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Accepted Manuscript
Title: Highly <!?<query id="Q1">Please check the
presentation of dochead R?esearch paper,? and correct if
necessary.</query>?>ordered TiO2 nanotube arrays wrapped
with g-C3 N4 nanoparticles for efficient charge separation and
increased photoelectrocatalytic degradation of phenol
Authors: Huan Wang, Yinghua Liang, Li Liu, Jinshan Hu,
Wenquan Cui
PII:
DOI:
Reference:
S0304-3894(17)30800-2
https://doi.org/10.1016/j.jhazmat.2017.10.044
HAZMAT 18949
To appear in:
Journal of Hazardous Materials
Received date:
Revised date:
Accepted date:
4-8-2017
17-10-2017
21-10-2017
Please cite this article as: Huan Wang, Yinghua Liang, Li Liu, Jinshan Hu, Wenquan
Cui, Highly ordered TiO2 nanotube arrays wrapped with g-C3N4 nanoparticles for
efficient charge separation and increased photoelectrocatalytic degradation of phenol,
Journal of Hazardous Materials https://doi.org/10.1016/j.jhazmat.2017.10.044
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Highly ordered TiO2 nanotube arrays wrapped with g-C3N4 nanoparticles
for efficient charge separation and increased photoelectrocatalytic
degradation of phenol
Huan Wang1, Yinghua Liang1,2*, Li Liu2, Jinshan Hu2, Wenquan Cui2*
1. School of Chemical Engineering and Technology, Hebei University of Technology,
Tianjin 300130, P.R. China;
2. College of Chemical Engineering, Hebei Key Laboratory for Environment
Photocatalytic and Electrocatalytic Materials, North China University of Science and
Technology, Tangshan Hebei 063210, P. R. China;
Highly ordered TiO2 nanotube arrays wrapped with g-C3N4 nanoparticles
for efficient charge separation and increased photoelectrocatalytic
degradation of phenol
Huan Wang1, Yinghua Liang1,2*, Li Liu2, Jinshan Hu2, Wenquan Cui2*
(1. School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, P.R. China;
2. College of Chemical Engineering, Hebei Key Laboratory for Environment Photocatalytic and Electrocatalytic
Materials, North China University of Science and Technology, Tangshan Hebei 063210, P. R. China;)
Graphical abstract:
1
Highlights:
?
?The g-C3N4 nanoparticle-wrapped TiO2 nanotube arrays are successfully synthesized.
?
?The g-C3N4/TiO2 increases the charge separation and photoconversion efficiency.
?
?The g-C3N4/TiO2 shows enhanced photoelectrocatalytic degradation of phenol.
Abstract
Novel graphitic carbon nitride nanoparticles (NPs)-wrapped TiO2 nanotube arrays
(NTAs) (g-C3N4/TiO2) were fabricated by a two-step method including an
electrochemical anodization technique followed by impregnation under vacuum using
urea as precursor. The as-prepared photoelectrode exhibited outstanding
photoelectric properties and excellent photelectrocatalytic (PEC) performance for the
degradation of phenol under stimulated solar light, which was due to the enhanced
light absorption property and improved charge separation efficiency. The introduction
of g-C3N4 NPs strongly decreased the charge transfer resistance and boosted the
charge separation efficiency of TiO2. The optimum ratio of the g-C3N4/TiO2 yielded a
pronounced 4.18-fold higher photocurrent density than TiO2. Besides, the
combination of g-C3N4 NPs could negatively shift for the flat band potential of TiO2,
resulting in an enhanced reduction property for the photoelectrocatalytic degradation
of organic pollutants. The PEC process for the degradation of phenol over g-C3N4/TiO2
was much higher than the sum of photocatalytic (PC) and electrocatalytic (EC)
processes indicating that a photoelectric synergy was achieved on the as-prepared
2
photoelectrode and resulting in an improved PEC performance for the composite
photoelectrode.
Keywords: g-C3N4 nanoparticles; TiO2 nanotube arrays; Charge separation efficiency;
Photoelectrocatalysis; Organic pollutants
1. Introduction
In recent years, environmental pollution and energy shortages have required
researchers to develop more efficient and highly active photo(electro)-catalytic
materials [1]. TiO2 is widely used in the field of photocatalytic degradation of organic
pollutants, gas sensors, solar cells, biological materials and hydrogen production due
to its excellent photoelectric conversion ability, photocatalytic oxidation-reduction
ability and fine chemical stability [2]. In particular, TiO2 nanotube arrays have attracted
more attention because of their highly oriented charge transport channels, large
specific surface areas and low photo-generated charge recombination efficiency [3,4].
Recently, many efforts have been devoted to improve the electric structure of TiO2
NTAs to reduce their band gap and increase their charge separation efficiency
including ion doping [5,6], dye sensitization [3], narrow band gap semiconductor
quantum dots (QDs) modification [7-10], new carbon nanomaterials [11,12]. Studies
on semiconductor QDs, carbon material QDs (carbon dots [13-15] and graphene QDs
[16]) modified TiO2 NTAs have aroused wide interest.
The quantum dot material has a unique small size effect that allows the
photoelectrode to expose more active sites and improve the light absorption
efficiency. It can also shorten the migration distance of the photo-generated carriers
in space and further promote the rapid separation of the charges and improve the
photoelectric conversion efficiency on the synergetic effect of the unique onedimensional confined effect of TiO2 NTAs. However, most of the narrow band gap
semiconductor QDs such as CdS [7,8], PbS [9,17], CdTe [10], ZnS [18], etc. can cause
health and environmental problems because of the presence of toxic metal elements.
Meanwhile, carbon dots and graphene QDs do not possess photocatalytic oxidation
3
and reduction performance [19], which limits further improvement of these
photoelectrodes.
Graphitic carbon nitride (g-C3N4) with small size effect is a new kind of nontoxic and
nonmetallic semiconductor combined with a medium band gap of 2.7 eV. It exhibits
outstanding charge transfer property as well as excellent physicochemical stability due
to the existence of a covalent bond effect between carbon and nitrogen atoms in its
unique triazine ring structure [20-22]. Combined with TiO2 NTAs can simultaneously
solve this problem of environmental pollution of narrow band gap semiconductor QDs
and low photocatalytic performance of carbon material QDs. Moreover, its suitable
band position can form a matched band structure with TiO 2 NTAs to further promote
the separation of the photo-generated charges and improve the photoelectric
conversion efficiency of g-C3N4 NPs or QDs-modified TiO2 NTAs. However, there are
still few studies on the g-C3N4 NPs or QDs-modified TiO2 NTAs [23,24] or TiO2 nanorods
[25], and the mechanism of charge transfer as well as the activity improvement of the
composite photoelectrode during PEC process remains unclear.
Hence, we describe here a novel composite with core-shell structure of g-C3N4 NPs
covering TiO2 NTAs. Large amounts of g-C3N4 NPs can be anchored on TiO2 nanotubes
in the as-prepared g-C3N4/TiO2 NTAs, which effectively improves the low visible light
utilization and low photoelectric conversion efficiency for the g-C3N4 QDs-modified
TiO2 NTAs in the present studies. The hybrid effect formed between g-C3N4 NPs and
TiO2 nanotubes as well as the influence of g-C3N4 NPs on the band structure of TiO2
were investigated by a series of characterization techniques and electrochemical
measurements. The mechanism of the activity improvement and the charge transfer
mechanism for g-C3N4/TiO2 NTAs under simulated solar light were also proposed.
2. Experimental
The
detailed
synthesis
photoelectrochemical
of
g-C3N4/TiO2 NTAs,
measurements
and
characterization
methodology
methods,
related
to
photoelectrocatalytic perofrmance are given in supporting information.
4
3. Results and discussion
The overall preparation process of the g-C3N4/TiO2 NTAs is shown in Scheme 1. First,
the TiO2 NTAs was fabricated by an anodization process. Secondly, the TiO2 nanotubes
were impregnated in precursor urea solution under vacuum. Finally, g-C3N4 NPs were
formed under calcination with the influence of unique one-dimensional confined
effect of TiO2 NTAs. Thus, core-shell g-C3N4/TiO2 NTAs with outstanding
photoelectrochemical performance were obtained.
The phase purity and crystal structure of the as-prepared g-C3N4, TiO2 and gC3N4/TiO2 NTAs were investigated by XRD (Fig. 1.). Two characteristic peaks appeared
at approximately 13.1o and 27.4o in the XRD spectrum of pure g-C3N4. The g-C3N4
characteristic peaks centered at 27.4o, and the peaks of anatase TiO2 (JCPDS No: 211272) all appeared in the g-C3N4/TiO2 NTAs indicating successful modification of TiO2
NTAs with g-C3N4.
The top-view and side-view SEM images of pure TiO2 and g-C3N4/TiO2 NTAs are
shown in Fig. 2. Fig. 2B to Fig. 2D and the side-view SEM image of g-C3N4/TiO2-2 (Fig.
2F) show that the g-C3N4 NPs were uniformly anchored on the surface of TiO2
nanotubes, and the average size of g-C3N4 NPs increased gradually with the
concentration of urea increasing. The average size of g-C3N4 NPs in the g-C3N4/TiO2-2
NTAs was around 11 nm. The low magnified SEM images of pure TiO2 and g-C3N4/TiO22 NTAs are shown in Fig. S1A and Fig. S1B. A uniform morphology of TiO2 NTAs was
observed on a large scale (Fig. S1). This cannot be changed by the g-C3N4 NPs in the gC3N4/TiO2-2 NTAs. Fig. 3 shows the TEM image of g-C3N4/TiO2-2 NTAs. Figs. 3B to 3D
show that the g-C3N4 NPs were distributed in the inner nanotubes as well as outer
tubes of TiO2 NTs. In addition, a compact contact was generated between g-C3N4 NPs
and TiO2 nanotubes that can promote the photo-generated charge transfer and
improve the charge separation efficiency.
Fig. 4A shows the SEM image of g-C3N4 particles formed on the Ti foil instead of TiO2
NTAs. The g-C3N4 particles with an irregular morphology were formed on the surface
5
of Ti foil, while g-C3N4 NPs were formed with an average size of 11 nm on the surface
of TiO2 NTAs due to the unique one-dimensional confined effect of TiO2 NTAs.
Compared with the EDX spectra of g-C3N4/Ti (inset Fig. 4A), the intensity of the C and
N peaks were much higher than that of g-C3N4/Ti indicating a higher g-C3N4 content in
the g-C3N4/TiO2-2 NTAs.
The optical properties of the as-prepared samples were confirmed by UV-DRS (Fig.
5A). The optical band edge of TiO2 nanotubes was 390 nm corresponding to a band
gap of 3.17 eV. The weak absorption in the visible light region is mainly due to the light
scattering caused by pores or cracks in the TiO2 NTAs and surface defects formed
under heat treatment [26,27]. Versus TiO2 nanotubes, the g-C3N4/TiO2 NTAs showed
strong absorption in a wide visible light region because of the modification of g-C3N4
NPs. In addition, red shifts were observed for the three g-C3N4/TiO2 NTAs. According
to the Kubelka-Munk function [28], the relationships between (Ahv)1/2 and photo
energy indicated that the band gaps of g-C3N4/TiO2-1, g-C3N4/TiO2-2 and g-C3N4/TiO23 were 3.07 eV, 2.89 eV, and 2.66 eV, respectively. The band gaps of g-C3N4/TiO2 NTAs
decreased compared with that of TiO2 due to a probable compact interaction between
g-C3N4 NPs and TiO2 NTAs that could improve the charge separation efficiency and
enhance the PEC performance of the g-C3N4/TiO2 NTAs. As shown in Fig. 5B, the gC3N4/TiO2 NTAs showed lower PL intensities compared with pure TiO2 photoelectrode.
This indicates much lower photo-generated charge recombination efficiency for gC3N4/TiO2 NTAs.
The XPS survey spectrum of g-C3N4/TiO2-2 (Fig. 6A) indicates the existence of C, N, O
and Ti elements. Fig. 6B shows the Ti 2p XPS spectra of g-C3N4/TiO2-2. Two peaks
centered at 458.5 and 464.2 eV were found corresponding to Ti 2p3/2 and Ti 2p1/2 of
TiO2 [29], respectively. Besides, the characteristic peaks of Ti 2p in g-C3N4/TiO2-2
shifted to higher binding energies compared with pure TiO2 indicating that the
electron cloud density of the Ti element in g-C3N4/TiO2-2 was changed due to the
presence of g-C3N4 NPs. and a certain interaction was formed between g-C3N4 NPs and
TiO2 NTAs. The result agrees with the phenomenon reported by Li et al [30]. The O 1s
6
peaks at 529.8 and 531.4 eV (Fig. 6C) came from the Ti-O-Ti bond and surface -OH
functional groups [31], respectively. Fig. 6D shows that three peaks appeared at 284.6,
285.4 and 288.5 eV in the C 1s spectra of g-C3N4/TiO2-2 corresponding to the
adventitious hydrocarbon from the XPS instrument; sp2 C atoms bonded to N inside
the aromatic structure, and the sp3 C-N bond of the bonded composition [23]. The N
1s XPS spectra were divided into three peaks at 399.4, 400.3, and 401.7 eV, which
were identified as the C-N-C, tertiary nitrogen (N-(C)3), and amino functional groups
with a hydrogen atom (N-H), respectively [32].
FT-IR was used to further investigate the surface chemistry of g-C3N4/TiO2 composite
and the interaction between g-C3N4 NPs and TiO2 nanotubes (Fig. 7). For pure TiO2,
the peaks appeared at 400-700 cm-1 were ascribed to the stretching vibration of Ti-OTi and Ti-O in anatase TiO2 [33]. The other two peaks centered at 1632.5 and 3431.2
cm-1 were considered to the hydroxyl group and adsorbed water. From the g-C3N4
spectra, the characteristic peaks at 811.9 and 891.7 cm-1 belonged to the skeleton
vibration of triazine units [34], while some strong bands from 1200 to 1640 cm-1
corresponded to the typical stretching mode of CN heterocycles [35]. In the gC3N4/TiO2 spectra, the absorption peaks located at around 873 and 1384 cm-1 were
ascribed to the triazine units and CN heterocycles of g-C3N4 indicating that TiO2 NTAs
were successfully decorated with g-C3N4 NPs. In addition, compared with the TiO2
spectra, the characteristic peaks of Ti-O-Ti bonds in the g-C3N4/TiO2 composite spectra
shifted to lower wavenumbers indicating that a compact interaction was formed
between g-C3N4 NPs and TiO2 NTAs, that can sharply improve the charge transfer
efficiency and boost the PEC performance of g-C3N4/TiO2 NTAs. Hao et al. reported a
macro/mesoporous g-C3N4/TiO2 heterojunction photocatalyst with similar results
[33].
The photoelectric properties of the as-prepared photoelectrodes were investigated
by a series of photoelectrochemical measurements. Electrochemical impedance
spectroscopy is an effective means to explore the charge transfer properties in the
electrolyte and the electrode interface and charge separation efficiency. Fig. 8A shows
7
the EIS Nyquist plots of TiO2 and g-C3N4/TiO2 NTAs. Partial magnification of EIS plots
of TiO2 and g-C3N4/TiO2 NTAs at high frequencies are presented in Fig. S2. The
semicircular impedance spectra indicated that the charge transfer process at the
electrode interface played a leading role in the electrode reaction [36]. Fig. 8A shows
the semicircular Nyquist plots of g-C3N4/TiO2 NTAs. These obviously decreased
compared with that of TiO2 suggesting that the charge transfer properties and charge
separation efficiencies of g-C3N4/TiO2 NTAs were improved after decoration with gC3N4 NPs. In addition, the equivalent circuit (inset of Fig. 8A) of the devices was
conducted to analyze the reaction mechanisms involved in the electrode process. The
fitting results are presented in Table 1. R1, R2, R3 and CPE represent the series
resistance of the system, the resistance of semiconductor depletion layer, the charge
transfer resistance in Helmholtz layer, and the chemical capacitance [37], respectively.
Table 1 shows that the charge transfer resistance R3 of g-C3N4/TiO2 composite NTAs
was much lower than that of pure TiO2 NTAs. This indicated that g-C3N4 NPs could
effectively reduce the charge transfer resistance of TiO2 and boost the separation
efficiency of charge carriers. The g-C3N4/TiO2-2 NTAs showed the smallest R3 value of
122.51 ?.
The Bode-phase spectra of TiO2 and g-C3N4/TiO2 NTAs are shown in Fig. 8B. Versus
TiO2, the characteristic frequency peaks of TiO2 shifted from 1028.5 to 457.3 Hz after
being decorated with g-C3N4 NPs. It can be calculated from the relationship (??1/(2?f))
between frequency (f) and injected electron lifetime (?) that the electron lifetime of
g-C3N4/TiO2-2 (0.35 ms) was 2.33 times of that of TiO2 (0.15 ms) [38]. This
enhancement in electron lifetime was attributed to the suppressed recombination of
charge carriers and improved charge separation property for g-C3N4/TiO2-2 resulting
in an enhanced electron lifetime and promoted photoelectric conversion efficiency.
Besides, the effect of photo-irradiation and applied bias on the EIS Nyquist plots of
TiO2 and g-C3N4/TiO2-2 were also investigated (Figs. 8C and 8D). The arc radius of EIS
plots could be decreased under simulated solar light irradiation, and further decreased
by the applied bias for the two photoelectrodes suggesting that both photo-irradiation
8
and bias potential could promote the charge separation efficiency and improve the
photoelectrochemical performance for TiO2 and g-C3N4/TiO2 NTAs.
To study the photoelectric response of TiO2 and g-C3N4/TiO2 NTAs under light on
and off conditions, transient photocurrent densities were tested in 0.5 M Na2SO4 at
0.0 V vs. SCE (Fig. 9A). Obviously, the photocurrent densities of g-C3N4/TiO2 NTAs were
much higher than that of TiO2. The g-C3N4/TiO2-2 NTAs possessed the highest value of
0.83 mA cm-2. This was 4.18-fold higher than that of TiO2 (0.16 mA cm-2). The
enhancement of the photocurrent response of g-C3N4/TiO2 NTAs was attributed to
three points: (1) the extended light absorption and enhanced visible light absorption
property for the combination of g-C3N4 NPs and TiO2 NTAs; (2) the well matched band
energies and impact interaction between g-C3N4 and TiO2 highly promoted the charge
separation property; (3) the synergetic effect of one-dimensional confined effect of
TiO2 NTAs and small size effect of g-C3N4 NPs shorten the migration distance of the
photo-generated carriers and improved the charge transfer efficiency effectively.
Linear sweep voltammetry (LSV) is a common electrochemical technique for
studying the carrier property at the interface of semiconductor and electrolyte [39].
The variation of photocurrent densities with the bias potential was recorded by LSV
(Fig. 9B). The current response of g-C3N4/TiO2-2 NTAs in the dark was also investigated.
There was no current for g-C3N4/TiO2-2 NTAs in the dark with a bias potential less than
1.2 V. However, the photocurrent densities of TiO2 and g-C3N4/TiO2 NTAs sharply
increased with the increasing bias potential in simulated solar light irradiation. This
indicates that the increased bias potential could further inhibit the recombination of
photo-generated carriers, promote the photo-induced electrons transfer to the
external circuit and improve the photoelectrochemical performance of the
photoelectrodes.
Fig. 9C shows the variation of photocurrent densities vs. bias potential for TiO2
and g-C3N4/TiO2-2 NTAs under chopped light irradiation. Prompt and reproducible
photocurrent responses were observed for each switching on and off, and the
9
photocurrent density was observed to be 2.64 mA cm-2 at 1.0 V for g-C3N4/TiO2-2
NTAs, which was 3.43-fold of that of pure TiO2 (0.77 mA cm-2).
In photoelectrochemistry, the theoretical open circuit potential depends on the
difference value between the quasi-Fermi level of the photo-anode with light
irradiation and redox potential of the electrolyte [40,41]. A more negative open circuit
potential indicates a better charge separation property of the photoelectrode [42,14].
Fig. 9D is the open circuit potential of the as-prepared photoelectrodes under light on
and off conditions. The open circuit potential of g-C3N4/TiO2 NTAs showed negative
shift of about 0.1 V compared with TiO2. This demonstrates better photo-generated
electron-hole separation property for g-C3N4/TiO2 NTAs. In addition, no photocurrent
decreases were observed in 5000 s for TiO2 and g-C3N4/TiO2-2 NTAs at 1.0 V (Fig. 9E).
These results indicate that the as-prepared g-C3N4/TiO2 NTAs exhibited superior
photoelectrochemical performance and excellent photoelectric stability.
The Mott-Schottky plots in the dark further detail the semiconductor space charge
layer capacitance (C) depending on the applied potential (E) of TiO2 and g-C3N4/TiO2
NTAs and is given as follows [43,44]:
1
2
=
2
N D e? 0?
C
?
?T ?
? E - E FB ?
e ?
?
Here, C is the semiconductor space charge layer capacitance, ND is the donor
density, cm-3, e is the element charge (1.602�-19 C), ?0 is the vacuum permittivity
(8.86�-12 Fm-1), ? is the dielectric constant of the semiconductor (? = 48 for anatase),
E is the applied potential, EFB is the flat band potential, T is the absolute temperature
and ? is the Boltzmann constant. The flat band potentials of TiO2 and g-C3N4/TiO2 NTAs
could be calculated from the intercept by extrapolating the linear part of the MS plots
to the x-axis in Fig. 9F, the results are presented in Table 1. The flat band potential of
g-C3N4/TiO2 NTAs showed negative shifts than that of TiO2 indicating a stronger
reduction property for the photo-generated electrons. The donor density (ND) could
be calculated from the following equation:
10
ND
?
2 ? dE
=
e? 0 ? ?? d 1
C2
?
? ?
?
?
?
?
?
Table 1 shows that the donor density of g-C3N4/TiO2-2 exhibited the highest value
of 3.17�22 cm-3, which was 1.26-fold higher than that of TiO2 (1.40�22 cm-3). The
more negative flat band potential and the higher donor density of g-C3N4/TiO2 NTAs
improve the PEC activity of TiO2 NTAs.
Phenol was chosen as the targeted pollutant to investigate the PEC activity of the asprepared photoelectrodes under simulated solar light irradiation. The EC, PC, and PEC
process for phenol degradation was conducted on the g-C3N4/TiO2-2 NTAs,
respectively, as shown in Fig. 10A. Very little activity was observed for the EC process
over g-C3N4/TiO2-2 NTAs at 1.0 V, while the degradation efficiency for the PEC process
was much higher than the sum of PC and EC processes. This indicated that a synergetic
effect was achieved for the PEC degradation of phenol over g-C3N4/TiO2-2 NTAs at 1.0
V resulting in a boosted PEC performance for g-C3N4/TiO2 NTAs. Fig. 10B shows the
PEC activities for phenol degradation of TiO2 and g-C3N4/TiO2 NTAs at 1.0 V. The PEC
degradation efficiencies for g-C3N4/TiO2 NTAs were higher than that of pure TiO2 in
which g-C3N4/TiO2-2 NTAs showed the best PEC degradation efficiency of 98.40% in
150 min. The enhanced PEC performance was ascribed to the above mentioned three
points: (1) the extended light absorption and enhanced visible light absorption
property for the combination of g-C3N4 NPs and TiO2 NTAs; (2) the well matched band
energies and impact interaction between g-C3N4 and TiO2 highly promoted the charge
separation property; (3) the synergetic effect of one-dimensional confined effect of
TiO2 NTAs and small size effect of g-C3N4 NPs effectively shorten the migration
distance of the photo-generated carriers and improved the charge transfer efficiency.
Fig. S3 shows the two-dimensional HPLC spectra of phenol at different degradation
times over g-C3N4/TiO2-2 NTAs. Obviously, new absorption peaks appeared at the
retention time of 2.8 min indicating that the hydroxyl products of phenol were formed
during the PEC degradation. This could be completely degraded in 150 min. Besides,
11
the three-dimensional HPLC spectra of phenol (Fig. 10 C) also demonstrated that
phenol could be completely mineralized to small molecules such as CO2 and H2O over
g-C3N4/TiO2-2 NTAs in 150 min.
Fig. 10D shows the EC, PC and PEC processes for the degradation of phenol over TiO2
and g-C3N4/TiO2 NTAs in 150 min. Obviously, the PEC degradation efficiency of the four
NTAs were much higher than the sum of EC and PC processes for the phenol
degradation. This demonstrates that synergetic effects were achieved on the asprepared NTAs resulting in a highly enhanced PEC performance. The EC and PEC
processes for phenol degradation over g-C3N4/TiO2-2 NTAs under different bias
potential were conducted to study the influence of different bias potentials on the
synergetic effect of PEC performance for the g-C3N4/TiO2 composite NTAs as well as
to explain the cause of PEC improvement for phenol degradation (Fig. 10 E). The
degradation efficiency for the EC process was less than 5.0% when the bias potential
was below 3.0 V, while the PEC degradation efficiencies were higher than 90.0%, which
indicated that the improved PEC activity was ascribed to the promoted charge
separation efficiency by applied bias potential (below 3.0 V). When the applied bias
was above 3.0 V, the degradation efficiency for EC process increased with increased
bias potential. This suggests that the enhanced PEC activity was attributed to the
synergetic effect of direct electrocatalytic degradation and photocatalytic
degradation.
The effect of different light sources on the PEC degradation of the as-prepared
photoelectrodes was also studied as shown in Fig. 10F. The g-C3N4/TiO2 NTAs
exhibited higher PEC degradation efficiency than that of pure TiO2 under visible light
(?420 nm), UV light and simulated solar light irradiation in 120 min. In addition, the
g-C3N4/TiO2-2 NTAs still maintained a PEC degradation efficiency of above 90.0% at
1.0 V after used for five times (Fig. 11A). The used SEM image of g-C3N4/TiO2-2
photoelectrode was also investigated (Fig. 11B). Compared with Fig. 2C, the
morphology was almost not changed after used for five times. These results indicated
12
that the as-prepared g-C3N4/TiO2 NTAs possessed superior PEC performance and
excellent stability for phenol degradation.
To detect the active radicals during the PEC degradation process over g-C3N4/TiO2
NTAs, a series of quenching experiments were carried out using isopropanol (IPA) for
quenching 稯H, EDTA for holes, and p-Benzoquinone for O2�, respectively [45,46]. Fig.
12 A shows that the degradation efficiency (42.7%) of phenol over g-C3N4/TiO2-2 NTAs
was dramatically inhibited with the addition of IPA indicating that 稯H was the main
active radical during the PEC process. The PEC activity could also be suppressed by
addition of EDTA and p-Benzoquinone, confirming the existence of holes and O2�. To
further testify the produced active radicals involved in the PEC process, the DMPO
spin-trapping ESR spectra were investigated to characterize the O2�- and 稯H radicals
as shown in Figs. 12B and 12C [47]. No signal peaks of DMPO--O2�- and DMPO--稯H
were observed in the dark, while the characteristic peaks of both DMPO--O2�- and
DMPO--稯H were observed under simulated solar light irradiation. This is in
accordance with the results reported by Su et al [23,48]. These results confirmed that
O2�- and 稯H radicals were generated during the PEC process.
Based on the above results, a schematic diagram for PEC degradation of phenol
over g-C3N4/TiO2 NTAs was proposed (Fig. 13). The TiO2 NTs and g-C3N4 NPs could both
be excited to generate photo-induced electrons and holes under simulated light
irradiation. The electrons on the conduction band (CB) of g-C3N4 could be injected to
the CB of TiO2 due to the well matched band energies between TiO2 and g-C3N4 [49],
and then transferred to the counter electrode along with the TiO2 NTs vertically under
the applied bias potential, realizing fast charge transfer and separation. The electrons
on the counter electrode would combine with dissolved oxygen to generate O2穜adical. Meanwhile, the holes left on the VB of TiO2 would migrate to the VB of g-C3N4
and combine with H2O to produce 稯H radical. Some of the un-reacted holes on the
13
VB of g-C3N4, 稯H, and O2�- radicals on the counter electrode were involved in the PEC
process for the degradation of phenol. Thus the photo-generated electrons and holes
achieved an effective separation in space and strongly promoted the PEC degradation
efficiency for the degradation of organic pollutants of g-C3N4/TiO2 NTAs.
4. Conclusion
In summary, g-C3N4 NPs-wrapped TiO2 NTAs (g-C3N4/TiO2) were successfully
fabricated. A series of photoelectrochemical characterization results and the results
for PEC degradation of phenol under simulated light irradiation indicated that the asprepared g-C3N4/TiO2 NTAs possessed superior charge separation efficiency, excellent
photoelectrochemical stability, and high PEC activity for PEC degradation of organic
pollutants. The improved PEC activity of g-C3N4/TiO2 NTAs was mainly ascribed to the
broadened light absorption region, enhanced visible light absorption, and promoted
charge separation efficiency caused by the well matched band energies between gC3N4 and TiO2, and the synergetic effect of one-dimensional confined effect of TiO2
NTAs and small size effect of g-C3N4 NPs.
Acknowledgements
This work was supported by the National Natural Science Foundation of China
(grant No. 51672081), Key Program of Natural Science of Hebei Province (grant No.
B2016209375), and Hebei Provincial Foundation for High-level Scholars (grant No.
A201501001).
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Figure Caption
19
Figure captions
Scheme 1. Scheme for the fabrication of core-shell g-C3N4/TiO2 NTAs.
Fig. 1. XRD spectra of g-C3N4, TiO2 and g-C3N4/TiO2 NTAs.
Fig. 2. Top-view SEM images of (A) pure TiO2, (B) g-C3N4/TiO2-1, (C) g-C3N4/TiO2-2, (D) gC3N4/TiO2-3; side-view SEM images of (E) pure TiO2 and (F) g-C3N4/TiO2-2.
Fig. 3. (A) TEM image of g-C3N4/TiO2-2 composite, (B, C, D) TEM images of microdomain
between g-C3N4 NPs and TiO2 NTs.
Fig. 4. SEM images and EDX spectra of (A) g-C3N4/Ti, (B) g-C3N4/TiO2-2.
Fig. 5. (A) UV-DRS spectra and (B) PL spectra of the as-prepared samples.
Fig. 6. (A) XPS survey spectrum of g-C3N4/TiO2-2, (B) Ti 2P XPS spectra of TiO2 and g-C3N4/TiO22, (C) O 1s, (D) C 1s, and (E) N 1s XPS spectra of g-C3N4/TiO2-2.
Fig. 7. FT-IR spectra of g-C3N4, TiO2 and g-C3N4/TiO2 composites.
Fig. 8. (A, B) EIS plots and Bode-phase plots of TiO2 and g-C3N4/TiO2 NTAs, (C, D) photoirradiation and bias potential on EIS spectra of TiO2 and g-C3N4/TiO2-2 NTAs.
Fig. 9. (A) Transient photocurrent responses of the as-prepared photoelectrodes at 0.0 V (vs.
SCE), (B) variation of photocurrent densities vs. bias potential, (C) current-potential curves for
TiO2 and g-C3N4/TiO2-2 NTAs irradiated with chopped light, (D) open circuit potential of TiO2
and g-C3N4/TiO2 NTAs, (E) photocurrent stabilities of TiO2 and g-C3N4/TiO2-2 NTAs at 1.0 V, (F)
Mott-Schottky plots of TiO2 and g-C3N4/TiO2 NTAs.
Fig. 10. (A) Different degradation processes of phenol over g-C3N4/TiO2-2 NTAs, (B) PEC
degradation of phenol over the as-prepared photoelectrodes at 1.0 V under simulated solar
light, (C) three-dimensional HPLC chromatographic spectra of phenol degradation (0 min and
150 min), (D) the PC, EC, and PEC processes over TiO2 and g-C3N4/TiO2 NTAs, (E) PEC
degradation of phenol over TiO2 and g-C3N4/TiO2-2 NTAs at different potentials, (F) PEC
20
degradation of phenol over TiO2 and g-C3N4/TiO2 NTAs under different light sources in 120
min.
Fig. 11. (A) The PEC activities over TiO2 and g-C3N4/TiO2-2 NTAs used for five times at 1.0 V, (B)
SEM image of the used g-C3N4/TiO2-2 photoelectrode.
Fig. 12. (A) PEC activities over g-C3N4/TiO2-2 NTAs with different quencher addition, DMPO
spin-trapping ESR spectra of (B) O2� and (C) 稯H in dark and simulated solar light irradiation.
Fig. 13. Schematic diagram for PEC degradation of phenol over g-C3N4/TiO2 NTAs under
simulated light irradiation.
Table 1. EIS fitting results and band structure parameters of different photoelectrodes.
21
Scheme 1.
22
Fig. 1.
?
anatase
Intensity (a.u.)
g-C3N4/TiO2-3
?
C3N4
?
??
?
g-C3N4/TiO2-2
?
g-C3N4/TiO2-1
?
? Ti
?
?
?
?
?
?
?
??
TiO2
C3N4
10
20
30
40
50
60
70
80
2 ? (degree)
23
Fig. 2.
24
A
B
200 nm
200 nm
D
C
200 nm
200 nm
E
F
200 nm
200 nm
25
Fig. 3.
A
B
TiO2
TiO2
g-C3N4
g-C3N4
200 nm
C
20 nm
D
g-C3N4
g-C3N4
TiO2
TiO2
20 nm
20 nm
26
Fig. 4.
A
B
200 nm
200 nm
27
Fig. 5.
B
1/2
2.0
300
400
500
TiO2
g-C3N4/TiO2-1
g-C3N4/TiO2-2
g-C3N4/TiO2-3
2.5
600
Wavelength (nm)
3.0 3.5 4.0
Energy (eV)
700
4.5
800
PL Intensity (a.u.)
Intensity (a.u.)
TiO2
C3N4/TiO2-1
C3N4/TiO2-2
C3N4/TiO2-3
C 3N 4
(Ahv)
A
250
275
300
325
350
Wavelength (nm)
28
Fig. 6.
29
A
B
Ti 2p
458.6
Intensity (a.u.)
Intensity (a.u.)
464.3
g-C3N4/TiO2-2
458.5
464.2
TiO2
454
1000
800
600
399
456
458
460
462
464
466
468
199
Binding Energy (eV)
Binding Energy (eV)
C
D
C 1s
O 1s
529.8
Intensity (a.u.)
Intensity (a.u.)
284.6
531.4
285.4
288.5
526
528
530
532
534
280
282
284
286
288
290
Binding Energy (eV)
Binding Energy (eV)
E
N 1s
399.4
Intensity (a.u.)
400.3
401.7
396
398
400
402
404
Binding Energy (eV)
30
Fig. 7.
TiO2
Ti-O-Ti
3431.2
g-C3N4/TiO2-1
1632.5
Intensity (a.u.)
530.9
3431.6
1384.4
873.7
1384.7
872.9
g-C3N4/TiO2-2
525.6
3429.3
g-C3N4/TiO2-3
523.9
3431.3
1384.5
g-C3N4
3442.8
4000
3500
873.5
522.2
891.7 811.9
1242.4
1569.6 1411.7
1321.3
1462.2
1639.7
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
31
Fig. 8.
A 140
100
g-C3N4/TiO2-3
Fitted
10
0
-10
80
60
-20
0
-10
-30
-40
TiO2
g-C3N4/TiO2-1
g-C3N4/TiO2-2
g-C3N4/TiO2-3
40
-50
R1
20
R2
R3
CPE1
CPE2
20
40
60
80
100
120
140
160
-30
-40
-50
-60
180
0
1000 2000 3000 4000 5000 6000
Frequency (Hz)
0
200
10000
20000
30000
40000
50000
Frequency (Hz)
z' (?)
D 800
C 1400
light on 0 V
light off 0 V
light on 0.5 V
light off 0.5 V
1200
light on 0 V
light on 0.5 V
light off 0 V
light off 0.5 V
TiO2
600
-z" (?)
1000
-z'' (?)
-20
-60
0
0
Phase (deg)
120
Phase (deg)
-z" (?)
N
P
B
TiO2
g-C3N4/TiO2-1
g-C3N4/TiO2-2
800
600
400
g-C3N4/TiO2-2
400
200
200
0
0
0
100
200
300
z' (?)
400
500
0
50
100
150
200
250
300
z' (?)
32
Fig. 9.
33
B 4
TiO2
g-C3N4/TiO2-2
2
Photocurrent density (mA/cm )
A 1.6
g-C3N4/TiO2-1
g-C3N4/TiO2-3
3
2
Photocurrent (mA/cm )
1.2
TiO2
g-C3N4/TiO2-1
g-C3N4/TiO2-2
g-C3N4/TiO2-3
g-C3N4/TiO2-2 light off
0.8
0.4
2
1
0.0
0
100
200
300
0
-0.6
400
Potential (V vs SCE)
4
2
Photocurrent (mA/cm )
3
D 0.2
TiO2
g-C3N4/TiO2-2
TiO2
g-C3N4/TiO2-2
0.1
Potential (V vs SCE)
C
1.8
1.2
0.6
0.0
Time (s)
2
1
g-C3N4/TiO2-1
g-C3N4/TiO2-3
0.0
-0.1
-0.2
-0.3
-0.4
Light on Light off
0
-0.6
0.0
0.6
1.2
-0.5
1.8
0
120
240
Potential (V vs SCE)
E 4
480
600
F 1.0
TiO2
g-C3N4/TiO2-2
3
0.8
TiO2
g-C3N4/TiO2-1
g-C3N4/TiO2-2
g-C3N4/TiO2-3
4
1/C (� F cm )
-2
0.6
6
2
0.4
2
2
Photocurrent (mA/cm )
360
Time (s)
1
0
0
1000
2000
3000
Time (s)
4000
5000
0.2
0.0
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
Potential (V vs SCE)
34
Fig. 10.
B 1.0
A 1.0
0.8
0.8
0.6
C/C0
C/C0
0.6
0.4
Dark reaction
Blank test
EC
PC
PEC
0.2
0.4
TiO2
g-C3N4/TiO2-1.0%
g-C3N4/TiO2-10.0%
g-C3N4/TiO2-20.0%
0.2
0.0
0.0
0
20
40
60
80
100
120
140
0
20
40
Irradiation time (min)
60
80
100
120
140
Irradiation time (min)
D 100
C
B
C
EC
PC
PEC
150 min
Degradation rate (%)
0 min
80
60
40
20
0
TiO2
F 100
E
EC
100
PEC
Degradation rate (%)
80
Degradation rate (%)
2-1
2-2
iO2-3
N4/TiO
N4/TiO g-C3N4/T
g-C3
g-C3
80
60
40
20
0
0.5
1.0
1.5
2.0
2.5
Potential (V)
3.0
3.5
4.0
TiO2
g-C3N4/TiO2-1
g-C3N4/TiO2-2
g-C3N4/TiO2-3
60
40
20
0
Visible light
UV irradiation Simulated solar
35
Fig. 11.
A
TiO2
Degradation rate (%)
100
g-C3N4/TiO2-2
B
80
60
40
20
200 nm
0
1 st
2 nd
3 rd
4 th
5 th
36
Fig. 12.
37
A 1.0
B
DMSO
0.8
Intensity (a.u.)
8 min
C/C0
0.6
0.4
Blank
IPA
EDTA
p-Benzoquinone
No addition
0.2
4 min
Dark
0.0
0
30
60
90
120
150
318.1
318.2
318.3
318.4
318.5
B (mT)
Irradiation time (min)
C
H2O
Intensity (a.u.)
8 min
4 min
Dark
318.1
318.2
318.3
318.4
318.5
B (mT)
38
Fig. 13.
39
Table 1.
Photoelectrode
R1
R2
R3
CPE1
CPE2
EFB
ND
(?)
(?)
(?)
(F)
(F)
(V)
(cm-3)
TiO2
2.68
0.98
254.90
4.49�-4
5.59�-4
-0.22
1.40�22
g-C3N4/TiO2-1
4.05
2.13
222.89
4.22�-4
9.83�-5
-0.51
1.47�22
g-C3N4/TiO2-2
3.48
0.89
122.51
5.23�-4
9.70�-4
-0.57
3.17�22
g-C3N4/TiO2-3
3.99
1.03
179.34
4.99�-4
1.09�-3
-0.52
2.35�22
40
41
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