Synthesis and Characterization of ZnO. Nanoparticles

Transcription

1 Chapter 3 Synthesis and Characterization of ZnO Nanoparticles This chapter describes the synthesis of nanocrystalline ZnO particles using sol-gel route and solid state reaction method. Section A describes the synthesis and characterization of ZnO nanoparticles via sol-gel route and Section B describes the synthesis and characterization of ZnO nanoparticles via solid state reaction method. In Section-C the effect of ph variation on the sol-gel synthesized ZnO nanoparticles is studied. The sol-gel synthesized ZnO nanoparticles are used in the fabrication of QDSSCs as described in later chapters. 75

2 3.1 Introduction ZnO is a member of II-VI semiconducting compounds and occurs naturally as the mineral zincite. It is a hexagonal wurtzite type crystal exhibiting anisotropy. ZnO is a well known n-type semiconductor having piezoelectric, dielectric properties with a wide direct bandgap of 3.37 ev at room temperature (300 K) and a large exciton binding energy of 60 mev, which is 2.4 times the effective thermal energy (KBT=25meV) at room temperature. ZnO is considered a good candidate for TCO electrodes in solar cells because it is transparent to the visible light (>80%) [1]. It is also considered a prime candidate for UV and blue light emitting devices such as blue LED and LASERs due to its large exciton binding energy [2]. Due to large exciton binding energy, the excitons remain dominant in optical processes even at room temperature. Due to its vast industrial applications such as electro-photography, electroluminescence phosphorus, pigment in paints, flux in ceramic glazes, filler for rubber products, coatings for paper, sunscreens, medicines and cosmetics, ZnO is attracting considerable attention in powder as well as thin film form. Its resistance to radiation damages also makes it useful for space applications. The fabrication of ZnO nanostructures have attracted intensive research interests [3-4] as these materials have found uses as TCO [5-9]. Since ZnO is the hardest of the II-VI semiconductors due to the higher melting point of 2248 K and large cohesive energy of 1.89 ev, its performance is not degraded as easily as the other compounds through the appearance of defects. Since Zinc, the main constituent is cheap, non-toxic and abundant, ZnO has become commercially viable. Some of the important properties of ZnO are listed in table 3.1. Earlier TiO 2 was used extensively in solar cell applications [10, 11]. But now a days, in place of TiO 2, ZnO is extensively used. The bandgap of ZnO is almost same as that of TiO 2 and the electron mobility and electron diffusion coefficient of ZnO showed 77

3 much higher values than TiO 2, that would be favourable for electron transport with reduced recombination loss when used in QDSSCs. To understand this issue, QDSSC technology based on ZnO has been explored extensively. Although the conversion efficiencies of % obtained for ZnO are much lower than that of 14% for TiO 2, ZnO is still thought of as a distinguished alternative to TiO 2 due to its ease of crystallization and anisotropic growth. These properties allow ZnO to be produced in a wide variety of nanostructures. Thus fabrication of ZnO based QDSSC presents unique properties for electronics, optics, or photocatalysis [12-15]. Properties Crystal structure Bandgap (ev) Electron Mobility (cm 2 Vs -1 ) Exciton Binding Energy Values Rock salt, Zinc blende and Wurtzite 3.37 at room temperature (Bulk ZnO), 1000 (Single nanowire) 60 mev Density g/cm 3 Refractive Index Electron Effective Mass (m e ) 0.26 Relative Dielectric Constant 8.5 Melting point Boiling point Electron Diffusion Coefficient 1975 o C 2360 o C 5.2 cm 2 s -1 (Bulk ZnO), cm 2 s -1 (Particulate Film) Table 3.1 Important properties of ZnO 78

4 In particular, recent studies on ZnO nanostructure based QDSSCs have delivered many new concepts, leading to a better understanding of photoelectrochemical based energy conversion. This, in turn, would speed up the development of QDSSCs that are associated with TiO 2. Moreover, these ZnO nanomaterials can be synthesized through simple chemical methods with the wide range of structural evolution; fabricating QDSSCs with ZnO nanostructured materials will be advisable and reliable in place of TiO 2, whose structural controllability is not easy in a conversional chemical synthetic route. The production of these structures can be achieved through sol gel synthesis, hydrothermal, physical or chemical vapour deposition, low-temperature aqueous growth, chemical bath deposition and electrochemical deposition etc. This chapter focuses on the synthesis of ZnO nanoparticles using sol-gel method and solid state reaction method Crystal and Surface Structure of ZnO At ambient pressure and temperature, ZnO crystallizes in the wurtzite structure, as shown in figure 3.1. This is a hexagonal lattice, belonging to the space group P63mc with lattice parameters a = and c = nm and is characterized by two interconnecting sublattices of Zn 2+ and O 2, such that each Zn ion is surrounded by tetrahedra of O ions, and vice-versa [16]. This tetrahedral coordination gives rise to polar symmetry along the hexagonal axis. This polarity is responsible for a number of the properties of ZnO including its piezoelectricity and spontaneous polarization, and is also a key factor in crystal growth, etching and defect generation. The four most common face terminations of wurtzite ZnO are the polar Zn terminated (0001) and O terminated (0001) faces (c-axis oriented), and the non-polar (1120) (a-axis) and (1010) faces both of which contain an equal number of Zn and O atoms [17]. The polar faces are known to possess different chemical and physical properties, and the O-terminated face possesses a slightly different electronic structure 79

5 compared to the other three faces. Additionally, the polar surfaces and the (1010) surface are found to be stable, however the (1120) face is less stable and generally has a higher level of surface roughness than its counterparts [18]. The (0001) plane is also basal. Apart from causing the inherent polarity in the ZnO crystal, the tetrahedral coordination of this compound is also a common indicator of sp 3 covalent bonding. However, the Zn O bond also possesses very strong ionic character, and thus ZnO lies on the borderline between being classed as a covalent and ionic compound, with an ionicity of fi = on the Phillips ionicity scale [19]. Fig. 3.1 ZnO crystallizes in the wurtzite structure Comparison of ZnO with its Chief Competitors ZnO was one of the first semiconductors to be prepared in rather pure form after Si and germanium (Ge). It was extensively characterized as early as the 1950 s and 80

6 1960 s due to its promising piezoelectric/acoustoelectric properties. Wide bandgap semiconductors have gained much attention during last few decades because of their possible uses in optoelectronic devices in the short wavelength and UV region of the electromagnetic spectrum. These semiconductors such as ZnO, ZnSe, ZnS, and GaN have shown similar properties with their crystal structures and bandgaps. Some of the important properties of these wide bandgap semiconductors are summarized in table 3.2. Initially, ZnSe based devices and the GaN based technologies obtained large improvements such as blue and UV LED and injection laser. ZnSe has produced some defect levels under high current drive. No doubt, GaN is considered to be the best candidate for the optoelectronic devices. However, ZnO has great advantages for LEDs and LASER diodes over the currently used semiconductors. Recently, it has been introduced that ZnO as II VI semiconductor is promising for various technological applications, especially for optoelectronic short wavelength light emitting devices due to its wide and direct bandgap. The most important advantage is the high exciton binding energy giving rise to efficient exitonic emission at room temperature. Since ZnO and GaN have almost identical lattice parameters and the same hexagonal wurtzite structure, ZnO can be satisfactorily used as lattice matched substrate in GaN based devices or vice versa. ZnO has excellent radiation hardness among all other semiconductors. This property supplies the uses of ZnO based devices in space applications and high energy radiation environments. Bandgap of ZnO energy can be varied from 3.3 up to 4.5 ev with alloying process. Hence it can be used as an active layer in the doubly confined hetero structured LEDs and quantum well lasers. These unique nanostructures unambiguously demonstrate that ZnO is probably the richest family of nanostructures among all materials, both in structure and properties [20-22]. 81

7 Wide bandgap Crystal Lattice E g Melting Exciton Dielectric semiconductor structure parameter (ev temperature binding Constant (Å) at (K) energy RT) (MeV) a b ɛ o ZnO Wurtzite GaN Wurtzite ZnSe Zin blende ZnS Wurtzite Table 3.2 Comparison of ZnO with its competing materials 82

8 3.2 Synthesis of ZnO Nanoparticles using Sol-Gel Method Zinc acetate dihydrate (Zn(CH 3 COO) 2.2H 2 O) was used as zinc precursor. ZnO nanoparticles were prepared by dissolving 0.2M zinc acetate dihydrate in methanol at room temperature and then mixing this solution ultrasonically at 25 o C for 2h. Clear and transparent sol with no precipitate and turbidity was obtained M of NaOH (0.1N NaOH) was then added to the sol and stirred ultrasonically for 60 min. The sol was kept undisturbed till white precipitates settled down at the bottom of sol. After precipitation, the precipitates were filtered and washed with excess methanol to remove starting material. Precipitates were dried at 80 ο C for 15 min on hot plate. Precipitates were then annealed at 400 ο C for 30 min [23]. The flowchart for the synthesis of ZnO nanoparticles using sol-gel method is shown in figure Synthesis of ZnO Nanoparticles using Solid State Reaction Method In solid-state reaction method, 0.2M of Zinc acetate dihydrate in methanol was first ground for by mortar pestle for 10 min and then mixed with 0.02M of NaOH. After the above mixture was ground for 30 min, the product was washed many times with deionized water. After that the product was again washed with methanol to remove the by-products. The final product was then filtered using micron filter paper and dried into solid powder at 80ºC for 15 min on hot plate. After that the powder was annealed at 400ºC for 30 min [24]. The flowchart for the synthesis of ZnO nanoparticles using solid state reactionmethod is shown in figure

9 Fig. 3.2 Flow chart for sol-gel synthesis of ZnO nanoparticles 84

10 Fig. 3.3 Synthesis of ZnO nanoparticles using solid state reaction method 85

11 (200) (004) (202) (104) (102) (201) (112) Intensity (a.u.) (110) (103) (002) (100) (101) Section A Characterization of ZnO Nanoparticles Synthesized using Sol-Gel Method 3.4 Structural and Morphological Properties XRD Analysis The XRD pattern of the nanoparticles obtained by sol-gel route is shown in figure 3.4. The nanoparticles showed crystalline nature with 2θ peaks lying at o <100>, o <002>, o <101>, o <102>, o <110>, o <103>, o <200>, o <112>, o <201>, o <004>, o <202>, o <104>, and o <203>. The preferred orientation corresponding to the plane <101> is also observed. These peak positions coincide with JCPDS card no for ZnO powder (degree) Fig. 3.4 XRD pattern of ZnO nanoparticles synthesized using sol-gel method 86

12 Crystallite size was obtained by Debye-Scherrer formula [25] given by equation (3.1). D 0.94λ βcosθ (3.1) Where D is the crystallite size, 0.94 is the particle shape factor which depends on the shape of the particles, λ is the CuK α radiations (1.54 Å), β is the full width at half maximum (FWHM) of the selected diffraction peak corresponding to <101> plane and θ is the Bragg angle obtained from 2θ value corresponding to maximum intensity peak in XRD pattern. The crystallite size obtained was nm [26] TEM Analysis TEM images of sol-gel synthesized ZnO are shown in figure 3.5. Clear hexagonal structures can be seen in the TEM image. Selected area electron diffraction (SAED) pattern is shown in figure 3.5(b). Bright and well aligned diffraction rings clearly indicates that the ZnO nanoparticles are crystalline in nature. Three bright fringes were observed in SAED which correspond to <100>, <002> and <101> planes of pure wurtzite hexagonal structure of ZnO. Hexagonal structures can be seen in the figure 3.5(c) having particle size ~ 24 nm. (a) (b) (c) Fig. 3.5 (a) TEM image (b) SAED (c) Hexagonal structure of the sol-gel synthesized ZnO nanoparticles 87

13 3.4.3 SEM Analysis SEM images of the ZnO nanoparticles prepared via sol-gel route are shown in figure 3.6. Clear nanostructures can be seen having grain size of ~ 70 nm. The crystallite size as observed from TEM in this case is ~ 23 nm. This shows that one grain in sol-gel derived nanoparticles is approximately equal to three crystallites. So it is clear that the nanoparticles seen by SEM image consist of a number of crystallites which are seen by TEM image. Fig. 3.6 SEM of ZnO nanoparticle synthesized via sol-gel route 3.5 Optical Properties UV-Visible Absorbance Analysis The absorbance curve of the sol-gel derived nanoparticles in the visible region is shown in figure 3.7(a). The graph shows that ZnO does not absorb light in the visible region. This result is in accordance with the bandgap value of the bulk ZnO (3.37 ev) according to which ZnO absorbs only a small portion of UV range. Bandgap is calculated 88

14 Absorbance (a.u.) ( h ) 2 (cm -1 ev) 2 using Tauc s plot (Fig. 3.7 b) which comes out to be 3.23 ev. Tauc s equation is given by equation (3.2) [27]. αhν A hν Eg n (3.2) Where α is the absorption coefficient, hυ is the photon energy, A is the constant, E g is the bandgap of the sample. The value of n is ½ or 2 depending upon whether the transition from valence band to conduction band is direct or indirect. The value is ½ in case of direct transition and 2 in case of indirect transition. Since ZnO has a direct band structure, the value of n is ½ in this case. So the equation (3.2) takes the form of equation (3.3). αhν 2 B hν Eg (3.3) In the above equation, B is a constant related effective masses of charge carriers associated with valence and conduction bands. Intersection of the slope of (αhυ) 2 Vs hυ curve provides bandgap energy of the samples. According to the experimentally calculated bandgap, the synthesized ZnO nanoparticles should absorb light below 383 nm and absorbance graph is in agreement with this. (a) 1.2x10-18 (b) 9.0x x x Wavelength (nm) h (ev) Fig.3.7 (a) Absorbance and (b) Tauc s plot of sol-gel derived nanoparicles in visible range 89

15 3.5.2 Photoluminescence Analysis PL spectra of the ZnO nanoparticles synthesized using sol-gel method is shown in figure 3.8. The first peak in PL spectra corresponds to band to band transition and the spectrum between nm is showing blue luminescence. As can be seen from the PL spectrum of sol-gel derived nanoparticles, the high intensity peak is observed at nm. If we calculate the bandgap value from this wavelength, it comes out to be 3.2eV. The bandgap calculated using PL spectra is approximately same as the one calculated using Tauc s plot. Intensity (a.u.) Wavelength (nm) Fig. 3.8 Photoluminescence peak of ZnO nanoparticle synthesized using sol-gel route 90

16 (200) (004) (202) (104) (102) (103) (201) (112) Intensity (a.u.) (110) (002) (100) (101) Section B Characterization of ZnO Nanoparticles using Solid State Reaction Method 3.6 Structural and Morphological Properties XRD Analysis The XRD patterns of the ZnO nanoparticles synthesized using solid state reaction method is shown in figure 3.9. The nanoparticles showed crystalline nature with 2θ peaks lying at o <100>, o <002>, o <101>, o <102>, o <110>, o <103>, o <200>, o <112>, o <201>, o <004>, o <202>, o <104>, and o <203>. The preferred orientation corresponding to the plane <101> is also observed in this case also. These peak positions coincide with JCPDS card no for ZnO powder. Crystallite size was calculated by Debye-Scherrer s formula (eq. 3.1) and was obtained to be 37 nm (degree) Fig. 3.9 XRD pattern of ZnO nanoparticles synthesized using solid state reaction method 91

17 3.6.2 TEM Analysis TEM image and SAED pattern of the ZnO nanoparticles synthesized using solid state reaction method are shown in figure 3.10 (a) and 3.10 (b) respectively. SAED pattern of the nanoparticles indicates that the ZnO nanoparticles prepared using solid state reaction method are crystalline in nature. However the diffraction rings in this case are not properly aligned as in the case of sol-gel derived nanoparticles. No clear hexagonal structures can be seen in the TEM image. Nanoparticles obtained in this case are adhering to one another. Agglomeration of nanoparticles is more in this case than the former one. As can be seen from the TEM image that the average particle size is ~ 37 nm which is in agreement with the crystallite size obtained from XRD. (a) (b) Fig (a) TEM image (b) SAED of ZnO nanoparticles synthesized using solid state reaction method SEM Analysis SEM image of ZnO nanoparticles prepared by solid state reaction method is shown in figure Grain size in this case is ~ 200 nm. Crystallite size as seen from TEM image is ~ 37 nm in this case. This shows that one grain in solid state reaction 92

18 derived nanoparticles consists of approximately five crystallites. XRD results are confirmed by the combined study of these SEM and TEM images. Fig SEM of ZnO nanoparticles synthesized via solid state reaction method 3.7 Optical Properties UV-Visible Absorbance Analysis The absorbance curve of the solid state reaction synthesized nanoparticles in the visible region is shown in figure 3.12 (a). Tauc s plot is shown in Figure 3.12 (b). The bandgap comes out to be 3.15 ev from the Tauc s plot. The bandgap values validates our crystallite size results according to which smaller crystallite size should have larger bandgap (23.59 nm, 3.23 ev for sol-gel derived nanoparticles) and large crystallite size should have smaller bandgap (37.34 nm, 3.15 ev for solid state reaction derived nanoparticles). 93

19 Intensity (a.u.) Absorbance (a.u.) ( h ) 2 (cm -1 ev) 2 (a) 7.50x10-19 (b) 5.00x x Wavelength (nm) h (ev) Fig (a) Absorbance and (b) Tauc s plot of ZnO nanoparticles synthesized using solid state reaction method Photoluminescence Analysis PL spectrum of the ZnO nanoparticles synthesized by solid state reaction method is shown in figure ZnO exhibits a strong luminescence around nm, which can be attributed to bound exciton emission [26]. The first peak in PL spectra corresponds to band to band transition and the spectrum between nm is showing blue luminescence. ZnO nanoparticles prepared using solid state reaction method show high luminescence than sol-gel derived nanoparticles. The PL intensity peak in case of solid state reaction synthesized nanoparticles is observed at nm. From this value, bandgap comes out to be 3.16 ev. The bandgap energies calculated using PL spectra are approximately same as the ones calculated using Tauc s plot Wavelength (nm) Fig Photoluminescence peak of ZnO nanoparticle synthesized using solid state reaction metod 94

20 Section - C Effect of ph Variation on Properties of ZnO Nanoparticles In the previous sections, ZnO nanoparticles were synthesized and characterized by two different methods: (i) Sol-gel route and (ii) Solid state reaction method. The ZnO nanoparticles prepared using sol-gel route had smaller crystallite size (~ 24 nm) as compared to the one prepared by solid state reaction method (~ 37 nm). As compared to other techniques sol-gel technique has advantages such as simplicity, low cost, excellent homogeneity as well as purity of the product, relatively low processing temperatures such as room temperature [27]. The other advantages of sol-gel process are that the composition of the sol can be controlled and doping can be achieved easily. Sol-gel processing has been found to be an economical, convenient and non- vacuum method to synthesize homogenous and high quality nanoparticles as we ll as films on large scale and on different types and shapes of substrates. This technique is especially useful for growth of ZnO nanoparticle and films, since zinc belongs to the group of elements that form polymeric hydroxides easily, a fundamental requirement for sol-gel chemistry. The main factors affecting the sol-gel der ived nanoparticle properties are sol concentration, sol s chemical equilibrium, ph value of the sol, time, temperature and order-time-temperature of reagent mixing. The stepwise methodology of synthesis of ZnO nanoparticles with ph variation is shown in figure The only difference in this case is the variation in ph value (7 to 12) of the sol using different concentration of NaOH. 95

21 Fig Flow chart for sol-gel synthesis of ZnO nanoparticles with ph variation 96

22 Intensity (a.u.) 3.8 Structural and Morphological Properties XRD Analysis The XRD pattern of the ZnO nanoparticles with varying ph is shown in figure Intensity peaks are showing that nanoparticles are highly crystalline in nature. The preferred orientation corresponding to the plane <101> is observed in all the samples. These peak positions coincide with JCPDS card no for ZnO powder. (112) (103) 7 PH (110) 8 PH (102) 9 PH (101) 10 PH (002) 11 PH (100) 12 PH (degree) Fig XRD pattern of ZnO nanoparticles for ph ranging from 7 to 12 Crystallite size (D) in the orientation <101> was calculated by Debye-Scherrer s formula given by equation (3.1). Crystallite size of the prepared samples increased from 28 nm to 34 nm with increasing ph value from 7 to 11. At 11 ph, the size was maximum (34 nm). After that, on increasing ph to 12, crystallite size decreased to 32 nm. When the 97

23 FWHM Crystallite Size (nm) concentration of OH - ions i.e. ph is low, ZnO nanoparticles do not grow in size due to the lack of Zn(OH) 2 formation in the sol [28, 29]. Since sol with ph < 7 would not have sufficient OH - concentration, no formulation of nanoparticles has been observed. For ph > 7 formulation of nanoparticles is observed. The reason for decrease in size after a certain ph level (ph 11 in present work) is that precipitates start to dissolve due to higher reaction rates. When ZnO reacts with OH -, the dissolution of OH - occurs. Figure 3.16 shows the variation in FWHM of the dominating 2θ peak <101> with increasing ph. It can be clearly seen that FWHM decreases on increasing ph. The decrease in FWHM with increase in ph implies the increase in crystallite size. After 11 ph, FWHM increases thereby decreasing the crystallite size decreases (a) (b) ph Value ph Value Fig Variation of (a) FWHM and (b) crystallite size with ph As the ph of the sol is increased, the intensity value of the dominant 2θ peak also increases (Fig. 3.17). This implies that the number of crystallites in the orientation <101> is also increasing with increase in ph. 98

24 Intensity (a.u.) ph Value Fig Variation in intensity with increasing ph TEM Analysis TEM images confirm the results obtained by XRD (Fig. 3.18). As the ph is increased from 7 to 11 the particle size increases from 28 to 34 nm. At 11 ph, the size was maximum 34 nm. After that, on increasing ph to 12, crystallite size decreased to 32 nm. Hexagonal structures of the nanoparticles can be seen in the TEM images. This range of size (28-34 nm) is suitable for making front wide bandgap semiconductor electrode in QDSSC. 3.9 Optical Properties UV-Visible Absorbance Analysis The optical absorption spectra of ZnO nanoparticles in the visible region are shown in figure The graph shows that ZnO does not absorb light in the visible region which means that prepared ZnO samples are transparent to visible light. Bandgap values are calculated using Tauc s plot as explained earlier by equations (3.2) and (3.3). 99

25 Absorption (a.u.) Fig TEM images of ZnO nanoparticles wih increasingt ph value 7-12 (a-f) (a) ph7 ph8 ph9 ph10 ph11 ph ( h ) 2 (cm -1 ev) 2 1.2x x10-19 ph7 ph8 ph9 ph10 ph11 ph12 (b) 6.0x x Wavelength (nm) h (ev) Fig (a) Absorption spectra and (b) Tauc s plot of ZnO nanoparticles 100

26 Bandgap energy (ev) Bandgap energy (ev) The variation in bandgap energy with increasing crsytallite size (ph value) is shown in figure Bandgap decreases from 3.25 to 3.21 ev with increase in crsytallite size from 28 to 34 nm [30] (a) (b) Crystallite size (nm) ph value Fig Variation in bandgap with (a) crystallite size and (b) ph value 3.10 Growth Mechanism of ZnO Nanoparticles The growth of ZnO from zinc acetate dihydrate precursor using sol-gel process generally undergoes four stages which are 1. Solvation 2. Hydrolysis 3. Polymerization and 4. Finally transformation into ZnO The zinc acetate dihydrate precursor was first solvated in methanol, and then hydrolyzed, regarded as removal of the intercalated acetate ions and results in a colloidal-gel of zinc hydroxide (Eq. 3.4). The size and activity of solvent have obvious influence on the reacting progress and product. Methanol has smaller size and a more active OH and OCH 3 groups. Methanol can react more easily to form a polymer precursor with a higher 101

27 polymerization degree, which is required to convert sol into gel [31]. These zinc hydroxide splits into Zn 2+ cation and OH - anion according to reactions (Eq. 3.5) and followed by polymerization of hydroxyl complex to form Zn-O-Zn bridges and finally transformed into ZnO (Eq. 3.6) [30]. Zn (CH 3 COO) 2. 2H 2 O + 2NaOH Zn (OH) 2 + 2CH 3 COONa + 2H 2 O (3.4) Zn(OH) 2 + 2H 2 O Zn(OH) H 2 + (3.5) Zn(OH) 2+ 4 ZnO + H 2 O + 2OH - (3.6) When the concentration of OH - i.e. ph is low, the growth of ZnO particle does not proceed because of the lack of Zn(OH) 2 formation in the solution. Therefore in sol gel technique, there is a threshold ph level above which the nanostructure may be formed. In this study, the growth of ZnO nanoparticles in zinc acetate solution was observed from a solution having ph of 7. A solution with a ph < 7 would have insufficient OH - concentration to form ZnO. Since ph controls the rate of ZnO formation, it affects the size and their way of combination to get stable state. As the freshly formed nuclei in the solution are unstable, it has a tendency to grow into larger particles. The largest crystallite size was observed when the ph of the solution was 11. Further increase in the concentration of OH - from this point, reduced the crystallite size of ZnO. This is presumed to be because of the dissolution of ZnO via back reaction of equation (3.6). When ZnO reacts with OH -, the dissolution of ZnO occurs [32]. The decrease in crystallite size above 11 ph level is the evidence of the acceleration of ZnO dissolution during competitive ZnO formation. 102

28 3.11 Summary ZnO nanoparticles were synthesized via sol-gel route and solid state reaction method. Under the same growth conditions, the crystallite size was ~24 nm in case of solgel method and ~37 nm in case of solid state reaction method. Crystallinity was better in case of sol-gel method. Because of ease of preparation and control over crystallite size in sol-gel synthesis, the effect of ph variation on sol-gel synthesized ZnO nanoparticles was studied. The size of the ZnO nanoparticles increased with increasing ph. After a certain ph level, the size started decreasing. Minimum and maximum crystallite size was ~27 nm and ~32 nm for ph 7 and 11 respectively. A blue shift in the bandgap was observed with decrease in the size of the ZnO nanoparticles. The growth mechanism of ZnO has been discussed in terms of solvation, hydrolysis and polymerization. Aggregation has been found to be dominant growth mechanism of ZnO nanoparticles. 103

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