Chapter 4

Facile Synthesis of Face Oriented ZnO Crystals:

Tunable Polar Facets and Shape Induced Enhanced

Photocatalytic Performance**

This chapter accounts for the synthesis of unique oriented ZnO structures

such as face oriented hexagonal discs, 3D-trapezoids, rings, doughnuts, and

hemispheres with tunable number of exposed polar facets. The successful

realization of morphologies was achieved by a simple hydrothermal route in

aqueous base environment without using any templates/structure directing

agents. The Photocatalytic degradation of methylene blue as a model system

was used to showcase the morphology-dependent enhanced photoactivity

under UV-light.

**Part of the published article: J. Phys. Chem. C, 2013, 117 (9), 4597–4605

Chapter 4

91

4.1. INTRODUCTION:

Semiconductor photocatalysts have attracted attention worldwide owing to

their envisaged potentials in mitigating environmental issues in addition to their

prospects in next generation energy conversion/storage devices.1−3 Photocatalysis

offers a great advantage for environmental remediation with possibilities to detoxify

noxious organic pollutants. Primarily, the elimination of toxic and hazardous

substances effectively facilitating recycling of wastewater has remained a major

challenge. In this perspective, semiconductor based photocatalysis is being projected

as a viable cost-effiective method for environmental abatement.3 Over the years,

several types of micro or nanostructured semiconductor metal oxide photocatalysts

such as SnO24 Fe2O3,5 ZnO,1 and TiO2

3 have been developed and studied for their

remediation efficiencies using model systems. Among them, TiO2 and ZnO are the

two most extensively investigated materials because of their lower cost, nontoxicity,

biocompatibility, and high thermal and chemical stability.6,7 ZnO presents

impressive photocatalytic activity and is recognized as a suitable alternative to

TiO2.8,9 Though ZnO is a wide-band-gap material (Eg= 3.37 eV), nevertheless it has

attracted attention for its ability to generate hydrogen peroxide, photocatalytically

which effectively degrades several pollutants and viruses.

It is now well understood that the photocatalytic reaction occurs primarily at

the interface where the pollutants come in contact with the active surface of the

photocatalyst. Hence, the photocatalytic properties of ZnO largely depends on the

shape and surface morphologies effected by the surface atomic rearrangements.1,10,11

Surface atomic arrangements and coordination modulate the crystal facets in

different orientations. Consequently, the reactivity of the photocatalyst significantly

Chapter 4

92

varies on the exposed crystal facet.12,13 Recently, Jang et al. have reported that

terminal polar facets are much more reactive than thermodynamically stable

nonpolar facets due to the higher surface energy of the former.1,11 Owing to the

intrinsic anisotropy in the growth rate of ZnO, one-dimensional hexagonal rods have

been predominantly synthesized that offer fewer exposed polar (0001) and (0001)̅

facets.14,15 In such cases, however, it is difficult to assess an explicit relationship

between the face orientation and the photocatalytic activity. Assuming that a ZnO

rod could be sliced into many shorter discs, the relative number of polar facets

(0001) and (0001)̅ can be increased significantly.16 Thus, synthesizing ZnO with

preferential growth patterns to obtain higher percentage of exposed polar facets is

the key to enhance its photocatalytic activity. Successful tuning of morphology to

obtain higher polar to nonpolar facets ratio is desirable to increase the percentage of

reactive sites, and this remains a synthesis challenge. Control of the size, shape, and

preferred orientation of ZnO nanostructures to tailor its chemical and physical

properties for optimum reactivity and selectivity is also very important.17−20

Although considerable efforts are being devoted to synthesize ZnO crystals with

controlled morphologies that can possibly provide highly reactive facets, relatively

few attempts to correlate the effect of oriented ZnO morphology on the

photocatalytic activity have been reported.13,21 In the present chapter, we describe in

detail our successful attempts in achieving appreciable control on the fraction of

exposed polar facets during the synthesis of a series of hexagonal prismatic ZnO

structures with unique morphologies.

Hierarchical structures including twin discs, 3D-trapezoids, self-assembled

rings, doughnuts, and hemisphere morphologies could be achieved by simple

Chapter 4

93

hydrothermal routes depending on the choice of precursor, reactant concentration,

pH, time, temperature, and pressure without the use of any additives/templates. Of

particular interest was our successful realization of morphologies with tunable polar

to nonpolar facet ratio; the twin discs and 3D-trapezoids that provide predominantly

exposed (0001) and (0001)̅ planes. The precursor and the pH of reaction mixtures

are found to play a critical role in the formation of self-stacked hexagonal ZnO, twin

discs, 3-D trapezoids, and hemispheres. The degradation of methylene blue under

the UV-illumination is used as a model system to showcase the photocatalytic

activity, and the results are compared with the conventional ZnO microrods. The

twin discs, 3-D trapezoids, and hemispheres exhibit significant enhancement in the

photocatalytic activities. A detailed study on the optical properties and reactivity

reveals a strong dependence on the morphology induced photoactivity.

4.2.EXPERMENTAL SECTION:

4.2.1. Materials and Methods:

All chemicals zinc acetate dihydrate (Zn(Ac)2, Zn(CO2CH3)2·2H2O), zinc

nitrate hexahydrate (Zn(NO3)2 · 6H2O), and Trizma base ((HOCH2)3C−NH2),

hereafter referred to as TB) were purchased from Sigma-Aldrich and used as

received. In a typical preparation of zinc oxide, the required amount of zinc acetate

dihydrate was dissolved in 22 mL of water under vigorous magnetic stirring to

obtain a precursor solution. A requisite amount of TB was dissolved separately in

another beaker with 20 mL of water, which was then added to the above zinc

precursor solution. A white precipitate formed instantly, and the solution was stirred

for 30 min to obtain a homogeneous mixture. The resulting mix was transferred into

a 100 mL Teflon autoclave and heated to 120 °C for 12 h. Subsequent to the

Chapter 4

94

completion of the reaction time, the autoclave was allowed to oven cool to room

temperature; the precipitate was collected, washed repeatedly with water to get rid of

organic components, and finally dried at 60 °C in an oven. In another method of

ZnO synthesis, zinc nitrate hexahydrate was used as the zinc precursor while the rest

of synthetic protocol described above was followed as such. Samples synthesized

using Zn(Ac)2 and Zn(NO3)2 are coded as ZTR and ZNTR, respectively.

4.2.2. Evaluation of Photocatalytic Activity:

The photocatalytic activity of the as-prepared ZnO was evaluated by the

photo-assisted degradation of methylene blue (MB) aqueous solution at room

temperature under UV-light (400W, 360 nm, high pressure Hg vapor lamp, SAIC,

India). In a typical reaction, 0.03g of catalyst was dispersed in 30 mL of aqueous

MB (1 × 10−5M) in a glass reactor vessel equipped with a water jacket for effective

heat dissipation. Prior to irradiation, the suspension was magnetically stirred for 30

min in the dark to stabilize and equilibrate the adsorption of MB on the surface of

ZnO. The stable aqueous dye-ZnO suspension was then exposed to UV-light

irradiation under continual stirring. Aliquots of 5 mL were drawn at regular time

intervals to be analyzed on a Varian Cary 5000 UV−Vis−NIR spectrophotometer to

quantify the dye concentration in the suspension. The concentration of MB (C/C0)

was estimated from the absorbance obtained. A blank run without ZnO reaction

carried out under similar conditions was used for comparative evaluation.

4.3. RESULTS AND DISCUSSIONS:

Controlled synthesis of a series of hexagonal prismatic ZnO structures with

unique morphologies was successfully achieved under hydrothermal conditions.

Detailed analyses on the synthesized structures are carried out using electron

Chapter 4

95

microscopy in the scanning and transmission mode along with selected area electron

diffraction patterns. Figure 1a-d and Figure 1a-d details a series of low-resolution

scanning electron microscopy (SEM) images obtained for different precursors

(Zn(Ac)2, Zn(NO3)2) and concentrations of Trizma base (TB) in the reaction mix. As

can be observed, the fine-tuning of the preferred polar face orientation for the ZnO

crystals in aqueous solution of Zn(Ac)2 and Zn(NO3)2 can be achieved by varying

the concentration of TB at 120 °C. Different morphologies can be selectively

obtained within a certain pH range, where TB exercises control on both the pH and

as a shape-directing agent (Table 1).

The use of Zn(Ac)2 precursor as a function of increasing TB concentration is

sequenced in Figure 1a-d. Micrograph Figure 1a shows formation of hexagonally

prismatic structures when the zinc acetate to TB mole ratio was 1:1.5. The diameters

of hexagonal discs are ∼5-6 μm, and these discs are observed to be stacked

symmetrically in pairs along with some irregular shapes interspersed. Interestingly,

cover-slips like overgrowth are seen a-top the exposed hexagonal sides almost

uniformly throughout the samples. The pairs together apparently resemble layered

hexagonal rods with an average length of ∼7-9 μm. When the TB concentration was

increased to 1:2 mol, high aspect ratio well-formed hexagonal discs were obtained

with defined edges and are self-stacked symmetrically to form conjoined twin discs

(Figure 1b). The diameters of these hexagonal discs are ∼7 μm with edge length of

∼3−3.5 μm. Upon further increase of TB to 1:2.5 (Figure 1c) and 1:3 (Figure 1d)

mole ratios, a noticeable change in the morphology was observed with formation of

truncated hexagonal cone like shapes approaching 3D-trapezoids. There is also a

significant reduction of the axial growth along with the tendency to pair up.

Chapter 4

96

Figure 1. Low resolution SEM images of as synthesized ZnO hexagonal microstructures (a)

ZTR-1, (b) ZTR-2, (c) ZTR-3, (d) ZTR-4. The scale bar is 4m in all figures.

On the other hand, contrasting morphologies in the shape of ring,

doughnuts and hemispheres of ZnO crystals were obtained from zinc nitrate

precursor. The effect of TB concentration in the reaction medium can be observed in

Figure 2 a−d. When the Zn(NO3)2 to TB ratio was 1:1.5, ringshaped ZnO was

observed with outer diameter of ∼640 ± 50 nm and inner diameter of 100 ± 50 nm.

For the ratios 1:2 and 1:2.5, the ZnO particles still maintained their ring-like

morphology, but the inner diameter of the ring decreased, giving a doughnut-like

appearance, while at higher Zn(NO3)2/TB ratio (1:3) resulted in the formation of

uniformed hemispheres. Upon further increase in the TB concentration, the size of

the hemispheres appeared to increase considerably with significant agglomeration

and irregular morphology. Overall, it could be inferred from the SEM studies that

Trizma base (TB) concentration does play a very crucial role in the growth of ZnO

Chapter 4

97

crystals and mediates the orientation of the planes. The morphology and size of the

synthesized ZnO crystals were further investigated using transmission electron

microscopy and selected area electron diffraction pattern to confirm the crystal

structure.

v

Figure 2: Ring like structures (a) ZNTR-1, doughnuts (b) ZNTR-2 (c) ZNTR-3, and hemi

spheres (d) ZNTR-4. The scale bar is 4m in all figures

Figure 3a,b shows the TEM images of ZTR-2 and ZTR-4 samples,

while Figure 3c,d corresponds to the ZNTR-1and ZNTR-3 ZnO samples. The inset

SAED patterns provided in each micrograph confirm the wurtzite ZnO structures

with hexagonal lattices. The diffraction pattern of samples ZTR-2 and ZTR-4 exhibit

hexagonal shapes having plane (10̅10) at the edge and plane (0001) as the top and

bottom facet. Close inspection of Figure 3c,d in the transmission mode reveals

further details on the microstructure which was not apparent in the SEM studies.

(a) (b)

(c) (d)

Chapter 4

98

Hierarchical self-assemblies of smaller ZnO particles in micrometer-sized structures

(ring and doughnut shapes) are very evident from these images.

Figure 3. TEM and their corresponding SAED pattern of as synthesized ZnO structure (a)

ZTR-2, (b) ZTR-4, (c) ZNTR-1, (d) ZNTR-3. Inset shows the SAED of corresponding

samples.

Figure 4a represents typical X-ray diffraction patterns for the hexagonal

faceted twin discs (ZTR) and rings (ZNTR) of the synthesized materials. All the

diffraction peaks of synthesized samples can be ascribed to (100), (002), (101),

(102), (110), and (103) planes corresponding to the hexagonal wurtzite ZnO crystals

and are in good agreement with the standard JCPDS Card No. 36-1451. Raman

studies also confirm the phase purity and degree of crystallization for the as-

synthesized samples. Figure 4b shows the representative Raman spectra collected at

room temperature for the synthesized samples (ZTR, ZNTR) in the range of 50−800

cm−1. All samples exhibit similar scattering, which corresponds to the characteristic

bands of the hexagonal wurtzite phase of ZnO. The two high-intensity peaks

Chapter 4

99

observed at ∼99.7 and ∼441.02 cm−1 are attributed to the low and high E2 modes of

nonpolar optical phonons, respectively, typical of wurtzite phase.22,23 Weaker peaks

observed at ∼332.7 and ∼383.11 cm−1correspond to the E2H−E2L multiphonons

and A1T modes, respectively. The broad and suppressed peak at ∼578.4 cm−1is

assigned to the E1L mode, which possibly relates to the structural defects owing to

the interstitial oxygen vacancies in the ZnO samples.24,25

100 200 300 400 500 600 700 800

Inte

nsity

(a.u

.)

Wavenumber (cm-1)

ZTR ZNTR

332.

738

3.11

441.

02

578.

4

664.

61

(b)

Figure 4. (a) X-Ray Crystallographic studies of the ZnO particles obtained with 1:2 ratio of

different Zn precursor to of TB base; (b) Corresponding Raman spectral analysis.

The composition and oxidation states of the synthesized materials were

further confirmed by XPS. As-synthesized ZnO exhibits peaks at binding energies of

1021.8 and 1044.8 eV, which corresponds to Zn 2p3/2 and Zn 2p1/2 electrons in the

Zn2+ oxidation state, respectively (Figure 5). The O 1s spectrum shows the existence

Chapter 4

100

of oxygen in two different states, with binding energies at 530.7 and 532.8 eV that

can be assigned to the lattice-oxygen and the oxygen deficient species within the

ZnO matrix, respectively.26,27

Figure. 5 XPS spectra indicating the binding energies of Zn2p and O1s in samples (a), (b)

corresponds to ZTR and (c), (d) corresponds to ZNTR samples, respectively.

4.3.1. Growth Process and Structural Evaluation:

To evaluate the growth mechanism and effect of TB concentration on ZnO

formation, we carefully carried out reaction/process optimization by monitoring the

parameters involved following multiple experiments and using a wide range of

techniques. The key reaction parameters that are necessary to control and the

underlying mechanism to obtain the desired morphology of ZnO (hexagonal disks,

hemispheres, ring shapes) can be rationalized as follows. The immediate formation

of a white turbid complex formed initially at room temperature when TB was added

(a) (b)

(c) (d)

Chapter 4

101

to the zinc precursor was analyzed by XRD and FT-IR (see figures 6a and 6b,

respectively). Our deduction shows the formation of Zn(OH)2 particles unlike an

earlier report on the formation of zinc amine complex with polyamines.27

3500 3000 2500 2000 1500 1000 500

Inte

nsity

(a.u

)

ZTR

ZNTR

Wavenumber(cm-1)

(b)

Figure 6: X-Ray difraction pattern of Zn(OH)2 formed when the TB was initially mixed

with zinc precursor solution at room temperature. (b) FT-IR spectra of Zn(OH)2. ZTR and

ZNTR samples are treated with trizma base at room temperature with zinc acetate

hexahydrate and zinc nitrate dihydrate respectively. The peak at 1648 cm-1 in ZTR samples,

indicates the presence of acetate group

As discussed in the preceding section, the concentration of TB was found to

play a significant role in determining the morphology of ZnO formation (Figures 1

and 2). Our investigations indicates that the choice of zinc precursor also influences

the formation of the ZnO structure. Zinc salts such as Zn(Ac)2 used in this study

yields hexagonal structures while ring-shaped ZnO was obtained when Zn(NO3)2 is

used as the precursor. This observation strongly indicates the structure directing

effect of Trizma base during the nucleation and growth processes. To demonstrate

that the reaction process in presence of TB is exclusive, we have carried out multiple

experiments in different sets of conditions, reactants, and structure-directing agents.

In the thermal treatment of aqueous zinc precursors (Zn(Ac)2, Zn(NO3)2) under

identical experimental conditions without the presence of TB, no ZnO could be

obtained. The addition of NaOH to aqueous zinc acetate (pH ∼11.2) and zinc nitrate

Chapter 4

102

(pH ∼10.3) in Zn2+ to NaOH mole ratio of 1:2 resulted in dumbbell shapes and

microrods of ZnO, respectively (see figure 7). Further increase in NaOH

concentration leads to the formation of irregular shaped ZnO microstructures. This

drastic change in the morphology could be ascribed to a large difference in the initial

and final pH of the reaction system.28

Figure 7. SEM images of (a) dumbell and (b) rod shaped ZnO obtained from Zn(ac)2.6H2O

and Zn(NO)3.2H2O as precursors when treated with NaOH solution in 1.0 : 2.0 moles ratio

followed by hydrothermal treatment at 120 oC, respectively. The scale bar provided is 4m

On the grounds of the above observation, a plausible mechanism on the

formation and growth process of the hexagonal disks and rings can be proposed. The

amino group of TB is basic in nature, which can be effectively used to control the

desired pH of the reaction media. Thus, increasing TB concentration leads to an

increase in the pH of the reaction system. Additionally, TB in aqueous medium

provides the necessary hydroxide ions for the formation of ZnO along with the

presence of a cationic counterpart (ammonium cation).

(HOCH2)3C-NH2+ H2O (HOCH2)3C-NH3+ + OH-

Zn2++ 4OH- Zn(OH)42-

Zn(OH)42- ZnO+H2O+2OH-

(a) (b)

Chapter 4

103

Zinc cations are known to readily react with hydroxide anions to form stable

tetrahedral Zn(OH)42−complexes, which act as the growth units for the ZnO

structures.29,30 The solution pH controls the rate of hydrolysis of the zinc precursor.

Shinde et al. suggested that, in the intermediate pH range between 7 and 10,

insoluble Zn(OH)2 is formed which redissolves at higher pH (>10).29 In our protocol

for synthesis, the pH of reaction system was maintained between ∼7 and 9. Hence,

we do observe the initial formation of Zn(OH)2, which subsequently condenses and

transforms into ZnO at higher temperatures. In addition, pH of the reaction mixture

shows an obvious influence on the final shape of the particles. Large change in the

pH values not only alters the nucleation rates but also accelerates the crystal growth.

Any difference in the growth rates of different crystallographic planes would be

reflected in the final geometry of crystals. When the pH variation in initial and final

stages of the reaction is small, the nucleation rate is appreciably slower, leading to

well-defined particle shapes and sizes.28,31 Subsequent to our significant observations

made from the electron microscopy studies, the realization of diverse morphology

can be comprehended based on the controlled nucleation and directed growth under

the specific reaction conditions (choice of precursors, reactant concentrations, pH,

counterions, etc.). With increasing concentration of the TB and its corresponding

pH, the morphology of the synthesized ZnO changed from hexagonal stacked discs

with interspersed irregular shapes-to-uniform hexagonal twin discs and finally into

truncated hexagonal cones (3D trapezoids). This shape transformation can be

understood in terms of the differences in the growth rates of various crystal faces at

different pH values.

Chapter 4

104

Table 1. Samples synthesized at different concentration of trizma base, corresponding pH

change observed as function of trizma base concentration and their surface area.

In general, depending on the structural anisotropy and surface electric

polarity of ZnO, the growth rate is higher for [0001], decreases for (10̅10), (1010̅),

(110̅0), and (11̅00), and is lowest for (0001)̅ under normal conditions.32,33 In the pH

range 6−9, ZnO nuclei are hexagonally prismatic, and the crystals grow in both axial

and equatorial direction simultaneously.19,34 Under this pH condition, the ZnO

crystals grow preferentially along the (1010̅) or (0110̅) equatorial crystal planes that

forms the side edges of the hexagonal discs (prismatic faces).35 Although with

increase in the base concentration, the particles hold the hexagonal prismatic

structure, they show significant changes radially. This is probably due to the small

difference between the initial and final pH value of the reaction mixture that alters

Sample Name Zn/TB

moles ratio

pH of the reaction BET Surface

Area (m2g–1) Initial Final

ZTR-1 1:1.5 7.28 6.5 4.3

ZTR-2 1:2 7.96 6.7 8.4

ZTR-3 1:2.5 8.23 7.1 27.55

ZTR-4 1:3 8.65 7.2 32.02

ZNTR-1 1:1.5 6.11 6.1 32.46

ZNTR-2 1:2 6.62 6.2 94.7

ZNTR-3 1:2.5 6.96 6.2 15.82

ZNTR-4 1:3 7.23 6.4 14.87

ZNR - - - 16.55

Chapter 4

105

the nucleation rate and growth kinetics leading to well-defined but different

morphologies.

Moreover, in our experiments, the Trizma base possibly also serves as an

effective capping agent for the ZnO nuclei and plays an important and parallel role

in the growth orientation. The -OH groups of TB molecule can preferentially get

adsorbed on the positive surface of (0001) Zn2+ plane and prevents the rapid growth

along the ⟨0001⟩ direction.30 This condition promotes ZnO crystal growth along the

six equatorial directions symmetrically, which leads to the formation of hexagonal

discs. Further, the choices of precursor do show the obvious influence as observed in

our studies and also discussed in earlier reports.36 Liang et al. reported the inhibition

effect of adsorbed acetate ions on the surface of Zn2+ substituting hydroxyl anions

along the ⟨0001⟩ direction. It is now well understood that acetate ions in the solution

have high affinity of adsorption on the ZnO surface compared to other anions.37−39

The high proportion of polar plane can thus be related to the strong suppression of

crystal growth along ⟨0001⟩ and relative enhancement of crystal growth along the

⟨0110̅⟩ direction.11 Although the exact mechanism of stacking or pairing of discs is

not clear, it is proposed that the surface charge of these polar faces with opposite

polar direction interact and assemble together during initial growth and form stacked

discs, resulting in neutralization of local polar charges.11,40 However, the positively

charged Zn2+-terminated (0001) and negatively charged O2− -terminated polar

(0001)̅ surfaces have high surface energies.41 Hence, it is possible that coming

together of these polar surfaces would be energetically unfavorable unless the

surface charge is compensated by a passivating agent, in the present case, possibly

the combination of both Trizma base and acetate anions.40,42

Chapter 4

106

That the crystal growth depends strongly on the structure of the material, the

surface chemistry of the particles resulting from ions in the solution, the interface

between the crystals, and surrounding solution28,43 is also reflected in the formation

of ZnO rings and hemispheres. Zeshan et al. stated that the tendency of anion

adsorption on surface of ZnO, affect the growth kinetics and resulting particle

morphologies.43 These adsorbed anions can dictate the material structure, shape, and

size by altering the nucleation rate and growth kinetics. This leads to the formation

of rings and hemisphere-shaped ZnO crystals comprised of smaller primary particles

as observed in TEM images.

The growth mechanism and resulting morphology of these ZnO rings and

hemispheres can be interpreted in the context of nucleation, Ostwald ripening, and

oriented attachment. Oriented attachment crystal growth phenomena have been often

observed for nanoparticles, where primary crystallites can be joined together into

larger ones.44−46 Unfortunately, it was noticed that during this process Ostwald

ripening also usually occurred simultaneously.47,48 The Ostwald ripening mechanism

driven by minimization of the overall surface energy47 results in the rings,

doughnuts, and hemispherical shapes consisting of larger particles expense of

smaller particles. The higher magnification TEM images (Figure 8) of rings,

doughnuts, and hemispherical shaped ZnO reveal that the structures are composed of

several aggregated smaller crystallite domains (primary particles) yielding larger

morphologies.

Initially, ZnO nuclei are understood to grow via the diffusive mechanism,

resulting in smaller particles, and the growth process starts with self-assembly of

these ZnO nanocrystallites through an oriented attachment mechanism.28 The

Chapter 4

107

nanocrystallites possess higher surface energies and easily aggregate at an early

stage of the growth process under hydrothermal conditions.

Figure 8. High magnification TEM images of the as synthesized samples using zinc nitrate

precursor as a function of trizma base concentration in mole ratios. (a) 1.0 : 1.5, Rings; (b)

1.0 : 2.0, (c) 1.0 : 2.5; doughnuts (d) 1.0 : 3.0, hemispheres. The images of synthesized

samples indicate that smaller crystallites (primary particles) adjoin into larger shapes

through oriented growth and Ostwald ripening process. Scale bar provided in all

micrographs is 100 nm.

Analysis of the SAED patterns provided as the inset in Figure 3c,d reveals

that the polycrystalline domains are oriented differently for the ring-like and

hemispherical ZnO assemblies. The formation of these new structures as rings and

hemispheres can be attributed to an imperfect oriented attachment mechanism.49

Different but adjacent crystallographic planes oriented randomly self-assemble and

adjoining to yield spherical structures via an Ostwald ripening process form the

basis of this mechanism.50 It is reported that the inner region possessing significantly

higher surface energies has a stronger tendency to partially redissolve, reorient,

redeposit, and adjoin via Ostwald ripening process to help formation of ring-like

structures. With the increase in concentration of TB we observe that the dissolution

Chapter 4

108

effect of inner region reduces, reflecting the final morphology (formation of

doughnuts/hemispheres) with indication of considerable surface roughness.

4.3.2. Optical Properties and Photocatalytic Activity:

The effect of morphology on the optical properties of synthesized ZnO

samples was evaluated from the diffuse reflectance (UV-DRS) studies. As can be

observed from Figure 9a, the absorption edge of the ZTR-1 sample is ∼414 nm,

which corresponds to a band gap of 2.99 eV. For all other samples, the observed

absorption band edge is found to be ∼398 nm, corresponding to a band gap of ∼3.11

eV. Overall, no noteworthy change was inferred in the optical assessment that

relates to the change in morphology of the synthesized samples. Interestingly, even

though the crystals synthesized are notably larger in size (micrometers), all the

samples show a considerable red shift of the optical band gap when compared to that

of bulk ZnO (∼3.34 eV). This observation can be rationalized considering the

increased percentage of polar facets present in the synthesized samples,

consequently with more oxygen defects. Earlier reports have suggested that large

fraction of polar planes can generate significant number of oxygen defects which can

appreciably influence the optical properties.21,51 It has been reasoned that these

oxygen defects can form shallow levels within the band gap, leading to the

redshift.51 Photoluminescence (PL) spectra of hexagonal discs were measured using

a He−Cd laser as an excitation source presented in Figure 9b. The room temperature

PL spectra of the ZnO samples excited at 360 nm showed two intense emission

bands centered at ∼398 and 409 nm along with a less intense band centered at ∼468

nm. While the UV emission corresponds to the near-band-edge emission resulting

from the excitation recombination, the blue emission peak is usually attributed to the

Chapter 4

109

zinc vacancy, with instances of being referred to oxygen vacancy as well.26,52,53

However, the origin of defect emissions in ZnO is still an unresolved question, in

spite of a large number of reports. The polar planes easily generate oxygen

vacancies and are expected to prevent electron−hole recombination, thereby

resulting in an increased photoinduced activity.54 That in our synthesized samples

oxygen vacancies exists was evident and confirmed by Raman and XPS analysis as

discussed in the preceding section.

Morphology induced enhanced photocatalytic performance of the hexagonal

prismatic ZnO and zinc oxide rings were studied following the degradation kinetics

of methylene blue (MB) under the UV light as a model system. The results of the

MB degradation in a series of experimental conditions are summarized in Figure 10.

In the absence of catalyst, only a minimal decrease in the concentration of MB was

detected. The characteristic absorption of the methylene blue seen at 663 nm was

selected for monitoring the adsorption and photocatalytic degradation process. A

rapid decrease in the initial absorbance and finally disappearance of this peak in the

presence of synthesized ZnO samples could be observed (Figure 10a). The

photocatalytic effect and degradation of MB were obvious under UV irradiation in

the presence of synthesized ZnO, and the samples do exhibit different

photoactivities, depending on the morphology and surface area. Comparative

analysis shows, that for the ZTR-3 sample, the dye degradation was 97% within 40

min while for the ZTR-1 and ZTR-2, the degradation was 93% and 88%,

respectively. ZNTR sample, on other hand, degrades 80% of dye within 40 min.

Among all the synthesized samples, ZTR-4 shows the best photocatalytic activity

and completely degrades the MB solution within 40 min, under UV illumination.

Chapter 4

110

300 350 400 450 500 550

ZTR-1 ZTR-2 ZTR-3 ZTR-4 ZNTR-2

Abs

orba

nce

Wavelength (nm)

(a)

400 450 500 550

Inte

nsity

(a.u

)

Wavelength(nm)

ZNTR-2 ZTR-4 ZTR-3 ZTR-2 ZTR-1

(b)

Figure 9. (a) Diffuse reflectance UV spectra of the as synthesized samples recorded in the

absorbance mode. The band edge provides an estimate of the optical band gap; (b)

Photoluminescence spectra of as synthesized samples recorded at an excitation wavelength

of 360 nm and room temperature.

500 550 600 650 700 7500.0

0.1

0.2

0.3

0.4

0.5

0.6

Ab

sorb

ace

Wavelength (nm)

0 min 10 min 20 min 30 min 40 min

(a)

0 10 20 30 40 50 60

0.0

0.2

0.4

0.6

0.8

1.0

C/C

o

ZTR-1 ZTR-2 ZTR-3 ZTR-4 ZNTR-2 ZNR MB

Time (min)

(b)

Figure 10. (a) UV/Vis spectra of methylene blue over ZTR-4 sample at different irradiation

times under UV light; (b) Change in methylene blue concentration (photocatalytic

degradation) as a function of UV light irradiation time over different ZnO samples as

catalysts. The catalyst-free condition is denoted by MB in the figure.

To demonstrate the morphology induced superior photocatalytic

performances of hexagonal ZnO discs and ring-like microstructures over the other

morphologies synthesized, photocatalytic activity of ZnO rods (ZNR) was also

studied. It is evident from our findings that these ZnO rods show relatively poor

photocatalytic activity when compared to hexagonal discs and rings. The schematic

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illustration (Figure 11) represents the trend of increasing photocatalytic activity as

observed for the synthesized samples studied.

The superior photoactivity of hexagonal discs can be attributed to their exotic

structural features with predominantly exposed active polar facets. It has been earlier

demonstrated that highly exposed terminal polar surfaces of ZnO crystals could

induce high photocatalytic activity. Jang et al. and Mclarn clearly suggested that

compared to other planes the exposed (0001) and (0001)̅ polar faces are more active

surfaces for photocatalysis.8,17 As demonstrated in the present study, all the

synthesized ZTR samples show the presence of highly ordered and cleanly exposed

polar faces (Figures 1 and 3). Thus, in our observation, ZTR-1, ZTR-2, and ZTR-3

all show better photocatalytic efficiency with ZTR-4 exhibiting the best activity

(ZTR-4 ≥ ZTR-3 > ZTR-2 > ZTR-1), consistent with their surface area and

percentage of exposed polar facets.

The comparison of BET surface area of the ZNR (16.55 m2/g), ZNTR-2

(94.7 m2/g), and ZTR-1 (4.5 m2/g) reveals the importance of polar planes of ZnO.

The BET surface area of ZNTR-2 is ∼20 times higher than the ZTR-1 and ∼4 times

higher than ZNR. Conventionally, the higher surface area is beneficial for the higher

catalytic activity. Yet the photoactivity of ZTR-1 was found to be better than that of

ZNTR-2. Though the ZNR sample possesses higher surface area compared to that of

ZTR-1, the photocatalytic activity was significantly less than the ZTR-1 sample.

This too can be understood on the basis of percentage of polar facets present, which

definitely decreases for ZNR (ZnO microrods) due to their larger particle size

compared to that of ZTR-1.8 Analysis of photoluminescence spectra along with

Raman and XPS also confirms the presence of more oxygen vacancies in samples

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that have higher ratio of polar planes which supports higher photocatalytic

efficiencies. Exposed surfaces of polar facets induce rapid generation of H2O2 in situ

under light irradiation that accounts for the high photoactivity of ZnO. These results

clearly demonstrate that ZTR samples with higher percentage of exposed polar

facets possess superior photocatalytic activity than the ZnO rods with fewer exposed

polar facets (Figure 11).

Figure 11. A schematic illustration representing the correlation between polar facets and

photocatalytic activity trends observed experimentally for the synthesized ZnO structures.

0 10 20 30 40 50 600.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce(

a.u.

)

Wavelength(nm)

ZNTR-1 ZNTR-2 ZNTR-3 ZNTR-4

Figure 12: Photocatalytic activity studies on aseries of as synthesised ZNTR samples on the

degradation of methylene blue. The synthesised samples exhibit almost similar

photoactivities irrespective of their surface area and morphology.

These observations together provide conclusive evidence that the surface

area is not the only parameter that determines the photoactivity of ZnO. The

percentage of exposed polar faces bears more influence on the photoactivity of ZnO

than its surface area. In addition, the ZnO particles were found to be structurally

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very robust and stable even when treated with 0.1 M HCl solution at 90 °C for 12 h.

No apparent change was observed (Figure 13) and these findings indicate that the

synthesized samples are appreciably resistant to acid corrosion. It has been

suggested that such chemical stabilities of the ZnO samples can also be partially

held responsible for the higher photocatalytic activity of the ZnO observed.8

Figure 13. SEM images of ZTR-2, ZTR-3, ZTR-4 and ZNTR-2 samples were treated with

0.1M HCl for 12 hrs at 90 oC. Scale bar is 4m for all samples

4.4. CONCLUSIONS:

In summary, we demonstrate a successful strategy for synthesis of ZnO with

preferentially exposed polar facets in unique morphologies such as self-stacked

hexagonal discs, rings, and hemispheres. An appreciable control on the fraction of

polar facets is achieved by a simple hydrothermal method in aqueous base

environment. Choice of precursor, concentration of reactants, use of Trizma as a

base and structure-directing agent, mild basic conditions, slower rate of hydrolysis,

nucleation and growth all play equally important roles in determining the final

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114

morphology and fraction of the exposed crystal facets of the hexagonal ZnO. The

photocatalytic activities of these controlled ZnO superstructures were evaluated

albeit with a model system, methylene blue. The photocatalytic studies reveal not

only enhanced performance but also a significant dependence on the shape and

fraction of polar faces present in the samples. We believe that this approach along

with our findings on shape and directionality control, can possibly be adopted and

extended to other materials as well, to create unique morphologies yielding exotic

physicochemical properties.

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