Unique morphologies of zinc oxide synthesized by thermal decomposition and co-precipitation routes:...

Cryst. Res. Technol. 50, No. 5, 379–388 (2015) / DOI 10.1002/crat.201400484

Orig

inalPaper

Unique morphologies of zinc oxide synthesized by thermaldecomposition and co-precipitation routes: Ultravioletabsorption and luminescence characteristicsHassan Waqas1, Muhammad Saad Salman2, Asim Riaz2, Naeem Riaz3, and Saima Shabbir2,∗

Received 31 December 2014, revised 11 April 2015, accepted 15 April 2015Published online 5 May 2015

Two facile and efficient methods, to synthesize zinc oxide(ZnO) particles with different morphologies, have been re-ported here. Thermal decomposition route yielded micronsized irregular shaped ZnO particles. While co-precipitationmethod rendered transparent flakes which then trans-formed to hexagonal discs with relatively more uniform sizeand shape. These hexagonal discs were further converted tothe cone type morphology when hexamethylenetetraminewas added in the precursor solution. However, sphericaltype ZnO nanoparticles were obtained by incorporatingpolyvinyl alcohol during co-precipitation strategy. XRD con-firmed the formation of wurtzite structure in all the sam-ples. FTIR spectroscopy revealed the presence of ZnO char-acteristic peaks. Moreover, 3-D directional growths and thepresence of UV-Vis broadband multi-absorption peaks, andgreen to orange photoluminescence emissions confirmedthe potential application of the synthesized ZnO particles invarious piezoelectric and luminescence applications.

1 Introduction

Zinc oxide (ZnO) compounds have white or lemon yel-low appearance and can adopt three different types ofcrystalline arrangements i.e. wurtzite, zinc blende androck salt structure. However, among these arrangements,hexagonal wurtzite is most common and stable crys-talline lattice due to the higher electronegativity of oxy-gen content. Wurtzite crystal has three lattice parameterssuch as two base plane axis a = b = 0.3249 nm and thirdone is perpendicular axis which is denoted by c = 0.5204nm. c/a ratio i.e. 1.633 is key geometric factor which con-firms the formation of hexagonal wurtzite structure intypical zinc oxide powder.

Moreover, ZnO is a versatile electronic and semi-conducting material with a direct wide band gap of3.37 eV and has higher excitation binding energy of 60meV at room temperature than TiO2 [1]. ZnO nanopar-ticles have enormous applications in electronic, opto-electronic, electrochemical and electromechanical de-vices etc. [2–15]. The multi functionality of ZnO canbe correlated with the morphology or size of nanos-tructures which grow under definite synthesis condi-tions. Previously, various kinds of ZnO nanostructureshave been synthesized such as dots, rods, wire, belts,bridges/nails/walls/helixes, seamless rings, mesoporoussingle-crystal, polyhedral cages etc. [16]. The fabricationof these nanostructures was attained by employing vari-ous synthetic routes ranging from very simple precipita-tion process [17], gas condensation [18], sol-gel method[19], hydrolysis in polyol medium [20], hydrothermalsynthesis [21] to somewhat complex methods such asphysical vapor deposition [22], metal–organic chemicalvapor deposition (MOCVD) [23], molecular beam epitaxy(MBE) [24], pulsed laser deposition (PLD) [25], and sput-tering [26] etc. However, most of these methods are notbeing used extensively because of their complexity andcost factors. So far, enormous work has been reportedon the facile synthesis of ZnO nanoparticles by solutionmethods, which employ hydrolysis and condensation ofzinc salts in alcoholic (methanol, ethanol etc.) [27–29] oraqueous media [30].

∗ Corresponding author: e-mail: [email protected], Phone:+92-51-9075423, Fax: +92-51-92733101 Materials Division, Directorate of Technology, PINSTECH,Islamabad, Pakistan

2 Department of Materials Science and Engineering, Institute ofSpace Technology, Islamabad 44000, Pakistan

3 Federal Urdu University of Arts Science and Technology,Islamabad, Pakistan

379C© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

H. Waqas et al.: Unique morphologies of zinc oxide synthesized by thermal decomposition . . .

Orig

inalPaper

ZnO has enormous applications in energy harvest-ing like in solar cells, and photovoltaics. Recently moreemphasis has been paid on dye sensitized solar cells(DSSCs) as future energy materials. Among several ma-terials, ZnO and TiO2 have been proposed as the mostsuitable anode materials for solar cell conversion beca-sue they are inexpensive, easy to synthesize and pos-sess extaordinary electrical and optical semiconductingproperties [7]. Law et al., [7] demonstrated that the so-lar cell efficiecny and electrical conductivity could beincreased by using ZnO paste on the TiO2 anode sur-face. Moreover, another report by Wang et al., showedthat ZnO could be used as a future alternative energymaterial. They employed ZnO nanowire arrays for thegeneration of electricity. Becasause of the one dimen-sional (directional) growth and hence piezoelectric ef-fects, these ZnO nanowire arrays could be used for al-ternative energy harvesting if the power is generated bymechanical means [8]. The usefulness of ZnO based onits peizoelectric effects can be observed by the work ofWang et al., [9] who employed piezoelectric and semi-conducting properties of ZnO nanowires accompaniedwith nanobelts as electrode materials for harvesting andrecyling energy from the environment. Moreover, com-prehensive work done by Sakthivel et al., [31] exploitedTiO2, ZnO, SnO2, ZrO2, α-Fe2O3, WO3, CdS and ZnS forprobing the solar light conversion efficiencies of thesematerials as photocatalysts. In this context, photocat-alytic efficiency of each material was determined, com-pared and evaluated to prove that ZnO was superior thanthe rest of the materials. This greater actitvity of ZnOwas attributed to the ability of ZnO to absorb greateramount of light quanta. Additionally, ZnO also absorbedlarge fraction of light spectrum and demonstratedbetter quantum efficiency than TiO2 which renderedZnO a prospective material for the future solar energyharvesting.

Researchers have also observed that ZnO has largerband gap excitation energy (60 meV) and therefore ab-sorbs UV light more efficiently than TiO2 [1]. Moreover,its UV absorption can be further enhanced by creatingadditional surface defects [31]. Besides UV absorption,the luminescence response of ZnO particles in visible re-gions has also been investigated extensively. These stud-ies indicated that ZnO exhibits visible emission bandsin the UV, yellow, green and rarely in the red regions[32]. UV emissions are described as the direct recom-bination of photo generated holes with the electrons.The deep level luminescence in the green and yellowregions has been attributed to the defects present onthe surface of ZnO nanostructures. Whereas, green lu-minescence (GL) is endorsed to the oxygen vacancies

and yellow luminescence (YL) is recognized as the effectof oxygen interstitials or zinc vacancies [33]. Neverthe-less, these investigations are still controversial. Addition-ally green and yellow luminescences, emissions in theorange and red regions have not been well understoodyet.

In the context of various applications it is observedthat facile synthetic strategies that produce better yieldsof nanomaterials are very rare. Most of the methods arevery expensive and cannot be used for large scale pro-duction as has been mentioned earlier [17–26]. The mostcommon, low cost, laboratory scale method adopted sofar is chemical precipitation (wet chemical) route. Thereare several researches that use this method as the coremethod for the synthesis of materials of industrial im-portance. Zahra et al. reported that the nano scale ZnOcan be obtained by micro size ZnO in the chemical pre-cipitation route [34]. This procedure yielded flower likestructures using simple and cheap reagents in the solu-tion. It is also reported in another work by L. Chen et al.[35] that spherical nano structures of ZnO of less than12 nm sizes can be obtained by a simple precipitationmethod under suitable temperature in the presence ofoleic acid (OLA), methanol, ethanol and hexadecylamine(HDA).

On the other hand, there are very rare reported meth-ods on the synthesis of ZnO particles by direct ther-mal decomposition from the pure salt precursor. One ofthe thermal decomposition methods was employed byRaghvendra et al. [36]. This method was reported as theprecise method for the mass-scale production of ZnOnanoparticles but the microscopic view of the particleswas not observed by this research group. It was furtherreported in the same work that the photoluminescencefrom the particles was yellowish in color and ascribed tobe purely dependent on defects surface state indepen-dent of particle sizes.

Generally it is difficult to synthesize small sized ZnOespecially with nano morphology in solution because theparticle growth is too fast to control the crystal size. Someorganic substances used as structure directing reagents,such as specific surfactants, organic molecules, catalystsetc., are introduced into the reaction process in orderto control the crystal size. It is well understood that themorphologies of the particles are greatly dependent onthe pH and concentration of the precursor ions in thesolutions. The pH and concentrations of the ions can bevaried to get desired morphologies. Pal et al. [37] showedthat different morphologies of ZnO can be obtainedusing zinc acetate di-hydrate and sodium hydroxide,ethylenediamine and water at two different pH valuesby the hydrothermal method. In another work, a fluffy

380 www.crt-journal.orgC© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Cryst. Res. Technol. 50, No. 5, 379–388 (2015)

Orig

inalPaper

type pompom like ZnO nano-structures by zinc nitratehexahydrate and ammonia were achieved by Hochepiedet al. [38].

According to the previously published work, zinc saltprecursors, especially Zn(Ac)2 and Zn(NO3)2 have beenextensively used for preparation of ZnO nano-structuresvia precipitation in basic solutions such as NH4OH,(NH4)2CO3, KOH, and NaOH with different surfac-tants like polyvinyl chloride (PVC), polyvinyl alcohol(PVA), cetyltrimethylammonium bromide (CTAB), orhexamethylenetetramine (HMTA) etc.), which yielded;rods, spherical particles, nanorods and hexagonal longrods, respectively [39–41]. In another detailed workreported by Barreto et al. [42], the effects of Zn salt pre-cursors, temperature, solvent washing, precursor base,stoichiometry, capping agent or surfactant have beenthoroughly investigated. In this work it was reported thathexagonal rod type morphologies could be obtained dueto strong effects of bases, zinc nitrate precursors andnonionic surfactants.

In the context of aforementioned, unique ZnO mor-phologies being reported here have never been achievedearlier using the two methods presented in the cur-rent work. To the best of our knowledge the use ofHMTA and PVA for the conversion of morphologieslike spherical nanoparticles and disk shapes to mi-cro 3-D hexagonal cones, utilizing NH4OH as a base,is exhibited for the first time and such a type of re-port is not available in the literature. Therefore, in thepresent endeavor, ZnO macro, micro and nano struc-tures were synthesized by thermal decomposition andco-precipitation methods, respectively. More specificallythe effects of HMTA and PVA on the size and morphol-ogy of ZnO particles have been thoroughly investigated.The optical properties such as ultraviolet absorbanceand photoluminescence of the synthesized ZnO parti-cles have also been probed. It is known that morpholo-gies have phenomenal effects on the functional proper-ties of the materials. Hence, depending on the uniquemorphologies of ZnO obtained in this work, it has beenproposed that directional 3-D growths of ZnO struc-tures can be potentially employed in piezo-electronicapplications.

2 Experimental

Zinc nitrate hexahydrate (>99.9% purity), ammoniasolution (35%), hexamethylenetetramine (HMTA) andpolyvinyl alcohol (PVA) were purchased from Fluka.Demineralized water (DMW) was used as a solvent. Allthe reagents were utilized without further purification or

modifications. Direct and indirect methods, i.e. thermaldecomposition and co-precipitation, have been chosento synthesize ZnO particles.

2.1 Thermal decomposition method

25 g of zinc nitrate hexahydrate was thermally decom-posed in a tube furnace at 500 °C with holding time of 45min in the presence of air. ZnO particles having lemonyellow appearance (sample a) were collected after cool-ing the furnace to room temperature.

2.2 Co-precipitation method

25 ml of 0.5 M zinc nitrate solution was prepared inDMW. A 1:1 ratio of 35% solution of aqueous ammo-nia (NH4OH) diluted by DMW was added drop wise inthe nitrate solution. White precipitates were instantlyformed with constant stirring. This reaction was allowedto complete for 5 hrs (sample b). Then separately cap-ping agents 0.53 wt% HMTA (sample c) and 0.22 wt%PVA (sample d) were also added to the nitrate solution.The precipitation reactions were completed by addingfew drops of aq. NH4OH to the above solutions (contain-ing PVA or HMTA) under vigorous stirring and the syn-thesized precipitates in all the experiments were washedthree times with DMW and then dried for 24 h at 80 °Cin vacuum oven. Finally, dried powders were calcined at300 °C for 1 h in air.

2.3 Measurements

ZnO particles, synthesized using aforementioned routes,were characterized by X-ray Diffractometer X’pert PRO(Cu- Kα , λ = 0.1542 nm, 40 kV, 30 mA) within diffrac-tion range of 10–80° and 0.02°/s step scan. Thermalgravimetric analysis (TGA) of nitrate precursor was per-formed by NZTECH-429 in nitrogen environment ata rate of 10°/min up to 500 °C. Nicolet-6700 Fouriertransform infrared spectroscopy (FTIR) was used to getfunctional group information in ZnO powders. JEOL-6490LA Scanning Electron Microscope (SEM) was em-ployed to investigate particle morphology and aver-age sizes. UV-Vis spectroscopy was done by AnalytikJena-206 and luminescence response from all the sam-ples in powder form was measured using Labram-3 spectrophotometer He-Cd laser (325 nm) at roomtemperature.

www.crt-journal.org 381C© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim


Orig

inalPaper

Fig. 1 XRD patterns of ZnO structures.

3 Results and discussion

Figure 1 shows the XRD patterns of ZnO particles ob-tained by thermal decomposition and co-precipitationmethods.

The presence of multiple peaks at specific Bragg’spositions in all the XRD patterns indicates that synthe-sized powders are polycrystalline in nature and havewurtzite structure. Peak positions of each diffraction pat-tern were matched with the data provided by Joint com-mittee on powder diffraction standards (JCPDS) and itwas observed that ZnO powder synthesized by ther-mal decomposition method (sample a) closely matchedwith the indexing present in JCPDS (75–576). While theXRD peaks of sample b synthesized by co-precipitationmethod matched with JCPDS (79–0205). Using the cap-ping agents HMTA and PVA shifted the peaks in XRDspectrum and the new diffraction data accorded withJCPDS (75–1526). XRD peaks in each pattern were prop-erly indexed. (hkl) planes were calculated by employ-ing crystallographic knowledge, c/a ratio and d-spacing(Table 1).

In all the XRD patterns, three most intense peaks be-long to (101), (100) and (002) planes, respectively. In ther-mal decomposition method, c/a ratio was calculated as1.6020. However, in case of co-precipitation route, in thepresence of capping agents like HMTA or PVA, the pa-rameters of hexagonal lattice (i.e. d-spacing, a and c)slightly changed as given in Table 1.

It can also be observed from Table 1 that all samples(b–d) synthesized by co-precipitation method have slightpeak shifting towards higher side at each 2θ position ascompared to thermal decomposition method due to the

Table 1 Comparison of c/a ratio of ZnO particles synthesized bythermal decomposition and co-precipitation methods.

ID d-spacing c/a Intensity(I/Io)

1st 2nd 3rd 1st 2nd 3rd

a 2.4786 2.8152 2.6041 1.602 100 60 36

b 2.4799 2.8065 2.5969 1.6004 100 52.1 48.8

c 2.4622 2.7980 2.5859 1.5965 100 63 37

d 2.622 2.980 2.5859 1.5965 100 59 46

*c/a ratio = 1.602 (JCPD# 75–0576); c/a ratio = 1.600 JCPD (79–0205); c/a ratio = 1.596 (JCPD# 75–1524).

Fig. 2 FTIR spectra of ZnO structures.

formation of smaller crystallite sizes (nm) as comparedto thermal decomposition method (μm size). Howeverthe average crystalline size in the entire samples is foundto be �38 nm using the Debye–Scherer equation. Fur-thermore, creation of vacancies and crystalline defectsin ZnO lattice by different processing routes also con-tributed to define the lattice parameters.

Therefore the observed XRD patterns can be explic-itly attributed to the presence of ZnO hexagonal wurzitecrystallites. No excess peaks detected, which indicatesthat all the precursors have been completely decom-posed and no other complex products were formed.

Figure 2(a–d) shows FTIR analysis of all ZnO parti-cles synthesized via different strategies. The characteris-tic peaks [43] for Zn-O bonds are observed from 419 to468 cm−1 in all the samples (figure 2) indicating the suc-cessful conversion of Zn(OH)2 to ZnO crystals.

Vibrational peaks of samples a and b are somehowalike; which confirm that without the addition of cap-ping agent i.e. HMTA, PVA, similar FTIR spectrum of ZnO



Orig

inalPaper

Fig. 3 TG plot of zinc nitrate hexahydrate.

powder was observed in both the thermal decomposi-tion and co-precipitation routes.

In samples a and b, the absorbance peaks appearedat 514 and 550 cm−1 due to the stretching vibration ofZnO particles. Whereas the peaks at 1221 and 1370 cm−1

positions are due to C–O and C = O symmetric stretch-ing vibrations, respectively. Their presence in the IR ab-sorption spectrum is due to the chemisorption of carbondioxide (CO2), by ZnO particles, from the surrounding at-mosphere. Minor symmetric stretching vibrations from1221 to 1370 cm−1 indicate the presence of NO3

−1 vibra-tions in the IR spectra. The peak appearing around 1741cm−1 can be a major indicative of the humidity in theform of adsorbed water molecules [44].

In order to study the conversion of zinc nitrate hex-ahydrate precursor to ZnO, nitrate precursor was ana-lyzed by TGA as shown in figure 3.

Thermal decomposition behavior of zinc nitrate hex-ahydrate can be explained in the following steps; initiallywith the rise of temperature, water molecules start toevaporate form zinc nitrate salt and at 150 °C hexahy-drate zinc salt is converted to monohydrate nitrate asmentioned in equation 1 [45].

Zn(NO3)2.6H2O → Zn(NO3)2.H2O (1)

At this stage, it can be deduced, from the TGA curve,that about 27% weight has been lost in the first segment.In the second step at 160 °C, partial conversion of zincnitrate to ZnO started. Monohydrate water molecule isnot instantly eliminated but preferably forms zinc nitrate

and zinc hydroxide complex by creating Zn+2 ions as de-picted in the equation 2.

Zn(NO3)2.H2O → Zn(NO3)2.2Zn(OH)2 (2)

When this metastable phase Zn(NO3)2.2Zn(OH)2 wassubjected to higher temperature conditions, it graduallytransformed to ZnO phase. Hence the thermal decompo-sition of zinc nitrate is completed at 250 °C according toequation 3.

Zn(NO3)2.2Zn(OH)2 → ZnO (3)

This analysis led to an important conclusion that inorder to get pure and stable ZnO particles, thermal treat-ment of nitrate precursors must be performed above 250°C. During this decomposition step, further 38% reduc-tion in weight was observed and hence total weight lossduring the whole thermal cycle was �67%. Further in-crease in temperature showed that TG curve approachedzero slope conditions which confirm the thermal stabil-ity of ZnO powder. This thermal investigation concludesthat heating below 250 °C may yield incomplete conver-sion of monohydrate complexes into ZnO crystals. Thusconventional methods employing heating or drying of assynthesized powders below 250 °C are not suitable to getcrystalline structures.

Figure 4a shows the SEM images of ZnO particles atdifferent magnifications produced by thermal decompo-sition method. It can be observed that particles are lack-ing uniformity both in shape and size. In this simpleheating method, operating temperature and surround-ing environment are the synthesis parameters that influ-ence particle morphology. In the present work, air waschosen as synthesis environment and temperature wasincreased from room temperature to 500 °C. As the tem-perature reached 100 °C evaporation of water moleculesstarted and left behind water free nitrates. With fur-ther rise in temperature, the creation of thermal gradi-ent across the surface to interior of nitrate salt resultedin, non-uniform decomposition of zinc nitrate. This pro-cess gave yellowish irregular shaped ZnO particles hav-ing variable sizes as shown in figure 4a. Therefore it isworth mentioning here that higher temperatures (�500)thermodynamically favor anisotropic growth of ZnO par-ticles. Thus, thermal decomposition process being asimple method yielded irregular micron sized particles.However, in case of co-precipitation method, in whichnitrate solution was mixed with ammonium solution,more uniformity and homogeneity was obtained andhexagonal discs type morphology was observed as aftercalcination shown in figure 4b. The reaction chemistry



Orig

inalPaper

Fig. 4 SEM images of ZnO micro and nanostructures.

and mechanism behind the formation of ZnO hexago-nal discs can be described as; initially, dissolution of zincnitrate in DMW gave Zn+2, OH−1 and NO3

−1 ions. Thezinc ions further reacted with hydroxyl ion (OH−1) to givezinc hydroxide (Zn(OH)2) as mentioned in equation 4.Whereas ammonia was converted to its ionic form i.e.NH4

+1 according to equation 5.

Zn2+ + 2OH−1 → Zn(OH)2 (4)

NH3 + H2O → NH+14 + OH−1 (5)

The NH4+1 ions were responsible for the formation of

ZnO disc type morphology from transparent flakes (sim-ilar to HMTA refer to equation 6, i.e. NH3 dissolved inH2O). Figure 4c shows the magnified image of ZnO par-ticles having hexagonal cone morphology. Such type ofmorphology was obtained when HMTA was added as acapping agent during the co-precipitation process of ni-trate precursors. HMTA is nonionic and water soluble

amine and usually considered to synthesize ZnO nanos-tructures [47]. Even though the exact function of HMTAfor the synthesis of ZnO nanostructure is still controver-sial, however, it has been suggested that the addition ofHMTA in Zn(NO3)2.6H2O solution acts as a base and pro-duces excess ammonium ions by radially undergoing hy-drolysis process according to equation 6 [46, 48].

HMTA + 6H2O → 4NH3 + 6HCHO (6)

The formation of ammonia (NH3) during this reactionfurther converted to NH4

+1 and OH–1 ions which tendto accelerate preferential growth of Zn(OH)2 along [0001]direction [49]. Under the action of similar phenomenon,hexagonal cone morphology shown in figure 4c of ZnOcrystals was obtained with the addition of HMTA surfac-tant. Therefore, it can be concluded that the ionizationof both the ammonium base and HMTA give quite simi-lar effects on ZnO morphology.

Moreover it has been observed, in case of PVA addi-tion, the effect of OH−1 ions in the nitrate solution dom-inates over NH4

+1 ions. Excess OH−1 ions favor the 3-Dparticle growth process and act as nano-patterning cen-ters to provide active sites for a specific pattern [50]. PVAalso helps to reduce the particle size and inhibits thegrowth process. In a typical synthesis of ZnO nanopar-ticles, PVA provides active OH−1 ions [51]. These ionscreate electrostatic hindrance against the agglomerationof ZnO particles by residing at the surfaces and ulti-mately giving nano size ZnO particles as it was observedin the present case shown in figure 4d. Thus, in order toget ZnO nanoparticles, PVA should be added as an ef-fective capping agent during co-precipitation syntheticstrategy. Additionally, temperature dependent morpho-logical transformation of Zn(OH)2 transparent flakes toZnO particulates is depicted in figure 5. In figure 5, firstZn(OH)2 flakes were formed by precipitation and dry-ing only. Moreover, when these transparent flakes weresubjected to calcination at 300 °C then the flakes trans-formed to ZnO hexagonal disc type morphology (figure5 and figure 4b). It is worth mentioning here that, ini-tially flakes grew into hexagonal discs and then hexag-onal cones built on these discs, confirming that discs actas a growth template for ZnO hexagonal cones. But theeffect on the formation of 3D hexagonal cones in thecase of the aqueous ammonium has been observed tobe less pronounced by merely employing NH4OH thanusing HMTA as a surfactant. This indicated that NH4

+1

ions generated in case of HMTA surfactant had greaterconcentration and better ability to direct 3D growth ofZnO than NH4OH alone. All morphological transforma-tions were obtained only after calcination accompanied



Orig

inalPaper

Fig. 5 Temperature dependent transformation of Zn(OH)2 trans-parent flakes to ZnO particulates.

with directional growth effect of NH4+1 ions as explained

earlier for figure 4 (b, c).To investigate ultraviolet absorbing property, UV-Vis

spectroscopy was employed. The synthesized ZnO par-ticles were well dispersed (0.4 w/v%) in ethanol by son-ication and then absorbed wavelengths were recordedby spectrophotometer at room temperature as shown infigure 6. The related sharp absorption edges and opticalband gap energies calculated by energy-wavelength re-lation are mentioned in Table 2. ZnO powder obtainedby simple thermal decomposition method (sample a)showed UV absorbance at �380 nm. It is in accordancewith the typical cut off absorption wavelength of ZnOcrystals which is a narrow peak near the band edgeabsorption. Whereas the sample b synthesized by co-precipitation method showed absorbance at �325 nm.This absorption is blue shifted shown in figure 6 (a–b)which is mainly due to the formation of smaller size par-ticles having high optical band gap energy (Table 2) thatrequires lower UV wavelength.

Fig. 6 UV-Vis spectra of ZnO structures.

Table 2 Optical band gap measurements from UV-Visabsorption spectra of ZnO particles.

Sample Absorption(nm) Band gap energy (eV)

a 370 3.35

b 330 3.70

c 350 3.54

d 318 3.87

However, when HMTA was added multiple weak(�305 nm) and sharp (�370 nm) absorption peaks wereobserved as shown in figure 6c. Particle aggregation wasalso observed and can be seen in figure 4c (pointed ar-row). Therefore, the appearance of multiple absorbancesare due to the presence of larger and smaller particles inthe sample c as shown in figure 4c. Moreover, higher res-olution SEM micrograph (indicated in figure 4c) clearlyshows 3D hexagonal cone type morphology of ZnO.

Each size has specific absorption edges. The blueshift edge (�305 nm) in the spectrum shown in fig-ure 6c represents the absorption due to smaller parti-cles whereas red shift indicated the edge (�370 nm) forlarger particles. On the other hand, when HMTA was re-placed by PVA i.e. figure 6d shows that absorption peakis shifted to �318 nm. In this case, the blue shift effectwas more pronounced. It was due to the formation of ho-mogenized and uniform size ZnO nanoparticles whichwere obtained by adding PVA as capping agent in co-precipitation synthesis as observed in figure 4d.

The variations in the particle shape and size cre-ate different quantum confinement effects that play avital role in deciding the absorption behavior of ZnO[52]. Therefore, in the present work addition of different



Orig

inalPaper

capping agents rendered range of absorption edges andoptical band gap energies.

From aforementioned explanation, it can be inferredthat the selection of synthesis technique and the natureof capping agents are important parameters in tailoringthe optical band gap characteristics of ZnO particles withmorphologies. Thermal decomposition method showsmore bulk effects due to clustering and agglomeration ofparticles. These effects are thermodynamically more fa-vorable at high temperature synthesis conditions. Thus,larger particles having low surface to volume ratio andpossessing lower energy state project red shifting in theabsorption spectra [53]. Whereas, blue shifted trend wasobserved in co-precipitation method due to the involve-ment of solution treatment and lower room temperaturesynthesis conditions that led to the production of nanosize ZnO particles.

The nature of the surfactants with nitrate precursorcan be used to explain why each surfactant gives rise toits particular morphology. Crystal morphology is deter-mined by the relative growth rates of the crystal planes ina particular direction, which differs greatly due the sur-face states of the systems i.e., surface free energies.

It is believed that varying the surfactants (i.e HMTAand PVA) with varying concentration, ratios or percent-ages with the precursor (Zinc Nitrate) have changedmorphologies of the systems due to their surface con-ditions and directional effects. Here particular to ourmethodology in this paper, molarity of nitrate precur-sor was held constant (i.e 0.5 M) and different w/v per-centages of HMTA and PVA as surfactants were usedwith the prescribed precursor. This gives an importantfact that only surfactants accompanied with the precur-sors led to the unique morphologies with certain opticalproperties.

Luminescence spectra of ZnO particles obtained withthe excitation wavelength (λex) of 325 nm at room tem-perature is given in figure 7(a–d). These visible emissionsin all the samples a-d are usually related to various in-trinsic defects especially those positioned at the surfaceof ZnO crystallites [54].

Green and yellow emissions between 502 to 570 nmare ascribed as the recombination of photo-generatedholes with electrons that belong to singly or doubly ion-ized oxygen vacancies. Their emission intensity may beattenuated due to the presence of over populated defectsdensity inside the ZnO crystals [55, 56].

The emission spectra of samples a, b, c, and d areshown in figure 7(a–d). Figure 7a shows that sample (a)has second highest PL intensity because it was synthe-sized by thermal decomposition of zinc nitrate hexahy-drate at 500 °C.

Fig. 7 Luminescence spectra of ZnO structures.

This high temperature synthesis tends to minimizethe interior defects in ZnO crystals by merging smallercrystallites with a resulting decrease in PL intensity.Amongst all, the intensity of sample d (figure 7d) is thehighest one. It is due to the production of nanocrystalshaving optimum amount of defect concentration re-quired for strong photoluminescence emission. It wasreported elsewhere [57] that the decrease in internaldefects density helps to enhance PL intensity by lessrecombination trapping within the band gap region. Thisis not observed to be true in our case infact, decreasein defects states led to the weak emissions. Whereas,samples a and b in figure 7 have relatively lower PLintensity due to the presence of micron size particles.These particles produce low intensity photolumines-cence (PL) emission (towards lower band gap energies)in the range up to 595 nm due to lower surface to volumeratio.

Regarding the morphologies of ZnO it can also be in-ferred that the pH has a greater effect. Its effect can beattributed similar to the effect of temperature; however,the former can be controlled more precisely. Therefore,morphology, crystallinity, growth and particle sizes re-vealed by SEM in figure 4 can be attributed well to the ef-fect of pH of the solution. The bulk effect of the particlesin figure 4c and quantum effect in figure 4d can also beexplained by considering the combine effect of pH andconcentration of ions in the solution as well as the pos-sible nucleation sites available. Solution with HMTA sur-factant has higher concentration and solution with PVAsurfactant was lower in concentration.

The bulk particles seen in figure 4c nucleatedfrom the higher concentration of HMTA having higher



Orig

inalPaper

nucleation sites of Zn ions available than PVA in figure4d demonstrating nanostructures.

It is, however, assumed that quantum confinementcannot contribute to the luminescence in the samples a,b, and c because the sizes of these particles are not inthe nanometer regime and their crystalline sizes greatlyexceed the Bohr exciton radii of ZnO. Moreover, this as-sumption is true except for sample d because sample dis in nanometer regime which favors high level of sur-face unstability and also traps greater number of sur-face recombination within the band gap region [58, 59].Orange or red spectral emission bands in the ranges of590–595 nm are red shifted and are supposed to occurbecause of the lattice distortion as explained by XRD inTable 1. This shows that with higher temperatures thered shifting in the crystals occurs and this tends to shiftemissions towards lower band gap energies figure 7a.From the results it is also observed that visible emis-sions towards the lower ends of band gap energies tendto decrease the luminescence intensity due to the re-duction of the surface defect states, and hence; qual-ity of the crystal improves subsequently as mentionedearlier.

It is well known that luminescence intensity dependsupon the surface condition and sizes of the particles [59].Figure 7d shows highest possible luminescence intensitythat could be achieved. This intensity is greater than thefinest crystals in figure 7a which is because the nanosizesovercome the ‘bulk effect’ of fine crystal in luminescencebecause of their higher surface to volume ratio withmore defects states, hence, stronger visible emissionsare obained (figure 7d). It is also an important fact tomention here that samples (b, c, and d) were subjected tosame calcination temperature i.e., 300 °C but regardlessof this temperature their surface states, morphologieswere changed due to specific surfactants as shown infigure 4(b, c, d). Accordingly, different morphologieshaving different surface defect states produced varyingluminescence. The intensities of sample-a and sample-care slightly differed by 200 (units) this is an indicationthat sample-c has better crystal quality than sample-adue to the effect of HMTA as shown in figure 4c andfigure 7c. Therefore, HMTA has pronounced effect toshape the morphology and yielded better quality crys-tals. The PL intensity of sample-a is higher as comparedto sample-c mainly due to the effect of temperaturedifference also, because as the temperature is increasedthe probability of oxygen to vacate the Zn sites increasesand hence defects lead to enhancement in the PL in-tensity. Similarly, sample-b bears comparatively moresurface defects and PL intensity is second highest tosample-d.

4 Conclusions

In summary, different unique 3-D hexagonal conesand nanoparticles morphologies of ZnO have beensynthesized by simple thermal decomposition andco-precipitation with HMTA and PVA. The 3-D hexag-onal cone morphology has been observed as a novelmorphology of ZnO. The addition of PVA gave nearlyspherical shape nanoparticles while 3-D hexagonal conemorphology was obtained by HMTA addition. Photolu-minescence studies confirmed that ZnO nanocrystalssynthesized by co-precipitation method followed by themodification with PVA have highest emission intensitiesin green to orange regions due to the quantum confine-ment effect and presence of optimum concentration ofsurface defects. In the similar lines luminiscences wereobserved as highly dependent on ZnO morphologies.It was also observed that the thermal decompositionmethod could be adopted for mass-scale productionbut the morphologies of the materials would be hard tocontrol. Moreover, the directional growth feature of ZnOin the case of 3-D conic structures permits their usage inpiezoelectric and luminescent applications.

Acknowledgements. Authors are grateful to PINSTECH for pro-viding synthesis and characterization facilities to accomplish thiswork.

Key words. ZnO nanostructure, morphology, crystal growth, lu-minescence, optical properties.

References

[1] D. C. Look, J. Mater. Sci. Eng. B-Adv. 80, 383–387(2001).

[2] Z. L. Wang, J. Chin. Sci. Bull. 54, 4021–4034 (2009).[3] M. H. Huang, S. Mao, and H. Feick, Science 292, 1897–

1899 (2001).[4] W. I. Park and G. C. Yi, Adv. Mater. 16, 87–90 (2004).[5] Y. W. Zhu, H. Z. Zhang, and X. C. Sun, J. Appl. Phys. 83,

44–146 (2003).[6] W. T. Yu, Y. P. Hung, and L. S. Yuan, J. Am. Chem. Soc.

131, 17690–17695 (2009).[7] M. Law, L. E. Greene, and J. C. Johnson, Nat. Mater. 4,

455–459 (2005).[8] Z. L. Wang and J. Song, Science 312, 242–246 (2006).[9] Z. L. Wang, Adv. Mater. 19, 889–892 (2007).

[10] Z. L. Wang, Mater. Today 10, 20–28 (2007)[11] X. L. Cheng, H. Zhao, and L. H. Huo, Sens. Actuators B

102, 248–252 (2004).[12] E. Topoglidis, A. E. G. Cass, and B. O’Regan, J. Electr-

analyt. Chem. 517, 20–27 (2001).



Orig

inalPaper

[13] W. Jun, X. Changsheng, and B. Zikui, Mater. Sci. Eng. B95, 157–161 (2002).

[14] P. Sharma, K. Sreenivas, and K. V. Rao, J. Appl. Phys. 93,3963–3970 (2003).

[15] P. V. Kamat, R. Huehn, and R. Nicolaescu, J. Phys.Chem. B 106, 788–794 (2002).

[16] Z. L Wang, Mater. Today. 7, 26 (2004).[17] Z. Y. Fan and J. G. Lu, J. Nanosci. Nanotechnol. 5,

1561–1573 (2005).[18] W. Jin and I. K. Lee, J. Eur. Ceram. Soc. 27, 4333–4337

(2007).[19] A. Gudkova, and K. Kienskava, Russ. J. Appl. Chem. 78,

1757–1760 (2005).[20] J. Liu, X. Huang and J. Duan, Mater. Lett. 59, 3710–

3714 (2005).[21] C. H. Lu, W. J. Hwang, and S. V. Godbol, J. Mater. Res.

20, 464–471 (2005).[22] Z. W. Pan, Z. R. Dai, and Z. L. Wang, Science 291, 1947–

1949 (2001).[23] H. Yuan and Y. Zhang, J. Cryst. Growth 263, 119–124

(2004).[24] Y. W. Heo, V. Varadarajan, and M. Kaufman, App. Phys.

Lett. 81, 3046–3048 (2002).[25] Y. Sun, G. M. Fuge, and M. N. R. Ashfold, Chem. Phys.

Lett. 396, 21–26 (2004).[26] W. Chiou, W. Wu, and J. Ting, J. Diam. Relat. Mater. 12,

1841–1844 (2003).[27] S. Monticone, R. Tufeu, and A. V. Kanaev, J. Phys.

Chem. B 102,2854–2862 (1998).[28] E. Meulenkamp, J. Phys. Chem. B 102, 5566–5572

(1998).[29] Z. Hu, D. J. E. Ramirez, and B. E. H. Cervera, J. Phys.

Chem. B 109, 11209–11214 (2005).[30] S. C. Zhang and X. G. Li, Colloids Surf. A 226, 35–44

(2003).[31] S. Sakthivel, B. Neppolian, and M. V. Shankar, Sol. En-

ergy Mater. Sol. Cells 77, 65–82 (2003).[32] U. Ozgur, Ya. I. Alivov, and C. Liu, J. Appl. Phys. 98,

041301–041404 (2005).[33] F. Hai-Bo, Y. Shao-Yan, and Z. Pan-Feng, Chin. Phys.

Lett. 24, 2108–2109 (2007).[34] Z. M. Khoshhesab, M. Sarfaraz, and M. A. Asadabad,

Synthesis and Reactivity in Inorganic, Metal-Organic,and Nano-Metal Chemistry 41, 814–819 (2011).

[35] L. Chen, J. D. Holmes, and S. R. Garcı́a, J. Nano. Mater.2011, 9 (2011).

[36] R. S. Yadav, R. Mishra, and A. C. Pandey, J. Exp.Nanosci. 4, 139–146 (2009).

[37] U. Pal, J. G. Serrano, and P. Santiago, Opt. Mater. 29,65–69 (2006).

[38] J. F. Hochepied, A. P. A. Oliveira, and V. G. Ferreol, J.Cryst. Growth. 283, 156–162 (2005).

[39] H. J. Zhai, W. H. Wu, and F. Lu, Mat. Chem. Phys.112,1024–1028 (2008).

[40] H. Zhang, J. Feng, and J. Wang, Mater. Lett. 61, 5202–5205 (2007).

[41] R. Hong, T. Pan, and J. Qian, Chem. Eng. J. 119, 71–81(2006).

[42] G. P. Barreto, G. Morales, and M. L. Quintanilla, J.Mater. 2013, 11 (2013).

[43] H. M. Xiong, R. Z. Ma, and S.F Wang, J. Mater. Chem.21, 3178–3182 (2011).

[44] R. Wahab, Y. S. Kim, and H. S. Shin, Mater. Trans. 50,2092–2097 (2009).

[45] B. Małecka, R. Gajerski, and A. Małecki, Thermochim.Acta 404, 125–132 (2003).

[46] S. Baruah and J. Dutta, Sci. Technol. Adv. Mater. 10,013001 (2009).

[47] L. Vayssieres, Adv. Mater. 15, 464–466 (2003).[48] I. S. Ahuja, C. L. Yadava, and R. Singh, J. Mol. Struct.

81, 229–234 (1982).[49] H. E. Unalan, P. Hiralal, and N. Rupesinghe, J. Nan-

otechnol. 19, 255608 (2008).[50] M. N. R. Ashfold, R. P. Doherty, and N. G. N. Angwafor,

Thin Solid Films 515, 8679–8683 (2007).[51] A. Bera and D. Besak, Appl. Mater. Interfaces 1, 2066–

2070 (2009).[52] N. Ashkenov, B. N. Mbenkum, and C. Bundesmann, J.

Appl. Phys. 93, 126–133 (2003).[53] P. S. Hale, L. M. Maddox, and J. G. Shapter, J. Chem.

Educ. 82, 775–778 (2005).[54] Z. W. Li, W. Gao, and R. J. Reeves, Surf. Coat. Technol.

198, 319–323 (2005).[55] A. K. Singh, V. Viswanath, and V. C. Janu, J. Lumin. 129,

874–878 (2009).[56] Z. L. Wang, J. Phys. Condens. Matter 16, 847 (2004).[57] D. C Iza, D. M. Rojas, and Q. Jia, Nanoscale Res. Lett.

7, 655 (2012).[58] P. Yang, H. Yan, and S. Mao, Adv. Funct. Mater. 12, 323–

331 (2002).[59] I. Shalish, H. Temkin, and V. Narayanamurti, Phys.

Rev. B 69, 245401–245408 (2004).


Unique morphologies of zinc oxide synthesized by thermal decomposition and co-precipitation routes:...

Documents

Transcript of Unique morphologies of zinc oxide synthesized by thermal decomposition and co-precipitation routes:...