Construction of Ag-modified TiO2/ZnO heterojunction nanotree arrays with superior photocatalytic and photoelectrochemical properties

Article abstract of DOI:10.1039/d0ra06596j

The work involves the preparation of TiO₂/ZnO heterojunction nanotree arrays by a three-step: hydrothermal, sol-gel, and secondary hydrothermal method, and then modification of Ag quantum dots (QDs). In the above process, the ZnO nanoparticles attached to the TiO₂surface were subjected to secondary growth by a hydrothermal method to form a unique nanotree structure with TiO₂, followed by Ag quantum dot modification by quantum dot deposition. In summary, TiO₂/ZnO nanotree arrays are cited for the first time. The prepared Ag-modified TiO₂/ZnO heterojunction nanotree arrays were found to exhibit enhanced photoelectrochemical and photocatalytic properties. The photocurrent of the Ag-modified TiO₂/ZnO heterojunction nanotree arrays is increased by 8-fold compared to the pure TiO₂nanorod arrays, the photocatalytic degradation rate within 180 minutes increased from 37% to 77% and the kinetic rate constants for the degradation of methyl orange were three times higher than the pure TiO₂nanorod arrays. The improved performance is partly due to the introduction of the TiO₂/ZnO heterojunction nanotree arrays which provide Ag QDs with greater adhesion area. Localized surface plasmon resonance (LSPR) leads to an increase in the intensity of absorbed light due to the modification of Ag QDs. On the other hand the generation of the TiO₂/ZnO heterojunction decreases the forbidden band width, resulting in the redshift of the light absorption edge. Therefore, TiO₂/ZnO heterojunction nanotree arrays are expected to play an important role in solar cells and photocatalytic materials.

Full text of DOI:10.1039/d0ra06596j

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PAPER
Construction of Ag-modified TiO2/ZnO
heterojunction nanotree arrays with superior
photocatalytic and photoelectrochemical
properties
Cite this: RSC Adv., 2020, 10, 34702
Kang Xu,
and Zhaoqi Sun
Zhu Liu, Shihan Qi, Zhuangzhuang Yin, Shangkun Deng, Miao Zhang
*
The work involves the preparation of TiO2/ZnO heterojunction nanotree arrays by a three-step:
hydrothermal, sol–gel, and secondary hydrothermal method, and then modification of Ag quantum dots
(QDs). In the above process, the ZnO nanoparticles attached to the TiO2 surface were subjected to
secondary growth by a hydrothermal method to form a unique nanotree structure with TiO2, followed
by Ag quantum dot modification by quantum dot deposition. In summary, TiO2/ZnO nanotree arrays are
cited for the first time. The prepared Ag-modified TiO2/ZnO heterojunction nanotree arrays were found
to exhibit enhanced photoelectrochemical and photocatalytic properties. The photocurrent of the Ag-
modified TiO2/ZnO heterojunction nanotree arrays is increased by 8-fold compared to the pure TiO2
nanorod arrays, the photocatalytic degradation rate within 180 minutes increased from 37% to 77% and
the kinetic rate constants for the degradation of methyl orange were three times higher than the pure
TiO2 nanorod arrays. The improved performance is partly due to the introduction of the TiO2/ZnO
heterojunction nanotree arrays which provide Ag QDs with greater adhesion area. Localized surface
plasmon resonance (LSPR) leads to an increase in the intensity of absorbed light due to the modification
of Ag QDs. On the other hand the generation of the TiO2/ZnO heterojunction decreases the forbidden
band width, resulting in the redshift of the light absorption edge. Therefore, TiO2/ZnO heterojunction
nanotree arrays are expected to play an important role in solar cells and photocatalytic materials.
Received 30th July 2020
Accepted 10th September 2020
DOI: 10.1039/d0ra06596j
rsc.li/rsc-advances
TiO2 and ZnO have been proved to be the best photocatalysts
for the oxidative degradation of organic compounds in many
1. Introduction
semiconductor oxides under ultraviolet (UV) irradiation. TiO2 is
the most widely used semiconductor photocatalytic material
due to its suitable band structure (3.2 eV), stable chemical
properties, high corrosion resistance, non-toxic and low
cost.14–18 ZnO (3.37 eV) and TiO2 have similar band gaps. The
higher electron mobility enable ZnO to exhibit a higher photo-
catalytic efficiency than TiO2. Moreover, the unique electrical
and optical properties of ZnO make it promising for applica-
tions in photocatalysis. However, ZnO is less stable than TiO2
(which is more prone to photocorrosion), which severely limits
the use of ZnO. The combination of TiO2 and ZnO can give an
oxide mixed heterostructure having good photocatalytic prop-
erties.19,20 As a result, the TiO2/ZnO heterostructure has also
been extensively studied. For instance, Wang et al. applied TiO2
to the surface of ZnO and observed that the charge recombi-
nation was inhibited and UV absorption was increased
compared to bare TiO2.21 Furthermore, Johra et al. found that
the TiO2/ZnO heterojunction was better than that of ZnO
alone.22 The coupled synergistic effect of the metal oxide TiO2
and ZnO can change their electronic state to produce
Semiconductor oxides comprise a class of compounds with
unique optical and electrical properties. Many semiconductor
oxides have been found to be ideal photocatalysts for the
decomposition of organics.1,2 However related reports show that
in a single semiconductor, the recombination of electron–hole
pairs occurs very rapidly, which signicantly reduces the pho-
tocatalytic efficiency of these materials and makes them diffi-
cult to use.3–6 For nearly a decade, in order to increase
photocatalytic activity by promoting the separation of photo-
excited charges or improving the surface of photocatalysts,
various strategies have been employed. For example, suitable
structural designs, ion doping, and the formation of semi-
conductor heterojunctions by combining with metals or other
semiconductors.7–11 Among them, the construction of semi-
conductor heterojunctions has attracted much attention due to
their signicant improvement in photocatalytic activity.12,13
School of Physics & Materials Science, Anhui University, Hefei 230601, PR China.
E-mail: szq@ahu.edu.cn
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photogenerated charge carriers that facilitate the separation of mixed in the ratio of 1 : 30 : 30 to obtain a culture solution, the
the electron pairs.23 As a result, the electrons are transferred FTO conductive glass was placed in an autoclave, and the
from the conduction band (CB) of ZnO to the CB of TiO2,24 culture solution was added to immerse the conductive glass.
likewise, holes can be transferred from the valence band (VB) of Then, aer 12 hours of reaction at 150 ꢀC, cool to room
TiO2 to VB of ZnO.23 The heterostructure simultaneously temperature, rinse with deionized water and absolute ethanol,
accelerates the decrease in the electron–hole pair recombina- and dry to obtain a transparent uoride-doped tin oxide (FTO)
tion in the semiconductor and increases the lifetime of conductive glasses with TiO2 nanorods arrays on its surface.
carriers.25 Although TiO2/ZnO heterostructures can impede the
The ZnO seed layer was rst prepared by the sol–gel method.
recombination of electron–hole pair, there are also many 0.4 mol of zinc acetate dihydrate (Zn(CH3COO)2$2H2O) was
disadvantages, such as poor response of visible light and high dissolved in 100 mL of ethylene glycol monomethylether
photocorrosiom. Metal quantum dots (QDs) own premium (HOCH2CH2OCH3), magnetically stirred for 10 min at 60 ꢀC,
optical characteristics, involving adjustable bandgap, high and then diethanolamine (C4H11NO2) having a concentration of
extinction coefficient, and optical absorption range.26–28 Among Zn2+ or the like was added dropwise as a stabilizer to the solu-
metal quantum dots, Ag QDs have attracted more and more tion and continued stirring for 30 min to obtain Zn2+ seed
attention because of their longitude surface plasmon resonance solution at a concentration of 0.4 mol Lꢁ1. Preparation of ZnO
(LSPR) and show the best photocatalytic properties in semi-
conductor heterojunctions.29,30
According to the advantages of Ag QDs of TiO2/ZnO heter-
ojunction, in order to better improve photoelectrochemical
performance and photocatalytic performance, ZnO nanorods
were grown on TiO2 nanorods by hydrothermal method to
form TiO2/ZnO heterojunction nanotree arrays, then modify
TiO2/ZnO heterojunction nanotree arrays with Ag QDs
(Scheme 1). The photocatalytic properties and photo-
electrochemical of the samples were tested. It was found that
Ag-modied TiO2/ZnO nano-dendritic heterostructures have
enhanced
photoelectrochemical
and
photocatalytic
properties.
2. Experimental
2.1 TiO2/ZnO heterojunction nanotree arrays preparation
TiO2 nanorod arrays was directly synthesized by a simple
hydrothermal method. The tetrabutyl butoxide (C16H36O4Ti,
CP), deionized water (DIW) and hydrochloric acid (HCl) were
Fig. 1 XRD patterns of (a) TiO2 nanorod arrays, (b) Ag-modified TiO2
nanorod arrays, (c) TiO2/ZnO heterojunction nanotree arrays and (d)
Ag-modified TiO2/ZnO heterojunction nanotree arrays.
Scheme 1 Schematic diagram of fabrication of Ag-modified TiO2/ZnO heterojunction nanotree arrays.
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seed layers of TiO2 nanorod arrays on FTO glass substrates by hours. Finally, a TiO2/ZnO heterojunction nanotree arrays is
the dipping-liing method, pulling and drying every 5 minutes formed on the FTO glass.
and repeating three times. Finally, it was calcined in a muffle
ꢀ
furnace at 480 C for 1 hour to nally obtain an array of TiO2
2.2 Ag-modied TiO2 nanorod and TiO2/ZnO heterojunction
nanotree arrays preparation
nanorods surrounded by a seed layer of ZnO.
Then, the seed layer is grown into a rod-like structure by
Ag-modied TiO2 nanorod arrays and Ag-modied TiO2/ZnO
heterojunction nanotree arrays were formed by photo-
deposition. The TiO2 nanorod arrays and the TiO2/ZnO heter-
ojunction nanotree arrays composite nanostructure FTO
substrate were placed in a transparent glass Petri dish, and
30.0 mL of a 0.1 M silver nitrate solution was added and irra-
diated for 1 hour under ultraviolet irradiation, wherein the color
of the sample turns gray. Finally, rinse the surface with deion-
ized water and dry it to obtain the desired sample.
hydrothermal method. First, 50 mL of 0.02 mol Lꢁ1 zinc nitrate
hexahydrate (Zn(NO3)2$6H2O) and 50 mL of hexamethylene-
tetramine (C6H12N4) group having
a concentration of
0.02 mol Lꢁ1 are prepared. The two solutions were mixed and
congured as a growth solution, and the FTO glass substrate
coated with the ZnO seed layer coated TiO2 nanorod array was
placed at a 45ꢀ angle downward in the reaction vessel, and
grown under the environmental condition of 90 ꢀC for four
Fig. 2 (a) Survey XPS spectra for Ag-modified TiO2/ZnO heterojunction nanotree arrays; (b) O 1s, (c) Zn 2p and (d) Ag 3d (e)Ti 2p XPS high
resolution spectrum for Ag-modified TiO2/ZnO heterojunction nanotree arrays.
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3d and Ti 2p. It can be seen from the enhanced line of Ti 2p
(Fig. 2(d)) that the binding energy of Ti 2p is 458.7 eV, corre-
sponding to Ti 2p3/2, which proves the existence of Ti4+. As shown
in Fig. 2(c) there are two peaks were found in the Zn 2p spectrum
at 1021.8 eV (Zn 2p3/2) and 1044.9 eV (Zn 2p1/2). Since in the XPS
spectrum no other Zn 2p peaks are observed, the Zn2+ ions exist
as separate ZnO phases.34 In Fig. 2(c), the nonlinear least-squares
tting procedure will t the O 1s spectrum to two peaks, corre-
sponding to peak a and peak b, that means there are two
different types of O2ꢁ in the sample. The low-energy peak at
530.7 eV corresponds to the O2ꢁ ions in the TiO2 and the ZnO
lattices. The second binding energy of 532.1 eV may be related to
surface hydroxyl adsorption of O2ꢁ. In Fig. 2(d), Ag 3d has two
weak peaks with binding energies of 367.7 and 373.7 eV. The
energy gap between the two peaks was 6.0 eV, which corresponds
to the peak of the metal Ag, showing the presence of Ag
element.35,36
Scanning electron microscopy (SEM) can characterize the
morphology and size of the TiO2/ZnO/Ag, TiO2, TiO2/Ag and
TiO2/ZnO samples. Fig. 3(a)–(e) is an SEM image of the prepared
sample. It can be seen from the Fig. 3(a) that the TiO2 grown by
hydrothermal method grows on the FTO substrate in a rod-like
structure. Fig. 3(b) shows the image of Ag QDs on TiO2 nano-
rods. The size of Ag NPs obtained by photodeposition has been
reported to be 5–15 nm.37 Fig. 3(c) is an SEM image of ZnO
nanoparticles xed on FTO glass grown from TiO2 nanorods by
sol–gel method, it can be seen from the image that the ZnO
nanoparticles are wrapped around the TiO2 nanorods. The
hydrothermal method causes the ZnO particles to grow into
a rod-like structure (the Fig. 3(d) can be seen). TiO2 nanorods
2.3 Characterization
The phase composition of the sample was determined using an
X-ray diffractometer (XRD MAC, M18XHF) with Cu Ka radiation.
Scanning electron microscopy (SEM Hitachi, S4800) uses the
principle of focused electron beam (1 pA to 2 nA) scanning
imaging on the surface of a sample to observe topography.
High-resolution transmission electron microscopy (HTEM
Hitachi, JEM-2100) uses a very short wavelength electron beam
for illumination and an electromagnetic lens for focused
imaging to further characterize the topography and physical
appearance of the sample. The X-ray photoelectron spectrom-
eter (XPS Thermo, ESCALAB250) uses Al Ka as the excitation
source and C 1s as the reference peak for calibration to analyze
the elemental composition of the sample. The UV-Vis absorp-
tion spectrum was measured by a UV-Vis spectrophotometer
(UV-2550, Shimadzu). A photoluminescence (PL) spectrum was
recorded using a uorescence spectrophotometer (FL, HITACHI
F-4500) under an environmental condition of an excitation
wavelength of 325 nm.
2.4 Photoelectrochemistry and photocatalytic
The time–current curve of photoelectrochemical properties was
measured using an electrochemical workstation in a polysulde
electrolyte (mixed solution of 0.35 M NaSO3 and 0.25 M NaS)
based on a three-electrode system based on a simulated solar
source AM 1.5 G and analyzed. The photocatalytic performance
test was carried out by disposing 50 mL of 15 mg Lꢁ1 methyl
orange (MO) solution as a dye reaction solution under a 15 W
ultraviolet lamp with a wavelength range of 200–270 nm. Aer
the photocatalytic reaction was over, the reection solution was
extracted and the light absorption spectrum was measured by
a UV-Vis spectrophotometer. Finally, the degradation rate was
obtained by using a formula.
3. Results and discussion
To ensure that the synthesized lm samples have the desired
phase composition, the prepared TiO2 (Curve a), TiO2/Ag
(Curve b) and TiO2/ZnO nano-dendritic heterostructures
(Curve c) and Ag-modied TiO2/ZnO nano-dendritic hetero-
structures (Curve d) samples were analyzed by X-ray diffraction.
The XRD results are shown in Fig. 1. Analysis of the XRD pattern
revealed that the peak of the TiO2 rutile phase was found at 2q ¼
36.1ꢀ [101], 54.4ꢀ [211], 62.7ꢀ [002],31 and the peak of the TiO2
rutile phase corresponding to PDF #21-1276 was well matched.
Also in 2q ¼ 31.7ꢀ [100], 34.4ꢀ [002] (the peak of the ZnO wurtzite
structure corresponding to PDF #36-1451 is well matched), the
presence of ZnO wurtzite structure can be found. However, no
diffraction peaks of Ag were found in the map due to the small
size of the Ag nanoparticles and the lower than the detection
limit of the instrument.32,33
To determine the chemical composition and sate of the Ag-
modied TiO2/ZnO nanodendritic structure, we measured its X-
ray photoelectron spectroscopy (XPS). The major signals of Zn,
O, Ti and Ag are found in survey spectra (Fig. 2(a)). Fig. 2(b)–(e)
Fig. 3 SEM images of (a) TiO2 nanorod arrays, (b) Ag-modified TiO2
nanorod arrays; (c) TiO2 nanorod arrays/ZnO nanoparticles, (d) TiO2/
ZnO heterojunction nanotree arrays; (e) Ag-modified TiO2/ZnO het-
erojunction nanotree arrays and (f) an EDS spectrum of Ag-modified
respectively shows the enhanced energy spectra of O 1s, Zn 2p, Ag TiO2/ZnO heterojunction nanotree arrays.
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Fig. 4 (a) TEM image of the Ag-modified TiO2/ZnO heterojunction nanotree arrays; (b) and (c) HRTEM image of the Ag-modified TiO2/ZnO
heterojunction nanotree arrays.
acts as the trunk of the tree, while ZnO nanorods forms the Ag. As can be seen from Fig. 4(a), the diameter of the surface Ag
branches of the tree. As shown in Fig. 3(e), Ag QDs were NPs distributed on the ZnO nanorods is about 10–20 nm, which
deposited in dendritic ZnO nanorods. Due to the short length of is in agreement with what has been reported in the literature.37
ZnO nanorods, Ag QDs make the dendritic structure less This can reect the ZnO nanorods to provide more adhesion
obvious. At the same time, it can be seen from the energy area for Ag quantum dots, which will be benecial to the SPR
spectrum(Fig. 3(f)) of the Ag-modied TiO2/ZnO heterojunction effect of Ag, photoelectrochemical performance and photo-
nanotree arrays that it contains four elements of Ag, Ti, Zn and catalytic efficiency.
O, which is consistent with the results in the X-ray diffraction
pattern.
The optical absorption intensity is the key factors of the
photoelectrochemical process. The UV-Vis absorption spectra of
The microscopic morphology of the Ag-modied TiO2/ZnO different specimens are presented in Fig. 5(a). Pure TiO2
heterojunction nanotree arrays sample was further character- exhibits the highest peak at 370 nm due to exciton absorption,
ized using high-resolution transmission electron microscope and its absorption edge is around 390 nm due to the transfer of
(Fig. 4(a)–(c)). The crystal plane having a lattice spacing of charge from the valence band formed by the oxide anion 2p
0.32 nm and were ascribed to the (110) crystal plane of the rutile orbital to the conduction band formed by the Ti cation 3d t2g
phase TiO2. A series of ordered lattice edges with lattice spac- orbital. Compared with pure TiO2, the red shi of the absorp-
ings of 0.260 nm and 0.236 nm can be seen in the branches and tion edge of TiO2/ZnO heterojunction nanotree arrays is due to
belong to the (002) plane of the ZnO and the (111) plane of the the introduction of impurity levels in the forbidden band of
Fig. 5 (a) UV-Vis absorption spectra of various photoelectrodes. (b) Bandgaps of different samples.
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TiO2, but the red shi phenomenon is not obvious due to the nm) of TiO2, TiO2/Ag, TiO2/ZnO and TiO2/ZnO/Ag prepared at
small difference in the forbidden band width of TiO2 and ZnO. room temperature. It can be seen from the gure that the two
Aer modication by Ag QDs, the absorption intensity of the main peaks, which is due to photoexergence caused by TiO2
sample is enhanced and the absorption is red-shied because edge free exciton recombination and various defects, are
of the LSPR effect produced by the Ag QDs and the presence of located at 452 nm and 469 nm. There are two small peaks at
electron transitions between the doping element and the base 479 nm and 492 nm due to photoluminescence caused by
material. Ag NPs have fcc crystal structure and spherical struc- binding excitons.47–49 The PL spectrum is mainly due to photo-
ture. The LSPR wavelength not only depends on the size, shape generated carrier recombination and the lower PL spectrum
of the particles, and the dielectric properties of the surrounding intensity indicates a decrease in recombination. As can be seen
medium, but also on interparticle coupling. For Ag nano- from the gure, the PL spectrum intensity of the Ag-modied
particles with a diameter of 5–15 nm, the LSPR effect is in the TiO2 nanorod arrays sample is lower than that of the pure
range of 430–700 nm.38–40 Photodeposition can be a method TiO2 nanorod arrays sample. This indicates that the PL spec-
where a large number of Ag NPs are deposited on the sample trum intensity of the Ag-modied TiO2 nanorod arrays sample
surface leading to dominant interparticle coupling, which has a lower recombination rate than the pure TiO2 nanorod
results in absorption of the optical edge redshi. Aer the arrays sample under illumination, because the photogenerated
introduction of the ZnO intermediate layer, it can be seen that carriers can be captured by the Ag 3d level under the conduction
the absorption edge of Ag-modied TiO2/ZnO heterojunction band, thereby inhibiting the recombination. Aer the intro-
nanotree arrays is further red-shied and the absorption duction of ZnO intermediate layer, the peak of Ag-modied
intensity is also increased. At the same time, the direct band gap
is shown in Fig. 5(b). It can be seen from the gure that the
bandgap width of the TiO2/ZnO heterojunction nanotree array
is 3.02 eV, which is only slightly reduced compared to pure TiO2
(3.07 eV). Due to the test environment, instrumentation and
light scattering, the TiO2 bandgap measurements differ from
those in the literature.41–43 The TiO2/ZnO bandgap width varies
little relative to that of TiO2, mainly due to the similar bandgap
structures of TiO2 and ZnO.44–46 Aer modication of the Ag
quantum dots, the bandgap width was greatly reduced to
2.70 eV for the sample of TiO2/ZnO/Ag and 2.78 eV for the
sample of TiO2/Ag. The reduction of the band gap greatly
facilitates light absorption.
Photoluminescence spectroscopy can be used to detect the
efficiency of charge transport, capture, transfer and separation.
Fig. 6 shows the PL spectra (the excitation wavelength is 325
Fig. 7 Transient photocurrent responses of different samples.
Fig. 6 PL spectra of the samples.
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the photoelectrochemical performance, a transient photocur-
rent response test was performed. As shown in Fig. 7, the initial
photocurrent is soared under transient illumination due to the
separation of electron–hole pairs at the semiconductor/
electrolyte interface. Aer reaching the maximum value, the
photocurrent will eventually stabilize as time passes. It can be
seen from the gure that aer 5 turn-on and turn-off cycles, the
photocurrent of the sample is relatively stable and exhibits good
reproducibility. Under light conditions, the photocurrent of
TiO2/ZnO heterojunction nanotree arrays is 0.07 mA cmꢁ2
,
which is 5 times that of pure TiO2 nanorod arrays. The photo-
current of Ag-modied TiO2 nanorod arrays is doubled
compared to pure TiO2 nanorod arrays, which is 0.047 mA
cmꢁ2. Aer introducing the middle layer of ZnO nanorods, the
photocurrent reaches 0.1 mA cmꢁ2, which is twice that of the
Ag-modied TiO2 nanorod arrays, and even is 8 times that of
pure TiO2 nanorod arrays. It can be proved that the ZnO
nanorods as an intermediate layer can contribute to the sepa-
ration and collection of photoinduced charge carriers.24
The method of measuring EIS can be used to verify the
charge transport impedance of the sample, the recombination
rate of photogenerated electron–hole pairs, and other charac-
teristics. Fig. 8 shows the Nyquist spectra of different samples at
0 V in the dark. At present, it is generally believed that the
smaller the radius of curvature of the curve, the smaller the
Fig. 8 Electrochemical impedance spectra of all samples.
TiO2/ZnO heterojunction nanotree arrays further decreases,
indicating that the ZnO intermediate layer makes the recom-
bination rate between photo-generated electron–holes lower
and the photoinduced charge carrier recombination is effec-
tively inhibited, thereby improving the photocatalytic activity.
Transient photocurrent testing has long been considered an
important evidence for the photoinduced electron–hole trans-
port mechanism of materials. In order to further demonstrate
Fig. 9 (a) Photodegradation percentages of different samples for MO; (b) the kinetics of different samples for MO; (c) three successive pho-
todegradation curves for Ag-modified TiO2/ZnO heterojunction nanotree arrays; (d) photodegradation ratio and rate constant of S1–S7.
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interface charge transfer ability of the sample. From the Fig. 8, it photocatalytic activity, which is consistent with the results of PL
can be observed that the resistance of electrons passing through and photocurrent. The linear exchange of pseudo rst-order
the interface is the smallest in the Ag-modied TiO2/ZnO model is used to quantify the reaction kinetics of WO degra-
nanotree array, which shows that the TiO2/ZnO nanotree array dation. As shown in Fig. 9(b), it is clear that the slope of Ag-
can effectively reduce the resistance of photogenerated charge modied TiO2/ZnO heterojunction nanotree arrays is the
transport on the surface and accelerate the transport of pho- highest, it was shown that the Ag-modied TiO2/ZnO hetero-
togenerated carriers. This conclusion is consistent with PL junction nanotree arrays had the highest photocatalytic degra-
analysis.
dation rate constant (Ka) and the best photocatalytic activity.
In order to test the photocatalytic performance, an experi- Fig. 9(d) shows the photodegradation efficiency constants and
ment in which the sample degraded methyl orange (MO) under photodegradation rates of different samples over 180 min in the
the solar light source was simulated with an ultraviolet lamp. form of a histogram, where the numbers S1–S4 represent TiO2
The Lambert–Beer law and the Langmuir–Hinshelwood model arrays, TiO2/ZnO heterojunction nanotree arrays, Ag-modied
were used to analyze the degradation rate of the sample and the TiO2 arrays and Ag-modied TiO2/ZnO heterojunction nano-
kinetics of degradation of methyl orange to evaluate the pho- tree arrays. According to the formula, the kinetic constant (Ka)
tocatalytic efficiency.50
bit 7.5 ꢂ 10ꢁ3 minꢁ1 of Ag-modied TiO2/ZnO heterojunction
nanotree arrays can be calculated, which is three times higher
than the original TiO2 (2.5 ꢂ 10ꢁ3 minꢁ1). Sample stability was
obtained by testing the cyclic stability of the samples through
three successive cycles of photodegradation of methyl orange.
As shown in Fig. 9(c), methyl orange was stably degraded in all
three cyclic reactions, indicating that the sample is an efficient,
stable decomposer of organic matter.
Fig. 10(a) is the illustration of the potential and band position
in the TiO2/ZnO/Ag system, which is based on the work function of
the metal Ag (ꢁ4.26 eV),51 the conduction band potentials of TiO2
(ꢁ4.21 eV)52 and ZnO (ꢁ4.05 eV).46,53 It can be seen that ZnO has
the highest conduction band position. Fig. 10(b) is the proposed
mechanism of charge-transfer interaction and photodegradation
of MO with TiO2/ZnO/Ag system. The coupling between the two
photocatalysts produces a heterojunction in which ZnO acts as an
n-type semiconductor, the electrons are transferred from the
conduction band (CB) of ZnO to CB of TiO2. In addition, TiO2 acts
as a p-type semiconductor to transfer positive holes from valence
band (VB) of TiO2 to VB of ZnO. Therefore, due to the illumination,
the photogenerated holes and electrons produced by pure TiO2 are
very easy to recombine in a short time, but the chance of recom-
bination the of photogenerated electrons and hole charge carriers
are greatly reduced because of the conformation of the TiO2/ZnO
ðC0 ꢁ CtÞ
A0 ꢁ At
h ¼
¼
ꢂ 100%
(1)
(2)
C0
A0
ꢀ
ꢁ
C0
ln
¼ Kat
Ct
h ¼ degradation rate; C0 ¼ the initial concentration; Ct ¼
concentration aer reaction time t; A0 ¼ initial absorption
luminosity; At ¼ absorption luminosity aer reaction time t; Ka
¼ rst order rate constant.
As shown in Fig. 9(a), in the case of monitoring the degra-
dation reaction using UV light, it was found that the concen-
tration of MO decreased with the irradiation time. Aer 180
minutes of exposure to 15 W UV light, the degradation rates of
TiO2 nanorod arrays and Ag-modied TiO2 nanorod arrays were
37% and 59%. Aer the introduction of ZnO, the degradation
rates of TiO2/ZnO and Ag-modied TiO2/ZnO were found to be
45% and 77%. It can be seen that the Ag-modied TiO2/ZnO
heterojunction nanotree arrays showed the highest degradation
rate, which was 40% higher than that of the pure TiO2 nanorod
arrays sample and 18% higher than the Ag-modied TiO2 het-
erojunction nanotree arrays. It is shown that the Ag-modied
TiO2/ZnO heterojunction nanotree arrays has high
Fig. 10 (a) Illustration of the potential and band position in the Ag-modified TiO2/ZnO heterojunction nanotree arrays and (b) proposed
mechanism of charge-transfer interaction and photodegradation of MO with TiO2/ZnO/Ag system.
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heterojunction nanotree arrays. For the case of Ag-modied TiO2/
ZnO heterojunction nanotree arrays, Ag QDs are deposited on
ZnO. The equilibrium Fermi level (Ef) of ZnO/Ag heterojunction is
lower than the bottom energy level of CB of ZnO.54,55 Therefore, the
photoexcited electrons can be transferred from the CB of the ZnO
nanorods to the Fermi level of the metal Ag as the electron trap,56
thereby effectively promoting the separation of the photo-
generated electron–hole. Photogenerated electrons eꢁ are
captured by O2 on the surface of TiO2 and Ag. The electrons and O2
undergo a reduction reaction to generate reactive oxygen radical
O2cꢁ. The h+ le on the valence band of ZnO will react with H2O
and produce the sexual oxygen radical OHc. So eventually the
organic pollutant methyl orange (MO) will be oxidized to CO2 and
H2O by h+, O2cꢁ and OHc.44,46 In addition, due to the LSPR effect of
Ag nanoparticles, the light absorption intensity increases and the
absorption edge red shis. Therefore the sample Ag-modied
TiO2/ZnO heterojunction nanotree arrays showed good catalytic
properties. Taking MO as a class, the important reaction steps in
the photodegradation process are summarized as follows:
Acknowledgements
This work was successfully completed with the support of the
National Natural Science Foundations of China (No. 51772003,
51472003 and 51701001). The authors also would like to thank
to Zhongqing Lin and Yonglong Zhuang of the Experimental
Technology Center for help with microstructure testing.
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The TiO2/ZnO heterojunction nanotree arrays was successfully
prepared by hydrothermal, sol–gel and secondary hydrothermal
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Conflicts of interest
There are no conicts to declare.
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