2D Ti3C2 as electron harvester anchors on 2D g-C3N4 to create boundary edge active sites for boosting photocatalytic performance

Article abstract of DOI:10.1016/j.apcata.2019.117367

Control and insight into active sites and charge transport behaviors are challenging topics for fabricating heterojunction photocatalyst and investigating photocatalytic reaction mechanism. Herein, the 2D/2D Ti₃C₂/g-C₃N₄ heterojunctions are fabricated in the semi-closed system, in which the interaction at heterointerfaces is enhanced to improve the stability. Furthermore, 2D Ti₃C₂ not only serves as electron harvester to propel the directed transfer of photogenerated electrons from 2D g-C₃N₄ to 2D Ti₃C₂ and the high-efficiency separation of charge carriers, but also creates the boundary edge active sites on its surface, thus enhancing the photocatalytic performance. The photocatalytic reaction mechanism for H₂ production and degrading tetracycline hydrochloride (TH-HCl) are also revealed in detail, including identification of active sites, transport behaviors of charge carriers, generation of active species and intermediates, reaction ways, etc. This work provides a significant reference for the design, fabrication and mechanism insight of 2D/2D heterojunction materials in the photocatalytic application.

Full text of DOI:10.1016/j.apcata.2019.117367

Journal Pre-proof
2D Ti3C2 as electron harvester anchors on 2D g-C3N4 to create
boundary edge active sites for boosting photocatalytic performance
Hongjun Dong, Xiaoxu Zhang, Yan Zuo, Ning Song, Xi Xin, Binghui
Zheng, Jingxue Sun, Gang Chen, Chunmei Li
PII:
S0926-860X(19)30522-8
DOI:
https://doi.org/10.1016/j.apcata.2019.117367
APCATA 117367
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To appear in:
Applied Catalysis A, General
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Revised Date:
Accepted Date:
27 September 2019
21 November 2019
27 November 2019
Please cite this article as: Dong H, Zhang X, Zuo Y, Song N, Xin X, Zheng B, Sun J, Chen G,
Li C, 2D Ti3C2 as electron harvester anchors on 2D g-C3N4 to create boundary edge active
sites for boosting photocatalytic performance, Applied Catalysis A, General (2019),
doi: https://doi.org/10.1016/j.apcata.2019.117367
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2D Ti3C2 as electron harvester anchors on 2D g-C3N4
to create boundary edge active sites for boosting
photocatalytic performance
Hongjun Donga, Xiaoxu Zhanga, Yan Zuoa, Ning Songa, Xi Xina, Binghui Zhenga,
Jingxue Sunb, Gang Chenb and Chunmei Li*a
aInstitute of Green Chemistry and Chemical Technology, School of Chemistry and
Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China.
bMIIT Key Laboratory of Critical Materials Technology for New Energy Conversion
and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of
Technology, Harbin 150001, PR China.
E-mail: lichun_mei_happy@126.com.(C.Li).
Graphical Abstract
Highlights
1






2D/2D Ti3C2/g-C3N4 heterojunctions are fabricated in the semi-closed system,
which improve the photocatalytic activity and stability.
2D Ti3C2 serves as electron harvester to propel directed transfer of electrons
from 2D g-C3N4 to 2D Ti3C2 and high-efficiency separation of charge carriers.
The boundary edge active sites are created on 2D Ti3C2 structure unit, thus
enhancing the photocatalytic performance.
The photocatalytic reaction mechanism for H2 production and degrading
TH-HCl are revealed in detail.
This work provides a significant reference for design, fabrication and
mechanism insight of 2D/2D heterojunctions in the photocatalytic
application.
Abstract
Control and insight into active sites and charge transport behaviors are challenging
topics for fabricating heterojunction photocatalyst and investigating photocatalytic
reaction mechanism. Herein, the 2D/2D Ti3C2/g-C3N4 heterojunctions are fabricated
in the semi-closed system, in which the interaction at heterointerfaces is enhanced to
improve the stability. Furthermore, 2D Ti3C2 not only serves as electron harvester to
propel the directed transfer of photogenerated electrons from 2D g-C3N4 to 2D Ti3C2
and the high-efficiency separation of charge carriers, but also creates the boundary
edge active sites on its surface, thus enhancing the photocatalytic performance. The
photocatalytic reaction mechanism for H2 production and degrading tetracycline
hydrochloride (TH-HCl) are also revealed in detail, including identification of active
sites, transport behaviors of charge carriers, generation of active species and
intermediates, reaction ways, etc. This work provides a significant reference for the
design, fabrication and mechanism insight of 2D/2D heterojunction materials in the
photocatalytic application.
Keywords：2D/2D Ti3C2/g-C3N4 heterojunction; Photocatalytic degradation of and H2
production; Directed transfer of photogenerated electrons; Control of active sites;
Photocatalytic reaction mechanism
2

1 Introduction
Two-dimensional (2D) semiconductor materials have brought about enormous
attention in the last few decades for their superlative physicochemical characteristics.
The atomically thin sheets in the crystal structure usually display covalent in-plane
bonding and weak interlayer bonding, which endows 2D semiconductor materials
with the unique electronic, optical and mechanical properties so that they exhibit
prominent potential for catalysis, energy storage, adsorption, electron device and
other technological applications [1, 2].
As a typical 2D semiconductor material, graphitic carbon nitride (g-C3N4)
nanosheet has manifested the great potentials in the photocatalytic solar energy
conversion and environmental purification, such as photocatalytic H2 production,
reduction of CO2, degradation of pollutants in water, removal of volatile organic
compounds and so on [3-10]. However, the major issue still to be addressed is how to
accelerate charge transfer and improve the separation efficiency of photogenerated
electron-hole pairs. As a consequence, a great deal of endeavors have been carried out
to exploit the new strategies for enhancing photocatalytic activity of g-C3N4, such as
doping metal/nonmetal elements [11, 12], loading noble metals [13], optimizing
energy band structure [14], controlling micro-structure/morphology [15], modifying
functional groups [16], fabricating semiconductor heterojunction [17], etc. These
works have been made great advancements on the improvement of photocatalytic
3

activity of g-C3N4. It has been reported that fabricating the 2D/2D heterojunction is
one of the effective strategies to suppress recombination of charge carriers and
increase separation efficiency, which can be achieved by embellishing other 2D
material on g-C3N4 nanosheets. As reported 2D/2D g-C3N4/graphdiyne heterojunction,
it obviously improves the hole mobility of g-C3N4 and motivates separation of charge
carriers [18]. In addition, by means of constructing 2D/2D heterojunction between
oxygen-doped carbon nitride (OCN) and ultrathin CoAl-layered double hydroxide
(CoAl-LDH), the formed strong electronic coupling effect on the heterostructure
interface can induce photogenerated charge transfer from CoAl-LDH to OCN and
make for the construction of an interfacial internal electric field between CoAl-LDH
and OCN, thus improving the photocatalytic activity [19].
Recently, Ti3C2 as a member of the 2D MXene family materials has been
extensively used for improving the photocatalytic activity of 2D photocatalysts via
constructing 2D/2D heterojunction. For instance, Cao et al. report a 2D/2D
heterojunction of ultrathin Ti3C2/Bi2WO6 nanosheets, which exhibits a short charge
transport distance and a large contact interface, ensuring the outstanding
bulk-to-surface and interfacial charge transfer abilities [20]. The recent researches
reveal that 2D Ti3C2 served as a structure unit constructs composite heterojunctions
with g-C3N4 nanosheet, which can distinctly improve the photocatalytic H2 production
and degradation activity owing its superior visible light harvest ability and metal-like
nature [21, 22]. However, in these contributions, the obtained Ti3C2/g-C3N4
heterojunctions are usually prepared by simple low-temperature treated mechanical
mixing of Ti3C2 and g-C3N4, which may result in decreasing stability and recyclability.
Moreover, it also needs further discussions and investigations for the mechanism of
photocatalytic H2 production and organic pollutant degradation. As a result, we design
a new preparation strategy that 2D/2D Ti3C2/g-C3N4 heterojunctions are fabricated by
introducing 2D Ti3C2 into the thermal polymerization of g-C3N4 in the semi-closed
system. On one hand, NH3 released at the polymerization of g-C3N4 can make 2D
Ti3C2 in a reductive atmosphere to avoid oxidization. On the other hand, the surface
termination groups such as –O, –F and –OH on 2D Ti3C2 may be partly removed at
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the high temperature [23], and so the generated surface dangling bonds not only take
part in polymerization of g-C3N4 to enhance the interaction at the heterointerfaces
between 2D Ti3C2 and 2D g-C3N4, improve the stability of heterostructure and
accelerate the separation of charge carriers, but also serve as the active sites to
promote photocatalytic reaction.
Therefore, in this work, we fabricate the 2D/2D Ti3C2/g-C3N4 heterojunctions by
adding 2D Ti3C2 into urea raw material before performing thermal polymerization
reaction in the semi-closed system. The enhanced interfacial interaction obviously
improves the stability of heterostructure. Moreover, 2D Ti3C2 not only acts as electron
harvester to induce the directed transfer of photogenerated electrons from 2D g-C3N4
to 2D Ti3C2 and increase the separation efficiency of charge carriers, but also creates
the boundary edge active sites on its surface, which dramatically result in the
enhanced photocatalytic performance for H2 production and degrading tetracycline
hydrochloride (TC-HCl).
2. Experimental section
2.1. Preparation of samples
Ti3AlC2 of 2 g was slowly whisked into 80 mL hydrofluoric acid (HF, ≥40%) and
stirred constantly at 60 ℃ for 24 h. Then the etched suspension was centrifuged and
washed with distilled water and ethanol in turn. Finally, the obtained black 2D Ti3C2
powder was dried in vacuum at 80 ℃ for 4 h. Moreover, the correct amount of urea
was firstly dried at 80 ℃ for 12 h in the oven. A certain amount of as-obtained Ti3C2
(0 g, 0.01 g, 0.02 g, 0.05 g and 0.1 g) were added in urea of 20 g, respectively. After
grinding thoroughly, the obtained uniform mixtures were putted in an unsealed and
covered crucible, subsequently annealed at 550 ℃ with a ramping rate of 2.5 ℃ min-1
for 4 h. Finally, Ti3C2/g-C3N4 samples were obtained and denoted as g-C3N4,
Ti3C2/g-C3N4-1, Ti3C2/g-C3N4-2, Ti3C2/g-C3N4-3 and Ti3C2/g-C3N4-4, respectively.
2.2. Characterization of samples
The phase compositions of samples were examined by X-ray diffraction (XRD)
on SHIMADZU XRD-6100 diffractometer equipped a Cu Ka radiation source
(λ=1.54178 A). The morphology of samples was observed by Hitachi JFM-7800F
5

scanning electronic microscopy (SEM). The micro- structure and morphology of
samples were identified by JEOL-2100F transmission electron microscopy (TEM)
and high-resolution transmission electron microscopy (HRTEM). The fourier
transform infrared (FT-IR) spectra of samples were analysed by a Nicolet is 50
spectrometer using KBr as the dispersion medium. The X-ray photoelectron
spectroscopy (XPS) was detected by Thermo ESCALAB 250X electron spectrometer
with 150 W Al Kα radiations. UV-Vis diffuse reflectance spectroscopy (DRS) of
samples was obtained on SHIMADZU UV-3600 Plus spectrophotometer using BaSO4
as the reflectance material. The steady-state photoluminescence (SS-PL) spectra and
transient-state photoluminescence (TS-PL) decay curves were performed on a
QuantaMasterTM 40 spectrofluorometer. The specific surface area and pore diameter
distribution of the samples was measured by a nitrogen adsorption BET method on a
TriStar Ⅱ3020 analyzer. Total organic carbons (TOC) were detected by an Analytik
Jena AG multi N/C 2100 analyzer. The active species were analyzed by electron spin
resonance (ESR) using Bruker A300-10/12 Electron paramagnetic resonance
spectrometer. The mass spectrometry (MS) was tested by hermo LXQ liquid
chromatgraphy-ion trap mass spectrometry.
2.3. Photocatalytic experiments
The photocatalytic H2 production was performed under the visible light provided
by 300 W Xe arc lamp equipped with a cut-off filter (λ > 420 nm, 150 mW cm-2).
Photocatalyst of 50 mg was dispersed in aqueous solution of 100 mL including 20
vol% triethanolamine (TEOA). Afterwards, a given mass of H2PtCl6·6H2O (Pt of 3
wt%) was added in above solution as co-catalyst. The reaction system was degassed
with Ar gas for 20 min to remove air before light on. The produced H2 was detected
on the gas chromatography GC-2030 with Ar as carrier gas. The apparent quantum
efficiency (AQE) was detected under the same experiment conditions by changing
cut-off filter to band-pass filters with 400 nm, 420 nm, 450 nm, 500 nm and 600 nm,
respectively.
The photocatalytic degradations of tetracycline hydrochloride (TC-HCl) were
performed under the visible light. First of all, as-prepared samples of 50 mg were
6

equably dispersed in the solution of TC-HCl (100 mL, 10 mg L-1) with stirring for 30
min in the dark to reach adsorption-desorption equilibrium. After that, the suspension
was irradiated with the visible light provided by a 250 W Xe arc lamp (λ ≥ 420 nm).
During given irradiation time intervals, the turbid liquid of 6 mL was removed and
centrifuged (9000 rmp, 5 min). The absorbance of supernatant was measured on
Agilent Cary 8454 UV-Vis spectrophotometer at the characteristic wavelength of 357
nm. Finally, the degradation efficiency of TC-HCl was calculated by Lambert-Beer
law.
For the sake of investigating the active species produced in the photocatalytic process,
three scavengers were added into the TC-HCl solution in which isopropanol (IPA),
EDTA-2Na and vitamin C (VC) were served as the scavengers for trapping hydroxyl
-
radicals (·OH), holes (h+ ) and superoxide radical (·O2 ), respectively.
2.4. Photoelectrochemical measurements
The transient photocurrent response and impedance spectra were detected on the
VersaSTAT3 electrochemical working station by using a standard three-electrode
system contained 0.5 M Na2SO4 electrolyte. Pt plate and saturated Ag/AgCl electrode
served as the counter electrode and reference electrode, respectively. The samples
coated on ITO glass (2 × 2 cm2) served as working electrodes.
3. Results and discussion
The structure and composition of as-prepared samples were investigated by XRD.
As displayed in Fig. 1a, the MAX phase Ti3AlC2 sample possesses the intense
diffraction peaks at 9.5°, 19.1°, 34.0°, 36.0°, 38.8°, 41.7°, 48.5°, 56.5°, 60.6°, which
are attributed to the lattice planes of (002), (004), (100), (103), (104), (105), (107),
(109) and (110), respectively [24, 25]. From the XRD pattern of Ti3C2 sample, the
characteristic peak of (104) lattice planes of Ti3AlC2 disappears, and the diffraction
peaks of (002) and (004) lattice planes take place obvious shift to the low angle
direction, which indicates that the interlayer Al atoms in Ti3AlC2 are efficiently
removed to transform into Ti3C2 [26]. In addition, the diffraction peaks at 8.9°, 18.2°,
27.8°, 35.9°, 41.7° and 60.5° originate from (002), (004), (006), (008), (010) and (110)
lattice planes of Ti3C2, respectively [27, 28]. Moreover, as shown in Fig. 1b, two
7

characteristic diffraction peaks at 13.2° and 27.4° are observed in the XRD pattern of
single g-C3N4 sample, which correspond to (100) and (002) lattice planes of the
tri-s-thiazine rings and the stacking of conjugated aromatic systems, respectively [29].
It is worth noting that two characteristic peaks of (008) and (010) of Ti3C2 are
detected in the XRD pattern of Ti3C2/g-C3N4 sample, implying the formation of
heterojunction between Ti3C2 and g-C3N4. Moreover, from FT-IR spectra (Fig. S1†),
no absorption peaks of C–C and C–Ti bonds are produced in FT-IR spectra of Ti3C2
and Ti3C2/g-C3N4-3 samples, which may be because Ti3C2 exhibits the weak infrared
absorption feature and has relative low content in Ti3C2/g-C3N4-3 sample [22]. FT-IR
spectra of g-C3N4 and Ti3C2/g-C3N4-3 samples present the similar absorption features.
The absorption peak at 810 cm-1 is attributed to tri-s-triazine units in g-C3N4, and the
several fingerprint peaks among 1200-1700 cm-1 result from aromatics stretching
modes of CN heterocycles, and the broad peaks between 3300 cm-1 and 3500 cm-1
originate from the vibration absorption of NHx groups, respectively [17].
Fig. 1 XRD patterns of Ti3AlC2, Ti3C2, g-C3N4 and Ti3C2/g-C3N4-3 samples.
The morphologies of samples were further investigated by SEM and TEM
techniques. As SEM image displayed in Fig. 2a, the MAX phase Ti3AlC2 raw material
shows irregular block, but the obtained Ti3C2 presents the typical 2D nanosheet
features after MAX phase Ti3AlC2 is etched by HF (Fig. 2b). In addition, comparing
with SEM images of g-C3N4 (Fig. 2c) and Ti3C2/g-C3N4-3 (Fig. 2d) samples, they
exhibit the similarly morphological features which show the macroscopical irregular
bulk shape composed of 2D nanosheets. The 2D nanosheet structure of g-C3N4 is
further revealed by TEM image in Fig. 2e. What is worth noting is that some 2D Ti3C2
nanosheets are distinctly loaded on the surface of 2D g-C3N4 nanosheets in TEM
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images of Ti3C2/g-C3N4-3 sample (Fig. 2f-g), which demonstrates that the 2D/2D
Ti3C2/g-C3N4 heterojunction is fabricated successfully, in favor of transfer and
separation of charge carriers between 2D Ti3C2 and 2D g-C3N4 nanosheets to improve
the photocatalytic activity. Furthermore, in order to determine the flow direction of
photogenerated electrons and the positions of active sites, the Pt deposited
Ti3C2/g-C3N4-3 sample are visually observed by TEM and HRTEM images after
photocatalytic reaction. As TEM image shown in Fig. 2h, Pt nanoparticles are mainly
deposited on the boundary edges of 2D Ti3C2 nanosheets, which indicates the
photogenerated electrons should transfer from 2D g-C3N4 to 2D Ti3C2 and then
migrate to the boundary edges of 2D Ti3C2 along the nanosheet planes inside 2D/2D
Ti3C2/g-C3N4 heterojunction, suggesting the boundary edges of 2D Ti3C2 nanosheets
can serve as active sites to take place the photocatalytic reaction. The results are
further revealed by HRTEM image in Fig. 2i, where Pt nanoparticles are anchored on
the boundary edge of 2D Ti3C2 nanosheet at the clear heterointerface between 2D
Ti3C2 and 2D g-C3N4, and the interplanar crystal spacing of 0.23 nm corresponds to
the (111) lattice planes of Pt. In addition, the EDS elemental mappings (Fig. 2j-m) of
Ti3C2/g-C3N4-3 sample demonstrate the C, N and Ti elements all are detected and
exhibit characteristic distribution, further implying the formation of 2D/2D
Ti3C2/g-C3N4 hybrid.
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Fig. 2. SEM images of Ti3AlC2 (a), Ti3C2 (b), g-C3N4 (c) and Ti3C2/g-C3N4-3 (d)
samples; TEM images of g-C3N4 (e) and Ti3C2/g-C3N4-3 (f, g) samples; TEM (h) and
HRTEM (i) images of Pt deposited Ti3C2/g-C3N4-3 sample; HAADR image (j) and
EDS elemental mapping (k, l, j) of Ti3C2/g-C3N4-3 sample.
XPS measurements were carried out to inquire into the surface elemental
compositions and chemical states of the samples. The survey XPS spectra in Fig. 3a
present the distinct binding energy peaks of C, N and O elements, in which O element
signal may be ascribed to the adsorbed H2O and CO2 molecules on the surface of
samples. No detected Ti element signal in XPS spectrum of Ti3C2/g-C3N4-3 sample
may be due to the low content Ti3C2 in the 2D/2D heterojunction. Moreover, the C 1s
XPS spectrum of g-C3N4 sample (Fig. 3b) is grouped into three binding energy peaks
at 284.6 eV, 286.3 eV and 288.0 eV, which originate from the hybridized sp2 C in
C=C, sp3 C in C-C(N) and sp2 C in N-C=N, respectively [30]. The N 1s XPS spectra
of g-C3N4 sample also can be fitted to three binding energy peaks at 398.5 eV, 399.8
eV and 400.9 eV (Fig. 3c), which correspond to sp2 hybridized N in C-N=C of
triazine rings, graphite N in N-C3 and amino N in C-NHx, respectively [30].
Furthermore, compared XPS spectra of Ti3C2/g-C3N4-3 with that of g-C3N4 sample,
the binding energy peaks of sp2 hybridized C in N-C=N and sp2 hybridized N in
10

C-N=C all shift 0.1 eV to the low energy direction and locate at 287.9 eV and 398.4
eV, respectively, which confirms the strong interaction existed between 2D Ti3C2 and
2D g-C3N4 nanosheets, availing to accelerate the transfer and separation of charge
carriers. Moreover, the Ti 2p XPS spectrum of Ti3C2/g-C3N4-3 sample in Fig. 3d
shows four sets of typical 2p3/2/2p1/2 spin-orbit coupling binding energy peaks at
454.6/460.6 eV, 455.4/462.1 eV, 456.5/463.3 eV, and 458.1/464.1 eV, which can be
well ascribed to Ti-C, Ti2+, Ti3+ and Ti4+ states in Ti3C2, respectively [31], suggesting
that the fabrication of 2D/2D Ti3C2/g-C3N4 heterojunction.
Fig. 3 XPS spectra of g-C3N4 and Ti3C2/g-C3N4-3 samples.
The photocatalytic activity of the fabricated 2D/2D Ti3C2/g-C3N4 heterojunction
was firstly evaluated by H2 production from water under the visible light (λ > 420 nm).
As can be seen from H2 production kinetic curves (Fig. 4a) and H2 production rates
(Fig. 4b), the single Ti3C2 sample displays the very weak photocatalytic activity and
the single g-C3N4 sample only exhibits a relative low photocatalytic H2 production
rate of 12.5 μmol h-1, respectively. However, the photocatalytic H2 production activity
is distinctly enhanced after forming 2D/2D heterojunction between 2D Ti3C2 and 2D
g-C3N4, and the H2 production rates increase and then decrease with increasing the
content of Ti3C2. Especially the highest H2 production rate rises to 26.7 μmol h-1 over
11

the Ti3C2/g-C3N4-3 sample, which is about 2.13 times as high as that of single g-C3N4
sample (12.5 μmol h-1). Meanwhile, Fig. 4c demonstrates the apparent quantum
efficiency (AQE) reaches up to 1.61% at 420 nm. Of particular note is that the
Ti3C2/g-C3N4-3 sample displays the considerably high stability. As presented in Fig.
4d, the cycle experiments demonstrate the Ti3C2/g-C3N4-3 sample still maintains the
stable H2 production rate when withstanding 12 cycle runs with total 60 h, indicating
that the 2D/2D Ti3C2/g-C3N4 heterojunction has the superior reusability and
durability.
Fig. 4 H2 production kinetic curves (a) and H2 production rate (b) over different
samples; wavelength dependent AQE (c) and cycle run kinetic curves (d) over
Ti3C2/g-C3N4-3 sample.
12

Fig. 5 Dynamic curves (a), plots of ln(C0/C) versus time (b), rate constants (c) for
degrading TC-HCl over different samples; absorbance variations of TC-HCl solution
over Ti3C2/g-C3N4-3 sample (d).
Furthermore, the photocatalytic activity of the fabricated 2D/2D Ti3C2/g-C3N4
heterojunction was also evaluated by degradation of antibiotic TC-HCl under the
visible light (λ ≥ 420 nm). As shown in Fig. 5a, the photocatalytic activity of single
Ti3C2 and g-C3N4 samples is comparatively low for removing TC-HCl, whereas that
of the fabricated 2D/2D Ti3C2/g-C3N4 heterojunctions is improved overtly. The
degradation activity of Ti3C2/g-C3N4 samples goes up and then down with increasing
the content of Ti3C2, and the highest degradation rate of TH-HCl over the
Ti3C2/g-C3N4-3 sample dramatically reaches up to 84.4% within 120 min which is far
higher than that of single g-C3N4 sample (29.0%). Besides, the pseudo-fist-order
model is used for the fitting of TC-HCl degradation reaction (Fig. 5b) [32], and the
obtained degradation rate constants are shown in Fig. 5c. We note that the largest rate
constant of degrading TC-HCl over the Ti3C2/g-C3N4-3 sample is 0.0141 min-1, which
runs up to about 5.04 times as high as than that of single g-C3N4 sample (0.0028
min-1). Moreover, Fig. 5d shows the absorbance variations of TC-HCl solution over
the Ti3C2/g-C3N4-3 sample during the degradation process, in which the characteristic
13

absorption peak of TC-HCl at 357 nm becomes weaker and weaker with prolonging
reaction times, which indicates that the TC-HCl molecule may be destructed to be
small molecules and ions [33].
Fig. 6 TOC removal rates of TC-HCl over g-C3N4 and Ti3C2/g-C3N4-3 samples (a),
cycle degradation kinetic curves (b) and cycle degradation rates (c) of TC-HCl over
Ti3C2/g-C3N4-3 sample; XRD patterns of Ti3C2/g-C3N4-3 sample before and after
cycle reaction (d)
In order to further confirm the outstanding photocatalytic degradation
performance of 2D/2D Ti3C2/g-C3N4 heterojunctions, the total organic carbon (TOC)
of TC-HCl solution was analyzed after performing photocatalytic reaction of 120 min
over the single g-C3N4 and Ti3C2/g-C3N4-3 samples. As shown in Fig. 6a, the TOC
removal rate of TC-HCl over the Ti3C2-g-C3N4-3 sample is 79.0%, which runs up to
about 3.48 times as high as that of the single g-C3N4 sample (22.7%), indicating that
the most of TC-HCl molecules are mineralized into small molecules/ions. Moreover,
the cycle degradation experiments of TC-HCl over the Ti3C2/g-C3N4-3 sample were
also performed. As can be seen from cycle degradation kinetic curves (Fig. 6b) and
cycle degradation rates (Fig. 6c) of TC-HCl, the Ti3C2/g-C3N4-3 sample can maintain
continuously stable running through five cycle experiments, and the slight reduction
14

of degradation rate may result from the loss of photocatalyst in the cycle reaction
process. In addition, the XRD pattern of the used Ti3C2/g-C3N4-3 sample has almost
no changes compared with that of the fresh one (Fig. 6d), further indicating the
fabricated 2D/2D Ti3C2/g-C3N4 heterojunction possesses the superior stability and
reusability.
Fig. 7 UV-vis DRS (a), plots of (ahv)2 versus hv (b), XPS valence band spectra (c)
and Mott-Schottky plots (d, e, f) of g-C3N4, Ti3C2 and Ti3C2/g-C3N4-3 samples.
The photocatalytic performance is closely associated with the light absorption
prosperity of photocatalysts which is usually investigated by UV-Vis DRS. As shown
in Fig. 7a, the Ti3C2 sample obviously displays the broad absorption feature in the
whole ultraviolet and visible regions, which has something to do with its dark color
and metal-like nature [20, 22]. In addition, the absorption edge of g-C3N4 sample
shows relative obvious red-shift and the light absorption over 500 nm is distinct
enhanced after forming 2D/2D Ti3C2/g-C3N4 heterojunction, which is directly
concerned with the modification effect of 2D Ti3C2 on the surface of 2D g-C3N4
nanosheets. The increased light absorption ability is more advantageous to the
photocatalytic H2 production and TC-HCl degradation reactions. Moreover, the band
gap energy of g-C3N4 sample before and after forming 2D/2D Ti3C2/g-C3N4
heterojunction are calculated by Tauc formula ahν=A(hν-Eg)n/2 [34], where v, a, h, A,
and Eg are light frequency, absorption coefficient, Planck constant, a constant and
15

band gap energy, respectively. It is well known that the value of n depends on the type
of optical transition (n=1 for direct transition and n= 4 for indirect transition) [34].
Therefore, on the basis of the direct band gap feature of g-C3N4 [34], the curves of
(αhν)2 versus hν are carried out (Fig. 7b). The band gap values of g-C3N4 are 2.90 eV
and 2.84 eV before and after modifying Ti3C2, respectively. In addition, the
conduction band (CB) and valence band (VB) positions of semiconductor can be
calculated by using empirical equations of EVB=X−Ee+0.5Eg and ECB=EVB-Eg, where
ECB, X, Ee, Eg and EVB are CB value, the geometric average of absolute
electronegativity of the constituent atoms (X=4.73 eV for g-C3N4), the energy of free
electrons on the hydrogen scale (4.5 eV), the band gap and VB value of
semiconductor, respectively [34]. Therefore, the values of CB and VB of original
g-C3N4 are -1.22 eV and 1.68 eV, as well as those of g-C3N4 modified with Ti3C2 are
-1.19 eV and 1.65 eV, respectively. The VB values are also identified via XPS valence
band spectra of g-C3N4 and Ti3C2/g-C3N4-3 samples (Fig. 7c). In addition, the flat
band potentials of Ti3C2, g-C3N4 and Ti3C2/g-C3N4-3 samples were also determined
by means of electrochemical measurements [20-35]. As the Mott-Schottky plots
shown in Fig. 7d-f, the Ti3C2, g-C3N4 and Ti3C2/g-C3N4-3 samples accord with the
characteristics of n-type semiconductors owing to the positive slopes, whose the flat
band potentials are measured to be -1.34 V, -0.94 V and -0.91 V, respectively.
The steady-state photoluminescence (SS-PL) spectra are usually applied to
investigate the separation degree of photogenerated electron-hole pairs [36, 37]. In
general, the weaker SS-PL intensity means the phocatalyst possesses less
recombination rate of charge carriers. As SS-PL spectra shown in Fig. 8a, compared
with single g-C3N4 sample, the Ti3C2/g-C3N4-3 sample exhibits the lower SS-PL
intensity, suggesting that the recombination rate of photogenerated electrons and holes
is reduced to promote interfacial charge transfer between 2D Ti3C2 and 2D g-C3N4
nanosheets. Besides, for the sake of exploring the dynamic electron migration process,
the transient-state photoluminescence (TS-PL) decay curves are measured [38]. As
displayed in Fig. 8b, the fluorescent lifetime (3.731 ns) of the Ti3C2/g-C3N4-3 sample
is much shorter than that of single g-C3N4 sample (5.408 ns), implying the additional
16

nonradioactive decay channel may be opened through the electron transfer from 2D
g-C3N4 to 2D Ti3C2 owing to the metal-like nature of Ti3C2, which can efficiently
accelerate the separation of photogenerated charge carriers [20-22]. Furthermore, the
migration and separation behaviors of charge carriers were also investigated by the
photoelectrochemical experiments [39, 40]. Fig. 8c exhibits the transient photocurrent
response curves of g-C3N4 and Ti3C2/g-C3N4-3 electrodes. It is obvious that both
Ti3C2/g-C3N4-3 and g-C3N4 electrodes produce relatively longstanding and stable
photocurrent responses under the visible light, and the photocurrent density of the
former is much larger than that of the latter, which indicates that the Ti3C2/g-C3N4-3
sample observably improves the transfer and separation efficiency of photogenerated
charge carriers. Meanwhile, the electrochemical impedance spectroscopy (EIS) is an
effective method to reveal the electron transfer efficiency at the electrode [41, 42]. It
is widely believed that the small EIS radius means the high mobility and separation of
charge carriers. In Fig. 8d, the Ti3C2/g-C3N4-3 electrode displays a smaller ESI radius
than the g-C3N4 electrode, demonstrating that the former possesses the lower interface
resistance and the higher transfer ability, in favor of the separation of charge carriers.
These results prove that 2D Ti3C2 in 2D/2D Ti3C2/g-C3N4 heterojunction serves as
electron harvester to realize the high-efficiency separation of photogenerated charge
carriers, thus enhancing the photocatalytic H2 production and degradation
performance. Besides, the specific surface areas and pore volumes of the
Ti3C2/g-C3N4-3 sample are 64.53 m2 g-1 and 0.1918 cm3 g-1, which all reach up to
about 1.5 times as high as 43.19 m2 g-1 and 0.1281 cm3 g-1 of the single g-C3N4,
respectively, thus also availing to enhance the photocatalytic performance (Fig. S2†).
17

Fig. 8 SS-PL spectra (a), TS-PL decay curves (b), transient photocurrent responses (c)
and Nyquist plots (d) of g-C3N4 and Ti3C2/g-C3N4-3 samples.
In order to clarify the photocatalytic degradation mechanism, the capture
experiments were performed to analyze the active species during the photocatalytic
degradation of THC-HCl. EDTA-2Na, VC and IPA are employed as the scavengers
-
for hole (h+), superoxide radical (·O2 ) and hydroxyl radical (·OH), respectively [43,
44]. As displayed in Fig. 9 (a-b), when EDTA-2Na and VC are added in the reaction
system, the degradation reactions of TC-HCl are strongly suppressed and the
degradation rates of TC-HCl are only 9.0% and 8.0% which is far less than that
-
without adding capture agents (84.4%). The results reveals that h+ and ·O2 play the
important roles in the degradation process. Furthermore, once IPA is used as the
capture reagent, the degradation rate of TC-HCl still reaches to 48.4%, indicating
that ·OH has the relative weak influence on the degradation of TC-HCl. Moreover, the
above results can be further investigated by electron spin resonance (ESR) technique,
-
which is used to detect ·O2 and ·OH generated during the degradation reaction
-
process [45, 46]. Fig. 9c displays the DMPO-·O2 ESR spectrum of Ti3C2/g-C3N4-3
sample, in which four sets of strong signals are produced when light on, suggesting
-
that ·O2 can be produced in large quantities during the degradation process. In
18

contrast, the DMPO-·OH ESR spectrum of Ti3C2/g-C3N4-3 sample (Fig. 9d) possess
four groups of weak peaks when light on, indicating that a small amount of ·OH is
produced in the photocatalytic degradation process. Grounded on the above results,
-
-
we conclude that ·O2 , h+ and ·OH all are the effective active species and ·O2 and h+
play the main roles for the photocatalytic degradation of TC-HCl.
Fig. 9 Kinetic curves (a) and degradation rates (b) of TC-HCl over Ti3C2/g-C3N4-3
sample with different scavengers, ESR spectra of DMPO-·O2- (c) and DMPO-·OH (d)
over g-C3N4 and Ti3C2/g-C3N4-3 samples
For probing into the degradation way of TC-HCl molecule, MS was used for
analyzing the generated intermediate products in the photocatalytic degradation
reaction [47, 48]. Fig. 10 shows the MS of TC-HCl solution over Ti3C2/g-C3N4-3
when the photocatalytic degradation reaction is performed for 0 min, 60 min and 120
min, respectively. According to the above MS analysis results, we present some
possible intermediates in the degradation of TC-HCl (Fig. 11). Firstly, TC-HCl in the
solution forms protonated tetracycline A (TC-H+, m/z=445) by losing Cl- ions, and
then tow unconjugated double bonds in rings are opened to produce B (m/z=481).
Subsequently, the methyl breaks away from the tertiary amino group of B to form C
+
(m/z=453). Then a series of continuous releasing -NH3 from C to D (m/z=453),
19

+
-COH and -OH from D to E (m/z=453), -CH3 from E to F (m/z=365) and -OH2 from
F to G (m/z=349) take place. Finally undergoing the ring-opening reaction from G to
H (m/z=305), losing –CH3CHO group from H to I (m/z=261) and the ring-opening
reaction from I to J (m/z=234), TC-HCl molecules are completely decomposed into
+
CO2, H2O, NH4 , etc.
Fig. 10. MS of TC-HCl at degradation time of 0 min (a), 60 min (b) and 120 min (c)
over Ti3C2/g-C3N4-3 sample.
Fig. 11. The possible intermediates and pathway of TC-HCl molecule at degradation
reaction process over Ti3C2/g-C3N4-3 sample.
20

The possibly transferred and separated behaviors of charge carriers and
photocatalytic reaction mechanism over the 2D/2D Ti3C2/g-C3N4 heterojunction are
put forward. According to the flat band potential analyses, the Fermi levels of the
g-C3N4 and Ti3C2 samples are measured to be -0.94 V and -1.34 V, respectively.
However, after 2D/2D Ti3C2/g-C3N4 heterojunction is fabricated by modifying 2D
Ti3C2 on the surface of 2D g-C3N4 nanosheets, the Fermi level changes to be -0.91 V
owing to the intense interface interactions. Meanwhile, on the basis of results of
UV-Vis DRS and XPS Valance spectra, the CB and VB positions of 2D g-C3N4
change from -1.22 eV and 1.68 eV to -1.19 eV and 1.65 eV, respectively. In
consequence, as illustrated in Fig. 12, once the 2D/2D Ti3C2/g-C3N4 heterojunction is
irradiated under the visible light, the photogenerated electrons and holes are produced
on CB and VB of 2D g-C3N4 through the band gap transition, respectively. Thereafter,
the photogenerated electrons are immediately collected by 2D Ti3C2 as electron
harvester and then directionally assemble on the boundary edges of 2D Ti3C2, which
certainly will accelerate transfer of charge carriers at the heterointerfaces between 2D
Ti3C2 and 2D g-C3N4 to increase the separation efficiency and effective numbers of
charge carriers dramatically, thus enhancing the photocatalytic performance. As far as
the photocatalytic H2 production reaction is concerned, the photogenerated electrons
directionally transfer to Pt nanoparticles deposited at the boundary edge of 2D Ti3C2,
and then reduce H+/H2O to produce H2. At the same time, the photogenerated holes
can be consumed by sacrificial agent TEOA. For the photocatalytic degradation
reaction of TC-HCl, the photogenerated electrons on the boundary edge of 2D Ti3C2
-
can reduce O2 molecules in solution to produce ·O2 because the Fermi level of 2D
-
Ti3C2 is more negative than the standard redox potential of O2/·O2 with -0.046 eV
-
[49], as well as a handful of generated ·O2 are further reduced to ·OH by reacting
with H+ because the holes cannot oxidize OH- owing to the higher VB position (1.65
eV) of 2D g-C3N4 than the standard redox potential of OH-/·OH (2.3 eV) [49]. Finally,
-
+
the OH, holes and O2 active species stepwise oxidize TC-HCl to produce NH4 , CO2,
H2O, etc.
21

Fig. 12 The transferred and separated behaviors of charge carriers and photocatalytic
reaction mechanism over the 2D/2D Ti3C2/g-C3N4 heterojunction.
4. Conclusions
To sum up, the boundary edge active sites are created on 2D Ti3C2 decorated on
2D g-C3N4 nanosheets by means of fabricating the 2D/2D Ti3C2/g-C3N4
heterojunctions in the semi-closed system. Ti3C2 not only extends the visible light
response range to increase the utilization rate of solar energy, but also can serve as
electron harvester to realize the directional transfer of photogenerated electrons from
2D g-C3N4 to 2D Ti3C2 and promote the high-efficiency separation of charge carriers,
thus dramatically boosting the photocatalytic performance. The optimal photocatalytic
H2 production rate reaches up to 26.7 μmol h-1 over the Ti3C2/g-C3N4-3 sample, which
is about 2.13 times as high as that of single g-C3N4 sample (12.5 μmol h-1).
Meanwhile, the TOC removal rate of TC-HCl over the Ti3C2-g/C3N4-3 sample is
79.0%, which runs up to about 3.48 times as high as that of single g-C3N4 sample
(22.7%), indicating that the most of TC-HCl molecules are mineralized into small
molecules/ions. Furthermore, the cycle experiments demonstrate that the
Ti3C2/g-C3N4-3 sample can maintain the stable H2 production rate and TC-HCl
degradation activity. All above results indicate the obtained 2D/2D Ti3C2/g-C3N4
heterojunctions have the superior activity, durability and reusability. This work affords
a typical exemplification for the fabrication and insight into photocatalytic reaction
mechanism of other new 2D/2D heterojunction materials.
22

Author Contributions Section
All authors contributed this work.
Acknowledgements
This work was supported by the NSFC-Shanxi Coal Based Low Carbon Joint
Fund (U1810117), National Natural Science Foundation of China (21606114,
21878131), Postdoctoral Science Foundation of China (2017M611712,
2017M611717), the Open Project Program of Key Laboratory of Preparation and
Application of Environmental Friendly Materials (Jilin Normal University)
(No.2019009), Scientific Research Foundation for Senior Talent of Jiangsu University
(17JDG020) and Young talent cultivate programme of Jiangsu University
(4111310017).
23

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27

Figure Captions
Fig. 1 XRD patterns of Ti3AlC2, Ti3C2, g-C3N4 and Ti3C2/g-C3N4-3 samples.
Fig. 2. SEM images of Ti3AlC2 (a), Ti3C2 (b), g-C3N4 (c) and Ti3C2/g-C3N4-3 (d)
samples; TEM images of g-C3N4 (e) and Ti3C2/g-C3N4-3 (f, g) samples; TEM (h) and
HRTEM (i) images of Pt deposited Ti3C2/g-C3N4-3 sample; HAADR image (j) and
EDS elemental mapping (k, l, j) of Ti3C2/g-C3N4-3 sample.
Fig. 3 XPS spectra of g-C3N4 and Ti3C2/g-C3N4-3 samples.
Fig. 4 H2 production kinetic curves (a) and H2 production rate (b) over different
samples; wavelength dependent AQE (c) and cycle run kinetic curves (d) over
Ti3C2/g-C3N4-3 sample.
Fig. 5 Dynamic curves (a), plots of ln(C0/C) versus time (b), rate constants (c) for
degrading TC-HCl over different samples; absorbance variations of TC-HCl solution
over Ti3C2/g-C3N4-3 sample (d).
Fig. 6 TOC removal rates of TC-HCl over g-C3N4 and Ti3C2/g-C3N4-3 samples (a),
cycle degradation kinetic curves (b) and cycle degradation rates (c) of TC-HCl over
Ti3C2/g-C3N4-3 sample; XRD patterns of Ti3C2/g-C3N4-3 sample before and after
cycle reaction (d)
Fig. 7 UV-vis DRS (a), plots of (ahv)2 versus hv (b), XPS valence band spectra (c)
and Mott-Schottky plots (d, e, f) of g-C3N4, Ti3C2 and Ti3C2/g-C3N4-3 samples.
Fig. 8 SS-PL spectra (a), TS-PL spectra (b), transient photocurrent responses (c) and
Nyquist plots (d) of g-C3N4 and Ti3C2/g-C3N4-3 samples.
Fig. 9 Kinetic curves (a) and degradation rates (b) of TC-HCl over Ti3C2/g-C3N4-3
sample with different scavengers, ESR spectra of DMPO-·O2- (c) and DMPO-·OH (d)
over g-C3N4 and Ti3C2/g-C3N4-3 samples
Fig. 10. MS of TC-HCl at degradation time of 0 min (a), 60 min (b) and 120 min (c)
over Ti3C2/g-C3N4-3 sample.
Fig. 11. The possible intermediates and pathway of TC-HCl molecule at degradation
reaction process over Ti3C2/g-C3N4-3 sample.
Fig. 12 The transferred and separated behaviors of charge carriers and photocatalytic
28

reaction mechanism over the 2D/2D Ti3C2/g-C3N4 heterojunction.
Fig. 1 XRD patterns of Ti3AlC2, Ti3C2, g-C3N4 and Ti3C2/g-C3N4-3 samples.
Fig. 2. SEM images of Ti3AlC2 (a), Ti3C2 (b), g-C3N4 (c) and Ti3C2/g-C3N4-3 (d)
samples; TEM images of g-C3N4 (e) and Ti3C2/g-C3N4-3 (f, g) samples; TEM (h) and
HRTEM (i) images of Pt deposited Ti3C2/g-C3N4-3 sample; HAADR image (j) and
29