lattice spacing 0.28 nmare shown in Fig. 1(c) and (d) respectively. A possible growthmechanism is shown in Fig. 2.

By structure, the wurtzite structure of ZnO has a number ofalternating planes, which comprises O2� and Zn2þ ions tetrahedrallycoordinated slanted along the c-axis [13]. These contrasting chargedions form the Zn (0001) and O ð0001Þ surfaces, resulting in the surfaceenergy divergence, spontaneous polarization and normal dipolemoment along the c-axis. Along with (0001) polar surface, ZnO alsohas ð1011Þ polar surfaces. It is also a cause of pyramid structureformation, and the crystal growth along the (0001) facet is faster thanthat on nonpolar facets [10]. ZnO cone shaped nanostructures have nocrystallographic defined side shape and exhibit excellent single crystalnature. It means that there is a step-edge present on ð0001Þ surface.The surface energy can be decreased by reducing dangling bonds onthe surface in cone like structures [11]. PVP is more anionic, as thenitrogen atom resides inside the PVP molecule and has a saturatedcovalent bond, whereas the oxygen locates outside the molecule.Because nitrogen atom exhibits less electronegativity than oxygen, itmeans a partial negative charge resides on oxygen atom and partialpositive charge on the nitrogen atom. Hence PVP shows more anionicproperties and causes the damping effect on (0001) polar faces. Thestrong adsorption ability of PVP in the nucleation of ZnO can acceleratethe dehydration reaction, even at low temperatures [9,12]. In addition,the coordination between Znþ and PVP–SDS� further suppresses theaggregation of nanostructures. Besides, SDS is also an anionic organic

solvent and is capable of disrupting non-covalent bonds. It also causesmolecules to lose their shape. The anionic reaction between SDS andPVP initiated the anisotropic growth; and growth along the (0001)direction was twice than in the vertical ð1100Þ direction [6,14]. ForZnO crystal, the negative ð0001Þ polar surfaces are more passive thanthe positive polar (0001) surfaces. It has been reported that while thecrystals growth occurs, the polar surface appears as growing surfacesduring the crystal growth because of its high surface energy. Thereby,the top end of the hexagonal cone is in the (0001) direction. The polar

5 nm

Fig. 1. (a) XRD pattern, (b) PVP–SDS assisted SEM images of the nanocones, (c) TEM images and insets of SAED pattern and (d) HRTEM image and conical faces.

Fig. 2. Proposed growth mechanism of ZnO nancones.

S. Hussain et al. / Materials Letters 128 (2014) 35–3836

faces (0001) and ð0001Þ play a vital role in the formation of hexagonalcone structure. Since the solution was more anionic, the surfaceenergy changes drastically and the crystal grows more along the polefaces (00001) [14]. Andelman has reported that change in ZnOmorphology by composure of certain planes can be easily achievedby changing the capping agents or precursor ratio [15]. In the presentstudy, the surface energy can be controlled by changing the surfacecharge by passivating the reagents PVP and SDS. So, a strongelectrostatic interaction occurs between ion and ionic liquid whichresults in decrease of surface energies of basal (0001) and polar facesð1011Þ, as compared to other crystal surfaces.

For the detection of toxic and harmful gases in air, a semiconductoroxide gas sensor plays a vital role. One of the harmful pollutants inindoor environments is formaldehyde gas, and is related to asthma,nasopharyngeal cancer, andmany other health grumbles [16]. Hence, asensitive detection method for formaldehyde is very important forhuman wellbeing. The sensor fabrication for ZnO nanostructures hasalready been discussed in our previous study [2]. The sensing responseof the sensor was observed for formaldehyde in the concentrations of50 ppm,150 ppm, 250 ppm and 350 ppm at the temperature of 275 1C(Fig. 3(a)). The maximum response was 12.5 at 350 ppm correspond-ing to 275 1C (Fig. 3(d)). Also, sharp response and recovery time wereobserved at 8 and 9 s respectively. The sensitivity of the sensorincreases with increasing temperature. The optimal temperature was

found to be 275 1C and a clear decrease in response was observed asthe temperature increased more than 275 1C. The decreased tempera-ture ensued because the exothermic reaction between gas moleculesand oxygen species increased the desorption rate and gas moleculesbecame less vibrant [2,16], as shown in Fig. 3(c). It is established thatthe ZnO based gas sensors chiefly encompass the chemisorption ofoxygen on their surface, the reaction and transfer of charge betweenchemisorbed oxygen and the target gas, and an escorted variation inresistance at the surface. When the sensor was exposed to air, oxygenion species (O� , O2� , and O2

�) adsorbed on its surface by capturingfree electrons from the particle. The potential barrier was increasedbecause of the thick space charge layer. When sensor was exposed toHCHO, these trapped molecules were released, resulting in a signifi-cant change in the surface resistance, thus sensitivity increased [17,18],

O2þ2e�-2O� ðadsÞ ð1Þ

HCHOþ2O� ðadsÞ-H2OþCO2þ2e� ð2Þ

4. Conclusion

We have successfully reported the preparation of ZnO hexago-nal nanocones, with the assistance of double anionic surfactants.

Fig. 3. Response and recovery curves for ZnO nanocones (a) at different gas concentrations, (b) repeatability at 275 1C and 350 ppm, (c) temperature ranges 100–400 1C for350 ppm and (d) response to concentration changes.

S. Hussain et al. / Materials Letters 128 (2014) 35–38 37

The XRD exhibited wurtzite ZnO nanostructures whereas FESEMshowed an average length and base-diameter of 1.3 and 1.2 μm,respectively. The HRTEM results showed that growth appearedmainly along (0001) and ð1011Þ polar surfaces. The as preparedZnO nanocones were exposed to formaldehyde for differentconcentrations (50–350 ppm) at temperature ranges of 100–400 1C. It was found that at concentration 350 ppm and 275 1C,the sensor showed a response 12.5, with a sharp response andrecovery time of 8 and 9 s respectively.

Acknowledgment

The authors gratefully acknowledge the financial support tothis work from the National Natural Science Foundation of Chinaunder Grant number 11332013.

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