Microstructure, growth process and enhanced photocatalytic activity of immobilized hierarchical ZnO nanostructures

Article abstract of DOI:10.1039/c3ra42902d

The hierarchical ZnO nanostructures, branched nanorods, were prepared through a multi-step growth method on glass substrates without any capping agents. The process includes magnetron sputtering deposition of ZnO seed particles and hydrothermal growth of ZnO nanorods. The morphology, growth process and microstructure were characterized using scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The hierarchical structure consists of large primal nanorods as the trunks and much smaller secondary nanorods as the branches. Both the trunks and branches are single crystal grown along the [0001] direction. The ZnO seed particles which experienced Ostwald-ripening are essential for the growth of branched nanostructures. Raman scattering, photoluminescence (PL) emission and UV-Vis absorption were applied to measure the defects and optical properties. The hierarchical ZnO nanostructures exhibited the enhanced photocatalytic performance in degradation of Rhodamine B (RhB) under UV light, which can be attribute to the increased surface area, improved light harvesting, active surfaces, dense branch network and large amount of defects. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra42902d

View Article Online
View Journal
RSC Advances
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: Y. Liu, N. Zhao and W. Gao, RSC Adv.,
2013, DOI: 10.1039/C3RA42902D.
This is an Accepted Manuscript, which has been through the RSC Publishing peer
review process and has been accepted for publication.
Accepted Manuscripts are published online shortly after acceptance, which is prior
to technical editing, formatting and proof reading. This free service from RSC
Publishing allows authors to make their results available to the community, in
citable form, before publication of the edited article. This Accepted Manuscript will
be replaced by the edited and formatted Advance Article as soon as this is available.
To cite this manuscript please use its permanent Digital Object Identifier (DOI®),
which is identical for all formats of publication.
More information about Accepted Manuscripts can be found in the
Information for Authors.
Please note that technical editing may introduce minor changes to the text and/or
graphics contained in the manuscript submitted by the author(s) which may alter
content, and that the standard Terms & Conditions and the ethical guidelines
that apply to the journal are still applicable. In no event shall the RSC be held
responsible for any errors or omissions in these Accepted Manuscript manuscripts or
any consequences arising from the use of any information contained in them.
www.rsc.org/advances
Registered Charity Number 207890

DynamDicOIA: 1r0t.1ic03Vle9ie/wCL3AiRrntAick4le2sO9n►0l2inDe
RSC Advances
Page 1 ofR10SC Advances
Cite this: DOI: 10.1039/c0xx00000x
www.rsc.org/advances
PAPER
Microstructure, growth process and enhanced photocatalytic activity of
immobilized hierarchical ZnO nanostructures
Yangsi Liu,*a Naiqin Zhaob and Wei Gaoa
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
The hierarchical ZnO nanostructures, branched nanorods, were prepared through a multi-step growth
method on glass substrates without any capping agents. The process includes magnetron sputtering
deposition of ZnO seed particles and hydrothermal growth of ZnO nanorods. The morphology, growth
process and microstructure were characterized using scanning electron microscopy (SEM), high
10 resolution transmission electron microscope (HRTEM) and X-ray diffraction (XRD). The hierarchical
structure consists of large primal nanorods as the trunks and much smaller secondary nanorods as the
branches. Both the trunks and branches are single crystal grown along [0001] direction. The ZnO seed
particles which experienced Ostwald ripening are essential for the growth of branched nanostructures.
Raman scattering, photoluminescence (PL) emission and UV-Vis absorption were applied to measure the
15 defects and optical property. The hierarchical ZnO nanostructures exhibited the enhanced photocatalytic
performance in degradation of Rhodamine B (RhB) under UV light, which can be attribute to the
increased surface area, improved light harvesting, active surfaces, dense branch network and large
amount of defects.
thermal vapour transport and condensation process.15 High-
density ZnO heterostructures were grown on various 1D
nanomaterials, including carbon nanotubes, GaN nanowires, SiC
nanowires, by thermochemical vapour deposition of Zn.16 The
50 vapour-liquid-solid protocols generally require high temperatures,
involve complicated equipment, and have a low yield.
1. Introduction
20 Semiconductor based nanostructures have stimulated a great
interest in contemporary materials researches due to their unique
property and huge potential in chemical, physical, electronic, and
optical applications.1,
As
a
II-VI compound n-type
2
Aqueous solution approaches are more attractive because of
their low-energy consumption, low cost and feasibility for
industrial-scale manufacturing. Pioneering hierarchical ZnO
55 crystals were fabricated via site-specific nucleation and growth in
solutions with sequential branching events by Zhang and Sounart
et al.17, 18 Ko et al. reported the "nanoforest" composed of high
density, long branched “treelike” multigeneration ZnO nanowire
photoanodes selectively grown by a hydrothermal method.6
60 Seeded growth of ZnO hexabranched nanostructures was
achieved by a laterally epitaxial solution growth strategy.14
Hierarchical tree-like ZnO arrays with increasing branching order
and complexities have been prepared by a simple hydrothermal
oxidation of Zn foils in an aqueous medium.19
semiconductor, ZnO has a direct wide band gap of 3.37 eV at
25 room temperature and large exciton binding energy of 60 meV.3
ZnO has shown considerable useful feature including high
photoreactivity, chemical and thermal stability and low
environmental toxicity.4, 5
The wide attractions to ZnO nanostructures, especially one
30 dimensional (1D) structures (e.g., nanorods, nanowires, nanobelts
and nanotubes), come from the ease of crystallization and
simplicity of fabrication process. In comparison with the normal
ZnO 1D nanostructures, the hierarchical configurations are
particularly desirable for their unique characteristics of increased
35 surface area, advanced geometric structures, specific morphology
and atomic arrangements, dense network, and favourable electron
transportation.6-8 These advantages are linked to the functional
properties of ZnO, making the hierarchical architectures more
attractive in the applications of dye-sensitized solar cells,6-9 gas
40 sensors10, 11 and photocatalysis.12-14
65
During the solution based preparation, capping agents (e.g.,
diaminopropane, citrate, ethylene diamine, diaminobutane, and
polyethylene glycol) have been frequently applied as structure
tuning additives.14, 17-19 The capping agent molecules can slightly
etch the surface of ZnO to produce nucleation sites for the
As a response, considerable efforts have been devoted to
fabricate hierarchical ZnO nanostructures. In general, two main
70 secondary growth. Without these structure directing agents,
hierarchical ZnO structures could not be produced owing to the
absence of the nuclei 19. However, reports on the physical
introduction of nucleation sites are very limited.6
approaches have been employed: gas phase processes15, 16 and
13, 14, 17, 18
solution based routes.6-9,
Hierarchical ZnO
45 nanostructures on In2O3 nanowires have been grown by using a
In this paper, we report a multi-step nucleation-growth process
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

View Article Online
DOI: 10.1039/C3RA42902D
RSC Advances
Page 2 of 10
without using capping agents to form ZnO branched nanorods. To
the best of our knowledge, although hierarchical ZnO
nanostructures have been fabricated as described above, there is
The crystal structure of the ZnO NRs, ZnO NRs-SPs and ZnO
BNRs was identified by X-ray diffraction (XRD, Bruker D2
phaser) using Cu Ka radiation. The morphology and
still no report on the realization of branched architectures via 60 microstructure of the ZnO samples were characterized by a field-
5
magnetron sputtering. We used such a technique to obtain the
seed particles, from which the branches of hierarchical ZnO
nanostructures have developed. A possible mechanism has been
suggested to elucidate the formation and growth process of the
emission gun scanning electron microscope (FEG-SEM, Philips
XL-30S) and a high resolution transmission electron microscope
(HRTEM, Philips Tecnai G2 F20, 200 KV). The composition of
the ZnO BNRs was conducted with energy dispersive
ZnO branches. The morphology, microstructure, crystallographic 65 spectroscopy (EDS) equipped on SEM. Raman spectra were
10 phase, optical property and photocatalytic performance of the
ZnO samples were investigated.
Furthermore, the prepared ZnO nanostructures were
immobilized on glass substrates, offering themselves the
observed on Renishaw Raman spectrometer (System 1000) with
Ar+-ion laser at 488 nm excitation. Room temperature
photoluminescence (PL) properties were measured by a Perkin-
Elmer LS55 luminescence spectrophotometer with a Xe lamp as
advantages of easy recycling over the free-standing powders 70 the excitation source at a wavelength of 325 nm. The optical
15 when they are served as photocatalysts. As expected, the
hierarchical ZnO nanostructures displayed the high photocatalytic
activity in organic dye degradation. The influencing factors for
the photocatalytic enhancement such as the branched structure,
active facets and instinct defects have also been discussed.
absorption spectra were recorded on an Agilent 8453 UV-vis
spectrophotometer.
2.3 Photocatalytic degradation
Degradation of Rhodamine B (RhB) dye aqueous solution was
75 used to evaluate the photocatalytic activity of the ZnO NRs, ZnO
NRs-SPs and ZnO BNRs. The photocatalysts grown on glass
slides of 25 mm × 10 mm were immersed into RhB solutions (3
ml, 2.5 mg L-1). Prior to the degradation test, they were kept in
dark for 30 min to establish an adsorption/desorption equilibrium
80 of RhB with photocatalysts. This guarantees that the measured
concentration changes during UV irradiation would be solely
caused by photocatalysis. Then the RhB solutions were exposed
to two 9 W UV lamps (λ = 254 nm, Philip).
The variation of RhB concentration with irradiation time was
85 measured using a UV-vis spectrophotometer (Perkin Elmer
Lambda 35). The intensity of the absorption band peak (553 nm)
was recorded at a certain time interval. The degradation rate was
estimated as C/C0, where C0 is the equilibrium concentration
before UV irradiation and C is the concentration at the sampling
90 time. According to the Beer-Lambert law, the concentration of
RhB is linearly proportional to the absorbance value (A) at 553
nm, thus C/C0 = A/A0.
20 2. Experimental
2.1 Preparation of hierarchical ZnO nanostructures
2.1.1. Growth of ZnO nanorods (NRs)
All chemicals of analytical grade and de-ionized (DI) water were
used throughout this study. Glass substrates were pre-coated with
25 ZnO seed layers by magnetron sputtering. The details are
described elsewhere.20 The ZnO NRs were grown from the ZnO
seed layers via a hydrothermal method.21, 22 An aqueous solution
containing 25 mM zinc nitrate (Zn(NO3)2·6H2O, 98%) and
hexamethylenetetramine (HMT) (C6H12N4, 99%) with an equal
30 molar ratio was prepared. Diluted nitride acid (HNO3, 5M) was
added dropwise to adjust the initial pH value to 5. The aqueous
solution was then transferred into a sealable glass jar, in which
the glass substrates with the ZnO seed layers were held face-
down. The whole set was put into an oven at 95°C for 4 h, after
35 which the glass substrates were withdrawn from the solution,
rinsed with DI water and dried at 60°C overnight.
2.1.2 Deposition of ZnO seed particles
3. Results and discussion
The glass substrates with the ZnO NRs were then loaded into a
magnetron sputtering unit installed with a ZnO target (99.99%).
40 The background vacuum pressure of the sputter chamber was
lower than 6.7 × 10-4 Pa. Argon gas (99.999%) was introduced
into the chamber as the working gas with a flow rate of 10 sccm
and a pressure of 1.33 Pa. Sputtering was performed with a DC
current of 0.25 A for 2 h to deposit ZnO seed particles onto the
45 ZnO NRs.
3.1 Morphology and composition
95 Fig. 1 shows the SEM images of the ZnO NRs, ZnO NRs-SPs
and ZnO BNRs. The ZnO NRs randomly tilt from the glass
substrate, which are 800-1000 nm in diameter and several micron
in length. These ZnO rods distribute separately and they are in the
hexagonal shape with sharp edges and smooth surfaces,
100 indicating their growth direction is along the c-axis (Fig. 1a). The
ZnO NRs-SPs are still in the hexagonal shape, but the surfaces
become uneven and the edges are blunt because they were
covered by many seed particles (Fig. 1b). The ZnO BNRs are
composed of the primal nanorods as trunks and the secondary
105 nanorods as branches. Abundant radial nanorods with much
smaller size emanate from the ZnO NRs can be observed on both
the top and side surfaces (Fig. 1c-d). The composition of the ZnO
BNRs was analysed by EDS. Apart from Si and Ca of the glass
substrates and Pt from the pre-SEM coating, Zn and O are the
110 only detected elements, proving no contamination was introduced
(Supporting Information Fig. S1).
2.1.3 Growth of ZnO branched nanorods (BNRs)
The ZnO BNRs were acquired via the second hydrothermal
method. The procedure was the same as the first hydrothermal
method in section 2.1.1, except that no HNO3 was added to the
50 aqueous solution and the initial pH value was 7. The ZnO
nanorods with seed particles (NRs-SPs) were immersed upside
down into an aqueous solution containing 25 mM
Zn(NO3)2·6H2O and HMT with an equal molar ratio. After
holding the samples at 95°C for 4 h, the hierarchical ZnO
55 nanostructures were obtained on the glass substrates.
2.2 Sample characterization
The formation process from the ZnO NRs to the ZnO BNRs is
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 2

View Article Online
DOI: 10.1039/C3RA42902D
Page 3 of 10
RSC Advances
3.3 Effect of ZnO seed particles
The necessity of ZnO seed particles was verified by
schematically illustrated in Fig. 2. The ZnO NRs were firstly
grown in an acid aqueous solution by a hydrothermal process,
and then the seed particles were placed by magnetron sputtering
thereon. After that the intermediates, ZnO NRs-SPs, were
undergone another hydrothermal protocol in a neutral solution
and finally developed into the hierarchical ZnO nanostructures,
ZnO BNRs.
a
60 counterevidence study. The second hydrothermal process was
directly applied to the ZnO NRs before the magnetron sputtering
in section 2.1.2, which means there were no intentionally-induced
seed particles. Fig. 4 gives the morphologies of the products after
the ZnO NRs were incubated in the identical aqueous solution for
65 4 h. As expected, very different nanostructures were obtained.
Some nanowires appear on the top surfaces of the primal
nanorods, but no secondary crystals can be observed on the side
surfaces. The nanowire tips are apt to “gluing” together and the
nanowires assemble into clustered aggregates (Supporting
70 Information Fig. S2). Since the polar {0001} planes of ZnO are
more energetic than the nonpolar {10-10} planes, nuclei for the
new crystals would favourably emerge on the top surfaces rather
than the side surfaces. As a result, without the seed particles, no
secondary nanorods have grown from the side surfaces.
5
3.2 Growth process of ZnO BNRs
The secondary hydrothermal growth was applied to the ZnO
10 NRs-SPs for different times and the growing process of the ZnO
BNRs was monitored as demonstrated in Fig. 3.
Before the ZnO NRs-SPs were immersed in the aqueous
solution, a lot of densely distributed crystalline nanoparticles can
be observed on the hexagonal bodies to form the rough surfaces.
15 These particles are the ZnO seeds introduced by magnetron
sputtering (Fig. 3a). In the early stage, the seed particles enlarge
but their quantity reduces. It looks like some seed particles merge
with each other and become big embryos with convex shape (Fig.
3b). This can be explained by Ostwald-ripening effect. Ostwald-
20 ripening is a spontaneous process triggered by energetic self-
decline, in which the larger crystals grow at the expense of the
smaller ones.23 Smaller crystals are thermodynamically less stable
and they rapidly vanish, transferring their mass to the surface and
onto larger crystals.24 In our case, the seed particles are
25 metastable due to the small size, hence they transform into larger
ZnO crystals through Ostwald-ripening. The embryos keep
expanding and gradually contact with each other to form new
layers on the primal nanorods (trunks). Meanwhile, the nascent
secondary nanorods (branches) with much smaller size emerge
30 from the trunk surfaces (Fig. 3c). Some energetically favourable
embryos with smaller size became the nuclei for the secondary
nanorods during Ostwald-ripening as indicated by the arrows in
Fig. 3b. As abundant Zn2+ and OH- existed in the neutral aqueous
solution, ZnO branches then grew from these nuclei.
75
This result confirms that the ZnO branches are not the
derivatives of the ZnO trunks but grown from the seed particles
introduced by magnetron sputtering. Thus, the seed particles are
essential for preparing the hierarchical ZnO nanostructures.
3.4 Crystal structure
80 XRD was performed on the ZnO samples to investigate their
crystal structure and orientation. As shown in Fig. 5, all the
diffraction peaks are indexed to a typical hexagonal wurtzite ZnO
crystal. The highest (100) peak for the ZnO NRs results from the
oblique nanorod bodies, which allow a large number of side
85 surfaces with {1010} planes to be exposed to the incident X-ray.
It is worth noting that the (002) diffraction enhances and becomes
the strongest one for the ZnO NRs-SPs, indicating that the seed
particles have a strong c-axis texture. This can be confirmed by
the sole (002) peak in the XRD pattern of the ZnO film, which
90 was deposited on glass substrates by the same magnetron sputter
(Supporting Information Fig. S3). The significant increase of the
(002) peak for the ZnO BNRs is contributed by the abundant
branches grown from the trunk surfaces. The overwhelming (002)
diffraction also reveals that the ZnO branches have a preferential
95 growth along c-axis in [0001] orientation. No peaks related to
other Zn complexes, such as Zn(OH)2 and Zn(NO3)2, are
observed, evidence of the pure ZnO phase.
35
With the progress of incubation, the branches grow
continuously and some parts of the new layers exceed the top
surfaces of the trunks (Fig. 3d). The continuing expansion of the
new layers also results from the adequate supplement of zinc and
oxygen species in the bulk solution. The protuberant parts then
40 gradually become the new branches sprouting from the top
surface edges, which are perpendicular to the branches on the side
surfaces (Fig. 3e). The secondary nanorods enlarge rapidly in
length after 3 h, and their hexagonal body shape become more
obvious (Fig. 3f-g), implying that the branches are in the same
45 growth direction as the primal nanorods. The driving force comes
from the intrinsic growth impetus of ZnO crystals, which favours
both the trunks and branches growing along the c-axis direction.
Some of the ZnO branches are not exactly vertical to their
substrates (the side surfaces of ZnO trunks), possibly because
50 their root parts were oppressed more or less as the new layers
outspreading in the early stage, making their bodies oblique
slightly.
3.5 TEM analysis
The morphology and crystallographic orientation of the ZnO
100 hierarchical nanostructures were further investigated by HRTEM
as shown in Fig. 6. It can be seen that a ZnO BNR includes a big
nanorod and several smaller nanorods with a straight shape and
uniform diameters to form a trunk-branch structure (Fig. 6a &
Fig. S4). Fig. 6b-c and 6d-e are typical HRTEM images recorded
105 from a segment of the ZnO trunk and a ZnO branch, from which
regular lattice fringes are parallel to their growth axis direction.
The distance between the lattice planes was measured to be 0.28
nm, corresponding to the interspacing of the (100) planes of
wurtzite ZnO. The results verify that both the trunks and the
110 branches have wurzite single-crystalline structure in a preferential
growth along the [0001] c-axis direction, even the two
components of the ZnO nanostructures did not form
simultaneously.
As such, the ZnO BNRs have been developed from the ZnO
NRs through the growth of the secondary nanorods. The resultant
55 individual nanostructure shows a hierarchical configuration,
which is made up of a big trunk and many smaller branches on its
top and side surfaces (Fig. 3h).
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 3

View Article Online
DOI: 10.1039/C3RA42902D
RSC Advances
Page 4 of 10
3.6 Raman studies
peaks of the ZnO NRs-SPs and the ZnO BNRs, however, are
centred at 408 and 423 nm, respectively. It is known that the deep
60 level emission in the visible range is related to the defect states
such as oxygen vacancies and zinc interstitials, which would act
as luminescence centres.25, 32 The dominant peaks for the ZnO
samples experience red shifts from NBE emission to deep level
emission, indicating that the combination of photo-generated
65 electrons and holes reduces from the ZnO NRs to the ZnO BNRs.
The highest relative intensity of the blue emission (423 nm) and
the blue-green emission (486 nm) of the ZnO BNRs also suggests
that the hierarchical ZnO nanostructures possess the largest
number of defects, which is consistent with the Raman analysis.
Raman scattering is a sensitive characterization method to assess
the microstructure and defects of materials. The hexagonal
wurtzite ZnO structure belongs to the space group C6υ (P63mc).
On the basis of group theory, Raman phonon modes at the Γ point
of the Brillouin zone for wurzite ZnO are A1+2B1+2E2+E1.
Among them, the B1 mode is normally silent and others are
Raman active.7 The A1 and E1 modes are polar and can split into
the transverse optical (TO) and longitudinal optical (LO) phonon
4
5
10 modes. The nonpolar E2 mode is composed of two optical phonon
modes with a low (E2L) and a high frequency (E2H).25
Fig. 7 shows the Raman spectra of the ZnO NRs, ZnO NRs-
SPs and ZnO BNRs. For all the samples, vibrational peaks can be
clearly observed at 334, 380, 439, 581 cm-1. The most intensive
15 one at 439 cm-1 is characteristic of the high frequency E2H mode
of ZnO hexagonal wurtzite structure. The peaks at 334 and 380
cm-1 can be assigned as the second order Raman scattering arising
70
With the reduction of electron-hole combination and most
defects, the hierarchical ZnO nanostructures would have a greater
potential in photocatalytic applications than the simple ZnO
samples (discussed thereinafter).
3.8 UV-Vis absorption
from zone-boundary phonons 3E2H- E2L and the transverse optical
75 As shown in Fig. 9, the absorption band edges for all the ZnO
samples locate in the UV region with an obvious red shift from
the ZnO NRs to the ZnO BNRs. Two edges appear for the ZnO
NRs-SPs, which may be caused by the light adsorption of the
rough surfaces resulting from the deposition of seed particles and
80 the multi-reflection among the seed particles. The absorption
edge of ZnO BNRs exhibits an obvious red shift to 380 nm,
indicating the best light utilization rate. This is because when
light passes through the hierarchical ZnO nanostructures, multiple
scattering and reflection could happen among the branches and
85 the trunks, favouring the light trapping and absorption. The
enhanced light harvesting of the ZnO BNRs would also benefit
their photocatalytic activity.
26
A1(TO) phonon mode, respectively.7,
The peak at 581 cm-1
20 corresponds to the longitudinal optical E1(LO) phonon mode,
which is generally believed to relate to the defects in ZnO, such
as oxygen vacancies and zinc interstitials.10, 26, 27
The most prominent feature, however, of the results presented
in this graph, is the three anomalous Raman vibrational modes at
25 275, 510 and 644 cm-1 for the ZnO BNRs. Some explanations
have been proposed for the origin of these Raman modes
observed in ZnO, but no agreement has been reached yet. As
these modes have been observed for the doped ZnO with various
elements such as Fe, Sb, Al, Ga, and N,28-30 they were linked to
30 the intrinsic host-lattice defects or doping element related
complexes.28, 29 Manjon et al. showed that the anomalous peaks
are attributed to the silent Raman mode B1 of wurtzite ZnO
allowed by the breakdown of the translational crystal symmetry
induced by defects and impurities.31 In our experiments, no
35 doping was applied, therefore, we deduce that the three Raman
peaks may be originated from the silent B1 mode activated by the
intrinsic defects, which is also evidenced by the huge peak of the
defect related E1(LO) mode of the ZnO BNRs.
It can be noticed that none of the three peaks are seen from the
40 ZnO NRs, and as for the ZnO NRs-SPs, the peaks are quite small
and only one at 275 cm-1 is observable. Interestingly, the
appearance number of the additional peaks from the ZnO NRs to
the ZnO BNRs is in accordance with the relative intensity of their
E1(LO) mode at 581 cm-1. The variation of the anomalous and
45 E1(LO) modes reveals that the intrinsic defects increase from the
ZnO NRs to the ZnO BNRs.
3.9 Photocatalysis
The photocatalytic degradation of RhB under UV irradiation was
90 conducted to evaluate the photocatalytic ability of the ZnO NRs,
ZnO NRs-SPs and ZnO BNRs. Fig. 10 displays the degradation
rate of RhB using different ZnO samples. The blank experiment
under identical UV light but without any photocatalyst shows the
self-degradation of RhB is very slow. It can be seen that 91.7% of
95 RhB is degraded by the ZnO BNRs after 4 h UV irradiation, in
contrast, 73.4% and 52.2% of RhB are removed by the ZnO NRs-
SPs and the ZnO NRs, respectively. Obviously, the ZnO BNRs
exhibits the best photocatalytic activity, followed by the ZnO
NRs-SPs and the poorest one is the ZnO NRs.
100
The photocatalytic degradation mechanism of semiconductor
photocatalysts, such as ZnO, has been well understood and can be
formulated as follows:11, 33, 34
ZnO + hν→ ZnO (e− + h+)
3.7 Photoluminescence properties
e− + O2→ O2•−
As an effective technique to investigate the intrinsic defects and
the separation efficiency of photo-generated electrons and holes
50 of ZnO, photoluminescence (PL) analysis was applied. The room
temperature PL emission spectra of the ZnO samples are shown
in Fig. 8.
The ZnO NRs exhibit a dominant UV emission around 395
nm, which corresponds to the near band edge (NBE) emission of
55 ZnO. The NBE emission is usually ascribed to the excitonic
recombination of the photo-generated holes in the valence band
and the electrons in the conduction band.3 The highest emission
105
h+ + H2O → OH• + H+
O2•− + H+ → HO2•
HO2• + H2O → H2O2 + OH•
H2O2 + hν →2OH•
OH• + RhB → CO2 + H2O
110
h+ + RhB → CO2 + H2O
e− + h+ → recombination
Generally, when ZnO nanocrystals are irradiated by UV light
with a photon energy hν higher or equal to the band gap, an
electron (e−) in the valence band (VB) can be excited to the
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 4

View Article Online
DOI: 10.1039/C3RA42902D
Page 5 of 10
RSC Advances
conduction band (CB) with the simultaneous generation of a hole
evaluated. Fig. 11 shows the cyclic degradation rate of RhB using
(h+) in the VB, so the electron-hole pairs will appear.33, 34 The 60 one ZnO BNRs sample. After four cycles, the photocatalytic
photo-generated electrons can be accepted by the surface-
absorbed O2 molecules or the dissolved oxygen to produce
superoxide radical anions (O2•−). O2•− will be further transformed
into hydroxyl radicals (OH•), which are powerful oxidants to
performance is stable and the decrease of the degradation rate is
negligible, suggesting that the ZnO BNRs maintain relatively
high photocatalytic activity. Thus, the ZnO hierarchical
nanostructures are expected to have a long service life as
5
decompose RhB molecules. Similarly, the photo-generated holes 65 photocatalysts with high efficiency and durability. Moreover, the
can be trapped by the H2O molecules and break them apart into
ZnO BNRs are supported on removable substrates, which
simplify the recycling process such as filtration or centrifugation,
making the hierarchical ZnO nanostructures a steady and
economical photocatalyst.
Throughout the sample preparation, the ZnO seed particles, the
“origin” of ZnO branches, were introduced by magnetron
sputtering, which is a key step to realize the hierarchical ZnO
nanostructures. Under different deposition time, sputtering energy
and/or working pressure, the size and/or the distribution of ZnO
OH•. Meanwhile, the holes can directly participate in the
34
10 decomposition of RhB as OH•.11,
However, the photo-
generated electrons and holes can readily recombine to dissipate
the energy as light or heat, which will inhibit the photocatalysis
process.
Based on the proposed mechanism, several advantages of the
15 hierarchical ZnO nanostructures may contribute to their superior
photocatalytic performance. The branched structure of the ZnO
70
BNRs enables the best light harvesting, hence more UV light can 75 seed particles would be varied and the microstructure of the ZnO
be utilized for the photocatalytic process. The branches not only
provide the additional surface area for the reaction sites for RhB
20 degradation, but also allow more O2 molecules to be absorbed,
therefore, more OH• can be produced by the ZnO BNRs.
BNRs can be changed subsequently. It would be interesting to
investigate the influence of magnetron sputtering condition on the
morphology and photocatalytic property of the hierarchical ZnO
nanostructures, which is under further study.
The photo-generated electrons and holes are essential for the
phtocatalytic reaction. However, the electron-hole recombination
80 4. Conclusion
is always unavoidable, leading to a reduction of subsequent redox
In summary, the immobilized hierarchical ZnO nanostructures,
branched nanorods, have been synthesized through a combination
of magnetron sputtering and hydrothermal growth without
capping agents. The ZnO branched nanorods compose of the
85 primal nanorods as trunks and the much smaller secondary
nanorods as branches, both of which are single crystal with
hexagonal wurtzite phase grown along the c-axis. ZnO seed
particles were introduced to the ZnO trunks by magnetron
sputtering, which is a crucial step for the formation of the trunk-
90 branch structure. The seed particles went through Ostwald-
ripening, new layer formation, and rod growth into the ZnO
branches in a hydrothermal process.
12
25 reactions.11,
If the electrons or holes can be separated or
captured by any means, the recombination can be prevented and
the photocatalysis will be improved accordingly. The 1D ZnO
branches attached to the ZnO trunks create a dense network with
direct conduction pathways, which supplies more diffusion paths
30 for electron quick transfer8 and facilitates the electron-hole
separation. Besides, ZnO intrinsic defects, such as oxygen
vacancies, can serve as electron acceptors and trap the
photoinduced electrons temporarily to restrain the undesirable
35
recombination.34,
Therefore, the largest amount of intrinsic
vital role in the predominant
35 defects also plays
a
photodegradation efficiency of the ZnO BNRs.
The ZnO branched nanorods possess a large amount of
intrinsic defects which would reduce the recombination of photo-
95 generated electrons and holes. Moreover, the trunk-branched
construction enhances the light harvesting, facilitates the electron
transport, and provides the additional surface area with exposed
active facets. All these jointly result in the superior photocatalytic
performance of the hierarchical ZnO nanostructures in RhB
100 degradation.
Furthermore, it has been reported that the highly polar and
energetic {0001} planes (top surfaces) of ZnO crystals are more
photoelectrically active than other non-polar planes such as {10-
40 10} planes (side surfaces).5, 36, 37 The ZnO BNRs actually have a
large areal proportion of exposed active surfaces, because the
ZnO trunks are covered by ample ZnO branches whose {0001}
facets are exposed as rod tips, which can be also indicated by the
huge (002) peak in the XRD pattern. Consequently, the
45 photocatalytic activity of the ZnO BNRs is much higher than that
of the ZnO NRs whose major exposed surfaces are the inert {10-
10} facets. Accordingly, with all the above merits, the ZnO BNRs
represent the highest photocatalytic activity.
Among these virtue, we are inclined to believe that the light
50 harvesting is the most significant factor for the improved
photocatalytic activity. Because of the unique multi-level
morphology, the light scattering and trapping would happen
repeatedly among the branches and the trunks, the utilization of
the UV light which is the “direct stimulation” of the
Acknowledgement
The authors gratefully appreciate the assistance from the staff
members in the Department of Chemical and Materials
Engineering and the Research Centre for Surface and Materials
105 Science at the University of Auckland. The authors would also
like to thank research group members for their support, especially
Dr. Yuxin Wang and Mr. Shanghai Wei.
Notes and references
55 photocatalytic reactions will be heightened. This feature is not
a Department of Chemical and Materials Engineering, the University of
110 Auckland, Private Bag 92019, Auckland 1142, New Zealand. Fax: +64 9
3737463; Tel: +64 9 3737599 ext. 89840; E-mail:
38,
only beneficial for photocatalysis
39, but also has vastly
contributed to the solar cell efficiency6-8, 40, 41
Finally, the photocatalytic reusability of the ZnO BNRs was
.
yliu403@aucklanduni.ac.nz
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 5

View Article Online
DOI: 10.1039/C3RA42902D
RSC Advances
Page 6 of 10
b School of Materials Science and Engineering, and Tianjin Key
Laboratory of Composite and Functional Materials, Tianjin University,
Tianjin 300072, China
† Electronic Supplementary Information (ESI) available: EDS spectrum
5 of the ZnO BNRs. SEM image recorded from the top section of a ZnO
NR after the second hydrothermal growth. XRD pattern of the ZnO film
produced by magnetron sputtering. TEM image of the ZnO BNRs. See
DOI: 10.1039/b000000x/
22. L. Vayssieres, Adv. Mater., 2003, 15, 464-466.
23. X. W. Lou, L. A. Archer and Z. C. Yang, Adv. Mater., 2008, 20,
3987-4019.
24. A. Krost, J. Christen, N. Oleynik, A. Dadgar, S. Deiter, J. Blasing, A.
Krtschil, D. Forster, F. Bertram and A. Diez, Appl. Phys. Lett., 2004,
85, 1496-1498.
25. P. Jiang, J. J. Zhou, H. F. Fang, C. Y. Wang, Z. L. Wang and S. S.
Xie, Adv. Funct. Mater., 2007, 17, 1303-1310.
26. D. F. Zhang, L. D. Sun, J. Zhang, Z. G. Yan and C. H. Yan, Cryst.
Growth Des., 2008, 8, 3609-3615.
27. G. J. Exarhos and S. K. Sharma, Thin Solid Films, 1995, 270, 27-32.
28. C. Bundesmann, N. Ashkenov, M. Schubert, D. Spemann, T. Butz, E.
M. Kaidashev, M. Lorenz and M. Grundmann, Appl. Phys. Lett.,
2003, 83, 1974-1976.
29. K. Y. Wu, Q. Q. Fang, W. N. Wang, M. A. Thomas and J. B. Cui, J.
Appl. Phys., 2012, 111.
30. A. Kaschner, U. Haboeck, M. Strassburg, M. Strassburg, G.
Kaczmarczyk, A. Hoffmann, C. Thomsen, A. Zeuner, H. R. Alves, D.
M. Hofmann and B. K. Meyer, Appl. Phys. Lett., 2002, 80, 1909-
1911.
55
60
65
70
10 1. A. G. Kanaras, C. Sönnichsen, H. Liu and A. P. Alivisatos, Nano
Lett., 2005, 5, 2164-2167.
2. J. Li, S. J. Guo and E. K. Wang, RSC Adv., 2012, 2, 3579-3586.
3. U. Ozgur, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S.
Dogan, V. Avrutin, S. J. Cho and H. Morkoc, J. Appl. Phys., 2005,
15
98.
4. B. Dindar and S. Içli, J. Photochem. Photobiol., A, 2001, 140, 263-
268.
5. A. McLaren, T. Valdes-Solis, G. Li and S. C. Tsang, J. Am. Chem.
Soc., 2009, 131, 12540-12541.
20 6. S. H. Ko, D. Lee, H. W. Kang, K. H. Nam, J. Y. Yeo, S. J. Hong, C.
P. Grigoropoulos and H. J. Sung, Nano Lett., 2011, 11, 666-671.
7. H. M. Cheng, W. H. Chiu, C. H. Lee, S. Y. Tsai and W. F. Hsieh, J.
Phys. Chem. C, 2008, 112, 16359-16364.
31. F. J. Manjon, B. Mari, J. Serrano and A. H. Romero, J. Appl. Phys.,
2005, 97.
75 32. K. Vanheusden, C. H. Seager, W. L. Warren, D. R. Tallant and J. A.
Voigt, Appl. Phys. Lett., 1996, 68, 403-405.
8. C. M. Yang, M. H. Hon and I. C. Leu, J. Electrochem. Soc., 2012,
25
159, H638-H643.
33. Y. S. Liu, J. Han, W. Qiu and W. Gao, Appl. Surf. Sci., 2012, 263,
389-396.
34. Y. Zheng, C. Chen, Y. Zhan, X. Lin, Q. Zheng, K. Wei, J. Zhu and
Y. Zhu, Inorg. Chem., 2007, 46, 6675-6682.
35. J. Wang, P. Liu, X. Fu, Z. Li, W. Han and X. Wang, Langmuir, 2008,
25, 1218-1223.
36. J. Gotzen and G. Witte, Appl. Surf. Sci., 2012, 258, 10144-10147.
37. E. S. Jang, J. H. Won, S. J. Hwang and J. H. Choy, Adv. Mater.,
2006, 18, 3309-3312.
9. S. B. Zhu, L. M. Shan, X. N. Chen, L. He, J. J. Chen, M. Jiang, X. L.
Xie and Z. W. Zhou, RSC Adv., 2013, 3, 2910-2916.
10. L. X. Zhang, J. H. Zhao, H. G. Lu, L. Li, J. F. Zheng, J. Zhang, H. Li
and Z. P. Zhu, Sens. Actuator B-Chem., 2012, 171, 1101-1109.
80
85
30 11. S. Ma, R. Li, C. Lv, W. Xu and X. Gou, J. Hazard. Mater., 2011,
192, 730-740.
12. F. Xu, Y. Shen, L. Sun, H. Zeng and Y. Lu, Nanoscale, 2011, 3,
5020-5025.
13. M. A. Kanjwal, F. A. Sheikh, N. A. M. Barakat, X. Q. Li, H. Y. Kim
38. M. Ahmad, E. Ahmed, Z. L. Hong, J. F. Xu, N. R. Khalid, A. Elhissi
and W. Ahmed, Appl. Surf. Sci., 2013, 274, 273-281.
39. M. M. Gao, C. K. N. Peh, W. L. Ong and G. W. Ho, RSC Adv., 2013,
3, 13169-13177.
90 40. S. S. Yang, Y. F. Wang, L. S. Bai, B. F. Liu, J. Fan, X. Yang, H. X.
Zhao, C. C. Wei, Q. Huang, X. L. Chen, G. C. Wang, Y. Zhao and X.
D. Zhang, RSC Adv., 2013, 3, 208-214.
35
and I. S. Chronakis, Appl. Surf. Sci., 2012, 258, 3695-3702.
14. R. Kozhummal, Y. Yang, F. Güder, A. Hartel, X. Lu, U. M.
Küçükbayrak, A. Mateo-Alonso, M. Elwenspoek and M. Zacharias,
ACS Nano, 2012, 6, 7133-7141.
15. J. Y. Lao, J. G. Wen and Z. F. Ren, Nano Lett., 2002, 2, 1287-1291.
40 16. S. Y. Bae, H. W. Seo, H. C. Choi and J. Park, J. Phys. Chem. B,
2004, 108, 12318-12326.
17. T. R. Zhang, W. J. Dong, M. Keeter-Brewer, S. Konar, R. N. Njabon
and Z. R. Tian, J. Am. Chem. Soc., 2006, 128, 10960-10968.
18. T. L. Sounart, J. Liu, J. A. Voigt, M. Huo, E. D. Spoerke and B.
41. Y. M. Meng, Y. Lin and J. Y. Yang, Appl. Surf. Sci., 2013, 268, 561-
565.
95
45
50
McKenzie, J. Am. Chem. Soc., 2007, 129, 15786-15793.
19. F. H. Zhao, J. G. Zheng, X. F. Yang, X. Y. Li, J. Wang, F. L. Zhao,
K. S. Wong, C. L. Liang and M. M. Wu, Nanoscale, 2010, 2, 1674-
1683.
20. X. D. Yan, Z. W. Li, C. W. Zou, S. Li, J. Yang, R. Chen, J. Han and
W. Gao, J. Phys. Chem. C, 2010, 114, 1436-1443.
21. X. D. Yan, Z. W. Li, R. Q. Chen and W. Gao, Cryst. Growth Des.,
2008, 8, 2406-2410.
100
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 6

View Article Online
DOI: 10.1039/C3RA42902D
Page 7 of 10
RSC Advances
Fig. 1 SEM images of (a) ZnO nanorods (NRs), (b) ZnO nanorods with seed particles (NRs-SPs), (c-d) ZnO branched nanorods (BNRs); Inset in (a), (b)
and (c) is a corresponding cross-sectional SEM image of each sample.
5
Fig. 2 Schematic illustration of the formation procedure of the immobilized hierarchical ZnO nanostructures.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 7

View Article Online
DOI: 10.1039/C3RA42902D
RSC Advances
Page 8 of 10
Fig. 3 SEM images of the ZnO BNRs for different incubation times: (a) 0 h; (b) 0.5 h; (c) 1 h; (d) 2 h; (e-f) 3 h; (g-h) 4 h.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 8

View Article Online
DOI: 10.1039/C3RA42902D
Page 9 of 10
RSC Advances
Fig. 5 XRD patterns of the ZnO NRs, ZnO NRs-SPs and ZnO BNRs.
Fig. 4 SEM image of the ZnO NRs after the second hydrothermal growth.
5
Fig. 6 (a) TEM image of an individual ZnO BNR; HRTEM images of a segment from (b-c) the ZnO trunk and (d-e) a ZnO branch marked with squares in
panel (a).
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 9

RSC Advances
View ArtiPcleaOgnleine10 of 10
DOI: 10.1039/C3RA42902D
10 Fig. 10 Photodegradation rate of RhB by different ZnO photocatalysts.
Fig. 7 Raman spectra of the ZnO NRs, ZnO NRs-SPs and ZnO BNRs.
Fig. 11 Cyclic photodegradation of RhB by the ZnO BNRs.
Fig. 8 Room temperature photoluminescence (PL) spectra of the ZnO
5
NRs, ZnO NRs-SPs and ZnO BNRs.
Fig. 9 UV- Vis absorption spectra of the ZnO NRs, ZnO NRs-SPs and
ZnO BNRs.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 10