New insight into the selective photocatalytic oxidation of RhB through a strategy of modulating radical generation

Article abstract of DOI:10.1039/c8ra01810c

Rhodamine B (RhB) has often been used as a model pollutant, but its photocatalytic mechanism is still controversial. Herein, Ag NPs were sandwiched between CdS QDs and amorphous-TiO₂ (a-TiO₂) with the intent to build a CdS/Ag/a-TiO₂ catalyst with highly selective oxidation activity. When rhodamine B (RhB) was used as the model organic compound, the CdS/Ag/a-TiO₂ composite can not only modulate radical generation but also improve the conversion ratio of RhB to rhodamine 110 (Rh-110) to as high as 82% at 80 min during the visible-light irradiation. A series of the radical scavenging experiments revealed that CdS/Ag/a-TiO₂ composites could modulate the effects of hydroxyl radicals (·OH) and superoxide anion radicals (·O₂^-) at different reaction stages so that the overoxidation of RhB and Rh-110 were repressed. Therefore, the transient state protection mechanism of selective oxidation of RhB was proposed to explain the reaction selectivity for Rh-110. Although the effects of both ·O₂^- and ·OH are important during the photocatalytic selective oxidation of RhB, it is shown that the selective oxidation of RhB would be performed when the effect of ·O₂^- is bigger than the ·OH, if not, RhB would be oxidized unselectively. Meanwhile, this may provide a new strategy for modulating radical generation in the photocatalysis of water phases.

Full text of DOI:10.1039/c8ra01810c

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New insight into the selective photocatalytic
oxidation of RhB through a strategy of modulating
Cite this: RSC Adv., 2018, 8, 13625
radical generation†
a
a
Huijun Liang,ab Shengnan Liu,a Hucheng Zhang,
Xiaobing Wang
*
a
*
and Jianji Wang
Rhodamine B (RhB) has often been used as a model pollutant, but its photocatalytic mechanism is still
controversial. Herein, Ag NPs were sandwiched between CdS QDs and amorphous-TiO2 (a-TiO2) with
the intent to build a CdS/Ag/a-TiO2 catalyst with highly selective oxidation activity. When rhodamine B
(RhB) was used as the model organic compound, the CdS/Ag/a-TiO2 composite can not only modulate
radical generation but also improve the conversion ratio of RhB to rhodamine 110 (Rh-110) to as high as
82% at 80 min during the visible-light irradiation. A series of the radical scavenging experiments revealed
that CdS/Ag/a-TiO2 composites could modulate the effects of hydroxyl radicals ($OH) and superoxide
anion radicals ($O2À) at different reaction stages so that the overoxidation of RhB and Rh-110 were
repressed. Therefore, the transient state protection mechanism of selective oxidation of RhB was
À
proposed to explain the reaction selectivity for Rh-110. Although the effects of both $O2 and $OH are
Received 1st March 2018
Accepted 13th March 2018
important during the photocatalytic selective oxidation of RhB, it is shown that the selective oxidation of
À
RhB would be performed when the effect of $O2 is bigger than the $OH, if not, RhB would be oxidized
DOI: 10.1039/c8ra01810c
unselectively. Meanwhile, this may provide a new strategy for modulating radical generation in the
photocatalysis of water phases.
rsc.li/rsc-advances
found the photoelectrochemistry of TiO2.10 So far, photo-
catalytic oxidation with O2 has been extensively used for unse-
1. Introduction
lectively obtaining the complete mineralization of a variety of
pollutants in gaseous and aqueous phases because of the
milder reaction conditions, ease of scaling up, environmentally
friendly properties, and relatively high economic efficiency.11–15
Beyond that, the selective photocatalytic oxidation of pollutants
or organics have also attracted attention in recent years because
it can also produce high-valued chemicals,11,16,17 such as the
selective oxidation of aromatic alcohols,11,17–20 aliphatic alco-
hols,21–25 alkenes and alkanes,26 amines,14,27 isoeugenol28 and so
on.
As a model organic compound, rhodamine B (RhB) is oen used
for examining the activities of photocatalysts,1,2 Up to now,
there have been two views on the photocatalytic degradation
mechanism of RhB, one is chemisorption of functional groups,
and the other is photosensitized degradation.3–5 Nevertheless,
the two views both assumed that the N-deethylation of RhB
would compete with the destruction of conjugated xanthene
structure resulting in the uncomplete (selective oxidation) or
complete degradation of RhB.3,4,6–8 However, according to the
two photocatalytic degradation mechanisms, it is still difficult
to control the oxidation process of RhB and obtain desired
products (e.g. rhodamine 110, Rh-110).9
However, it is well known that the photogenerated electrons
(eÀ) and holes (h+) could react with dissolved oxygen [(O2)aq] or
hydroxyl (–OH) on the surface of photocatalysts to produce
various reactive oxygen species (ROS) during the reactions, such
as hydroxyl radicals ($OH), superoxide anion radicals ($O2À),
Heterogeneous photocatalysis of semiconducting metal
oxides has been widely studied since Fujishima and Honda
À
and O2 protonation radicals ($OOH), and so on.4,29 For
aCollaborative Innovation Center of Henan Province for Green Manufacturing of Fine
Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of
Education, School of Chemistry and Chemical Engineering, Henan Normal
University, Xinxiang, Henan 453007, P. R. China. E-mail: hzhang@htu.edu.cn;
jwang@henannu.edu.cn
example, when the TiO2 is used as photocatalyst under ultra-
violet (UV) light irradiation, the formations of ROS can be
described as follow:30–34
TiO2 + hv / eÀ + h+
(1)
(2)
bCollege of Chemistry and Chemical Engineering, Xinxiang University, Xinxiang,
Henan 453003, P. R. China
(O2)aq + eÀ / $O2
À
† Electronic supplementary information (ESI) available: Synthesis of a-TiO2, size
distribution, TEM and elemental mapping images, element contents BET
measures, UV-visible spectrum,. See DOI: 10.1039/c8ra01810c
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h+ + H2O / H+ + $OH
h+ + OHÀ # $OH
(3)
(4)
(5)
(6)
(7)
exhibit the catalytic activity for the selective oxidation of
rhodamine B under Vis-light irradiation. The conversion ratio
of CdS/a-TiO2 is only about 9.7% in 100 min, however, the
conversion ratio of CdS/Ag/a-TiO2 can reach to 82.4% in 80 min.
Compare with our previous report, Ag/a-TiO2 (which was labeled
Ag/TiO2–I in previous work) also has the selective oxidation
performance, but the conversion ratio is only 23.8% in
180 min.9 In order to insight into the roles played by $O2À, $OH,
and h+, the scavengers of benzoquinone (BQ), isopropanol
(IPA), and triethanolamine (TEOA) were used respectively to
remove $O2À, $OH, and h+ during the photocatalytic reaction.
The scavenging experiments conrm that the ROS can be
modulated by CdS/Ag/a-TiO2 composite during the photo-
catalytic reaction, and the conversion ratio of Rh-100 can be
improved by the rational design of catalysts, so that the pho-
tocatalytic degradation mechanisms of RhB can be explained by
the transient state protection mechanism of RhB.
À
$O2 + H+ / $OOH
À
$OOH + $O2 + H+ / O2 + H2O2
H2O2 + $O2 / $OH + OHÀ + O2
À
This suggests that the photogenerated eÀ and h+ can be
converted into the different ROS. In fact, the nonselective auto-
oxidations of ROS involve in many parallel oxidation pathways
during photocatalytic oxidations,35 the photocatalytic selectivity
of desired products is usually very low when the produces or
substrates are seriously oxidized by these ROS in the water
phase.9,27,36,37 However, many reports have demonstrated that
ROS have different oxidation potential, such as 2.80 orÀ2.7 V for
$OH,29,33 1.76 V for O2 or H2O2,6 and À0.33 V for $O2
,
indi-
29
cating that ROS possess the different photocatalytic oxidation
activity, hereinto, $OH is the highest one according to the
oxidation potential. Therefore, it is possible to perform the
selective oxidation of RhB and improve the conversion ratio
towards the Rh-110 through the decrease of $OH during the
photocatalytic oxidations. However, to the best of our knowl-
edge, the mediating strategies for the selective generation of
ROS are very few in the previous literature, and one of the main
challenges is difficult to construct suitable framework for the
blocks of photocatalysts.11,21
CdS quantum dots (QDs) respond sensitively to visible (Vis)
light,19 and amorphous TiO2 (a-TiO2) efficiently captures h+.38,39
As the photogenerated h+ is directly trapped by a-TiO2, the $OH
radicals of higher oxidation activity would be decreased
according to equation (eqn (3) and (4)), further, the photoin-
duced stability of CdS QDs and the conversion ratio of selective
photooxidation would be improved during the photocatalytic
oxidation.40 Therefore, it is expected that CdS/a-TiO2 composite
could be more favorable for enhancing the effect of$O2À rather
than the $OH with higher oxidation activity in the
photooxidation.
However, the spectral response range of CdS QDs is relatively
narrow,41 and a-TiO2 in nature has the short range ordered
structure and the high defect density.38,42 These factors are very
detrimental to the photocatalytic activity of CdS/a-TiO2
composites. It is well known that the localized surface plasmon
resonance (LSPR) of noble metal nanoparticles (NPs) can induce
a local electric eld, increase the photocatalytic activity of a-
TiO2, and extend the light absorption range of CdS/a-TiO2.43
Therefore, CdS/a-TiO2 composites containing noble metal NPÀs
could be preferential catalysts for increasing the effect of $O2
and the conversion ratio of Rh-110.
In this work, the hydrosoluble CdS quantum dots (QDs) and
Ag nanoparticles were loaded on a-TiO2 with high specic
surface area, and respectively to prepare CdS/a-TiO2, CdS/Ag/a-
TiO2 and CdS/Ag/c-TiO2 composites. The selective photooxida-
tion activities of catalysts were evaluated by the yield of Rh-110.
It is shown that CdS/a-TiO2 and CdS/Ag/a-TiO2 composites all
2. Experimental section
Cadmium chloride (CdCl2$2.5H2O, AR) and absolute ethanol
were purchased from Beijing Chemical Works (China). Ethylene
glycol (EG, HOCH2CH2OH, AR) and tetrabutyl titanate (TBT,
98%) were purchased from Sinopharm Chemical Reagent Co.
Ltd. Acetic acid (CH3COOH, AR) was purchased from Tianjin
Deen Chemical Reagent Co. Ltd. Silver nitrate (AgNO3, AR) and
thioacetamide (TAA) were purchased from Aladdin. RhB (AR)
was purchased from Tianjin Kemiou Chemical Reagent Co.,
Ltd.
2.1 Synthesis of CdS QDs and a-TiO2
In a typical procedure, 2.08 g of CdCl2$2.5H2O and 0.75 g of TAA
were respectively dissolved into 100 mL absolute ethanol by
means of ultrasonication, then two solutions were mixed with
the ultrasonication for 10 min and let it stand for 4 h to form the
CdS precursor at room temperature.
The as-prepared CdS precursor was added into 300 mL
deionized water, and then heated to 40 ꢀC for 8 h under
magnetic stirring. The colour of solution changes from colorless
to yellow. The precipitate was separated by centrifugation at
20 000 rpm, and washed with deionized water and absꢀolute
ethanol several times, and nally dried in vacuum at 60 C to
obtain yellow hydrosoluble CdS QDs.
a-TiO2 was synthesized by the processes as reported in our
previous work,44 and also the details can be found in ESI S1.†
2.2 Synthesis of CdS/a-TiO2 and CdS/Ag/a-TiO2 composites
2 g a-TiO2 was dispersed into 300 mL deionized water with the
ultrasonication for 30 min, and then irradiated by UV-light
source for 2 h. The color of suspension evolved gradually from
white to black blue. Aer UV-light source was shut down, 50 mL
aqueous solution containing 0.6 g of CdS QDs was added into
the a-TiO2 suspension under vigorously magnetic stirring for
1 h. Then the precipitate was separated by centrifugation at
10 000 rpm, and washed with deionized water and absolute
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ꢀ
ethanol several times, and nally dried in vacuum at 40 C to Halenda (BJH, Micromeritics Instruments ASAP 2020). Their
obtain CdS/a-TiO2 composite. properties were measured by the UV-Vis diffused reectance
Aer UV-light source was shut down, similarly, 20 mL AgNO3 spectra (DRS, Lambda 950, PerkinElmer Inc, America, and
aqueous solution (19.4 mmol LÀ1) was mixed into the black blue BaSO4 was used as a reectance standard.), photoluminescence
a-TiO2 under vigorously magnetic stirring for 1 h to obtain gray spectrophotometer (PL, HITACHI FP-6500 spectrophotometer),
Ag/a-TiO2 composite. Then, 50 mL aqueous solution containing and UV-Vis spectrophotometry (TU-1900, Beijing). The
0.6 g of CdS QDs was added into under stirring for 1 h. Aer elemental compositions of samples were investigated by the
this, the precipitate was separated, washed, and dried following energy-dispersive X-ray spectroscopy (EDS) attached to the eld
the same processes as mentioned above to harvest CdS/Ag/a- emission scanning electron microscopy (FESEM, ZEISS SUPRA-
TiO2 composite.
40 VP).
CdS/Ag/c-TiO2 was prepared through annealing CdS/Ag/a-
TiO2 at 300 ꢀC for 4 h for comparison.
3. Results and discussions
3.1 Synthesis and characterizations of composites
2.3 Photochemical reactor and photocatalytic experiments
CdS QDs are usually synthesized by the organic phase or
aqueous phase routes. By contrast, aqueous phase syntheses of
CdS QDs are simpler and easily scaled up, but the resulting QDs
oen exhibit lower crystallinity and quantum yields.45,46 Here,
the uniform hydrosoluble CdS QDs were synthesized by a facile
hydrolysis method without surfactant. As shown from TEM
images in Fig. 1a and b, the as-prepared CdS QDs have good
dispersity and uniformity with the mean size about 5.4 Æ 0.2 nm
(Fig. S1a†). The lattice spacing of 0.336 nm corresponding to the
(002) plan of CdS in high-resolution transmission electron
micro (HRTEM) and the pattern of the selected area electron
diffraction (SAED) conrm the resulting QDs possess good
crystallinity.
When the CdS QDs were loaded on the surface of a-TiO2, the
images in Fig. 1c and d show the relatively uniform distribution
of CdS QDs with almost unchanged size of 5.6 Æ 0.2 nm
(Fig. S1b†). As reported in our previous work, the light chemical
reduction in aqueous AgNO3 solution can induce the growth of
Ag NPs on the a-TiO2 to produce Ag/a-TiO2 composite.9 It
suggests that the Ag NPs on Ag/a-TiO2 composite can provide
the good scaffolds to anchor CdS QDs through S–Ag chemical
interactions. As a result, CdS/Ag/a-TiO2 composite was facilely
prepared by mixing CdS QDs with Ag/a-TiO2 suspension.
Despite the stacking of CdS QDs on Ag NPs, the metal occur-
rence in CdS/Ag/a-TiO2 composite could still be identiable
from the lattice spacing of Ag (111) crystal plane (corresponding
to 0.236 nm in Fig. 1f) and a few Ag Nps with big particle size
(Fig. S2†). As shown in Fig. S2,† some representative Ag NPs
show that CdS QDs are loaded on the Ag NPs (white arrows), and
Ag NPs located at between CdS QDs and a-TiO2. The architec-
ture was further conrmed by the similar distribution of Cd and
Ag elements on a-TiO2 from the element mapping images of
CdS/Ag/a-TiO2 (Fig. S3†). Aer annealing CdS/Ag/a-TiO2,
however, the geometric arrangement in the composite could be
disarrayed to turn into the CdS/Ag/crystalline TiO2 (CdS/Ag/c-
TiO2) composite, and hence Ag NPs were exposed on TiO2 and
can be clearly observed from Fig. 1g and h. Simultaneously, the
SAED images and the lattice spacing of 0.352 nm corresponding
The photochemical reactor system was consisted of a 30 mL
quartz tube and a cylindrical water-cooled jacket quartz cell. A
500 W Xe-lamp or 500 W high-pressure mercury lamp was
placed inside the quartz cell, which was respectively used as the
Vis and UV-light source. The optical path length was 10 cm. The
cylindrical water-cooled jacket quartz cell was kept at 15 Æ 2 ꢀC
by an external cooling jacket with recycled water.
The photocatalytic degradation of 5 mg LÀ1 RhB was used to
evaluate the selective oxidation activities of as-synthesized
samples and the effect of $O2À, $OH, and h+. Typical, 25 mL
RhB solution and 0.05 g catalysts were dispersed into 30 mL
quartz tube under magnetically stirred and open to air, and
were held in dark for 30 min to reach adsorption–desorption
equilibrium. Aer irradiated by light for a certain time interval,
2 mL reaction mixture was centrifuged to remove the photo-
catalyst, and the degradation of RhB was evaluated by the
intensity of adsorption peak at 554 nm. The percentages of
degradation is calculated as C/C0 Â 100%, here, C is the
intensity of the RhB solution at each irradiated time interval,
and C0 is the intensity of the initial concentration.
In order to simplify the calculation, the concentration of
selective photooxidation product (Rh-110) was evaluated by the
intensity of adsorption peak at 498 nm according to the
molecular amount of converting RhB into Rh-110 (eqn (8)), and
correspondingly the conversion ratio is also calculated as C/C0
 100%. Especially, the conversion ratio was marked by blue
pentagrams at the degradation efficiency curves only when the
Rh-110 was generated. It means that the conversion ratio
increase with increasing the concentration of Rh-110, but the
degradation efficiency decrease.
In the same experimental conditions, a certain amount of
benzoquinone (BQ), isopropanol (IPA), and triethanolamine
(TEOA) were respectively added into the photocatalytic degra-
dation system of RhB to evaluate the effect of $O2À, $OH, and h+
according to the degradation percentage of RhB.
2.4 Structural characterizations
The as-prepared samples were characterized by the trans- to the (101) plan of TiO2 verify the transformation of a-TiO2 to
mission electron microscope (TEM, JEOL JEM-2100), X-ray crystalline TiO2 (c-TiO2) aer the annealing treatment of CdS/
diffraction (XRD, Bruker advance-D8), X-ray photoelectron Ag/a-TiO2.
spectroscopy (XPS, VG Scientic ESCALAB MKII), and Bru-
The phase structure and composition of as-prepared
nauer–Emmet–Teller (BET) method and the Barrett–Joyner– samples were determined by XRD patterns and XPS spectra.
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Fig. 1 The TEM and HRTEM images of as-prepared samples. (a) and (b) CdS QDs, (c) and (d) CdS/a-TiO2, (e) and (f) CdS/Ag/a-TiO2, (g) and (h)
CdS/Ag/c-TiO2. The insets are the ring patterns of the selected area electron diffraction (SAED) images.
As shown in Fig. 2a, the diffraction peaks show that CdS QDs of TAA ligands. The absence of N 1s peak in the survey spectra of
represent a hexagonal phase (JCPDS card no. 41-1049), but the CdS/a-TiO2, CdS/Ag/a-TiO2 and CdS/Ag/c-TiO2 indicates the
wide diffraction peaks of CdS/a-TiO2 and CdS/Ag/a-TiO2 remove of TAA ligands, and reects that the strong interactions
composites result from the a-TiO2 due to the low content of CdS of CdS with a-TiO2 and Ag NPs cause the QDs almost “naked” in
and Ag beyond the instrumental detection limit. Aer annealing these composites.
CdS/Ag/a-TiO2, the diffraction pattern indicates that the anatase
The chemical states of Cd 3d, S 2p, Ti 2p, and Ag 3d in the as-
TiO2 occurs in CdS/Ag/c-TiO2 (JCPDS no. 21-1272).
prepared samples show distinct differences in their high-
The survey scan XPS spectra of as-prepared samples clearly resolution XPS spectra (Fig. 2c–f), respectively. As shown in
show the occurrence of the expected Ti, O, Cd, S, C, and N Fig. 2c, the XPS spectra of Cd 3d in CdS QDs shows that the Cd
elements in the respective composite (Fig. 2b and Table S1†). It 3d3/2 and 3d5/2 peak can be divided into two peaks centered at
is noted that Cd and S elements can be detected from all ꢁ414.8/413.4 and ꢁ408.0/406.6 eV, and the peak separation of
samples, but N element only from CdS QDs due to the presence 6.8 eV corresponding to Cd2+. The peaks at 413.4/406.6 eV are
Fig. 2 The XRD patterns and XPS spectra of the as-prepared samples. (a) XRD patterns, (b) survey spectra, and the high-resolution XPS spectra of
(c) Cd 3d, (d) S 2p; (e) Ti 2p; (f) Ag 3d.
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ascribed to CdS, and the other at 414.8/408.0 eV are combined and electrons,58 and is expected to modulate ROS and to
with TAA ligands.47,48 When the CdS QDs are loaded on a-TiO2, perform selective photocatalysis. The excited state electron
the Cd 3d3/2 and Cd 3d5/2 appears as a single peaks in agree- radioactive decay lifetime of samples can also conrm the
ment with the remove of TAA, and the strong interactions cause inhibition of Ag in CdS/Ag/a-TiO2. As shown in Fig. S4b,† the
the peak shi to 411.6/404.8 eV. Furthermore, the Cd 3d peak is calculated average lifetime of CdS QDs, CdS/a-TiO2, CdS/Ag/a-
shied to 410.9/404.1 eV in CdS/Ag/a-TiO2 due to the chemical TiO2 and CdS/Ag/c-TiO2 are 3.5, 4.4, 4.3 and 4.9 ns, respectively.
bonding between S and Ag as the CdS QDs are loaded on Ag/a- The lifetime of CdS/Ag/a-TiO2 is longer than the CdS QDs.
TiO2, it means that the strong interactions between CdS QDs, Ag
In addition, the specic surface area, pore volume and
NPs and a-TiO2 cause the peak shi of Cd 3d.49,50 Correspond- average pore size of the composites were measured by the
ingly, the binding energy of S 2p exhibit the similar changes in nitrogen adsorption/desorption experiments (Fig. S5 and Table
pace with the enhanced interactions in the composites (Fig. 2d). S2†). Among these composites, CdS/Ag/a-TiO2 with the medium
By contrast, the Cd 3d peak in CdS/Ag/c-TiO2 reversely shis to pore size exhibits the largest BET surface area of 441 m2 gÀ1 and
412.3/405.5 eV, a binding energy more than that in CdS/a-TiO2, pore volume of 0.35 cm3 gÀ1, and can provide the optimal space
implying the collapse of stacking structure in CdS/Ag/c-TiO2 due for available photocatalystic reactions.
to the annealing treatment and the weaker interactions between
CdS and c-TiO2 in the resulting composite. Additionally, the
architecture transformation of the three blockings during
3.2 Selective photocatalytic oxidation of RhB
annealing can be veried from the high-resolution spectra of Ti When CdS/a-TiO2, CdS/Ag/a-TiO2, and CdS/Ag/c-TiO2 were
and Ag elements: the Ti 2p peak shis from 464.3/458.6 eV in respectively used as photocatalysts, and correspondingly the
CdS/a-TiO2 to 463.6/457.9 eV in CdS/Ag/a-TiO2, then to 464.7/ changes of UV-Vis spectrum and degradation efficiency with
459.0 eV in CdS/Ag/c-TiO2, the Ag 3d peak from 372.8/366.8 eV time were recorded from the RhB aqueous solution during the
in CdS/Ag/a-TiO2 to 373.8/367.8 eV in CdS/Ag/c-TiO2 (Fig. 2e and photooxidation (Fig. 4). As far as Vis-light irradiation is con-
f).51,52
cerned, the maximum absorption wavelength (lmax) of RhB
Fig. 3 shows several optical properties of as-prepared solution in the presence of CdS/a-TiO2 blue-shis from 554 to
samples. The pristine CdS QDs and CdS/a-TiO2 exhibit a Vis- 498 nm (Fig. 4a), indicating the formation of Rh-110 and the
light absorption edge at about 450 nm (Fig. 3a), resulting selective photocatalytic oxidation toward RhB.9 Although the
from the intrinsic band-gap transition of electron from the conversion ratio of Rh-110 is only about 9.7% at 100 min, CdS/a-
valence band (VB) to the conduction band (CB) in CdS.53 When TiO2 exhibits the highest degradation efficiency compared with
Ag NPs are added into CdS/a-TiO2, the synergistic effect of the the other photocatalysts (Fig. 4d). CdS/Ag/a-TiO2 represents the
LSPR with the light absorption of CdS QDs promotes the most efficient selective photocatalysis (Fig. 4b and d). The lmax
absorption of CdS/Ag/a-TiO2 and CdS/Ag/c-TiO2 composites of RhB solution blue-shis to 510 nm only in 20 min that is
extends over wider Vis-light region.47,54,55 The band gap (Eg) of attributed to the photoisomerization of dye chromophores, an
CdS QDs, CdS/a-TiO2, and CdS/Ag/a-TiO2 is respectively esti- intermediate for the conversion from RhB to Rh-110.1,2,4,9,33,59
mated to be 2.5, 3.1, and 2.7 eV by Tauc plot of (Ahn)2 vs. hn Aer 20 min, the peak intensity at lmax of Rh-110 increases with
(Fig. S4a†).52,56,57
irradiation time, and reaches the maximum (at 80 min) that
The emission intensity of photoluminescence (PL) spectra almost equates to the peak intensity of RhB in the adsorption–
demonstrates the concentration of charge carriers in the excited desorption equilibrium solution (corresponding to the absorp-
states of composites. As excited by 386 nm, the as-prepared CdS tion peak intensity at 0 min in Fig. 4b). At this phase, the
QDs exhibit two main emission peaks located at 487 and conversion ratio for Rh-110 is about 82%, subsequently, the
531 nm (Fig. 3b) that originate from the excitonic recombina- peak intensity at 498 nm reduces gradually due to the degra-
tion.55 The relative emission intensities of CdS hold strong in dation of Rh-110 with increasing irradiation time from 80 to
CdS/a-TiO2, but signicantly are weaken in CdS/Ag/a-TiO2 and 140 min. Unlike CdS/Ag/a-TiO2, however, no photocatalysis of
CdS/Ag/c-TiO2, and particularly in CdS/Ag/a-TiO2. This suggests the selective oxidation is observed from the CdS/Ag/c-TiO2
that the introduction of Ag NPs between CdS and a-TiO2 can (Fig. 4c and d), and also its photocatalytic degradation efficiency
moderately restrain the combination of photogenerated holes is lower than CdS/a-TiO2. The experimental facts suggest that
the special geometric arrangement in CdS/Ag/a-TiO2 is respon-
sible for the selective photocatalytic conversion from RhB to Rh-
110 with Vis-light irradiation.
When the RhB solution was degraded by UV-light irradia-
tion, both CdS/a-TiO2 and CdS/Ag/c-TiO2 exhibit the high pho-
tocatalytic degradation efficiency (Fig. 4e, g and h), and the RhB
solution almost completely become colorless in 140 min, but
the selective oxidation product of Rh-110 was not detected from
the photocatalysis. When the CdS/Ag/a-TiO2 is used as photo-
catalyst, by contrast, the blue-shi of lmax for RhB solution can
be observed, and the conversion ratio from RhB to Rh-110 is
Fig. 3 (a) UV-Vis DRS, and (b) PL spectra of as-prepared composites. estimated to be 43% at 140 min (Fig. 4f and h), a value much
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Fig. 4 Evolution of UV-Vis spectrum and photocatalytic degradation efficiency of RhB using the different photocatalysts and irradiation light
sources. (a) CdS/a-TiO2, (b) CdS/Ag/a-TiO2, (c) CdS/Ag/c-TiO2, (d) degradation efficiency under Vis-light; and (e) CdS/a-TiO2, (f) CdS/Ag/a-TiO2,
(g) CdS/Ag/c-TiO2, (h) degradation efficiency under UV-light. The conversion ratio of Rh-110 was marked by the blue pentagrams at the cor-
responding degradation efficiency curves (d and h).
less than that obtained from Vis-light irradiation. Therefore, depress the increase of $OH. In order to insight into the
CdS/Ag/a-TiO2 is a preferential photocatalyst for the selective controlling mechanisms of selective photocatalytic oxidation of
oxidation through UV- and Vis-light treatment, and demon- RhB, benzoquinone (BQ), isopropanol (IPA), and triethanol-
strates the great selectivity and activity under Vis-light amiÀne (TEOA) are used as scavengers respectively to remove
irradiation.
$O2 , $OH, and h+ under the different light-sources,60,61 and the
UV-Vis adsorption spectra of RhB solution and degradation
efficiencies are shown in Fig. 5, 6 and 8 when the CdS/a-TiO2,
CdS/Ag/a-TiO2, and CdS/Ag/c-TiO2 were respectively used as
photocatalyst.
3.3 Modulating ROS and the photocatalytic mechanism of
RhB
The photocatalysis of CdS/a-TiO2 in the presence of scaven-
À
gers is shown in Fig. 5. Aer BQ and TEOA scavenge $O2 and
h+, the obvious blue-shi of lmax is not observed from RhB
solution under Vis (Fig. 5a and c) and UV-light (Fig. 5e and g)
irradiation, and the photocatalytic degradation rate is remark-
ably decreased under Vis-light irradiation (Fig. 5d), suggesting
(8)
that both $O2 and h+ are important for the selective photo-
À
catalytic conversion of RhB to Rh-110. In addition, as shown in
RhB as a model organic compound has been widely used to
evaluate the photocatalytic degradation reaction because RhB
could be not only completely degraded but also selectively
photooxidized to produce Rh-110 (eqn (8)).1,2,4,7,33 In our
previous report, we demonstrated that RhB could be selectively
oxidized using Vis-light source and Ag/a-TiO2 photocatalyst to
harvest Rh-110 with the conversion ratio of 23.8% in 180 min.9
Under three light-emitting diode (LED, 1 W, radiant wavelength
of 455–460 nm), alternatively, it is shown that both Ag/a-TiO2
(Fig. S6a†) and Ag/c-TiO2 (Fig. S6b†) can act as photocatalyst for
selective oxidation, and the conversion ratio for Rh-110 is 39%
at 40 min using Ag/a-TiO2 and is 14% at 80 min using Ag/c-TiO2.
Unfortunately, it is the low conversion ratio that limits the
practical applications of the selective photocatalytic oxidations.
Herein, CdS/Ag/a-TiO2 not only exhibits the wider light
absorption range, but also the much higher selective oxidation
activity with the conversion ratio of 82% for Rh-110 under Vis-
light irradiation. Based on eqn (1) to (7), it is believed that
CdS/Ag/a-TiO2 could promote the generation of $O2À, but
À
Fig. 5d, the effects of h+ is almost consistent with the $O2
(green and blue line), it suggesting that the photooxidized
activities of h+ and $O2À in using CdS/a-TiO2 catalyst are almost
identical under Vis-light irradiation.
In fact, according to the eqn (3) to (7), we propose that the
effects of h+ can be considered together with the$O2À and $OH,
À
that is, the effects of $O2 and $OH are considered mainly.
Although h+ itself has certain photooxidized activity (which is
decided by the valence band position of materials.), the effects
of h+ can be nal converted into the $O2À and $OH according to
the eqn (3) to (7). It means that the effects of $O2À and $OH are
simultaneous changed with the h+. Therefore, in order to
simplify discussion, the effects (or amount) of $O2À and $OH are
considered mainly in this wÀork. Moreover, according to the
simultaneous changed of $O2 and $OH with the h+, the curve
overlapping of $O2À (Fig. 5d, BQ) and h+ (Fig. 5d, TEOA) is easy
explained. Under Vis-light irradiation, a-TiO2 is not excited by
the Vis-light, the photogenerated eÀ and h+ is mainly originated
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Fig. 5 Evolution of UV-Vis spectrum and photocatalytic degradation efficiency of RhB using CdS/a-TiO2 photocatalyst. Scavengers: BQ (a), IPA
(b), TEOA (c), and the corresponding degradation efficiency (d) under Vis-light; BQ (e), IPA (f), TEOA (g), and the corresponding degradation
efficiency (h) under UV-light. The degradation efficiency curves without scavenger is inserted into (d) and (h) for comparison. The conversion
ratio of Rh-110 was marked by the blue pentagrams at the corresponding degradation efficiency curves (d and h).
from CdS QDs. The photogenerated eÀ on the surface of CdS decreases the overoxidation of product (Rh-110) and substance
QDs can be scavenged by surface-adsorbed oxygen (eqn (2)), and (RhB) due to the high photooxidized activity of $OH.
h+ can be le in the CdS QDs.62 When the h+ is scavenged, the
According to the photocatalytic degradation curve of RhB
À
amount of $O2 and $OH also simultaneous decrease, so that (Fig. 5d and h), one rule of selective photocatalytic oxidation can
the curves of $O2 and h+ are accidentally overlapped.
also be obtained, that is, the selective photocatalytic oxidation
À
Using IPA as scavenger to remove $OH, by contrast, the lmax of RhB can be performed when the effect (or amount) of$O2À is
of RhB solution exhibit obvious blue-shis under both Vis bigger than the $OH in the reaction system, if not, RhB wilÀl be
(Fig. 5b) and UV-light (Fig. 5f) irradiation, and the conversion complete oxidized. As shown in Fig. 5d, the effect of $O2 is
ratio of Rh-110 can reach to 44% at 140 min under Vis-light bigger than the $OH under Vis-light irradiation, so that the
irradiation. Compared with the conversion ratio of 9.7% for selective photocatalytic oxidation of RhB can be performed by
CdS/a-TiO2 (Fig. 4a and d), the scavenging of $OH further the pure CdS/a-TiO2 (Fig. 4a).
improve the conversion ratio of Rh-110, suggesting that the
However, as shown in Fig. 4e, the blue-shi of lmax is not
scavenging of $OH with the high photooxidized activity observed. It can be attributed to the overoxidation of substance
Fig. 6 Evolution of UV-Vis spectrum and photocatalytic degradation efficiency of RhB using CdS/Ag/a-TiO2 photocatalyst. Scavengers: BQ (a),
IPA (b), TEOA (c), and the corresponding degradation efficiency (d) under Vis-light; BQ (e), IPA (f), TEOA (g), and the corresponding degradation
efficiency (h) under UV-light. The degradation efficiency curves without scavenger is inserted into (d) and (h) for comparison. The conversion
ratio of Rh-110 was marked by the blue pentagrams at the corresponding degradation efficiency curves (d and h).
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(RhB) at the initial stage of UV-lightÀirradiation. Because the should be suppressed during the complete photocatalysis
effect of $OH is bigger than the $O2 before 20 min, a large oxidation (or unselective oxidation). However, the selective
number of RhB could have been complete oxidized at the initial oxidation of RhB is the uncomplete photocatalysis oxidation
stage, and Rh-110 cannot be obtained. In order to test this process, we hope to reduce the effect of $OH and obtain more
speculation, we performed a controlled experiment at same $O2À, the combination of eÀ (derived from a-TiO2 and RhB) and
condition except for changing the concentration of RhB to h+ (derived from CdS QDs) at Ag NPs should be encouraged
10 mg LÀ1. As shown in Fig. S7(see ESI†), Rh-110 is generated at because it can improve the effect of $O2 and obtain the high
À
330 min. Therefore, the scavenging of $OH is benet to avoid conversion ratio. Meanwhile, RhB lost eÀ of the highest occu-
the overoxidation of substance or product and improve the pied molecular orbital (HOMO) to RhB+ with positively charged
conversion ratio of product. As shown in Fig. 5f, when the $OH (Fig. 7, 1st) due to the photosensitized of RhB. The result is that
is scavenged, the Rh-110 is obtained again at 140 min.
CdS/Ag/a-TiO2 shows the rich eÀ state. The photoisomerization
The similar phenomenon can be observed in the photo- of RhB chromophores can be seen as a feature of this
catalysis of CdS/Ag/a-TiO2. As shown in Fig. 6, the blue-shi of phenomenon (Fig. 4d, red line).9,59 Under UV-light irradiation,
À
lmax is not observed from RhB solution aer scavenging $O2
a-TiO2 is excited and h+ is captured, so that CdS/Ag/a-TiO2 also
(Fig. 6a and e) and h+ (Fig. 6c and g). In contrast, the lmax of RhB shows the rich eÀ state (Fig. 4 h, red line). According to eqn (1)
solution exhibit obvious blue-shi under Vis-light irradiation to (7), a large number of $O2À and a few $OH are formed. Due to
À
(Fig. 6b). However, the blue-shi of lmax disappear when the the electrostatic interaction, the reaction between $O2 and
$OH is scavenged UV-light irradiation (Fig. 6f). This phenom- RhB+ is easy to form a transient state of RhB (RhB*, Fig. 7, 2nd)
enon can be explained easily by the transient state protection rather than the oxidation of RhB, then RhB* is oxidized by $OH
mechanism, and obtaining the otheÀr rule of selective photo- (Fig. 7, 3rd) to form Rh-110.
catalytic oxidation. That is, both $O2 and $OH are important
The transient state protection mechanism can be further
for the selective photocatalytic conversion of RhB to Rh-110, the conrmed by the scavenging experiments, especially the scav-
conversion ratio of Rh-110 can be improved when the effect (or enging of $OH. Under Vis-light irradiation, as shown in Fig. 6b,
amount) of $OH is suitable increased at the later stage.
the lmax of RhB solution gradually shis from 554 nm (RhB) to
À
In the previous work, we proposed that the effect of $O2 is 498 nm (Rh-110). Compared to pure CdS/Ag/a-TiO2 (Fig. 4b), the
more important than the $OH during the process of forming shis rate is slowly. The reason is that N-deethylation reaction
Rh-110, because the Àconjugated xanthene structure of RhB of RhB* is also slowly when the $OH is scavenged, so that the
could react with $O2 to rst form a transient state.9 This serious overoxidation of RhB or Rh-110 is avoided, nally, the
transient state can protect the conjugated xanthene structure of conversion ratio of Rh-110 reach to 85% at 140 min (Fig. 6b and
RhB from damage, so that only N-deethylation reaction of RhB d).
can be performed by $OH to form Rh-110. In contrast, when the
However, under UV-light irradiation, although a large
effect of $OH is bigger than $O2À, the conjugated xanthene number of RhB* is generated, the effect (amount) of $OH is
structure of RhB is rst damaged by the $OH, so that the tran- always weak during photocatalytic oxidation (Fig. 6h, red line),
sient state of conjugated xanthene structure of RhB is not so the Rh-110 can be measured until 140 min (Fig. 4f). The
produced, Rh-110 can not be obtained. It suggests that the reason is that N-deethylation reaction of RhB* is more slowly
selective photocatalytic oxidation of RhB derives from the when the $OH is scavenged (Fig. 6f).
À
combined effect of $OH and $O2
.
It means that the effect of $OH cannot be neglected during
Therefore, the selective oxidation mechanism of CdS/Ag/a- the selective photocatalytic oxidation of RhB. When the effect of
TiO2 can be illustrated in Fig. 7. Under Vis-light irradiation, $OH is eliminated, the N-deethylation reaction of RhB* can not
CdS/Ag/a-TiO2 can provide a large number of the photo- be performed so that the selective photocatalytic oxidation of
generated eÀ and h+ due to the valence band (VB) electrons RhB disappears. Therefore, the rational controls of various ROS
excitation of CdS and the formation of Z-scheme.63,64 Because are an essential prerequisite for highly selective oxidation
the formation of Z-scheme, the combination of eÀ and h+ reactions.35 CdS/Ag/a-TiO2 certainly control radical generation
during the selective photocatalytic oxidation of RhB.
When the effect of $OH is bigger than the $O2À, the blue-shi
of lmax disappear, suggesting that RhB can not be selective
oxidized. As shown in Fig. 8, aer scavenging $O2À (Fig. 8a and
e), $OH (Fig. 8b and f), and h+ (Fig. 8c and g), the blue-shi of
lmax is not observed under both Vis- and UV-light irradiation.
The photocatalytic degradation efficiency curves (Fig. 8d and h)
demonstrate that the effect of $OH is always bigger than the
$O2À. Because c-TiO2 can not capture h+, the amount of eÀ is
almost equate with the h+, the amount (effect) of $OH increases.
This result further conrms that the selective oxidation of RhB
can be explained by the transient state protection mechanism.
Fig. 7 Schematic diagram of the possible process for CdS/Ag/a-TiO2
composites photocatalytic degrading RhB under light irradiation.
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Fig. 8 Evolution of UV-Vis spectrum and photocatalytic degradation efficiency of RhB using CdS/Ag/c-TiO2 photocatalyst. Scavengers: BQ (a),
IPA (b), TEOA (c), and the corresponding degradation efficiency (d) under Vis-light; BQ (e), IPA (f), TEOA (g), and the corresponding degradation
efficiency (h) under UV-light. The degradation efficiency curves without scavenger is inserted into (d) and (h) for comparison.
4. Conclusion
Notes and references
In summary, we designed and synthesized a CdS/Ag/a-TiO2
composite with highly selective oxidation activity for the pho-
tocatalytic degradation of RhB. Due to the light-responding
nature of CdS QDs and the LSPR effect of Ag NPs, CdS/Ag/a-
TiO2 composites can not only improve the conversion ratio of
RhB to Rh-110 in 82% at 80 min but also adjust the effect of ROS
under Vis-light irradiation. A serial of the radical scavenging
experiments conrm that CdS/Ag/a-TiO2 composite exhibits the
good photocatalytic selective oxidation activity, because CdS/Ag/
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