Preparation and photocatalytic performance of silver-modified and nitrogen-doped TiO2nanomaterials with oxygen vacancies

Article abstract of DOI:10.1039/d0nj04755d

The photocatalysis of titanium dioxide (TiO₂) exerts excellent degradation performance against contaminants in the environment. However, it prefers to absorb ultraviolet light rather than visible light, which significantly constrains its widespread use under visible light. Here, we prepared oxygen vacancy-containing TiO₂viaAg-modification and N-doping. The utilization of visible light for phenol degradation was significantly enhanced by Ag/N co-doping. The characterization results showed a shuttle-like material coupled with multiple oxygen vacancies, and a well-designed experiment demonstrated that the Ti?:?N?:?Ag ratio of 1?:?0.45?:?0.32 presented optimal performance for phenol degradation. The batch experiment results also proved the modified TiO₂as a potent photocatalyst against phenol degradation with an 80.8% degradation efficiency within 5 hours under visible light and with a 99.3% degradation efficiency within 2 hours under ultraviolet light. What is more, we also demonstrated that hydroxyl radical was the mainly effective radical in the mineralization of phenol and put forward a possible degradation pathway based on the observed intermediates. Lastly, the cycling tests indicated that the proposed photocatalyst is durable with a fair phenol degradation ability after recycling 5 times.

Full text of DOI:10.1039/d0nj04755d

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Preparation and photocatalytic performance
of silver-modified and nitrogen-doped TiO2
nanomaterials with oxygen vacancies
Cite this: New J. Chem., 2021,
45, 4694
a
ab
c
a
Hong Zhang,
Zhenya Zhu,
Yingyu Jiang,
Baiqin Zhou,
Zhuo Wei,
a
b
Lijuan Han, *b Ping Zhang *a and Yingying Hu
The photocatalysis of titanium dioxide (TiO2) exerts excellent degradation performance against
contaminants in the environment. However, it prefers to absorb ultraviolet light rather than visible light,
which significantly constrains its widespread use under visible light. Here, we prepared oxygen vacancy-
containing TiO2 via Ag-modification and N-doping. The utilization of visible light for phenol degradation
was significantly enhanced by Ag/N co-doping. The characterization results showed a shuttle-like
material coupled with multiple oxygen vacancies, and a well-designed experiment demonstrated that
the Ti : N : Ag ratio of 1 : 0.45 : 0.32 presented optimal performance for phenol degradation. The batch
experiment results also proved the modified TiO2 as a potent photocatalyst against phenol degradation
with an 80.8% degradation efficiency within 5 hours under visible light and with a 99.3% degradation
efficiency within 2 hours under ultraviolet light. What is more, we also demonstrated that hydroxyl
radical was the mainly effective radical in the mineralization of phenol and put forward a possible
degradation pathway based on the observed intermediates. Lastly, the cycling tests indicated that the
proposed photocatalyst is durable with a fair phenol degradation ability after recycling 5 times.
Received 26th September 2020,
Accepted 25th January 2021
DOI: 10.1039/d0nj04755d
rsc.li/njc
therefore pure TiO2 prefers to absorb ultraviolet light---accounting
for only about 4% of the entire solar spectrum—rather than
1. Introduction
Since the phenomenon of water decomposition by titanium visible light, significantly restricting its wider application in a
dioxide (TiO2) photocatalysts was first observed in 1970s, the realistic environment.11–13 A feasible method to solve this pro-
use of TiO2 in environmental applications has stimulated a lot blem is to dope precious metal particles (such as Au, Pt, and Ag)
of interest among scholars.1,2 Due to its high catalytic efficiency, onto the surface of TiO2, which extends the spectral response to
stable chemical properties and non-toxicity, studies on the use of the visible region. Bouhadoun et al.14 synthesized TiO2 and Au
TiO2 in wastewater treatment have continued for nearly 5 decades.3,4 modified TiO2 (Au–TiO2) nanoparticles. The photocatalytic activities
Under light source stimulation, TiO2 generates extremely active free of the two powders were evaluated by the photocatalytic degradation
radicals,5,6 which could despoil the electrons of organic matters of formic acid (FA) under ultraviolet and visible light irradiation.
and induce an ensuing chain reaction, thus efficiently degrading Under ultraviolet light irradiation, both samples showed a
pollutants and purifying water.7–10
higher catalytic efficiency compared with the commercial TiO2
However, some drawbacks of TiO2 also constrain its wide P25. In addition, Au modification, even at very low concentra-
application. The band gap of pure TiO2 is wide ranging from tions, increased the photocatalytic activity to a great degree, and
3.16 to 3.2 eV, which means that more light energy input is this performance was starker under visible light. Ge et al.15 used
needed to stimulate electron jumping from the valence band to an in situ deposition technology to uniformly disperse Ag parti-
the conduction band. Compounding the problem, the band cles inside and outside of vertically aligned TiO2 nanotubes. The
edge of pure TiO2 is located in the ultraviolet light region, and results showed that the deposition of Ag nanoparticles (AgNPs)
significantly enhanced the response of TiO2 nanotubes to visible
light. It was found that, in the AgNPs/TiO2 semiconductor system,
a School of Chemical Engineering, Northwest Minzu University, Lanzhou, 730070,
AgNPs played a key role in visible light absorption due to the
localized surface plasmon resonance (LSPR) effect.16 Wang et al.17
Gansu, China. E-mail: zhangping@xbmu.edu.cn
b Gansu Natural Energy Insititue, Gansu Academy of Sciences, Lanzhou, 730070,
also observed similar findings, demonstrating that Ag modification
is a powerful factor in photocatalysis with maximum H2 evolution,
as a result of strong LSPR. Simultaneously, the surface-formed
Gansu, China. E-mail: ljhanzhang@163.com
c School of Civil and Environmental Engineering, Harbin Institute of Technology
(Shenzhen), Shenzhen, 518055, China
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Schottky barrier of the material, which resulted from LSPR, resolution transmission electron microscope (HRTEM, JEOL,
strongly hindered photo-generated electron–hole recombination, Japan). The organic carbon content in wastewater was measured
thus improving its photocatalytic ability. Furthermore, other through a total organic carbon (TOC, Shimadzu, Japan) analyzer.
opinions were that metal-donated electrons efficiently filled The concentration of the solution was measured using a high
photo-generated holes, and then unrecombined photo- performance liquid chromatograph (HPLC, Shimadzu, Japan).
generated electrons can bombard the absorbed oxygen to form An ultrasonic oscillator (KQ-500E, Keqiao, Dongguan) was used
superoxide radical anion (ꢀO2À) on the surface, leading to a to achieve uniform dispersion of the materials. A high-speed
better photocatalytic performance.18 Besides metal-doping, other centrifuge rotator (TGL-20B, Anting, Shanghai) was used for
methods, such as non-metal doping, defect control, semiconductor centrifugal separation. A constant temperature blast drying oven
compounding, and dye sensitization, were also demonstrated to be (HS-9000A, Hesheng, Shanghai) was employed to dry the materials.
useful.19,20 Among these methods, nitrogen-doping has been A photo-chemical reaction instrument (PCX50A, Pofilai, Beijing)
regarded as the most promising one for improving catalytic perfor- was applied for the photocatalytic reaction.
mance.21 Numerous studies22 found that the substitution of O by
Ammonia solution (NH3ÁH2O, AR, Z25%) and titanium
element N in the TiO2 crystal lattice narrowed the band gap of butoxide (C16H36O4Ti, AR, Z99%) were purchased from Tianjin
TiO2 to some degree, enhancing its response to visible light.23 Guangfu Fine Chemical Research Institute; sodium hydroxide
Senthilnathan et al.24 found that N-doped titanium dioxide (NaOH, AR, Z96%) was obtained from Shanghai Wokai
(N-TiO2) degraded the insecticide lindane under visible light Biotechnology Co., Ltd; silver nitrate (AgNO3, AR, Z99.8%)
with a striking efficiency of B100%. Ibukun et al.25 prepared was purchased from Shanghai Zhongqin Chemical Reagent
N-TiO2 by a hydrothermal method and modified it by air plasma Co., Ltd; phenol (C6H6O, AR, Z99.5%) was purchased from
treatment. Compared with the band gap of pure TiO2 of 3.1 eV, Tianjin Fuchen Chemical Reagent Factory; deionized water
the band gap of N-TiO2 reduced to 2.7 eV, and the photocurrent (H2O) was purchased from Wahaha Beverage Co., Ltd, and
response by N-TiO2 increased 3.6 times. Lee H. U. et al.26 used an ethanol (C2H5OH, AR, Z99.9%) was purchased from Sino-
improved sol–gel method coupled with ultrasonic irradiation to pharm Group Chemical Reagent Co., Ltd.
synthesize nano-porous N-TiO2 at room temperature without
heat treatment, also making it more efficient to use visible light.
2.2. Experimental method
Herein, we balanced the merits of metal Ag and non-metal In a typical synthesis process, 7.8 ml tetra-n-butyl titanate and
N, and synthesized Ag-modified, N-doped and shuttle-like TiO2. 68 ml NH3ÁH2O solution were added into a flask to form a
We first reported an improved hydrothermal method for this mixture under magnetic stirring for 2 h. After that, the mixture
material synthesis. In the synthesis method of our experiment, was further processed hydrothermally in a polytetrafluoroethylene
NH3ÁH2O was used as the nitrogen source and also acted as a reactor for 24 h. After it was cooled down to room temperature,
settling agent and a solvent. This tactic greatly simplified the the precursor was washed with deionized water until a constant
operation and was more practical compared with other methods.27 supernatant pH was achieved. In order to investigate the effect of
Furthermore, once AgNO3 was added into NH3ÁH2O, silver ammonia synthesis temperature on the performance of N-doped TiO2, three
solution formed, which benefited in situ reactions between Ag and Ti heating temperatures were selected, namely, 150 1C, 180 1C, and
elements. Many other factors, for instance, temperature, addition 210 1C. The obtained samples were labelled as T1-N-TiO2, T2-N-
amount, and different additive ratios, were also considered in the TiO2 and T3-N-TiO2, respectively. Furthermore, we also selected
synthesis to simplify the whole process referred from Eryk Fer- several quantum 12 h, 24 h, 36 h, 48 h and 60 h to investigate the
nandes et al.27 Meanwhile, phenol was selected as the target for effect of residence time on catalyst structure and catalytic perfor-
catalytic performance testing, and multiple characterizations were mance. Additionally, the influence of Ag-doping was also investi-
also applied to elucidate the mechanisms including the effects of gated; AgNO3 solutions with different concentrations were
À
+
ꢀ
three free radicals (ꢀO2 , OH and h ). The purpose of this study separately added drop-wise into the flask after the solution of
was to design a rational, potent and economic semiconductor, and tetra-n-butyl titanate and NH3ÁH2O was stirred and mixed for
to offer some clues in exploring the mechanisms of photocatalysis. 2 h. The follow-up hydrothermal process was the same as
mentioned above.
2.3. Removal of phenol under different light sources
2. Materials and methods
The light source used was a Multi-channel Photo-chemical
Reaction Instrument (PCX50A, Beijing Bofeilai Technology
2.1. Instruments and chemicals
The crystal structures of the samples were obtained using an Co., Ltd) with a 420 nm LED light source as the visible light
X-ray diffraction (XRD, PANalytical, The Netherlands) instru- source and a 365 nm LED light source as the ultraviolet light
ment with Cu-Ka as the radiation source. The chemical states source. The incident intensities of ultraviolet light and visible
of elements in the materials were measured using an X-ray light were 7.4 mW cmÀ2 and 8.3 mW cmÀ2, respectively. Phenol
photoelectron spectroscopy (XPS, ESCALAB, USA) analyzer. The was chosen as the contaminant for the degradation study. The
morphology and structure of the sample were detected using a volume of the phenol solution was 50 mL with an initial
field emission scanning electron microscope (SEM, JEOL, Japan), concentration of 10 mg LÀ1, and the addition amount of
a transmission electron microscope (TEM, FEI, USA) and a high catalyst was 50 mg. After the adsorption equilibrium of phenol
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in a dark environment was obtained, the photocatalysis was very important. For example, anatase TiO2 is more active than
started by applying a light source for phenol degradation. rutile and brookite TiO2 in photocatalysis, meaning that a more
Samples were collected at 0 h, 0.5 h, 1 h, 1.5 h, 2 h, 3 h, 4 efficient catalyst was synthesized in this study.29 Furthermore, in
and 5 h. The remaining phenol concentration in the solution Fig. 1(b), several diffraction peaks located at 2y = 38.31, 43.91,
was measured using a HPLC analyzer and a TOC analyzer. The 65.01, and 77.01 appeared, which originated from the (111),
removal amount, removal rate, and mineralization rate were (200), (220), (211) and (311) crystal planes of Ag particles,
expressed using the following formula:
R = [(Cr0 À Crt)/Cr0] Â 100%
Q = [(C0 À Ct)/C0] Â 100%
demonstrating the successful synthesis of the Ag and N co-doped
TiO2 photocatalyst.
3.1.2. X-ray photoelectron spectroscopy (XPS) analysis. The
XPS results are shown in Fig. 2. It can be seen that Ag/N-TiO2
mainly contained five elements, namely, C, Ag, N, Ti and O.
Fig. 2(b) shows the Ti 2p spectrum of Ag/N-TiO2. The peaks at
(1)
(2)
where R is the phenol removal rate at the objective time (%); Cr0
and Crt are the concentrations of phenol at the initial and
objective times, respectively (mg LÀ1); Q is the mineralization of
phenol at the objective time (%); and C0 and Ct represent the
organic carbon contents of phenol at initial and objective
times, respectively (mg LÀ1).
458.5 eV and 464.3 eV corresponded to Ti 2p3/2 and Ti 2p1/2
,
respectively, which belong to the characteristic 2p peak of Ti4+
in TiO2.30 No Ti–N or Ti–C bond was observed in the Ti 2p
spectrum, indicating that the C and N elements did not enter
the TiO2 lattice during the synthesis.31 Fig. 2(c) shows several
decomposed peaks of O 1s, where the peak at 531.5 eV
corresponded to the Ag–O bond and the peak at 529.7 eV
corresponded to the Ti–O bond. The peak at 533.2 eV corresponded
to the O vacancy bond, and the generally large area of this peak
3. Results and discussion
3.1. Structures and morphology of photocatalysts
3.1.1. X-ray diffraction (XRD) analysis. Fig. 1(a) shows the also suggested the existence of a large number of O vacancies,
XRD patterns of N-TiO2 prepared at different hydrothermal which mainly originated from the TiO2 lattice. Fig. 2(d) shows
reaction temperatures. The standard comparison cards of the N 1s absorption peak, and the peak at a low binding energy
anatase and rutile TiO2 are also shown in the same figure for of 399.5 eV originated from sp hybridization. In TiO2, N formed
comparison. Fig. 1(b) presents the XRD patterns of Ag/N-TiO2 interstitial nitrogen to form Ti–N–O or Ti–O–N bonds. The
prepared at different reaction times, coupled with the standard nitrogen–oxygen bond, that is, the N element was doped into
comparison cards of anatase TiO2 and elemental Ag. The strong the TiO2 lattice by gap doping, and oxygen defects formed. The
diffraction peaks at 2y = 25.31, 37.81, 48.01, 53.91, 55.11, and result was consistent with many studies.32–34 From the analysis
62.71 corresponding to the (101), (004), (200), (105), (211) and of the Ag 3d spectrum in Fig. 2(e), it can be seen that the high-
(204) crystal planes of anatase TiO2, respectively, indicated that resolution XPS spectrum of Ag 3d was detected in the two strong
anatase TiO2 was the dominant TiO2 component produced in peaks of the composite catalyst located at around 367.6 eV and
our synthesis method. Additionally, the crystal phase of N-TiO2 373.8 eV, which are comparable to Ag 3d5/2 and Ag 3d3/2
,
(nTi : nN = 1 : 0.45) displayed the optimal crystal at a hydro- respectively.35,36 The appearance of these peaks was attributed
thermal temperature of 210 1C (shown in Fig. 1(a)). After Ag to the generation of metallic Ag during the modification.
modification, the (101) peak shifted slightly from 25.31 to 25.71,
Gannoruwa et al.37 reported that the presence of the Ag
implying that surface deposition occurred through Ag modifica- surface on TiO2 was conducive to the photocatalytic oxidation
tion.28 Also noticeably, both N-doping and Ag-deposition did not reaction of TiO2 and the separation of the photo-excited electron–
change the general crystallite of TiO2, revealing an unessential hole pair. Ag produced new electronic states below the TiO2
role of O vacancy influencing the TiO2 crystallite.23–26 For the conduction band. Surface plasmon resonance (SPR) enhances
effective photocatalytic activity of TiO2, the crystalline phase is visible light absorption. It was used as an electron trap to suppress
Fig. 1 X-ray diffraction patterns of (a) N-TiO2 prepared at different reaction temperatures and (b) Ag/N-TiO2 prepared at different reaction times.
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Fig. 2 Ag/N-TiO2 XPS spectrum (a), Ti 2p (b), O 1s (c), N 1s (d), and Ag 3d (e) XPS spectra.
the electron–hole recombination of the photocatalyst. Ag also and consequently results in a rod-like shape. Besides, small
provided catalytic sites to promote the reaction of photoelectrons particles were loaded with explicit Ag crystal lattice38,39 on the
with surface oxygen.
surface. In Fig. 3(c), Ag/N-TiO2l contains more nanorods. The
3.1.3. Topography analysis. SEM photographs show that size of the nanorods is nearly the same as that in Fig. 3(a and b).
the length of Ag/N-TiO2 is between 340 and 457 nm, and the Fig. 3(d) shows the corresponding HRTEM picture. The lattice
diameter of Ag/N-TiO2 is between 70 and 140 nm, while the fringe with a lattice spacing of 0.33 nm corresponds to the (101)
diameter calculated using Scherrer’s formula is 31.17 nm, crystal plane of anatase, and the lattice fringe with a lattice
obviously deviating from the results observed in SEM photo- spacing of 0.21 nm corresponds to the Ag element (200) crystal
graphs. These differences suggest that agglomeration occurs plane. The result indicates that the nanomaterial with rod-shaped
Fig. 3 (a and b) SEM image, (c) TEM image and (d) HRTEM image of Ag/N-TiO2.
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Ag/N-TiO2 was synthesized. Compared with the undoped TiO2, the
lattice strain of the doped sample has changed, indicating that
the introduction of N and Ag causes the distortion of or defects in
the TiO2 crystal.40,41 The cycled area in Fig. 3(d) presents a lot of
fractures on the lattice, implying abundant oxygen vacancies.
Milani Moghaddama and Nasirian42 also reported similar obser-
vations. The reason is that when the crystal lattice is affected by
the tensile (compressive) force from the surface to the crystal
volume, the strain of the crystal lattice is positive or negative,
resulting in the change of the crystal grain size and the change of
the octahedral block on the TiO2 lattice. The changes of grain size
and lattice strain can also confirm that the in situ solvent method
was successfully introduced into TiO2, and the doping of N and Ag
in each reaction was the main factor affecting the modification of
TiO2.43,44 Other studies45–47 also reported the effect of the crystal
structure and grain size of the catalyst on the photocatalytic activity.
Fig. 5 Color comparison among TiO2, N-TiO2, Ag–TiO2 and Ag/N-TiO2.
the lowest. N doping caused the N 2p band to form a new
middle band energy beyond the O 2p valence band coupled
with Ag-caused lower Eg, which led to an increase in the visible
light response ability.49
Photoluminescence (PL) spectroscopy is a useful method to
study the electronic structure and the electron–hole recombi-
nation rate during photocatalysis. Fig. 4(b) shows the PL emis-
sion spectra of pure TiO2, N-TiO2, Ag–TiO2, and Ag/N-TiO2. The
obvious absorption wavelengths of the 4 materials were in the
range of 480–490 nm. But it is notable that the PL intensity of
Ag/N-TiO2 is much lower than that of pure TiO2, a proposed
explanation for this is that the PL emission intensity of the
photocatalyst is directly related to the electron–hole recombina-
tion rate, and the introduction of Ag and N efficiently hinders
the electron–hole recombination, thus lowering the PL emission
intensity of Ag/N-TiO2. Some studies43,44,49 also found that this
hindrance of the electron–hole recombination could be attributed
to Ag modification. In addition, an assumption50,51 was put
forward that Ag acts as an electron captor, possessing a great
redox ability to generate various radicals. The overall mechanism
is shown in Fig. 6.
What is more, the doping of N and Ag exerts distinct influences
on the enhancement of the photocatalytic performance of TiO2. N
doping plays a role in the morphology of the photocatalyst. Ag
doping lowers the band gap energy, thereby shifting light absorp-
tion to the visible light region and simultaneously hindering the
electron–hole recombination. When Ag is modified and N is
doped on the TiO2 crystal lattice, a synergistic effect is produced.
Therefore, Ag/N-TiO2 exhibits a better response to visible light.
3.2. The optical properties and band gap energy
Fig. 4 shows the UV-vis absorption spectra and PL spectra of Ag/
N-TiO2, N-TiO2, Ag–TiO2 and pure TiO2. The intensity of the
absorbed radiation reveals the light absorption capacity of the
photocatalyst at different wavelengths. Obviously, Ag/N-TiO2
and Ag–TiO2 powders are darker in color and exhibit stronger
absorption in the wavelength range of 400–600 nm. The reason
is the surface plasmon resonance (SPR) effect of silver nano-
particles that appears in the wavelength range of 450–550 nm.48
As shown in Fig. 5, the calcined N-TiO2 powder was light yellow
and exhibited a low radiation absorption intensity in the visible
light region. Yet the white TiO2 powder displayed no light
absorption in this region, indicating that the change of the
absorption edge of the TiO2 semiconductor was related to the
type of dopant.
Fig. 4(a) lists the band gap energy in order to compare the
band gap energy (Eg) among pristine, Ag-modified and N-doped
TiO2. The calculated Eg value for pristine TiO2 was 3.2 eV. Once
doped by N or Ag, Eg declined to 3.0 eV for N-TiO2, 2.3 eV for
Ag–TiO2 and 2.1 eV for Ag/N-TiO2. This result indicated that Ag
modification has a more significant effect on reducing the Eg,
which corresponded to the red-shift absorption and surface
3.3. Photocatalytic activity of the catalyst
plasmon resonance (SPR) effects. Due to the synergy of N and Fig. 7 shows the relationship between the degradation efficiency
Ag, the values of Eg of Ag-modified and N-doped samples were of phenol and different doping ratios of Ag/N-TiO2 (a), vs. different
Fig. 4 (a) UV-vis absorption spectra and (b) photoluminescence (PL) emission spectra of TiO2, N-TiO2, Ag–TiO2, and N/Ag–TiO2.
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Fig. 6 Schematic diagrams of the band energy structure and the migration of photoexcited electrons in undoped TiO2, N-TiO2, Ag–TiO2, and
Ag/N-TiO2.
Fig. 7 (a) Under visible light, the degradation effect of Ag/N-TiO2 with different doping ratios on phenol, (b) the degradation effect of different
photocatalysts prepared under hydrothermal reaction for 36 h on phenol, and (c) the phenol removal effect of the Ag/N-TiO2 material prepared at
different hydrothermal reaction times.
doping agents (b) and different reaction times (c). For irradiation, performance of TiO2 was enhanced after N or Ag doping, and Ag
visible light with a wavelength of 420 nm was used. After 6 hours of doping had a greater impact on the photocatalytic performance of
visible light irradiation, the removal rates of phenol were 60.12%, TiO2. The photocatalyst doped with nTi : nN:nAg = 1 : 0.32 : 0.45
66.04% and 74.96% by 0.01 mol LÀ1 AgNO3, 0.05 mol LÀ1 AgNO3 displayed the best performance. After obtaining the optimal doping
and 0.15 mol LÀ1 AgNO3, respectively. Additionally, Ag/N-TiO2 doped ratio, we continued to find the optimal hydrothermal reaction time.
with 0.1 mol LÀ1 AgNO3 (nTi : nN:nAg = 1 : 0.45 : 0.32) exhibited the In Fig. 7(c), Ag/N-TiO2 prepared via a hydrothermal reaction for
best removal efficiency with an 80.21% removal rate against phenol, 36 hours exhibited the best phenol removal performance. After 6
and the corresponding removal amount was 8.02 mg gÀ1. In hours of visible light irradiation, the phenol removal rate reached
Fig. 7(b), after 6 hours of visible light irradiation, the removal rates 80.21%, while the phenol removal efficiencies of the catalyst sub-
of phenol by TiO2, N-TiO2, and Ag–TiO2 were 41.33%, 49.97% and jected to hydrothermal treatment for 24 h, 48 h and 60 h under
70.09%, respectively. The results showed that the photocatalytic visible light were 63.78%, 58.97% and 50.06%, respectively.
Fig. 8 (a) Under ultraviolet light, the degradation effect of different doping ratios of Ag/N-TiO2 on phenol, (b) the degradation effect of different
photocatalysts on phenol at a hydrothermal reaction time of 36 h, and (c) the phenol removal effect of the prepared Ag/N-TiO2 material at different
hydrothermal reaction times.
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Fig. 9 The Ag/N-TiO2 nanomaterials prepared at different hydrothermal reaction times are used as the first-order kinetic model for the removal of
phenol pollutants under (a) visible light and (b) ultraviolet light.
Fig. 8 shows the relationship of the phenol degradation than pristine TiO2 did, consistent with the discussions in
efficiency with different doping ratios of Ag/N-TiO2 and reaction section 3.3. Hence, the formation of a heterojunction between
times. A 365 nm light source was selected as the ultraviolet light Ag/N-TiO2 and the surface plasmon resonance effect of Ag
source. In Fig. 8(a), after 90 minutes of ultraviolet light irradiation, nanoparticles broadened the visible light response range, and
the removal rates of phenol were 77.53%, 81.34% and 82.41% by the band gap difference promoted photo-generated charge
0.01 mol LÀ1 AgNO3, 0.05 mol LÀ1 AgNO3 and 0.15 mol LÀ1 AgNO3, migration and improved the photocatalytic efficiency. With R2
respectively. Ag/N-TiO2 doped with 0.1 mol LÀ1 AgNO3 exhibited being the related linear fitting coefficient and k being the first-
the best removal efficiency toward phenol, and the phenol removal order reaction rate constant of the photocatalytic degradation
rate reached 91.5%. In Fig. 8(b), after 90 minutes of ultraviolet reaction, which reflects the degree of degradation, the reaction
light irradiation, the removal rates of phenol by TiO2, N-TiO2 and rate is proportional to the first power of the reactant concen-
Ag–TiO2 were 69.89%, 78.96% and 77.68%, respectively. It can be tration:53 dC/dt = kC, that is, ln(C0/Ct) = kt, where C0 and Ct are
seen from Fig. 8(c) that the Ag/N-TiO2 prepared via a hydrothermal the phenol concentrations at the initial time and objective
reaction for 36 hours exhibited the best phenol removal effect. time, respectively. Tables 1 and 2 present the detailed data of
A phenol removal rate of 98.85% was achieved after ultraviolet kinetics.
light irradiation for 90 minutes, while the phenol removal effi-
ciency of the catalyst subjected to hydrothermal treatment for 24 h,
48 h and 60 h was 81.98%, 98.86% and 90.06%, respectively, after
being irradiated with ultraviolet light for 5 h.
3.5. Factors affecting the photocatalysis of Ag/N-TiO2
Active free radicals are important oxidants produced by irradiation
upon photocatalysis. Current opinions consider them as the main
reagents to degrade contaminants. In order to detect the key
3.4. Kinetic study of the photocatalytic reaction
photocatalytic mineralization of phenol, three main strong oxidants,
À
In this study the first-order kinetic equation was used to fit the namely, superoxide radicals (ꢀO2 ), hydroxyl radicals (ꢀOH), and
photocatalytic performance of several materials.52 The results holes (h+), produced during the reaction were studied in detail.54
are shown in Fig. 9. The Ag/N-TiO2 material with Ag modifica- The reaction inhibitors, 100 mg LÀ1 benzoquinone, 150 mg LÀ1
tion had a significantly higher degradation rate against phenol isopropanol, and 150 mg LÀ1 EDTA-Na2, were selected as the radical
Table 1 Photocatalytic first-order kinetic parameters of phenol at different temperatures under visible light conditions
Material
First order dynamic model
K/min
R2
24 h Ag/N-TiO2
36 h Ag/N-TiO2
48 h Ag/N-TiO2
60 h Ag/N-TiO2
ln(C0/Ct) = 2.63 Â 10À3t + 1.78 Â 10À2
ln(C0/Ct) = 4.13 Â 10À3t À 1.27 Â 10À2
ln(C0/Ct) = 2.36 Â 10À3t + 5.71 Â 110À2
ln(C0/Ct) = 1.98 Â 10À3t À 9.15 Â 10À2
2.63 Â 10À3
4.13 Â 10À3
2.36 Â 10À3
1.98 Â 10À3
0.9871
0.9991
0.9819
0.9612
Table 2 Photocatalytic first-order kinetic parameters of phenol at different temperatures under ultraviolet light
Material
First order dynamic model
K/min
R2
24 h Ag/N-TiO2
36 h Ag/N-TiO2
48 h Ag/N-TiO2
60 h Ag/N-TiO2
ln(C0/Ct) = 6.34 Â 10À3t À 4.89 Â 10À3
ln(C0/Ct) = 2.38 Â 10À2t À 8.59 Â 10À3
ln(C0/Ct) = 1.99 Â 10À2t + 4.46 Â 10À1
ln(C0/Ct) = 7.59 Â 10À3t + 6.91 Â 10À1
6.34 Â 10À3
2.38 Â 10À2
1.99 Â 10À2
7.59 Â 10À3
0.9891
0.9993
0.9825
0.8539
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Fig. 10 Study on the mechanism of photocatalytic oxidation of phenol by Ag/N-TiO2 under (a) visible light irradiation and (b) ultraviolet light irradiation.
captors, and a preliminary test proved these concentrations as Under visible light irradiation, the mineralization rates toward
efficient for radical capture.54 Meanwhile, a blank group was also phenol were 4.20%, 24.50%, 38.85% and 51.20% by TiO2,
set for comparison. The LED light source (with a wavelength of N-TiO2, Ag–TiO2 and Ag/N-TiO2, while they were 19.62%,
420 nm and an incident intensity of 8.3 mW cmÀ2 as the visible light 45.65%, 78.92% and 90.86%, respectively, under ultraviolet light.
source, and with a wavelength of 365 nm and an incident intensity Among the 4 materials, Ag/N-TiO2 exhibited the best phenol
of 7.4 mW cmÀ2 as the ultraviolet light source) was used for irradia- mineralization under either visible or ultraviolet light. Considering
tion (for 6 h). In Fig. 10, the addition of the isopropanol inhibitor the phenol removal efficiency, Ag/N-TiO2 removed nearly 100%
had the strongest inhibitory effect on the reaction under either phenol while mineralizing 90.86% phenol under complex light
ultraviolet light irradiation or visible light irradiation, meaning that source illumination, denoting the potent photocatalytic perfor-
the hydroxyl radical (ꢀOH) was the main factor in phenol mance of Ag/N-TiO2. However, the inferior mineralization effi-
mineralization. Also, noticeably, some studies50,55 found that ciency obtained under visible light compared with that achieved
Ag addition significantly enhanced ꢀOH generation, which may under ultraviolet light (51.20% vs. 90.86) implied more active free
be one reason for the better performance of Ag/N-TiO2.
radicals produced under ultraviolet light irradiation. This observa-
tion agreed with commonly reported results.57,59
3.6. Mineralization performance
Phenol is a contaminant that is commonly being studied,
and its concrete degradation products are also being investigated
widely. Recently, 4 intermediates—catechol, resorcinol, hydro-
quinone, and benzoquinone—have been commonly proposed.57–60
In this regard, we detected the existence of the 4 intermediates
by HPLC and analyzed the concentrations of these intermediates
with irradiation time, as shown in Fig. 12. Under visible light
irradiation, hydroquinone, catechol and resorcinol were the
main degradation intermediates. Their concentrations increased
with irradiation at the early photocatalysis stage, gradually
reaching the peak after 240 minutes of irradiation, and then
decreased. Catechol and hydroquinone were the final products
after 300 minutes of irradiation, whereas benzoquinone molecules
were nearly eliminated. In contrast, phenol quickly degraded into
various intermediates under ultraviolet light. The intermediates
were hydroquinone, catechol, and benzoquinone, similar to those
observed under visible light irradiation. In addition, the concen-
trations of these intermediates were much lower than those of
their counterparts under visible light irradiation, implying that
phenol was degraded into nontoxic small molecules easily under
ultraviolet light. The results were in accordance with the analysis of
the mineralization properties. Based on the analysis of the results
obtained for intermediates, we proposed a potential degradation
pathway for phenol in our study as shown in Fig. 13 (mobile phase:
acetonitrile : water = 1 : 9, our test demonstrated that the flow
rate of 0.8 mL minÀ1 efficiently separated the aforementioned
4 intermediates, and the absorption wavenumber was set at
Mineralization toward contaminants is a critical factor in
determining the performance of a photocatalyst. In addition,
it is usually evaluated in terms of residual total organic carbon
concentration (TOC).56–58 Fig. 11 depicts the mineralization
performance against phenol by TiO2, N-TiO2, Ag–TiO2 and
Ag/N-TiO2 under 6 hours of visible/ultraviolet light irradiation.
Fig. 11 The mineralization properties of TiO2, N-TiO2, Ag–TiO2, and
Ag/N-TiO2 toward phenol.
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Fig. 12 Concentration changes of intermediate products in the process of Ag/N-TiO2 photocatalytic degradation of phenol under (a) visible light
irradiation and (b) ultraviolet light irradiation.
Fig. 13 Proposed degradation pathway for phenol.
Fig. 14 Reusability of Ag/N-TiO2.
280 nm; the retention times of hydroquinone, catechol, resorci-
nol, and benzoquinone were 3.116 min, 3.69 min, 4.5 min and
6.378 min, respectively).
was recycled 5 times. The used Ag/N-TiO2 was rinsed with deio-
nized water and ethanol several times, and then vacuum-dried at
60 1C. The recycling results are shown in Fig. 14. In the process of
UV degradation, the degradation in the first cycle was almost
3.7. Recycling performance
Regeneration performance is an important indicator for evaluating complete, and in the subsequent cycles it slowly declined down to
the economic operability of a catalyst.60 In this study, Ag/N-TiO2 89.84%, 84.12%, 83.33%, and 82.89%, respectively. In the process
Table 3 Comparison of the phenol removal ability of various TiO2 photocatalysts
Degradation time
for pollutants (min)
Degradation rate for
pollutants (%)
Mineralization
capacity (%)
Photocatalyst
Pollutant
Phenol
Light type
Ref.
61
H2O2/TNSs/TiO2
UV-A lamp
Visible light
UV light
Visible light
UV lamp
Natural sunlight
UV lamp
Visible light
UV lamp
Visible light
UV lamp
Visible light
UV light
—
—
—
350
120
—
180
450
—
90
—
180
—
350
300
350
97.12
89.7
—
88.00
62.80
—
61.00
—
97.00
—
TiO2/AC
Phenol
Phenol
Phenol
Phenol
Phenol
Phenol
—
—
—
62
4% PF/TiO2
63
Ag3PO4/TiO2-OV
[FeII(dpbpy)(phen)2]/TiO2
TiO2/H2O2
—
68.00
—
71.00
—
—
64
65
66
97.12
99.02
80.21
Ag/N-TiO2
90.86
51.20
This work
Visible light
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of visible light degradation, the degradation in the first cycle was Notes and references
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