Ag nanoparticles-decorated porous nanoplates for enhanced photocatalytic performance and SERS activity

Article abstract of DOI:10.1016/j.chemphys.2019.110556

Rational design of nanomaterials is of great significance for improving their properties and applications. This report demonstrates that both the photocatalytic performance and surface-enhanced Raman scattering (SERS) activity have been greatly enhanced after converting Ag-Ag₂O smooth nanoplates (nanoplates-A) to Ag nanoparticles-decorated porous nanoplates (nanoplates-B) through a facile thermal treatment. The photocatalytic efficiency towards degradation of methyl orange (MO) was improved two times. The limit of detection (LOD) of thiophenol (TP) molecules through SERS was upgraded from 5*10^?7 M to 1*10^?10 M. The improved properties of nanoplates-B can be attributed to the clean surfactant-free surface, the hierarchical porous structure and the large amount of Ag nanoparticles decorated on the nanoplate surface, which endorse potentially promising photocatalytic application and the high sensitivity of the SERS platform.

Full text of DOI:10.1016/j.chemphys.2019.110556

ChemicalPhysics529(2020)110556
Contents lists available at ScienceDirect
Chemical Physics
journal homepage: www.elsevier.com/locate/chemphys
Ag nanoparticles-decorated porous nanoplates for enhanced photocatalytic
performance and SERS activity
Yaoquan Mao⁎, Suke Yang, Mingyin Li, Tingting Dai
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Rational design of nanomaterials is of great significance for improving their properties and applications. This
report demonstrates that both the photocatalytic performance and surface-enhanced Raman scattering (SERS)
activity have been greatly enhanced after converting Ag-Ag2O smooth nanoplates (nanoplates-A) to Ag nano-
particles-decorated porous nanoplates (nanoplates-B) through a facile thermal treatment. The photocatalytic
efficiency towards degradation of methyl orange (MO) was improved two times. The limit of detection (LOD) of
thiophenol (TP) molecules through SERS was upgraded from 5*10−7 M to 1*10−10 M. The improved properties
of nanoplates-B can be attributed to the clean surfactant-free surface, the hierarchical porous structure and the
large amount of Ag nanoparticles decorated on the nanoplate surface, which endorse potentially promising
photocatalytic application and the high sensitivity of the SERS platform.
Ag porous nanoplates
Photodegradation
Surface-enhanced Raman scattering
1. Introduction
Ag2O composites enhanced photocatalytic property [22].
Besides the chemical composition, the morphology and structure of
Silver (Ag) nanomaterials have been the focus of intensive research
in the past few decades because of their prominent features of excellent
and stretchable conductivity [1,2], highly sensitivity [3], and anti-
bacterial activity [4]. Therefore, Ag and Ag-based nanomaterials have
wide applications in electrochemiluminescence [5], catalysis [6], bio-
sensor [7], surface-enhanced Raman scattering (SERS) [8], etc. So far
numerous Ag-based nanomaterials have been reported, including na-
nowires [9,10], nanoparticles [11,12], nanoplates [13,14], and other
nanostructures [15,16].
nanomaterials are also significant for determining their properties.
Among various nanostructures, two-dimensional (2D) nanoplates are an
important portion due to their amazing ability to harvest optical
properties. This is particularly true for the Ag-based nanoplates due to
their excellent localized surface plasmon resonance (LSPR) property
and high anisotropy in structure [23,24]. In this regard, if we can
combine the unique photocatalytic properties of Ag-based nano-
composites with the unique 2D nanoplate structure to rationally con-
struct Ag-based nanocatalyst, it will be highly desirable to display high
photocatalytic performance due to the large surface area and the strong
SPR response [25–27].
Apart from the well-studied Ag nanomaterials, silver oxide (Ag2O),
as a typical p-type semiconductor with narrow band gap (~1.3 eV), has
been developed as a new photocatalyst with self-stability and promi-
nent photocatalytic activity under visible light [17]. Comparing with
the wide bandgap semiconductors like zinc oxide (ZnO, ~3.2 eV), ti-
tanium dioxide (TiO2, ~3.2 eV), etc., Ag2O can be used as a favorable
sensitizer to broaden the light response from the ultraviolet (UV) region
to the visible region and show enhanced photocatalytic activities
[18,19]. Recently, more and more attentions have been attracted to the
Ag-Ag2O composite nanomaterials. Their photocatalytic activity and
enhanced photoeletrochemical performance have been reported
[20,21]. Moreover, the Ag-Ag2O composite could also well maintain the
surface plasmon resonance (SPR) effect of Ag nanostructures and the
photocatalytic stability of Ag2O nanostructures, which endowed the Ag-
In this contribution, we have explored the conversion of Ag-Ag2O
smooth nanoplates (nanoplates-A) to Ag nanoparticles-decorated
porous nanoplates (nanoplates-B) through programmed thermolysis
processes. The nanoplate morphology was well retained during the
thermal treatments except that the initial smooth surfaces changed to
be highly porous. Particularly interesting, the resulting nanoplates-B
was decorated with many Ag nanoparticles on their surfaces. It was
found out that both photocatalytic performance and SERS activity of
the nanoplates-B have been greatly enhanced in comparison with na-
noplates-A.
Impressively, it was discovered that comparing to the nanoplates-A/
ZnO heterostructures, the nanoplates-B/ZnO heterostructures exhibit a
⁎ Corresponding author.
E-mail address: m18362602876@163.com (Y. Mao).
https://doi.org/10.1016/j.chemphys.2019.110556
Received 21 September 2019; Received in revised form 8 October 2019; Accepted 10 October 2019
Availableonline12October2019
0301-0104/©2019ElsevierB.V.Allrightsreserved.

Y. Mao, et al.
ChemicalPhysics529(2020)110556
better performance for the photodegradation of methyl orange (MO).
The enhanced photocatalytic activities might be ascribed to the porous
structures providing a much larger surface areas that more oxygen (O2)
would be adsorbed, the great number of Ag nanoparticles on the sur-
faces of the Ag porous nanoplates might be another reason responsible
for the high photocatalytic activities. The SERS of thiophenol (TP)
molecules adsorbed on the substrates of nanoplates-A and nanoplates-B
were also conducted. The TP molecules can be detected at the con-
centration of 5*10−7 M with the substrate of smooth nanoplates-A, as a
contrast, the TP molecules can be detected at the concentration of
1*10−10 M with the substrate of nanoplates-B. The “hot spots” is the
key factor for nanoplates-B with a lower limiting of detection (LOD)
[26], their porous surfaces and decorated Ag nanoparticles will gen-
erate more “hot spots”.
nanoplate shape can be well retained at 350 °C as long as the thermal
treatment temperature was set below 350 °C.
2.4. Instrumentation and characterizations
The images of scanning electron microscopy (SEM) were collected
on a field-emission SEM (Hitachi S-4700, operated at 15 kV under high
vacuum) equipped with EDX. The images of transmission electron mi-
croscopy (TEM) and high-resolution TEM are obtained on a TEM
(Tecnai G20) and field-emission TEM machine, respectively. The crys-
tallographic and compositional information was obtained using powder
X-ray diffraction (XRD, PANalytical X‘Pert-Pro MPD, operated at 40 kV
and 40 mA) with monochromatic Cu Kα radiation (λ = 1.54 Å) and
selected area electron diffraction (SAED, integrated in TEM). The X-ray
photoelectron spectra (XPS) were recorded using an ESCALab220i-XL
electron spectrometer from VG Scientific with 300 W Al Kα radiation.
The UV–vis diffuse reflectance spectra (DRS) were collected in
200–700 nm using UV-3600 (Shimadzu).
2. Experimental section
2.1. Chemicals and materials
2.5. Photocatalytic degradation of methyl orange
Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, ≥99.0%) was purchased
from Sinopharm Chemical Reagent Co. Silver nitrate (AgNO3, ≥99.8%)
was obtained from Shanghai Lingfeng Chemical Reagent Co.
Thiophenol (TP, 98%) was purchased from Matrix Scientific Trade Co.
Methyl orange (MO, ≥99.5%) was purchased from Adamas Reagent
Co., Ltd. All the reagents were analytical reagent (AR) and used as re-
ceived without further purification. The deionized (DI) water was
purified by the Laboratory Water Purification System which was pur-
chased from Shanghai Hitech Instruments Co.
Photocatalytic activity experiments of the synthesized pure ZnO, the
nanoplates-A/ZnO and the nanoplates-B/ZnO were examined with MO
dye as the detective molecules under Xenon lamp (1.2 A). We dispersed
sixty milligrams of the sample within a 60 mL MO dye solution with a
concentration of 4*10−5 M in a 250 mL quartz beaker. The MO dye
with samples was continuously stirred in the dark without any irra-
diation for 30 min to build the adsorption-desorption equilibrium of the
MO dye on the surface of the sample before the photocatalytic activity
test. The xenon lamp were as optical resource and at a given time in-
terval, 4 mL of MO-sample suspension was withdraw and analyzed after
centrifugation at 5000 rpm for 5 min to remove the sample powders,
then the absorbance spectra of the centrifuged solutions were recorded
through UV–vis spectrometer. As comparisons, the photocatalytic ac-
tivities of the pure ZnO, the nanoplates-A/ZnO and the nanoplates-B/
ZnO were measured using the same parameters in ultraviolet–visible
light region, the nanoplates-B/ZnO and the nanoplates-A/ZnO were
also measured using the same parameters in visible light region.
2.2. Preparation of Ag-Ag2O smooth nanoplates (nanoplates-A)
The nanoplates-A were prepared by a photochemical method with
slight modifications from our recently published paper [25]. Typically,
the ZnO films covered by aliquots of AgNO3 aqueous solution (con-
centration: 1–8 M) was illuminated continuously with a UV lamp (wa-
velength: 365 nm) for 180 min. The samples were rinsed with plenty of
deionized water at the end of UV illumination. Obvious color change
from white to dark gray was observed, suggesting the growth of out-of-
substrate Ag-Ag2O nanoplate arrays on the ZnO film.
2.6. Measurement of SERS activity
2.3. Preparation of Ag nanoparticles-decorated porous nanoplates
(nanoplates-B)
The pure TP sample was dissolved by ethanol with different con-
centration. At the beginning of experiments, we prepared adequate
samples under two conditions. For samples of nanoplates-A, 2 mL of TP
solutions with varied concentrations from 1.0 × 10−3 mol/L to
1.0 × 10−6 mol/L were added to 5 mL of culture dish with the samples
of nanoplates-A. After 3 h, the TP solutions were completely adsorbed,
then lightly washed with ethanol and dried in desiccators. As for the
nanoplates-B, we used similar methods and only changed varying
concentration from 1.0 × 10−3 mol/L to 5.0 × 10−11 mol/L. Raman
tests were conducted at room temperature with 632.8 nm laser as ex-
citation source, about an area of 2 μm diameter was probed with a long
50 × objective lens, the acquisition time was 10 s and the incident
power was 1.4 mW.
To prepare the nanoplates-B, the photochemically prepared nano-
plates-A were subjected to programmed thermal treatments in oven
(Box Furnace, Thermal Fisher Scientific, USA) for different periods. The
thermal treatment includes two stages. The temperature was firstly
risen up from the room temperature (~20 °C) to 200 °C for 1 h and then
stayed at 200 °C for another 4 h.
During thermal treatments, the heating process was strictly con-
trolled. In a typically rapid heating condition, the temperature was
risen up to 200 °C within 15 min. On the other hand, in the slow heating
condition, the temperature was risen up to 200 °C within 2 h.
The slow heating condition will result in the Ag porous nanoplates
with “clean” surfaces; while the rapid heating condition will result in
the nanoplates-B. And a color change from dark gray to silver gray was
observed after the thermal treatments.
3. Results and discussion
The thermal treatments at the temperature above 200 °C will lead to
the decomposition of Ag2O and yielding the nanoplates-B. As a result,
the initially generated Ag-Ag2O nanoplates with smooth surfaces were
converted into the Ag nanoplates with porous surfaces due to the re-
lease of oxygen. On the other hand, the appearance of nanoparticles on
the surfaces of the nanoplates might also be ascribed to the structural
contraction from Ag2O (with the density of 7.143 g/cm3) to Ag (with
the density of 10.490 g/cm3) during the thermal treatments. While Ag-
Ag2O smooth nanoplates were converted into Ag porous nanoplates, the
The Ag-Ag2O composite nanoplates were produced by the later-by-
layer crystallization mechanism under long-time (180 min) UV illumi-
nation [25]. As shown in Fig. 1A, there were high-density nanoplates
producing and the nanoplates were growing vertically on the ZnO films
(Fig. 1B). Fig. 1C shows that the nanoplates are generated with smooth
surface, straight edges and sharp tips, being in combination with XRD
patterns (Fig. 1D). As seen, besides the diffraction peaks of the ZnO
substrate, there are also the diffraction peaks of both Ag and Ag2O
observed, indicating the generation of Ag-Ag2O composite nanoplates.
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Y. Mao, et al.
ChemicalPhysics529(2020)110556
Fig. 3. The close-up views of (A) the smooth nanoplates without any thermal
treatments. (B–D) The porous nanoplates obtained with different heating rate
(RB < RC < RD). (E) The porous nanoplates obtained by thermal treatment (@
250 °C) of a normal heating rate. (F) The porous nanoplates obtained by
thermal treatment (@350 °C) of a normal heating rate.
Fig. 1. (A) Low-magnification SEM image of high-density large nanoplates
grown on ZnO film, obtained after continuous UV illumination for 180 min. (B)
High-magnification SEM image of nanoplates. (C) The close-up view of the
nanoplates with characteristic smooth surface, straight edges and sharp tips. (D)
XRD pattern of the smooth nanoplates.
nanoplates were obtained. The smooth nanoplates as usual are shown in
Fig. 3A and then samples of the smooth nanoplates by different pro-
grammed thermal treatments with
a
different heating rate
(RB < RC < RD) were shown in Fig. 3B–D. As seen, the vibrations
about porous nanoplates were clearly observed, and there were more
and more nanoparticles decorating with it along with the increasing
heating rate. What's more, the nanoparticles were bigger than others
when the temperature was risen up to 200 °C within 15 min. As shown
in Fig. 3E, when the smooth nanoplates were risen up to 250 °C, the
surface of the porous nanoplate was rougher than before and the na-
noparticles fell off from it. When the temperature of thermal treatment
was further risen to 350 °C, the whole nanoplates tended to be the
cracked and fragile (Fig. 3E), this is due to the partial melting part of
nanoplates at 350 °C.
The samples were further investigated using TEM-related char-
acterization. Fig. 4A and B showed the typical TEM images of the na-
noplates-A and nanoplates-B. The smooth nanoplates obtained only
under long-time illumination were uniform and bright while the porous
nanoplates were unevenly and flickering. Besides these, some little
nanoparticles generated on the surface of the porous nanoplates were
shown in Fig. 4C. Fig. 4D showed the corresponding SAED pattern of
the porous nanoplates, in which the different spots correspond to
{1 1 1}, {2 0 0}, {2 2 0}, {3 1 1} and {2 2 2} of Ag. Fig. 4E shows the
HRTEM image of the porous nanoplates without any nanoparticles, the
lattice fringes with a d-spacing of 2.4 Å correspond to the (1 1 1) lattice
planes and a d-spacing of 2.0 Å correspond to the (2 0 0) lattice planes
of Ag, respectively. And the SAED pattern of another region without
any nanoparticles are shown in Fig. 4F, the lattice fringes were sepa-
rated by 2.5 Å, which could be ascribed to the (1/3){4 2 2} reflection
that is generally forbidden for a face-centered cubic (fcc) lattice. Fig. 4G
shows the HRTEM image of the nanoparticles, and the inset is the
corresponding fast Fourier transform (FFT) pattern of the square area.
As seen, the five sets of spots can be identified from this SAED pattern,
which can be indexed to the {1 1 1}, {2 0 0}, {2 2 0}, {3 1 1} and {2 2 2}
reflections of Ag, respectively. Fig. 4 further confirms that both the
porous nanoplates and the nanoparticles on the surface of it only cor-
respond to Ag and not to Ag2O.
Fig. 2. (A) Low-magnification SEM image of nanoplates-A obtained by pro-
grammed thermal treatment. (B) High-magnification SEM image of the nano-
plates by programmed thermal treatment. (C) The close-up view of the nano-
plates with characteristic porous and rough surface, straight edges, sharp tips
and new-generated nanoparticles. (D) XRD patterns of the porous nanoplates.
Then we conducted a programmed thermal treatment of the samples of
nanoplates-A. As for typically normal thermal treatment, the tempera-
ture was firstly risen up risen up from the room temperature (~20 °C) to
200 °C within 1 h and then stayed at 200 °C for 4 h, the resulting
morphologies with the low-magnification after thermal treatment are
shown in Fig. 2A, where no obvious morphology changes are observed.
From the local high-magnification image of the sample (Fig. 2B) and the
close-up view of the nanoplates (Fig. 2C), we can also easily find that
the nanoplates retain their sharp tips and straight edges. However, the
surface of the nanoplates turns to be obviously porous and a few small
nanoparticles are aroused. Besides these, as shown in Fig. 2D, the dif-
fraction peak of Ag2O disappeared from the XRD pattern of samples
after thermal treatment, which was due to the pyrolysis of Ag2O to Ag
nanoparticles. In comparison Fig. 1C with Fig. 2C, there were a few
nanoparticles appearing on the surface of the porous nanoplates except
for little holes were observed.
Basing on the results of SEM, XRD and TEM, it is a process from the
smooth nanopaltes to the porous nanoplates, and the nanoparticles
produced accompanied by the thermal treatments. Figs. 1D and 2D
express as mean that different chemical composition of the smooth and
porous nanoplates, from XRD spectrum, except ZnO, the samples before
thermal treatment were composed of Ag and Ag2O and samples after
thermal treatment (@200 °C) were only composed of Ag. As shown in
Interestingly, by varying the heating rate (RB < RC < RD) in the
programmed thermal treatments, three kinds of different porous
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Y. Mao, et al.
ChemicalPhysics529(2020)110556
Fig. 4. (A) TEM characterization of the nanoplates-A.
(B) TEM characterization of the nanoplates-B. (C)
Higher magnification TEM image taken from the
square area in panel B. (D) The SAED pattern image
taken from the square area in panel C. (E,F) The
HRTEM lattice fringe images at the different square
of porous nanoplates. (G) The HRTEM lattice fringe
image of the nanoparticles on the surface of the
porous nanoplates and the inset is the FFT pattern of
the square area.
Fig. 2D, after thermal treatment, the peaks of Ag2O (1 1 1) at about
32.78° and Ag2O (2 2 0) at about 54.80° disappeared as well as the
signals of Ag(1 1 1) at about 38.15°, Ag (2 0 0) at about 44.32°, Ag
(2 2 0) at about 64.50° and Ag (3 1 1) at about 77.45° were enhanced
correspondingly. Thermal treatment is a process of chemical reaction:
Ag2O → Ag + O2↑. To this end, we can describe the process that the Ag-
Ag2O smooth composite nanoplates convert to the Ag porous nano-
plates through a way of thermal conversion, and Ag2O decompose to O2
and Ag, the newly formed Ag assemble together in the shape of nano-
particles. A part of nanoparticles generated on the surface of the na-
noplates while others generated inside the nanoplates. And partial Ag
nanoparticles inside which have no time to combine with nanoplates
inside yet will be carried away by O2 when we accelerate the rate of
heating. Therefore, there will be more O2 produced at the same time,
which results in more nanoparticles having no time to combine with
nanoplates inside yet then carried out by O2, leading to the formation of
high-density enlarged Ag nanoparticles when the smooth nanoplates
annealed at a higher heat rate.
by the photochemical reaction [27], and the absorption of ZnO de-
creased irregularly on account of that the thickness of as-prepared
samples were heterogeneous. At the visible light region from 400 nm to
700 nm, the pure ZnO had no obvious absorption, while both nano-
plates-A/ZnO and nanoplates-B/ZnO had higher absorption of light in
the visible region than the pure ZnO. This can be assigned to the SPR
effects [28,29] of Ag and the Ag2O can act as a photosensitizer [19].
Due to the photochemical reaction conducted without adding any
protective agents, so the sizes of the nanoplates were uneven, which
resulted in the continuous absorptions of two samples. Moreover, the
porous surface can give rise to enhancing absorption from 400 nm to
500 nm and enhance the SPR effects of Ag because of the scattering
enhancing effects [30], and the generated nanoparticles cause the weak
peaks of the nanoplates-B. Therefore, the nanoplates-B had a better
absorption than the nanoplates-A, and it could induce them to possess
the higher photocatalytic performance.
To evaluate the photocatalytic activities of different samples of
blank (pure MO), the pure ZnO, the nanoplates-A/ZnO and the nano-
plates-B/ZnO, we take tests for degradation of MO dye. As shown in
Fig. 6, all the tested photocatalysts showed evidently enhanced pho-
tocatalytic activities except for the pure MO. The rate of the photo-
decomposition of MO is the nanoplates-B/ZnO > the nanoplates-A/
ZnO > pure ZnO > self-degradation. After 20 min xenon lamp irra-
diation, the percentages of MO degraded by all above samples were
78.86%, 60.71%, 10.89% and 0.42%, respectively. Furthermore, the
nanoplates-B/ZnO could decompose MO dye completely after about
30 min irradiation, while the nanoplates-A needed about 60 min. As
shown in Fig. 6D, the plots of ln(C/C0) versus irradiation time exhibited
a well linearity, which indicated that the photodegradation reactions of
MO dye over the tested photocatalysts all follow the first-order kinetics
precisely. According to Langmuir-Hinshelwood model equation of ln(C/
C0) = −kt, in which k denotes the degradation rate constant, while t
denotes the irradiation time. The crucial reaction rate constant k for
self-degradation, pure ZnO, nanoplates-A/ZnO and A nanoplates-B/ZnO
was 0.0001, 0.0055, 0.0419, 0.0824 min−1, respectively. The photo-
catalytic activities of both the samples before and after thermal treat-
ment were better than those of the pure ZnO and the self-degradation.
The recyclability is a principal trait for the practical application of
photocatalysts under xenon irradiation. All samples were repeatedly
used for three cycles. Fig. 7 showed the MO photodegradation stability
of the recycled samples, there was no obvious reduction in photo-
degradation efficiency after three cycles. Therefore, the nanoplates-A/
ZnO, especially the nanoplates-B were found to be extremely stable and
highly-efficient photocatalysts with potential applications.
In addition, the photochemical properties of the samples were also
thoroughly investigated. As shown in Fig. 5, UV–vis diffuse reflectance
analysis was carried out on the samples of pure ZnO, nanoplates-A/ZnO
and nanoplates-B/ZnO to study their optical properties. In comparison
with the pure ZnO, the light-absorption region for nanoplates-A/ZnO
and nanoplates-B/ZnO at the wavelength from 200 nm to 700 nm also
have absorption with obvious changes, which manifests the identical
band gap to the pure ZnO as well as to the ZnO-based Ag-Ag2O/Ag
nanoplates. At the ultraviolet region from 200 nm to 400 nm, the pure
ZnO had obvious absorption, as the photochemical reaction goes by.
After thermal treatment, the absorption decreased, which indicated that
the photocorrosion and the chemical corrosion occurring accompanied
As a crystalline material, ZnO is a wide band gap semiconductor
with 3.2 eV, and its optically active only express at irradiation wave-
length shorter than approximately 375 nm. For the pure ZnO, when
Fig. 5. Changing UV–vis DRS spectra of (a) pure ZnO; (b) nanoplates-A/ZnO;
(c) nanoplates-B/ZnO.
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Y. Mao, et al.
ChemicalPhysics529(2020)110556
Fig. 6. (A) The variation of UV–vis absorption of MO in the presence of the nanoplates-A/ZnO. (B) The variation of UV–vis absorption of MO in the presence of the
nanoplates-B/ZnO. (C) Time course of photodegradation efficiencies of MO dye and (D) kinetic curves for MO aqueous solutions under xenon lamp in the presence of
different photocatalysts: pure ZnO, the nanoplates-A/ZnO and the nanoplates-B/ZnO.
Fig. 7. The cyclic stability of the pure ZnO, the nanoplates-A/ZnO and the nanoplates-B/ZnO in MO dye.
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ChemicalPhysics529(2020)110556
light irradiation with a photon energy higher than the band gap of ZnO
(λ ≤ 375 nm), electron-hole (e−-h+) pairs are generated in ZnO upon
the absorption of photons. The holes can be captured by hydrones and
hydroxyl groups to produce hydroxyl radicals, therefore, a large
number of hydroxyl radicals are formed and then they decompose the
MO dye. Beyond that, there are also some superoxide radicals formed
by combining few adsorbed oxygen molecules with electrons. The main
mechanism of reaction process goes as follows:
ZnO + hv → e−+h+
H2O → H+ + OH−
OH− + h+ → %OH
O2 + e− → %O−
(15)
(16)
(17)
(18)
(19)
(20)
H2O → H+ + OH−
OH−+h+ → %OH
MO dye + %O2− → CO2 + H2O
MO dye + %O− → CO2 + H2O
MO dye + %OH → CO2 + H2O
(1)
(2)
(3)
(4)
Fig. 9A shows typical SERS spectra of TP molecule on the surface of
the nanoplates-A with different concentrations. As seen, the intensities
of TP signals decreased over the concentration of TP molecule, the
signals are weak at the concentration of 10−6 M. When further de-
creased the concentration of the TP probe to 5*10−7 M, as shown in the
inset of Fig. 9A, the Raman signals of TP molecule still can be detected
at the peaks of 999 cm−1, 1022 cm−1 and 1068 cm−1 though the in-
tensities had decreased to be weak. Therefore, the LOD of TP molecules
on the surface of the nanoplates-A is about 5*10−7 M. Then, the same
experiment methods was also used to handle the sample of the nano-
plates-B, as shown in Fig. 9B, the intensities of TP signals decreasing
with the decreasing of the concentration also was perceived. Finally,
the TP signals got at the concentration of 10−10 M, which means that
the LOD of TP molecules on the surface of the nanoplates-B is about
10−10 M.
MO dye + %OH → CO2 + H2O
However, when the nanoplates-A irradiated for full wavelength, in
addition to photocatalysis of ZnO in ultraviolet region, Ag2O nano-
particles are fully visible light sensitive because of its narrow band gap
(1.3 eV). As shown in Fig. 8, e−-h+ pairs generate in the smooth na-
noplates, there will be more pairs with the addition of ZnO. The gen-
erated electron-hole pairs react with the adsorbed oxygen molecules
and hydrones, respectively. The Ag nanoparticles in nanoplates can
adsorb more oxygen than ZnO and thus to form hydroxyl radicals and
superoxide radicals. The resulting radicals, being a strong reducing
agent and a strong oxidizing agent, can photodegrade MO dye to the
ending products. The main mechanism of reaction process goes as fol-
lows:
When immersing the samples of the nanoplates-A in TP solutions,
the TP molecules adsorbed on the surface of nanoplates, and the in-
tensities of molecules adsorbing on the surface of nanoplates also could
get certain enhancements because of the SERS effects of Ag. The den-
sities of the nanoplates is biggish and we can regard nanoplates gather
together as “nanoplates clusters”, owing to the well dispersity of the
smooth nanoplates, and when the intervals between adjacent nano-
plates is suitable and it can be seen as “hot spots” between adjacent
nanoplates. After Raman laser, the signals of TP molecules around two
areas will be enhanced. Therefore, the properties of SERS effect of Ag
and “hot spots” between adjacent nanoplates [31] are the main factors
explaining why we can detect the signals of TP molecules at the con-
centration of 5*10−7 M on the sample of the nanoplates-A. In com-
parison to the nanoplates-A, the nanoplates-B have the enhancement
factor (EF) of the nanoplates-A. Apart from the factors of the nano-
plates-A which make the intensities of TP signals enhanced, the fol-
lowing factors also lead to the intensities being enhanced: (a) Ragged
surface: firstly, there will more TP molecules adsorb on the surface of
porous nanoplates and it can enhance the intensities; secondly, the
porous nanoplates have stronger SERS EF effects [32,33], the holes of
the porous nanoplates can be seen as “hot spots”, lastly, ragged sharp
tips also bring about the better enhancement than common sharp tips;
(b) Ag nanoparticles: the adjacent nanoparticles can give rise to
stronger SPR effects and the nanoparticles also can form “hot spots”
with the porous nanoplates. In general, more adsorbent molecules and
more “hot spots” are the main factors [34] which make TP molecules
detected at a lower concentration. Moreover, more “hot spots” is the
key factor. To sum up, the LOD of the TP molecules for nanoplates-B is
lower than that of the nanoplates-A.
ZnO + hv → e− + h+
Ag2O + hv → e− + h+
(5)
(6)
−
O2 + e− → %O2
(7)
H2O → H+ + OH−
(8)
OH− + h+ → %OH
(9)
MO dye + %O2− → CO2 + H2O
MO dye + %OH → CO2 + H2O
(10)
(11)
In comparison the nanoplates-A with the nanoplates-B, there were
plenty of oxygen adsorbing around the porous nanoplates and which
result in producing plenty of superoxide radicals. On the other hand, as
shown in Fig. 8, the adsorbed O2 around new-producing Ag nano-
particles after thermal treatment exist an equilibrium of adsorption and
dissociated adsorption, and oxygen radicals generated in the process of
dissociated adsorption. The main mechanism of reaction process goes as
follows:
ZnO + hv → e− + h+
Ag2O + hv → e− + h+
O2 + e− → %O2
(12)
(13)
(14)
−
4. Conclusions
We have reported the photochemical synthesis of Ag-Ag2O smooth
nanoplates on ZnO films and gotten Ag nanoparticles-decorated porous
nanoplates through a way of programmed thermal treatment. Besides
these, the Ag nanoparticles-decorated porous nanoplates are better at
photodegrading of MO dye than the pure ZnO and the Ag-Ag2O smooth
nanoplates, and the Ag nanoparticles-decorated porous nanoplates also
have a lower LOD of TP molecules because its better SERS effect, so we
can apply the properties of Ag nanoparticles-decorated porous nano-
plates to agricultures like the detection of disease biomarkers [35],
Fig. 8. Schematic of the photocatalytic processes for the pure ZnO, the nano-
plates-A/ZnO and the nanoplates-B/ZnO.
6

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ChemicalPhysics529(2020)110556
Fig. 9. SERS spectra of TP molecule in different concentrations on the samples of (A) the nanoplates-A: (a) 10−3 M; (b) 10−4 M; (c) 10−6 M and (d) 5*10−7 M, and
the inset is the partially enlarged view of the rectangular area of the line (d). (B) The nanoplates-B: (e) 10−4 M; (f) 10−6 M; (g) 10−8 M and (h) 10−10 M, and the inset
is the partially enlarged view of the rectangular area of the line (h).
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