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
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 TiO23 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
91
Chapter 4
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 (0001̅)
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 (0001̅) 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
92
Chapter 4
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 (0001̅) 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
93
Chapter 4
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
94
Chapter 4
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.
95
Chapter 4
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 4m 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
96
Chapter 4
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.
(a)
(b)
v
(c)
(d)
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 4m 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 (1̅010) 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.
97
Chapter 4
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 assynthesized 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
98
Chapter 4
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
578.4
500
600
664.61
ZTR
ZNTR
383.11
332.7
Intensity (a.u.)
441.02
(b)
700
800
-1
Wavenumber (cm )
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 2p 3/2 and Zn 2p1/2 electrons in the
Zn2+ oxidation state, respectively (Figure 5). The O 1s spectrum shows the existence
99
Chapter 4
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
(a)
(b)
(c)
(d)
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
100
Chapter 4
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
(b)
Intensity (a.u)
ZTR
ZNTR
3500
3000
2500
2000
1500
-1
1000
500
Wavenumber(cm )
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
101
Chapter 4
(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
(a)
(b)
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 4m
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-NH3+ + OH-
(HOCH2)3C-NH2+ H2O
Zn2++ 4OHZn(OH)42-
Zn(OH)42ZnO+H2O+2OH102
Chapter 4
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.
103
Chapter 4
Table 1. Samples synthesized at different concentration of trizma base, corresponding pH
change observed as function of trizma base concentration and their surface area.
Sample Name
Zn/TB
pH of the reaction
BET Surface
moles ratio
Initial
Final
Area (m2g–1)
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
In general, depending on the structural anisotropy and surface electric
polarity of ZnO, the growth rate is higher for [0001], decreases for (1̅010), (101̅0),
(11̅00), and (1̅100), and is lowest for (0001̅) 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 (101̅0 ) or (011̅0) 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
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Chapter 4
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
⟨011̅0⟩ 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
(0001̅) 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
105
Chapter 4
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
106
Chapter 4
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
107
Chapter 4
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
108
Chapter 4
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.
109
Chapter 4
(a)
(b)
300
350
400
450
Wavelength (nm)
500
ZNTR-2
ZTR-4
ZTR-3
ZTR-2
ZTR-1
Intensity(a.u)
Absorbance
ZTR-1
ZTR-2
ZTR-3
ZTR-4
ZNTR-2
550
400
450
500
550
Wavelength(nm)
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.
0.6
(a)
1.0
ZTR-1
ZTR-2
ZTR-3
ZTR-4
ZNTR-2
ZNR
MB
0.8
C/Co
0.4
Absorbace
(b)
0 min
10 min
20 min
30 min
40 min
0.5
0.6
0.3
0.2
0.4
0.1
0.2
0.0
0.0
500
550
600
650
Wavelength (nm)
700
750
0
10
20
30
40
50
60
Time (min)
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
110
Chapter 4
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 (0001̅) 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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Chapter 4
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.
1.0
Absorbance(a.u.)
0.8
0.6
ZNTR-1
ZNTR-2
ZNTR-3
ZNTR-4
0.4
0.2
0.0
0
10
20
30
40
Wavelength(nm)
50
60
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
112
Chapter 4
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 4m 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
113
Chapter 4
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.
114
Chapter 4
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