The effect of complexing agent on the crystallization of ZnO

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C for 2 h. The result- ing product was a black–purple porous gel similar to aerogel which is called compound. (C) hereafter. 2.2 Synthesis of ZnO nanopowders ...
PRAMANA

c Indian Academy of Sciences 

— journal of physics

Vol. 77, No. 4 October 2011 pp. 679–688

The effect of complexing agent on the crystallization of ZnO nanoparticles S A KETABI∗ , A S KAZEMI and M M BAGHERI-MOHAGHEGHI School of Physics and Center for Solid State Physics Research, Damghan University, Damghan, Iran ∗ Corresponding author. E-mail: [email protected] MS received 12 October 2010; revised 10 March 2011; accepted 21 March 2011 Abstract. In this work, some structural and optical properties of the zinc oxide (ZnO) nanoparticles were studied. The highly crystalline ZnO nanoparticles were produced by the hydrothermal and sol– gel methods. The analyses of the XRD patterns, STEM images and UV spectroscopy showed that the size of the nanoparticles prepared by oxalic acid was smaller than the ones by urea. The properties of oxalic acid and urea were also investigated to determine the most effective crystallization process of ZnO nanoparticles. It has been shown that pH, decomposition temperature and activity coefficient of the complexing agent have certain effects on crystallization process. Keywords. ZnO nanoparticles; hydrothermal method; sol–gel method. PACS Nos 61.46.Df; 81.07.Bc; 81.20.Fw

1. Introduction ZnO is a unique material that exhibits semiconducting, piezoelectric and pyroelectric properties [1]. As a wide-band gap semiconductor (E g = 3.37 eV), it holds a great deal of promise for developing technological devices [2]. Various chemical, electrochemical and physical deposition methods have been used to prepare different ZnO nanostructures [3– 18]. These methods are generally carried out in water or alcohols using zinc salts as starting materials in the presence of basic or surfactant solutions, and may lead to nanomaterials of controlled morphology. In this work, ZnO nanoparticles were synthesized from zinc acetate reaction, using oxalic acid and urea as two different complex agents and then their characterizations following further stages (refluxing and annealing) were investigated. The procedures for synthesizing ZnO nanoparticles by hydrothermal and sol–gel methods are described in §2. The effect of each complexing agent on the size of the produced nanoparticles, the effect of decrease in size of the semiconducted samples on the optical direct band gap values and also the exhibition of blue shift in the band gap of the yielded nanoparticles compared with the band gap of bulk ZnO are investigated in §3, followed by the conclusions in §4. DOI: 10.1007/s12043-011-0135-y; ePublication: 19 August 2011

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S A Ketabi, A S Kazemi and M M Bagheri-Mohagheghi 2. Experimental procedure 2.1 Synthesis of ZnO nanopowders by sol–gel method A sol of 0.15 M zinc acetate and 100 ml double distilled water was prepared. Then, urea and ethylene glycol were subsequently added to this solution, and the resulting mixture was stirred and dissolved at room temperature for half an hour until a completely clear solution was obtained. This solution was refluxed in oil bath at T = 110◦ C for 6 h. During refluxing, the solution was turned into a metal–urea homogeneous complex with the colour changing from white to light yellow. After cooling down, to achieve the required chemical reactions for the polymerization and evaporation of the solvent, the sol was slowly heated at T = 90◦ C for 15 h in an open bath until an orange wet gel was attained. During continuous heating at this temperature, the urea, ethylene glycol and complexes were polymerized and finally the sol became more viscous as a wet gel. In the final step of the sol–gel process, the wet gel was fully dried by direct heating on a hot plate at T = 140◦ C for 2 h. The resulting product was a black–purple porous gel similar to aerogel which is called compound (C) hereafter.

2.2 Synthesis of ZnO nanopowders by hydrothermal method The slow addition of 0.15 mol/l (M) oxalic acid to the 0.1 M solution of zinc acetate along with the double distilled water was continued by magnetic stirring at T = 75◦ C for 45 min. Then, the solution was refluxed in an oil bath at T = 120◦ C for 6 h. The refluxed solution was then kept on hot plate at T = 80◦ C for 14 h and finally heated at T = 140◦ C for about an hour until it was fully dried. The resulting material is called compound (D) hereafter.

2.3 Further annealing The samples of compound (C) were annealed at 350, 400 and 450◦ C and those of compound (D) at 300, 350 and 400◦ C for an hour. The temperatures and the time for annealing were fixed because of our pre-experiences in synthesis procedures. ZnO phases were formed 50◦ C earlier in compound (D). At the temperature step of T = 50◦ C the characteristics of both compounds demonstrated certain differences.

2.4 Characterization Scanning transmission electron microscopy (STEM) micrographs of the prepared ZnO nanoparticles were taken by STEM (model: LM-10 C, Zeiss Germany). X-ray diffraction (XRD) patterns of ZnO nanoparticles prepared at various annealing temperatures were recorded by D8 Advanced Bruker system using CuKα (λ = 0.154056 nm) radiation to analyse the phase of the nanoparticles. The optical absorption measurements of nanoparticles were recorded using a UV–vis spectrophotometer (model: GBC 916) for calculating optical band gap values. 680

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Crystallization of ZnO nanoparticles 3. Results and discussion 3.1 XRD analysis XRD patterns (figures 1 and 2) confirm the hexagonal crystal structure of ZnO where a = b = 3242 Å; c = 5176 Å for compound (C) and a = b = 3242 Å; c = 5176 Å for compound (D), with strong reflections from (1 0 1), (1 0 0) and (0 0 2) planes. The formation of nanocrystalline ZnO is reflected through the broadening of the corresponding XRD 500

(a) T=350 oC

0 500

Intensity (a.u.)

(b) T=400 oC

(101) (100)

(002)

0 (101)

500

(c) T=450 oC (100) (002)

30

32

34 36 2θ (deg.)

38

0 40

Figure 1. XRD patterns of compound (C) at different temperatures.

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S A Ketabi, A S Kazemi and M M Bagheri-Mohagheghi 500

(a) T=300 oC

(101) (100)

(002)

0 500

(b) T=350 oC

Intensity (a.u.)

(101) (100) (002)

0 500

(101)

(c) T=400 oC

(100) (002)

30

32

34 36 2θ (deg.)

38

0 40

Figure 2. XRD patterns of compound (D) at different temperatures.

characteristic lines. There is no effect of annealing on the crystalline phase of ZnO as the XRD characteristic lines show. However, annealing has improved the crystallinity of ZnO nanoparticles which is in agreement with the literature [19]. Crystallization reduces the band gap of semiconductors and we expect an increase in the size of nanoparticles in higher annealing temperatures. By recording the full-width at half-maxima β (FWHM) of XRD peaks, the average size of nanocrystallites has been determined (table 1) using Debye– Scherrer equation (D = (kλ/β) cos θ , k = 0.99) [20] corresponding to (1 0 0), (0 0 2) and (1 0 1) planes for both compounds. 682

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Crystallization of ZnO nanoparticles Table 1 Full-width at half-maxima β (FWHM) of XRD peaks and the average size of the nanocrystallites determined by Debye–Scherrer equation corresponding to (1 0 0), (0 0 2) and (1 0 1) planes for compounds (C) and (D). Temperature (◦ C)

hkl

FWHM (rad.)

Crystallite size (nm)

Mean (nm)

Compound (C) 350

100 002 101

1.54 1.419 1.402

5.9 6.52 6.63

6.35

400

100 002 101

1.159 1.169 1.112

7.52 7.91 8.36

7.93

450

100 002 101

0.523 0.524 0.503

17.37 17.45 18.28

300

100 002 101

0.025 0.022 0.024

6.1 7.11 6.63

350

100 002 101

0.0028 0.003 0.0025

56.17 52.98 62.45

57.2

400

100 002 101

0.002 0.0021 0.0018

78.5 75.32 87.95

80.59

17.7

Compound (D) 6.61

3.2 STEM results At 400◦ C, particles of compound (C) are formed (figure 3a); they are small and close to each other. ZnO nanoparticles at 450◦ C (figure 3b) are larger and still close to each other but less overlapped. Figure 3c shows crystallites of compound (D) at 300◦ C close to one another, some already agglomerated and some still individual. As the temperature rises to 350◦ C, large non-uniform particles can be seen in figure 3d.

3.3 UV–vis spectroscopy and band gap determination Absorption spectroscopy is a powerful non-destructive technique for exploring the optical properties of semiconducting nanoparticles [21]. The absorption spectra of ZnO/ethanol solution in the UV and visible range are presented in figure 4. A blank solution of ethanol is taken as reference [22]. λ1/2 (wavelength at which the absorption is 50% of that at the excitonic peak [23]) in figure 4 moves toward higher wavelengths by increasing the annealing temperature. There is a significant blue shift in the excitonic absorption, for all nanoparticles compared to that of the bulk ZnO as the excitonic absorption for bulk ZnO is at 373 nm [24]. Pramana – J. Phys., Vol. 77, No. 4, October 2011

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(a)

(b)

(c)

(d) 400◦ C

Figure 3. STEM images of the annealed ZnO nanoparticles at: (a) and (b) 450◦ C related to compound (C); (c) 300◦ C and (d) 350◦ C, related to compound (D). As seen, the size and the shape of nanoparticles are slightly better controlled in compound (C).

A practical route selected among various methods of band gap E g determination [25,26] is to equate E g with λ1/2 through the equation E g = 1240/λ1/2 . The band gap values for all nanoparticles have been calculated. To achieve the size of nanoparticles by UV–vis analysis, we have used a convenient method described by Meulenkamp [26] which involves equating diameter D (Å) with λ1/2 (Å): 1240 B C (1) = A+ 2 − , λ1/2 D D where A, B and C are constants whose values are A = 3.301, B = 294 and C = −1.09 for ZnO nanoparticles. Our results on particle size and band gap energy are compared with the results of other investigators [25–29], in figure 5. Good conformity between our results and theirs is depicted. 3.4 Crystallization and chemical properties of complex agents The melting point of dihydrate (2H2 O) oxalic acid is 101–102◦ C, i.e. it loses its water of crystallization at about this temperature, and begins to sublime at about 150–160◦ C. On 684

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Crystallization of ZnO nanoparticles

Absorbance (a.u.)

o (C); 400 C o (C); 450 C o (D); 300 C o (D); 350 C

λ 1/2

280

300

320

340

360

380

Wavelength (nm) Figure 4. UV–vis absorbance spectra of ZnO nanoparticles at the various annealing temperatures. λ1/2 moves towards higher wavelengths as the temperature increases.

heating to a still higher temperature it partially starts to decompose [30]. Urea decomposes on heating above the melting point, crystallizes in long needles or prisms which melt at 132.7–135◦ C and sublime when heated in vacuo. Zinc acetate dihydrate (2H2 O) loses water at 100◦ C and starts to decompose at 237◦ C. Ethylene glycol, the material having contributed in both compounds, melts at −13◦ C and boils at 197.3◦ C. Briefly, the materials

4.0

.

. .

.

.

3.9

.

Band Gap (eV)

.

.

. .

3.8

. .

.

3.7

(D) (C) [25] [26] [27] [28] [29]

. .

.

3.6

. .

3.5

.

.

. . .

.

3.4

.

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

.

6.0

Particle Diameter (nm) Figure 5. Size-dependence of the optical band gap, defined as λ1/2 (a comparison between our work and other references). Both compounds are in good agreement with the references.

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S A Ketabi, A S Kazemi and M M Bagheri-Mohagheghi in compound (D) start to decompose at 132–237◦ C, while the start of decomposition for compound (C) is at 150–237◦ C, which means that compound (D) can form new chemical bonds at temperature at least 18◦ C less than compound (C). Furthermore, Zn oxidation with air is faster for Zn in compound (D) at further stages. Faster crystallization and less control on the growth process of nanoparticles is therefore expected in compound (D). Compound (C) was prepared at a pH of 2.63 and compound (D) at pH 1.3. Lepkov’a et al [31] found that higher pH value of the aqueous phase ends with lower sized nanoparticles. ZnO nanorods of different dimensions were synthesized through appropriate control of the pH by Baruaha and Dutta [32]. According to the literature, we expected finer ZnO nanocrystallites for compound (C) with higher pH. This is in agreement with TEM results as a matter of crystallite distribution but not as a matter of size. Öncül et al [33] have found that the supersaturation ratio, containing the activity coefficient in its definition, is the driving force for the crystallization process and even relatively small variations of this coefficient can considerably affect the results, especially for fast reactions. Here, the specific ion interaction model (SIT) is applied to determine the activity values [34]. Also the activity coefficient vs. temperature has been depicted in figure 6. As we see in this figure, the mean activity coefficient of H2 C2 O4 has lower value at room temperature (the start of the synthesis) and as the temperature is increased, it decreases. The mean activity coefficient of urea is unity because its ionic strength is zero and remains constant when the temperature increases. It is found that the activity coefficient is more important among the effective properties on the crystallization.

1.0

Mean Activity Coefficient

0.9

Urea HOOCCOOH

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 25

50

75

100 125 150 175 200 225 250 275 300

Temperature (oC) Figure 6. The variation of the mean activity coefficient with temperature for oxalic acid and urea. The mean activity coefficient of urea is unity, while that of oxalic acid is about 0.5 and decreases with increase in temperature.

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Crystallization of ZnO nanoparticles 4. Summary and conclusion The ZnO nanoparticles were synthesized successfully by hydrothermal and sol–gel methods accompanied by the complexing agents, oxalic acid and urea. The analyses of XRD graphs, TEM images and UV–vis analysis show that use of a certain complexing agent with a similar initial material to synthesize the ZnO nanoparticles have certain impacts on the crystallization of these nanoparticles. The optical direct band gap values slowly changed from 4 eV to 3.6 eV and 3.45 eV in oxalic acid and urea as complexing agents, respectively. Higher pH, higher decomposition temperature and lower activity coefficient relate to oxalic acid. Comparing the two complexing agents, one may conclude that the oxalic acid reduces the particle size more efficiently than urea. So the blue shift is higher at the second annealing temperatures and, as far as the applications are concerned, the sensing properties of the nanoparticles (e.g. in gas sensing) are more improved when oxalic acid is used. References [1] Z L Wang, Mater. Res. Bull. 32, 109 (2007) [2] S J Pearton, D P Norton, K Ip, Y W Heo and T Steiner, Prog. Mater. Sci. 50, 293 (2005) [3] F Grasset, O Lavastre, C Baudet, T Sasaki and H Haneda, J. Colloid Interface Sci. 317, 493 (2008) [4] X Ren, D Han, D Chen and F Tang, Mater. Res. Bull. 42, 807 (2007) [5] Y H Ni, X W Wei, J M Hong and Y Ye, Mater. Sci. Eng. B, Solid-State Mater. Adv. Technol. 121, 42 (2005) [6] N Scarisoreanu, D G Matei, G Dinescu, G Epurescu, C Ghica, L C Nistor and M Dinescu, Appl. Surf. Sci. 247, 518 (2005) [7] M Ristiæ, S Musiæ, M Ivanda and S Popoviæ, J. Alloys Compounds 397, L1 (2005) [8] S Chang and A Sakai, Mater. Lett. 53, 432 (2002) [9] T Liu, O Sakurai, N Mizutani and M Kato, J. Mater. Sci. 21, 3698 (1986) [10] B Chiou, Y J Tsai and J Duh, J. Mater. Sci. Lett. 7, 785 (1988) [11] J-J Wu and S-C Liu, Adv. Mater. 14, 215 (2002) [12] Y F Chen, D M Bagnall, H J Koh, K T Park, K Hiraga, Z Q Zhu and T Yao, J. Appl. Phys. 84, 3912 (1998) [13] D G Lamas, G E Lascalea and N E Walsoc, J. Eur. Ceram. Soc. 18, 1217 (1998) [14] S Badhuri and S B Badhuri, Nanostruct. Mater. 8, 755 (1997) [15] R E Juarez and D G James, J. Eur. Ceram. Soc. 20, 133 (2000) [16] P R Patil and S S Joshi, Mater. Chem. Phys. 105, 354 (2007) [17] B Baruwati, D K Kumar and S V Manorama, Sensors and Actuators B119, 676 (2006) [18] I V Markevich and V I Kushnirenko, Solid State Commun. 144, 236 (2007) [19] N Goswami and D K Sharma, Physica E42, 1675 (2010) [20] M M Bagheri-Mohagheghi, N Shahtahmasebi, M R Alinejad and A Youssefi, Physica B: Cond. Mater. 403, 2431 (2008) [21] J Han, H Su, C Zhang, Q Dong, W Zhang and D Zhang, Nanotechnol. 19, 365602 (2008) [22] M L Singla, M Shafeeq and M Kumar, J. Lumin. 129, 434 (2009) [23] Z L S Seow, A S W Wong, V Thavasi, R Jose, S Ramakrishna and G W Ho, Nanotechnol. 20, 045604 (2009) [24] N S Pesika, J. Phys. Chem. B107, 10412 (2003) [25] G Ramakrishna and H N Ghosh, Langmuir 19, 3006 (2003)

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