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Figure 4.3.8 FESEM images of the products obtained from the control reactions performed by varying time i.e., after:
(A) 40 minute, (B) 60 minute, (C) 90 minute and (D) 120 minute. Insets show corresponding higher magnification FESEM images for better clarity.
Morphology of the product at 40 min of reaction time is represented in the image (A) and inset to it, figure 4.3.8. Cage like ZnO with hollow characteristics are observed in this case. However, partially developed narrower and smaller size cages are also observed [indicated with red arrows in image (A)]. This is an indicative of incompletion of reaction and is confirmed from the low crystallinity of the product compared to the other two cases, i.e., reactions with 60 min and 90 min.
Please note that, variation of crystallinity is also probed for all the products from PXRD patterns and is depicted in figure 4.3.9. We have tabulated the values of Full Width Half Maximum (FWHM), calculated crystallite size and intensity in counts per second (c.p.s) of the highest intense peak in the PXRD patterns in table 4.3.1. Scherrer equation is used to calculate the crystallite size of the products. The Scherrer equation is represented as:
𝐷 = 0.95 × 𝜆
∆𝑊 × 𝑐𝑜𝑠𝜃 (1)
B
80
Where, D is the crystallite domain diameter, λ is the wavelength of the incident X-ray beam (1.54 Å for the Cu K), θ is the Bragg’s diffraction angle, W the width of the X-ray pattern line at half peak-height in radians.
Figure 4.3.9 PXRD patterns obtained for the synthesized ZnO by varying reaction time, i.e. (a) 40 min, (b) 60 min, (c) 90 min and (d) 120 min
Table 4.3.1. Calculated crystallite size and intensity in counts per second (c.p.s) of the highest intense peak obtained from the PXRD patterns of figure 4.3.9.
Name of the compound
FWHM Crystallite size Dp
(nm)
Intensity maximum (c.p.s.)
2 value (degree)
Crystal plane
(a) ZnO_40 min 0.57 20.0 165.6 36.2 (101̅1)
(b) ZnO_60 min 0.55 21.0 192.2 36.4 (101̅1)
(c) ZnO_90 min 0.46 25.7 215.4 36.4 (101̅1)
(d) ZnO_120 min 0.52 19.5 181.4 36.3 (101̅1)
It is observed from figure 4.3.9 that the PXRD peaks for the ZnO cages obtained after 90 min [trace (c)] are sharp (FWHM ~0.46) and intense (~215.4 c.p.s) as compared to the products recovered after 40 min [trace (a)] and 60 min [trace (b)] of reaction time. Moreover, the crystallite size (Dp) distribution (~26 nm; from table 4.3.1) is also found to be maximum for the product recovered after 90 min which is a clear reflection of higher crystallinity of the product as compared
0 100 200
0 100 200
0 100 200
10 20 30 40 50 60 70 80
0 100 200
(d) 120 min (c) 90 min (b) 60 min
Int ensit y (c .p. s )
2 (degree)
(a) 40 min
(2021)(1122)(2020)
(1013)
(1120)
(1012)
(1011)
(0002)
(1010)
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to others. Although, the morphology of ZnO cages perceived in the reaction time at 60 min and 90 min are very similar as we have seen from the FESEM images 4.3.8 (B) and (C) respectively, the crystallinity of the later is superior to the former. This implies that an adequate time is achieved by the product, for aging in the reaction with time at 90 min which allows most possible regular arrangement of the ZnO crystal planes. Further, with an increase in the reaction time to 120 min, dissolution of the superstructures is noted, as evident from the high magnification FESEM image portrayed in the inset of the image (D), figure 4.3.8. Dissolution of ZnO in wet chemical synthetic process is an inherent characteristic of the material, which leads to decrease in crystallinity.29 The dissolution effect becomes more dominant in case of prolonged reaction time. Growth process reaches a certain equilibrium with the growth units; Zn(OH)42 and Zn(NH3)42+ after the formation of ZnO cages. As the reaction progresses, concentration of Zn(OH)42 and Zn(NH3)42+ is decreased. In order to maintain the equilibrium, dissolution process of ZnO cages in the reaction occurs and results in degraded ZnO cages with poor crystallinity. The PXRD analysis of the material is also in good agreement with the FESEM observations as shown in the figure 4.3.9 and table 4.3.1. The increase in population of broken ZnO cages in this case, as indicated with red arrows in image (D), infers excess reaction time. Therefore, the optimum time for the completion of the reaction is established to be at 90 min.
Based on all the above discussions, the plausible formation mechanism of ZnO cages at optimized reaction conditions can be explained as follows.
4.3.3 MECHANISM OF ZnO SUPERSTRUCTURE FORMATION
Alginic acid is a linear copolymer of 1→4 covalently linked β-D-mannuronic acid and its C-5 epimer α-L-guluronic acid residues in different sequences to construct the backbone chain.
Sodium salt of alginic acid, i.e., sodium alginate (ALG) is a naturally abundant polysaccharide or a “biopolymer” extracted from the cell walls of brown seaweed. It is soluble in water and ionizes to its anionic form, i.e., the alginate ion. 0.1 wt % solution of ALG in water is found to be negatively charged with zeta potential () ≈ 60 mV. These negatively charged alginate ions with carboxylate groups interact with the positively charged Zn2+ ions in the solution due to electrostatic forces and form a complex (1): [Zn2+ALGI] as shown in scheme 4.3.2.26 This complex appears in the reaction mixture as a gelatinous precipitate which is formed immediately after the addition of Zn(NO3)2.6H2O to the solution of ALG.
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Scheme 4.3.2 Schematic representation of step by step evolution process of the ZnO cage formation in the reaction
On addition of NH4OH, initially water insoluble white precipitate possibly of Zn(OH)2 is observed. Further addition of NH4OH upto the optimized amount, i.e., 1.5 mL per 25 mL, the aqueous solution becomes transparent and pH is found to be 10 which infers the formation of water soluble Zn(OH)42 and Zn(NH3)42+ ions. It is well known that, Zn(OH)42 and Zn(NH3)42+
act as the growth units of ZnO crystal in the wet chemical reactions carried out in basic conditions.30 Subsequently, due to Coulombic interactions, the Zn(OH)42 and Zn(NH3)42+ growth units are believed to be attached with the negatively charged alginate ions to form an adduct complex (2): [Zn(OH)42ALGIZn(NH3)42+]. In hydrothermal reaction conditions, the growth units initiate the nucleation process and form numerous nuclei of ZnO. These ZnO nuclei tend to grow along (±) c-axes, i.e., along Zn2+ populated +(0001) and O2 populated (0001) high energy polar crystal facets promoted by Zn(OH)42 and the Zn(NH3)42+ growth units respectively which are leading to the formation of ZnO NRs.31 Since, the ZnO crystal growth along () c-axis is the slowest growth direction, predominant growth units are Zn(OH)42 ions.32 Simultaneously, the negatively charged alginate ions with (–COO) groups anchors to the positive Zn2+ populated +(0001) crystal facets of ZnO and adheres on the surface of ZnO NRs. Thus, the growth of ZnO NRs in presence of ALG is inhibited due to the minimized contact between the growth units and (0001) crystal surfaces. Additionally, the free (OH) and (COO) groups in the ALGI backbone
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chain, triggers the assembling process of ZnO NRs and arrange hierarchically to form ZnO superstructures as shown in the step 4. At this step, the existence of optimum amount of NH4OH and an equilibrium between gas–liquid in the autoclave are the crucial factors to acquire the anticipated size and shape of the product. As the reaction proceeds, some of the multi-cage super- structures fracture at the adjoining points and result in single cage superstructures.
4.3.4 UV-VISIBLE ABSORPTION AND PHOTOLUMINESCENCE ANALYSES
Figure 4.3.10 (A) UV-Visible absorption profiles for all the ZnO heterostructures (ZnO PNPs, ZnO NRs and ZnO Cages). Inset to it shows corresponding Tauc’s plots. (B) Room temperature steady state photoluminescence (PL) spectra for all the ZnO heterostructures. Inset to it shows the corresponding electronic transitions of the peaks indicated in the spectra with green arrows.
1
2 3
CB
VB 1392 nm
Zni
410490 nm 2
Oi 3
500580 nm
400 450 500 550 600 650
(500580 nm) 465 nm
410 nm