Controlled synthesis of ZnO nanostructures with assorted morphologies via simple solution chemistry

Journal of Alloys and Compounds 551 (2013) 233–242

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Review

Controlled synthesis of ZnO nanostructures with assorted morphologies via simple solution chemistry Prashant Kishor Baviskar a,c, Pratibha Rajaram Nikam a, Sandip Shankarrao Gargote b, Ahmed Ennaoui c, Babasaheb Raghunath Sankapal d,⇑ a

Thin Film and Nano Science Laboratory, Department of Physics, School of Physical Sciences, North Maharashtra University, Jalgaon 425001 (M.S.), India Department of Chemistry, New Arts, Commerce and Science College, Shevgaon 414502 (M.S.), India Helmholtz-Zentrum Berlin for Materials and Energy, Institute Heterogeneous Material Systems, D-14109 Berlin, Germany d Nano Materials and Device Laboratory, Department of Applied Physics, Visvesvaraya National Institute of Technology, South Ambazari Road, Nagpur 440010 (M.S.), India b c

a r t i c l e

i n f o

Article history: Received 7 August 2012 Received in revised form 9 October 2012 Accepted 9 October 2012 Available online 22 October 2012 Keywords: Zinc oxide Simple solution chemistry Low temperature annealing Nanostructures Assorted morphologies

a b s t r a c t In the present investigation, we report the reproducible simple solution chemistry towards synthesis of ZnO nanostructure on fluorine doped tin oxide (FTO) coated glass substrate in aqueous medium at low temperature (>100 °C). The different preparative parameters such as deposition time, bath temperature, concentration of precursor solution, pH of the bath etc. were optimized to get fibrous nanoflakes, nanobeads, nanoparticles, cactus and highly crystalline 1D nanoneedles & hexagonal nanorods of ZnO. However, the as-deposited film consists of mixture of zinc hydroxide [Zn(OH)2] and ZnO. In the present work, to get pure ZnO phase, the as-deposited films were air annealed at relatively lower temperature (200 °C). The annealed ZnO films deposited by CBD and SILAR were characterized for structural, surface morphological, optical properties and surface wettability studies etc. The simplicity of the synthesis route coupled with the variety of ZnO nanostructures is manifest to be an important candidate for nanoscale devices. Ó 2012 Elsevier B.V. All rights reserved.

Contents 1. 2.

3. 4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Synthesis of ZnO film by soft chemical route . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Synthesis and reaction mechanism of chemical bath deposited ZnO on FTO coated glass substrate . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Synthesis of ZnO film by CBD on FTO coated glass substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3. Deposition and reaction mechanism of ZnO film by SILAR on FTO coated glass substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4. Synthesis of nanostructured ZnO film by CBD over seed ZnO layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.5. Synthesis of ZnO by CBD over seed ZnO layer using acetate bath . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.6. Synthesis of ZnO by CBD over seed ZnO layer using nitrate bath . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Structural characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Optical absorption and band gap measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Wettability test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Morphological studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. Tel.: +91 (712) 2801170; fax: +91 (712) 2223230. E-mail address: [email protected] (B.R. Sankapal). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.10.052

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1. Introduction Zinc oxide (ZnO) has probably the richest variety of nanostructures [1] and in materials science; it is a key technological material. The lack of a center of symmetry in wurtzite, combined with large electromechanical coupling, results in strong piezoelectric and pyroelectric properties. This semiconductor has several favorable properties: good transparency, high electron mobility, wide bandgap with high exciton binding energy, strong room-temperature luminescence, etc. ZnO has occupied an inevitable role among all the other metal oxides for many applications, due to its unique combination of interesting properties; such as nontoxicity, good electrical conductivity and low cost [2]. With the help of discovery of the carbon nanotubes [3], nanostructures and nanomaterials attracted great interest. ZnO is the next important material which possesses variety of nanostructures for plenty of applications [4]. Furthermore, ZnO can be effortlessly synthesized using low-cost chemical methods with wide surface architecture and with easily accessible raw materials for large scale production. To date, the anisotropy inherent in the ZnO wurtzite can also be tailored by various methods to various nanostructures, such as nanobelts by thermal evaporation of ZnO powder [5], hierarchical nanostructures [6], nanocages [7] using thermal evaporation, nanorings [8] by solid–vapor process, needle-like ZnO nanowhiskers have been grown directly from aqueous solution [9], nanotubes [10] on zinc foil by low-temperature solution route, nanowires [11], nanorods [12] using MOCVD reactor, nanobeads [13] by pyrolytic decomposition, nanotubes by atomic layer deposition [14], polydispersive ZnO aggregates using hydrolysis of zinc salt and film formation was done by drop-cast method [15], nanoflowers films were grown by a hydrothermal method [16] as well as by simple wet chemical method (CBD) [17], tetrapod-like nanopowders using metal vapor transport-oxidation method and film formation was done by doctor blade technique [18], nanocombs using noncatalytic thermal evaporation process [19], nanosheets by electrodeposition method [20], nanoforest using hydrothermal growth approach [21] and spherical and bundles of ZnO were prepared by the thermal decomposition [22]. All such morphologies possesses variety of practical applications which triggers a wide range of subsequent research on the synthesis of ZnO by various methods, including methods such as a thermal evaporation [23], chemical vapor deposition (CVD) [24], molecular beam epitaxy [25], magnetron sputtering [26], and pulsed laser deposition [27]. Most of the methods require high temperature, vacuum, complex process control and sophisticated expensive equipments which are unfavorable for an industrialized process. To reduce the cost of synthesis, solution methods have attracted increased interest and have been employed to grow nano- and micro-structured ZnO. It has been seen that, there are few reports on the growth of ZnO nanoforms by chemical method [28,29]. However, despite the great progress in the synthesis, the room temperature synthesis of ZnO nanoforms is still a remarkable challenge. The ZnO nanoforms provides large surface area; useful for surface related applications [30]. Chemical bath deposition (CBD), referred as solution deposition method, has particularly attractive because of its own advantages such as simplicity, easy controllability, and cost effectiveness etc. The CBD method is well suited for producing large area thin films. The reaction mechanism during CBD is explained in detail by Lokhande et al. [31]. The physiochemical processes involved in the CBD for growth of ZnO film were well summarized by Govender et al. [32]. Yan et al. found that the morphology of ZnO has a strong function of the reaction conditions which include the solvent, molar ratio of reagents, and reaction temperature with capping agent (ethylene glycol) [33]. Table 1 summarized the reported morphologies for ZnO synthesized by various deposition methods with different precursor solutions at different synthesis temperature.

In the present study, simple solution chemistry is employed for the synthesis of various ZnO nanoforms on fluorine doped tin oxide (FTO) coated glass substrate in aqueous medium at room temperature. Different preparative parameters such as deposition time, concentration of precursor solution, bath temperature, pH of the bath etc. were optimized to get fibrous nanoflakes, nanobeads, nanoparticles, cactus, nanoneedles and hexagonal nanorods of ZnO in thin film form. 2. Experimental section All the chemicals were purchased from Glaxo, Loba and s. d. fine Chem., from India, and were used as it is without further purification. The preparation of fluorine doped tin oxide (FTO) coated glass substrates were done by using spray pyrolysis technique. The FTO coated glass substrate were cleaned in dilute HCl for 5 s, and then ultrasonically cleaned with soap solution followed by rinsing with ethanol for 5 min. in ultrasonic bath and then finally rinsed with mili-Q-water. 2.1. Synthesis of ZnO film by soft chemical route Different precursors are available for synthesis of ZnO film using chemical route. Depending on the precursors, film parameters plays vital role and produces different morphologies. The ZnO film were grown directly over FTO (15 O/h) coated glass substrate (without seeding) by CBD method and/or also on seeded/compact layer of ZnO deposited by SILAR over FTO prior to CBD using different precursors at room temperature (RT = 27 °C) and at higher temperature (HT = 90 °C) as shown in Fig. 1. 2.1.1. Synthesis and reaction mechanism of chemical bath deposited ZnO on FTO coated glass substrate Synthesis of ZnO thin film was based on the immersion of the substrate in alkaline bath of complexed zinc salt at room temperature (27 °C). Precursor solution of zinc acetate dihydrate [Zn(CH3COO)22H2O], hexamethylene-tetaramine (HMTA) [(CH2)6N4] and 25% ammonia (NH3) as a complexing agent was used as a deposition bath. The bath was prepared by adding ammonia to the bath containing equimolar (0.3 M/L) aqueous solutions of Zn(CH3COO)22H2O and HMTA with a constant stirring which resulted into the formation of precipitate. This precipitate was dissolved by further addition of excess ammonia with the formation of clear zincate ½ZnðNH3Þ2þ 4  solution having resultant pH 12.0. The solution was stirred for few seconds and then transferred into another beaker containing cleaned FTO coated glass substrate (sheet resistance 15 O/h) placed horizontally at the bottom of the beaker. The bath was kept at room temperature (27 °C) for 40 h. After the deposition, the substrate coated with films was taken out, washed with mili-Q-water, and then dried in air. However, the as-deposited film consists of mixture of zinc hydroxide [Zn(OH)2] and ZnO [67,68]. Also, addition of ammonia in aqueous solution makes possible to co-deposit ZnO and Zn(OH)2 in a pH range between 7 and 12 [69]. In present case, the as-deposited films were annealed at 100 °C for 1 h in air for conversion of mixed phase to pure ZnO phase [70]. In chemical method, the small degree of supersaturation of the solution causes the heterogeneous nucleation of the metal oxide on the substrates [71]. Eq. (2) represents the decomposition of hexamine (HMTA) in water to form formaldehyde and ammonia. As cation (Zn+2) was complexed with ammonia when kept in open atmosphere, the free-cation concentration was gradually increased as the ammonia content decreased in the bath which is clarifies from Eq. (3). So, as time passes the concentration of ammonia decreased and solution becomes supersaturated i.e. equilibrium state was established in the reaction bath as shown in Eq. (5). This leads to the decrease in resultant pH. In the earliest stages of the reaction, when the ionic product starts to exceed the solubility product, the highly anisotropic zinc hydroxide and zinc oxide nuclei with small size particle are probably to be produced on the surface of substrate as well as in the solution. Such small–small nuclei assembled together on agglomeration to form nanoporous structures with average particle size of 20–50 nm. As the time passes, ionic product of reaction progressively increases; small size particles are collected on the surface of substrate and collectively grow to form uniform thin film by coalescence. Film formation occurs when high-surface-energy particles of Zn(OH)2 and ZnO (single nanocrystals or small aggregates) reach the substrate before they precipitate out in the form of large aggregates. Thus, while extensive precipitation suggests a hydroxide mechanism, since colloids from the solution stick to the substrate surface, the crystal size is not expected to change greatly with film thickness. The as-deposited film consist of Zn(OH)2 and ZnO mixed phases which were formed as a competitive process in the CBD bath at room temperature (Eq. (5)). For conversion of Zn(OH)2 to pure ZnO phase, it is necessary to anneal the film at higher temperature to improve the crystallinity of the film and the interfacial structures [72]. In the present investigation, the obtained as-deposited film was air annealed at 100 °C for 1 h to get the pure ZnO (Eq. (6)) and then used for further characterizations.

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P.K. Baviskar et al. / Journal of Alloys and Compounds 551 (2013) 233–242 Table 1 Summary of different morphologies, results and methods reported for ZnO with different precursors and temperature. Deposition Method

Precursor Solution

Synthesis Temperature

Resulting Morphology

Remark

Ref.

Hydrothermal

Zinc acetate dihydrate and Urea

90–180 °C

Ethylene glycol used as an organic capping reagent

[33]

Hydrothermal

Zinc nitrate hexahydrate, Sodium hydroxide and ethylenediamine Metallic zinc powder and Sodium hydroxide

180 °C

2-D petal-like ZnO nanostructure Nanorods

Mixture of deionized water/pure alcohol as the solvent

[34]

70 °C

ZnO nanowhisker clusters formed on Zn microspheres via oxidize metallic Zn powder in concentrated NaOH solution TiO2 films on Si wafer was prepared by dip-coating method Synthesis of ZnO seed by drop casting at 60 °C

[35]

Zinc nitrate hexahydrate and Hexamethylenetetramine Zinc nitrate hexahydrate, PVA and diethylenetriamine Zinc (II) oleate, oleic acid and noctadecene solvent

Urchin-like multidimensional ZnO nanowhisker Tubular ZnO

[38]

Zinc acetate dihydrate and Hexamethylenetetramine

90 °C

Dumbbell-like ZnO microcrystals

Zinc (II) oleate precursor was prepared by ion exchange reaction between Zn chloride hexahydrate and potassium oleate The paste of dumbbell-like ZnO microcrystals was coated on a ceramic tube

Zinc nitrate hexahydrate and Hexamethylenetetramine Zinc nitrate hexahydrate & Potassium Chloride Zinc acetate dihydrate

90 °C

1-D ZnO nanostructures were grown on Si wafers using pulse laser deposition ITO coated glass substrate

[40]

Microscope glass substrates

[42]

Zinc chloride and Sodium hydroxide

85 °C

Anionic surfactant sodium dodecyl sulfate was used

[43]

DC plasma reactor

Pure Zn powder

–

High-purity Zn metal

Room temperature 120 °C

Nanorods

Screen-printing of ZnO film by mixing tetrapod-like ZnO powder, Ethyl cellulose, and terpineol n-type Si with a coated thin layer of silicon dioxide of about 1 um thickness Zinc foil a substrate

[44]

Reactive RF sputtering Wet chemical

Nanorods and nanotube Nanospikes & Nanopillars Islands with different sizes Needle- and flowerlike ZnO microstructures Tetrapod-like ZnO nanopowders Nanostructure grains

Hexagonal faceted ZnO quantum dots

Lithium hydroxide ethanolic solution with addition of ethyl acetate or heptane

[47]

Interconnected flakes Vertically oriented ZnO nanowires Well-defined branched ZnO nanorods Flowerlike ZnO nanostructure Nanowire and nanosphere Hexagonal- and spherical-shaped ZnO nanostructures Nanorods and nanospines

Deposition time was 30–120 h and annealed at 400 °C

[17]

Use of ITO coated glass substrate

[48]

Zinc substrate was used for deposition

[49]

The as-prepared flowerlike ZnO nanostructures were directly coated on the outer surface of an alumina tube. Seed ZnO layer on the FTO glass was formed by thermal decomposition zinc acetate at 350 °C The morphology could be controlled by changing the composition of solutions

[50]

ZnO seed layer was deposited by SILAR using aqueous zinc-ammonia complex as cation precursor and deionized water kept at 85 °C as anionic precursor Seed layer of ZnO on FTO substrate by spin-coating

[53]

[54]

Anionic surfactant sodium dodecyl sulfate was used

[55]

Zinc foil as substrate

[56]

Electrodeposited ZnO seed layer at 95 °C

[57]

Mixture of water/ethylene glycol as the solvent

[58]

ZnO seed layer was deposited by RF-reactive magnetron sputtering at 100 °C Soda-lime glass and ITO-covered glass substrate Seeded ZnO were deposited on Si(1 0 0) substrates using

[59]

Wet chemical route

Hydrothermal Chemical solution route Pyrolysis

Facile solution method under mild conditions (Refluxing) Hydrothermal Electrodeposition Ultrasonic spray pyrolysis technique Simple aqueous solution route

Sol–gel route

Zinc nitrate hexahydrate and aqueous ammonia Zinc acetate dihydrate and Oleic acid

200 °C

95 °C 317 °C

70 °C 300 °C

40 °C under ultrasonic stirring Room temperature 85 °C

Dendrite-like ZnO nanostructures 2-D ZnO nanopellets

Soft chemical deposition route Electrochemical deposition Chemical bath deposition

Zinc sulfate and Sodium hydroxide

Simple chemical solution route Hydrothermal

Zinc nitrate hexahydrate and Sodium hydroxide Zinc nitrate hexahydrate and Hexamethylenetetramine Zinc nitrate hexahydrate, Hexamethylenetetramine, sodium hydroxide and absolute ethanol Zinc acetate dihydrate, ammonia and triethanolamine

50 °C

Zinc acetate dihydrate and Sodium hydroxide Zinc chloride and Sodium hydroxide

20–65 °C

Sonochemical assisted hydrothermal route Chemical etching using 0.1 M alkaline solution (KOH) Hydro/solvothermal

Zinc nitrate hexahydrate and Hexamethylenetetramine

85 °C

Zinc nitrate hexahydrate and Hexamethylenetetramine

85 °C

Zinc acetate dihydrate

130–150 °C

Hydrothermal

Zinc nitrate hexahydrate and Hexamethylenetetramine Zinc chloride Zinc nitrate hexahydrate and

100 °C

Nobullets and nanoflakes Nanorods

450–550 °C 90 °C

Hexagonal nanorods Vertical-aligned

Chemical route

Chemical bath deposition (CBD) Hydrothermal Simple solution route (Powder form)

Spray pyrolysis Hydrothermal

Zinc chloride and Potassium chloride Zinc nitrate hexahydrate and Hexamethylenetetramine

95 °C

92 °C 85 °C

80 °C

85 °C

Bunch-Shaped ZnO Nanowires Rod-, Needle-, Rugby-and Flowerlike ZnO Pin-cushion cactus, Nanopencil and Hexagonal nanodisc Nanotubes

[36] [37]

[39]

[41]

[45] [46]

[51] [52]

[60] [61]

(continued on next page)

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Table 1 (continued) Deposition Method

Precursor Solution

Hydrothermal

Hexamethylenetetramine Zinc nitrate hexahydrate and Hexamethylenetetramine

Chemical bath deposition (CBD) Chemical bath deposition (CBD) Plasma assisted reactive pulsed laser deposition Chemical bath deposition

Synthesis Temperature 90 °C

Resulting Morphology

Remark

Ref.

nanorod arrays Vertically wellaligned ZnO nanorods Vertically aligned ZnO nanorods Mesoporous ZnO

Diethylzinc as precursor by atomic layer deposition Seed ZnO layer was directly grown on the Au electrode of the quartz crystal microbalance via wet chemical route

[62]

The crystallite orientation was controlled by varying content of H2O2 in the bath solution Direct bad gap with energy 3.24 eV

[64]

[63]

Zinc nitrate hexahydrate, aqueous ammonia and hydrogen peroxide Zinc sulfate, Urea and dil. H2SO4

75 °C

A metallic zinc target (99.999% in purity)

80 °C

c-axis oriented nanocrystal

Polished n-type single crystalline Si (1 0 0) wafers

[65]

Zinc nitrate hexahydrate and aqueous ammonia

30–90 °C

2-D flakes and 1-D rods

Deposition on glass substrates and annealed at 400 °C for 2h

[66]

80 °C

Fig. 1. Schematic representation for the synthesis of ZnO film via various route using different precursors at different temperature.

The complete reaction mechanism for synthesis of ZnO film by CBD method at room temperature using acetate bath is as follows

ZnðC2 H3 O2 Þ2 ! Zn2þ þ 2CH3 COO ðCH2 Þ6 N4 þ 6H2 O ! 6HCHO þ 4NH3

ð1Þ ð2Þ

Zn2þ þ 4NH3 ! ½ZnðNH3 Þþ2 4

ð3Þ

CH3 COO þ H2 O ! CH3 COOH þ OH

ð4Þ

4OH þ 2Zn2þ ! ZnðOHÞ2 þ ZnO þ H2 O

ð5Þ

ZnðOHÞ2 þ ZnO ! ZnO

ð6Þ

2.1.2. Synthesis of ZnO film by CBD on FTO coated glass substrate For the synthesis of ZnO thin films; the complex solution was prepared in the chemical bath containing an equimolar (0.03 M/l: 10 times less than previously used) solution of zinc acetate dihydrate and HMTA in double distilled water (100 ml). Initially the precipitate was formed with slow addition of ammonia and then dissolved by further addition of excess ammonia (13 ml) with the formation of clear zincate solution having resultant pH 12. The solution was stirred for a few seconds and then transferred to the another beaker containing cleaned FTO coated glass substrate placed vertically in the beaker kept at room temperature (27 °C). After the deposition, the substrate coated with ZnO thin films were taken out with suitable time intervals (5–25 h), washed with double distilled water, dried in air. It is observed that the uniform growth was takes place after 20 h on the surface of the FTO as well as on the wall of the beaker. As-deposited films were air annealed at 200 °C for 1 h in air and used for further characterizations [73].

ammonium zincate complex ion {[Zn(NH3)4]2+} was prepared by mixing 0.02 M/l Zn(CH3COO)22H2O with NH3 and the pH value of the resultant solution was maintained in between 11 and 12. Seeded ZnO layer was deposited on FTO coated glass substrate as (1) immersion of the substrate in ammonium zincate {[Zn(NH3)4]2+} bath kept at 27 °C for complex adsorption; (2) rinsing the substrate in mili-Q-water kept at room temperature (27 °C) to remove the unabsorbed species; (3) immersion of the withdrawn substrates in hot water (90 °C) to form solid ZnO layer; (4) again rinsing with mili-Q-water and drying the substrate in air for few sec. before starting of the next deposition cycle. Surface modification of the seed layer was carried out by altering the preparation conditions. Numbers of immersion cycles and concentration of zinc precursor were optimized to get appropriate seed layer. After the deposition, the film was washed with mili-Q-water, and dried in air. ZnO films were annealed at 200 °C for 1 h to improve particle adhesion to the substrate and removal of hydroxide phase if any. Annealed seeded ZnO film was used for characterization and further deposition of nanoporous ZnO via CBD method. Zinc acetate was complexed with a basic solution of ammonia, a white precipitate of zinc hydroxide (Zn(OH)2) appears first, according to the reaction [74].

Zn2þ þ 2OH ! ZnðOHÞ2

ð7Þ

Further, addition of ammonia, Zn(OH)2 dissolves to form a soluble complex of zinc hydroxide [Zn(OH)4]2, which leads to the formation of zincate (Zn[(NH3)4]2+ ions:

Zn2þ þ 4NH3 ! Zn½ðNH3 Þ4 2þ

ð8Þ

Zn½ðNH3 Þ4 2þ þ 4OH $ ZnO2 2 þ 2H2 O þ 4NH3

ð9Þ

Therefore, the overall reaction leading to the formation of zincate ions is 2.1.3. Deposition and reaction mechanism of ZnO film by SILAR on FTO coated glass substrate In details, the ZnO seeded/compact layer was deposited by successive ionic layer adsorption reaction (SILAR) method using aqueous ammonium zincate complex as cationic precursor and hot mili-Q-water as anionic precursor. The aqueous

2þ

Zn

þ 4OH $ ZnO2 2 þ 2H2 O

ð10Þ

Further, zincate ions react with hot water to give ZnO.  ZnO2 2 þ H2 O $ ZnO þ 2OH

ð11Þ

237

P.K. Baviskar et al. / Journal of Alloys and Compounds 551 (2013) 233–242 It has also been suggested that when NH3 is used as a complexing agent, first the tetra ammine zinc ions react with hot water to deposit a solid film of Zn(OH)2 on the substrate. However, at water temperatures above 50 °C, the Zn(OH)2 film transforms to ZnO [75]. 2.1.4. Synthesis of nanostructured ZnO film by CBD over seed ZnO layer The nanostructured ZnO films were deposited using a chemical bath placed at room temperature under the normal environmental condition on modified FTO coated with seeded ZnO as described by the process in Section 2.1.3. The seeded ZnO coated substrates were immersed horizontally in an aqueous solution bath for specific time interval in order to deposit films of desired thicknesses. Zinccomplex containing a mixture of 0.2 M/l zinc acetate dihydrate and 0.02 M/l HMTA, with 25% ammonia as a complexing agent were used for synthesis of films. Initially the precipitate was formed with slow addition of ammonia and then dissolved by further addition of excess ammonia (10 ml) with the formation of clear zincate solution having resultant pH 12. After the deposition (25 h), as-deposited films were taken out from the bath, washed with mili-Q-water, and dried in air. The uniform film growth was takes place on the surface of the modified FTO with ZnO as well as on the wall of the chemical bath. As-deposited films were annealed at 200 °C for 1 h in air and used for further characterizations. The reaction mechanism for the growth of ZnO using CBD is similar to that of described in Section 2.1.1. 2.1.5. Synthesis of ZnO by CBD over seed ZnO layer using acetate bath ZnO films were synthesized using a chemical bath deposition. High-density ZnO nanorods were grown on seeded ZnO substrates. Formation of the seed layer on FTO was conducted by SILAR method using the similar procedure as describe in Section 2.1.3. ZnO nanorods were grown on the seeded layer using a chemical bath deposition method in an equimolar solution (0.025 M) of zinc acetate dihydrate and HMTA with resultant pH  8 at 90 °C for 2 h. To increase the length of nanorods the deposited film were introduce in fresh solution of similar concentration after every 2 h. Then, the substrate with deposit was rinsed with mili-Q-water, and dried in air. As-deposited ZnO nanorods were annealed further in air at 200 °C for 1 h to improve the crystallinity of the nanorods and the interfacial structures. For deposition of vertically aligned 1-D ZnO nanorods, the mechanism of ZnO film formation can be clarified as follows. Initially, the thermal decomposition of HMTA in water was takes place to form formaldehyde and ammonia (Eq. (12)). Zinc acetate was used as a source of Zn2+ ions with aq. ammonia as a complexing agent resulting the formation of zincate ([Zn(NH3)4]2+). At higher temperature decomposition of [Zn(NH3)4]2+ leading to release of Zn2+ and OH ions into solution and result in the formation of Zn(OH)2 and/or ZnO particles. Eqs. ()()()(13)–(15) illustrate the chemical reaction related to this process

ðCH2 Þ6 N4 þ 6H2 O ! 6HCHO þ 4NH3

ð12Þ

ZnðNO3 Þ2 þ 2NH4 OH ! ZnðOHÞ2 þ 2NH4 NO3

ð13Þ

ZnðOHÞ2 þ 2NH4 OH ! Zn½ðNH3 Þ4 2þ þ 4H2 O þ 2OH

ð14Þ

2+

The conversion of [Zn(NH3)4] into Zn(OH)2 and/or ZnO with the release of ammonia gas in the solution at higher temperature. This can be presented by the following reaction:

4. Characterizations 4.1. Structural characterization Fig. 2 shows the X-ray diffractogram (XRD) of ZnO films prepared by different routes. Pattern (a) for as-deposited film on FTO coated glass substrate prepared by CBD method at room temperature consist of mix phases (hydroxide and oxide). The asdeposited phase shows orthorhombic crystal structure [JCPDS card No. # 89-0138]. Pattern (b) supports the complete removal of Zn(OH)2 phase and its conversion to pure ZnO by the air annealing of the as-deposited films at just 100 °C. The annealed film shows pure ZnO phase with hexagonal (wurtzite) crystal structure [JCPDS card No. # 36-1451]. The mean values of a = 3.22 A, c = 5.20 A are in good accordance with reported values. The measured ‘‘c/a’’ ratio of 1.6149 showed good match with the value 1.6330 for ideally close packed hexagonal structure. The mean crystallite size of the ZnO film was calculated using well known Debye–Scherrer’s formula

D¼

kk b cos h

where, k = 1.5406 A and b is the full width at half maxima. The average crystallite size of annealed ZnO film is found to be 41 nm and hence the crystallite dimensions were confirmed to the nanoscale. It should be noted that there is a correlation between the crystallite size along one direction and preferential orientation along that direction. It is seen that crystallites grow much faster in (1 0 1) plane compared to the other planes. The growth velocity in the (1 0 0) and (0 0 2) planes should be slightly slower with the increase in liquid pressure in deeper regions due to gravity. Relatively, the growth velocity along (1 0 1) plane become a bit faster [76]. Pattern (c) is for ZnO film grown by using CBD for 20 h at room temperature on FTO substrate followed by air annealing at 200 °C. All the diffraction

3. Instrumentation Crystal structure and crystallite size of the annealed ZnO films were analyzed by using an X-ray Diffractometer (D8 AdvanceBruker axs) with CuKa1 radiations (k = 1.5406 Å) in 2h range from 20° to 80°. The morphology and dimension of ZnO nanoparticles

20

o FTO

o

30

(103)

(101)

As-deposited Flower Nanobeads Nanoparticles Cactus Nanoneedles

40

50

2θ (degree)

o

o (112) (201)

(110)

o (102)

o

(100)

2.1.6. Synthesis of ZnO by CBD over seed ZnO layer using nitrate bath Hierarchical ZnO structure was synthesized via a two-step route on FTO coated glass substrate. In the first step, the FTO substrate was successively immersed in the ammonium zincate solution kept at room temperature and in hot water near boiling temperature. The ammonium zincate bath was prepared by mixing 0.02 M/l zinc nitrate hexahydrate in water and aq. ammonia with resultant pH  11 and 12. The as-deposited seeded layer of ZnO by SILAR on FTO was annealed at 200 °C to remove the hydroxide phase and improve the crystallinity of the layer. The way of preparation of seeded ZnO layer with nitrate precursor is similar as describe Section 2.1.3. In the second step, the seeded substrate was incubated at 90 °C in a equimolar solution (0.025 M/l) of Zn(NO3)2 and HMTA with resultant pH  8 for 2 h. To increase the length of nanorods, the deposited film was introduced in fresh solution of similar concentration after every 2 h. Then, the substrate with deposit was rinsed with mili-Q-water, and dried in air. As-grown ZnO films were annealed further in air at 200 °C for 1 h to improve the crystallinity of the nanorods and the interfacial structures. The mechanism for ZnO film formation is similar to that of describe in Section 2.1.5 except the source of Zn2+ ions. In this case nitrate source was used.

(002)

ð15Þ

Intensity (counts)

½ZnðNH3 Þ4 2þ þ 2OH ! ZnðOHÞ2 þ ZnO þ H2 O þ 4NH3

were characterized by using a LEO 1530, Gemini scanning electron microscope (SEM) unit operated at 5 kV. Wettability test was done by using the contact angle meter (Rame-hart, Model 500-F1, USA) at room temperature. The optical absorbance was performed with a UV–VIS (ParkinElmer, L950) spectrophotometer.

60

70

(f) (e) (d) (c) (b) (a)

80

Fig. 2. The X-ray diffraction pattern for (a) as-deposited, (b) annealed (100 °C) and (c) annealed (200 °C) ZnO film synthesized by only CBD at room temperature on FTO substrate, (d) XRD pattern of annealed (200 °C) ZnO film on FTO substrate deposited by SILAR method. Pattern (e) is of ZnO film deposited by CBD using acetate bath at room temperature on seeded ZnO by SILAR and (f) XRD pattern of annealed ZnO film deposited at high temperature (90 °C) by CBD using acetate bath.

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4

Nanoparticles

3 Absorbance (αt)

2

Cactus Nanoneedles

3

2

9

Nanobeads

(α hν) x 10 (eV-cm )

As-deposited

-1 2

4

1

0 1.5

2.0

2.5

3.0

3.5

4.0

Energy (eV)

2

1

0 300

400

500 600 Wavelength (nm)

700

Fig. 3. The variation of optical absorbance (at) with wavelength (k) for as-deposited (solid circles) & annealed (200 °C) film (hollow squares) synthesized by only CBD method at room temperature, absorption spectra for annealed ZnO film deposited by SILAR on FTO substrate (hollow circles). The hollow stars spectra is for annealed ZnO film deposited using CBD at room temperature on seeded ZnO by SILAR and ZnO film deposited at high temperature (90 °C) by CBD using acetate bath (hollow triangles). Inset shows the plot of (aht)2 vs ht for as-deposited and annealed ZnO films.

peaks can be indexed to the pure ZnO phase showing hexagonal (wurtzite) crystal structure [JCPDS card No. # 36-1451]. The XRD pattern of optimized annealed seeded ZnO film deposited by SILAR is depicted in Fig. 2(d). Strong orientation along (0 0 2) plane is observed for seeded ZnO films which shows growth of ZnO perpendicular to the substrate surface. The crystallite size of the annealed ZnO film was calculated along (0 0 2) orientation and is found to be 40 nm. The X-ray diffraction pattern (e) is for nanoporous ZnO deposited by CBD over seeded ZnO layer by SILAR, sharp peaks are observed along (1 0 0), (0 0 2) and (1 0 1) planes which are in agreement with the typical wurtzite structure of ZnO (JCPDS card No. # 36-1451). The crystallite size for annealed (200 °C) ZnO film deposited using CBD was calculated along (0 0 2) orientation and is found to be 62 nm. The X-ray diffraction (XRD) pattern of annealed ZnO film deposited by chemical bath deposition method using acetate bath at 90 °C on seeded ZnO is shown in Fig. 2(f). The as-deposited seeded ZnO films was annealed at 200 °C in order to avoid any hydroxide contain in it and to improve the crystallinity of the ZnO nanorods. Strong orientation is observed along (0 0 2) plane for ZnO films grown using acetate bath which shows growth perpendicular to the substrate surface and indicates the nanorods are highly c-axis oriented. 4.2. Optical absorption and band gap measurement The absorption spectra of the annealed ZnO films were studied at room temperature without taking into considerations of reflection and transmission losses. Annealing in air will oxidize any hydroxide to oxide and this may explain the effect of annealing on optical absorption studies [77]. The spectrum in Fig. 3 shows that annealed ZnO film (hollow squares) has low absorbance in the visible region of the solar spectrum than that of as-deposited film (solid circles). The absorption data were analyzed and the band gap was estimated using the Tuac’s relationship between the absorption coefficient (a) and the photon energy (hm) [78].

Fig. 4. The water contact angle measurement for annealed ZnO films deposited by using: (a) CBD only followed by annealing at 100 °C, (b) CBD only but, annealed at 200 °C, (c) SILAR on FTO substrate, (d) combination of SILAR and CBD at room temperature, combination of SILAR and CBD at high temperature (90 °C) using – (e) acetate bath and (f) nitrate bath.

a¼

a0 ðhm  Eg Þn hm

where, a0, h and m are the constant, ‘Eg’ is the optical band gap of the material. The linear variation of (ahm)2 versus hm (inset) at the absorption edge, confirmed the semiconducting behavior of the film with direct band gap. Extrapolating the straight-line portion of the plot (ahm)2 versus hm for zero absorption coefficient value gives the optical band gap (Eg). The ‘Eg’ value of annealed ZnO thin film is 3.28 eV and that for as-deposited film is 3.82 eV (inset). The shifting of band gap towards higher energy value confirm the conversion of Zn(OH)2 to pure ZnO. The spectrum with hollow circles shows that an annealed ZnO film prepared by SILAR method has low absorbance in the visible region of the solar spectrum. The absorption data were analyzed and the band gap was estimated. The linear variation of (ahm)2 versus hm (inset) at the absorption edge, confirmed the semiconducting behavior of the film with direct band gap (Eg) value of 3.25 eV which is formally reported for highly resistive ZnO polycrystalline films [79]. The absorption spectrum for annealed ZnO film synthesized by combination of SILAR and CBD method (hollow stars) has low absorption coefficient value (below 0.5) in the visible region of the solar spectrum. The value of band gap was determined using plot showing linear variation of (ahm)2 versus energy (hm) and found to be 3.22 eV (inset) [80]. Optical absorption spectra for nanorods ZnO film prepared by CBD using acetate bath at high temperature (90 °C) is shown in Fig. 3 (hollow triangles). Annealed ZnO nanorods film has absorbance below 2 in the visible

P.K. Baviskar et al. / Journal of Alloys and Compounds 551 (2013) 233–242

239

Fig. 5. (a) The surface morphology of ZnO film on FTO substrate annealed at 100 °C, (b) at higher high resolution showing flower like morphology and (c) the cross-section of the same film. (d) The surface morphology of ZnO film annealed at 200 °C, (e) the agglomerated ZnO nano particles at high resolution and (f) the cross-section of the same film. (g) Surface morphology from top view, (h) at higher magnification and (i) cross-section view of annealed ZnO film deposited by SILAR. (j) SEM micrograph of cactus ZnO film deposited by room temperature CBD (k) at higher magnification and (l) cross-sectional view. (m) The surface morphology of needle like ZnO film deposited by high temperature (90 °C) CBD using acetate bath on FTO substrate annealed at 200 °C, (n) at higher high magnification and (o) the cross-section of the same film. Inset shows the high-resolution TEM images of the nanoneedle. (p) The surface morphology of hexagonal ZnO nanorods film deposited by high temperature (90 °C) CBD using nitrate bath on FTO substrate annealed at 200 °C, (q) image at higher high magnification and (r) the cross-sectional view of the same film.

region of the solar spectrum with direct band gap 3.2 eV (inset) which is in good agreement with reported value [80].

4.3. Wettability test Wettability involves the interaction between liquid and solid in contact. The wetting behavior is characterized by the value of contact angle, a microscopic parameter. The contact angle is an important parameter in surface science and its measurement provides a simple and reliable technique for the interpretation of surface energies and well describe by Lokhende et al. [81]. Both superhydrophilic and super-hydrophobic surfaces are important for practical applications [82]. The annealed ZnO films deposited by only CBD shows water contact angles of 37° and 54° for nanoflakes and nanobeads, respectively (Fig. 4a and b), expressing hydrophilic nature of the surface. The annealed ZnO film deposited by SILAR showed water contact angle of 132°. The water drop forms droplet on the surface of ZnO film, as seen in the Fig. 4c. Since the morphology is dense and compact the water lies on the top of surface showing hydrophobic nature of surface. Chemical bath deposited

nanoporous ZnO film over seeded ZnO layer showed water contact angle of 66° (Fig. 4d). The water drop lies flat on the surface of ZnO films in sheets instead of forming droplet, as seen in the figure. Since the morphology is porous (pores of the size of few microns) the water goes into the pores and craves making contact angle hydrophilic. The hydrophilic nature of the film has been attributed to the fibrous morphology of the films [83]. The annealed ZnO films deposited by CBD at HT (90 °C) using acetate and nitrate bath shows a semi-spherical water droplet with a water contact angle of 85° and 87°, respectively. Fig. 4e and f indicates that the surface of ZnO nanorods are partially wets the solid.

4.4. Morphological studies Fig. 5(a) shows surface morphology of the annealed ZnO film. The morphology consists of flower like structure. Also small crystallites have observed at the bottom of the flowers with sizes ranges in a few microns, it indicates that initially rod like structure was grown but when the optimum deposition time was exceeded, further nucleation was observed on top of nanorods resulted into

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Fig. 5. (continued)

secondary nucleation. This secondary growth was randomly oriented which then leads to the formation of flower like structure [84]. Furthermore, the flower like structure consists of petals and the petals are made up of fibrous network with inter connected nanoparticles and some fibers are coming out, which can offers large surface area as shown in Fig. 5(b). The growth seems to be aggregates of small nanoparticles growing in two dimensional surfaces. Similar type of morphology has been reported by Wang et al. using high temperature chemical method [29]. The average thickness of annealed ZnO film is about 40 lm; seen from the cross-sectional SEM (Fig. 5(c)). In this case, the deposition parameters and rate of reactions were controlled by complexing agent (ammonia) in the bath and the reaction was carried out for 20 h instead of 40 h. This leads to the formation of small size nanoparticles (3–5 nm) at the FTO interface and then turns to bigger size particles (nanobeads, 20– 30 nm) due to agglomeration of smaller once and limits flower like morphology which was due to secondary growth as reported in first case. Fig. 5(d) shows surface morphology of the annealed ZnO thin film. The surface morphology consists of porous structure having nanobeads. Furthermore, the interconnected nanobeads consisting of small size particle resulted in agglomeration to form bigger ones. This can offer large surface area as shown in magnified

SEM image (5(e)). Fig. 5(f) shows the cross-section of nanobeads ZnO thin film showing thickness of 2 lm deposited over FTO substrate. Fig. 5(g and h) shows surface morphology of seeded ZnO nanoparticles uniform and densely grown on the FTO coated glass substrate using a simple SILAR method. Seeded ZnO with a slight vertical off-alignment were grown perpendicularly to the substrate surface. Fig. 5(i) shows the thickness of seeded ZnO film which is about 4 lm measured from SEM cross-section view. The scanning electron micrograph images of annealed ZnO thin film on modified FTO coated glass (seeded) substrate using room temperature CBD. The SEM micrograph of ZnO film showed Fig. 5(j) fibrous surface morphology with interconnected flakes constitutes thin solid film and 5(k) image at higher resolution. It is believed that this fibrous network has been developed on the small crystallites. At higher magnification the film demonstrates the formation of small particles with average grain size of 40–60 nm leading to ‘cactus’ like morphology. From the micrograph, it is easily seen that flakes-like structure with some fibrous coming out, just like leaf with small fibrous inside, which offers large surface area. The thickness of nanoporous ZnO was found to be ca. 24 lm measured from cross-sectional view (Fig. 5(l)). Surface morphology as Fig. 5(m) top-view, 5(n) at higher magnification and 5(o) the cross-sectional view of the aligned ZnO

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P.K. Baviskar et al. / Journal of Alloys and Compounds 551 (2013) 233–242 Table 2 Deposition conditions, annealing temperature, crystallite size, contact angle, band gap and thickness with resultant morphologies. Deposition conditions Method

Bath

Temperature (°C)

pH

CBD CBD SILAR SILAR + CBD SILAR + CBD SILAR + CBD

Acetate Acetate Acetate Acetate Acetate Nitrate

27 27 27 27 90 90

12 12 11–12 12 8 8

Annealing temperature (°C)

JCPDS card No.

Crystallite size (nm)

Contact angle (°)

Band gap (eV)

Thickness from SEM cross-section (lm)

Resulting morphology

100 200 200 200 200 200

36–1451 36–1451 05–0664 36–1451 05–0664 05–0664

41.20 35.25 40.22 61.77 87.90 82.96

37 54 132 66 85 87

3.26 3.28 3.25 3.22 3.2 3.2

40 2 4 24 6 5

Nanoflakes Nanobeads Nanoparticles Cactus Nanoneedles Hexagons

nanorods synthesized using chemical bath deposition method on seeded ZnO layer in acetate bath at 90 °C is shown. It can be seen that the nanorods are homogeneously grown on seeded ZnO substrate forming needle like structure with average diameter of 150 nm and length of several micron (6 lm). Inset shows the high-resolution TEM (HRTEM) image taken from the edge of nanoneedle. Fig. 5(p) shows surface morphology from top view, 5(q) at higher magnification and 5(r) cross-sectional SEM images of ZnO nanorods grown on seeded layer in nitrate bath at 90 °C. The SEM micrograph of annealed ZnO shows the hexagonal crystals facets of about 300–400 nm in diameter as shown in higher magnification image and about 5 lm in length calculate from cross-sectional view. The deposition conditions, annealing temperature, crystallite size, contact angle, band gap and thickness with resultant morphologies are summarized in Table 2.

5. Conclusions Simple solution chemistry was employed successfully to deposit ZnO nanostructures on FTO coated glass substrate in aqueous medium at low temperature (>100 °C) with controllable morphology. It is found that the morphology of nanostructure ZnO has a strong dependence on the reaction kinetics to get fibrous nanoflakes, nanobeads, nanoparticles, cactus and highly crystalline 1D nanoneedles & hexagonal nanorods. The presented chemical routes are simple and inexpensive to get variety of ZnO nanostructures with annealing at relatively lower temperature (200 °C) for diverse applications.

Acknowledgement BRS is thankful to DST Projects (SR/FTP/PS-03, 2006) & (Sr/S2/ CMP-0026/2010), PKB is thankful to DAAD Foundation, Germany for DAAD Fellowship & CSIR, New Delhi for SRF (09/728 (0025)/ 2010-EMR-I) and PRN is thankful to DST for WOS-A project (SR/ WOS-A/PS 16/2011).

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