Synthesis and photocatalytic properties of visible-light-responsive, three-dimensional, flower-like La-TiO2/g-C3N4 heterojunction composites

Article abstract of DOI:10.1039/c8ra06466k

We prepared a new three-dimensional, flower-like La-TiO₂/g-C₃N₄ (LaTiCN) heterojunction photocatalyst using a solvothermal method. Analysis and characterization were performed by conducting scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform-infrared spectroscopy, ultraviolet-visible spectrophotometry, and nitrogen adsorption and desorption. The prepared g-C₃N₄ nanosheets could reach 100 nm in size and covered the TiO₂ surface. A tightly bound interface formed between the g-C₃N₄ and TiO₂, speeding up the effective transfer of photo-induced electrons. In addition, the incorporation of La³⁺ reduced the electron-hole recombination efficiency. Consequently, the prepared La-TiO₂/g-C₃N₄ composite material exhibited better visible-light catalytic activity than pure TiO₂.

Full text of DOI:10.1039/c8ra06466k

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PAPER
Synthesis and photocatalytic properties of visible-
light-responsive, three-dimensional, flower-like
La–TiO2/g-C3N4 heterojunction composites
Cite this: RSC Adv., 2018, 8, 29645
a
a
a
*
*
Jinlong Li,
Lijuan Du, Shuaiqiang Jia, Guozhe Sui, a Yulin Zhang,a Yan Zhuang,a
a
b
*
Boxin Li and Zhiyong Xing
We prepared a new three-dimensional, flower-like La–TiO2/g-C3N4 (LaTiCN) heterojunction photocatalyst
using a solvothermal method. Analysis and characterization were performed by conducting scanning
electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron
spectroscopy, Fourier transform-infrared spectroscopy, ultraviolet-visible spectrophotometry, and
nitrogen adsorption and desorption. The prepared g-C3N4 nanosheets could reach 100 nm in size and
covered the TiO2 surface. A tightly bound interface formed between the g-C3N4 and TiO2, speeding up
the effective transfer of photo-induced electrons. In addition, the incorporation of La3+ reduced the
electron–hole recombination efficiency. Consequently, the prepared La–TiO2/g-C3N4 composite
material exhibited better visible-light catalytic activity than pure TiO2.
Received 1st August 2018
Accepted 14th August 2018
DOI: 10.1039/c8ra06466k
rsc.li/rsc-advances
a uniform monodisperse relationship, but the particle size is
within the micron range. Through the high-temperature calci-
1. Introduction
nation of a TiO2 hollow sphere with a particle size of 50–100 nm
and urea, Zou et al.15 obtained a TiO2/g-C3N4 composite mate-
rial. However, TiO2 microspherical particles can easily disperse
in g-C3N4 particles, mainly because it is difficult to control the
bulk-g-C3N4 coating on the surfaces of nanosized TiO2 particles.
To control this coating effectively and obtain a well-dispersed
TiO2/g-C3N4 composite material, the key is to prepare nano-
sized g-C3N4 lamellae.
Adding metal ions can enable catalytic materials to absorb
visible light more effectively.16–18 It has been found that La is
rich in electronic energy levels and that the introduction of
electronic energy levels can reduce the energy gaps of catalytic
materials and further increase the visible light absorption.19–21
Rong et al.22 reported that a g-C3N4 photocatalyst doped with La
exhibited higher photocatalytic activity in the degradation of
organic pollutants. Khalid et al.23 reported that a TiO2-graphene
composite material doped with La resulted in remarkably
slower electron–hole pair recombination than pure TiO2.
The main purposes of this study were to prepare a nanosized
TiO2/g-C3N4 composite material and further improve the pho-
tocatalytic activity of the TiO2/g-C3N4 composite material by
incorporating La3+. First, we prepared a g-C3N4 nanosheet with
plenty of nanopores via thermal spalling. Then, we achieved
molecular self-assembly by employing a one-step solvothermal
method and successfully prepared a La (x%)–TiO2/g-C3N4
Photocatalytic degradation of organic pollutants and hydrogen
production from water decomposition can improve the utiliza-
tion of solar energy. Therefore, photocatalysis is considered to
be an ideal green technology for solving the energy crisis and
reducing environmental pollution.1,2 In recent decades, TiO2
and g-C3N4 have been widely used in photocatalysis due to their
non-toxicity, chemical stability and photocatalytic activity.3–5
The close interface formed between the components of
a composite semiconductor material can effectively promote
the transport of photogenerated carriers, increasing the pho-
tocatalytic activity relative to those of pure semiconductors.6–8
Therefore, the development of novel TiO2/g-C3N4 hetero-
junction photocatalysts with excellent catalytic performance
has become a topic of signicant research interest.
Currently, most researchers use high-temperature calcina-
tion methods9–11 or solvothermal methods12–14 to recombine
TiO2 and g-C3N4. For instance, Chen et al.10 heated modied
TiO2 particles and protonated g-C3N4 particles at 70 ꢀC to
prepare a TiO2/Ag/g-C3N4 composite microsphere with a particle
size of 2–3 mm. Li et al.9 prepared a TiO2/g-C3N4 composite
hollow sphere via a solvothermal method. In these TiO2/g-C3N4
composite materials, g-C3N4 is always uniformly attached to the
TiO2 particle surface and the composite particles exhibit
aCollege of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar 161006,
China. E-mail: jinlong141@163.com; suiguozhe@163.com; Fax: +86-452-2738205;
Tel: +86-452-2738205
composite material with
a three-dimensional, ower-like
structure. During the molecular self-assembly process, the
formed TiO2 crystals acted as scissors, and the g-C3N4 nano-
sheet was cut into increasingly smaller slices. Actually, the
bCollege of Science, Northeast Agricultural University, Harbin, 150030, China. E-mail:
zyxing@neau.edu.cn; Fax: +86-451-55191810; Tel: +86-451-55191810
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g-C3N4 can also loaded on many other semiconductors with equilibrium prior to irradiation. The samples were taken from
matched energy band structure to construct high-efficient the reactor at regular time intervals during irradiation and
heterojunctions, and thus providing a new idea for fabricating centrifuged using
a high-speed centrifuge (RG-TGL-16C,
g-C3N4 based composite photocatalytic materials.
Ruijiang Co.) to remove the photocatalyst. The samples were
collected by centrifugation every 15 min to measure the RhB
degradation by performing ultraviolet-visible (UV-vis) spectro-
photometry, using the absorbance at 552.0 nm as the repre-
sentative peak to determine the RhB concentration.
2. Experimental
2.1 Materials and chemicals
Melamine (C3H6N6) was used to synthesise the g-C3N4 nano-
sheets. Tetrabutyl titanate (Ti(OC4H9)4) was utilized as the
titanium precursor in the synthesis of nanocrystalline TiO2.
Lanthanum nitrate (La(NO)3$6H2O) was employed as the metal
precursor for doping TiO2. These chemicals, together
with hydrouoric acid (HF, 40 wt%), ethanol (CH3CH2OH),
sodium bicarbonate (NaHCO3), tert-butyl alcohol (C4H10O),
r-benzoquinone (C6H4O2), and RhB (C28H31N2O3Cl), were of
analytical grade and purchased from Sigma Aldrich Co. All of
the reagents were of analytical grade and were used as obtained
without further purication. Distilled water was employed in all
of the experiments requiring water.
3. Results and discussion
3.1 Characterisation of synthesized catalysts
Fig. 1 shows the X-ray diffraction (XRD) patterns of TiO2, g-C3N4,
TiO2/g-C3N4 (TiCN), and LaxTiCN. Evidently, the TiO2 in all of
the samples was of the anatase phase. The diffraction peaks at
25.3ꢀ, 38.0ꢀ, 48.0ꢀ, 54.1ꢀ, 62.6ꢀ, 69.6ꢀ, and 75.2ꢀ respectively
correspond to the (101), (004), (200), (211), (204), (220), and
(215) crystal faces of anatase (JCPDS-21-1272). Meanwhile, the
peaks at 13.2ꢀ and 27.7ꢀ respectively correspond to the (100) and
(002) crystal faces of g-C3N4 (JCPDS no. 87-1526). In the
composite samples of TiO2/g-C3N4 and LaxTiCN, the charac-
teristic diffraction peaks of TiO2 and g-C3N4 are observable,
indicating that TiO2 and g-C3N4 constituted a composite
structure.24 In addition, as compared with TiO2/g-C3N4, the
diffraction peaks of La (x%)–TiO2/g-C3N4 composite samples are
not shied higher or lower, indicating that La3+ did not enter
the TiO2 lattice, because the ionic radius of La3+ (0.115 nm) is
much larger than that of Ti4+ (0.064 nm), and La3+ cannot
replace Ti4+ in a TiO2 lattice and form lattice distortion,25 which
means that La3+ ions may exist in the form of La2O3 or LaF3.
However, the relevant information about La2O3 and LaF3 is not
observable, because the contents of La2O3 and LaF3 were rela-
tively small. The lattice parameters of the samples were calcu-
lated from the XRD results, which are summarized in Table 1.
Compared with the TiCN, the lattice parameters of La1TiCN
were almost unchanged (a little decrease), which is consistent
with the above results.
2.2 g-C3N4 nanosheet preparation
Yellow bulk g-C3N4 was obtained through the calcination of
5.0 g of melamine in a muffle furnace at a heating rate of
2
ꢀC minꢁ1 at 550 ꢀC for 4 h. Aer that, which was further
oxidized and peeled off. The obtained light yellow powder was
tiled in aꢁs1quare porcelain boat and calcined at a heating rate of
5 ꢀC min at 530 ꢀC for 2 h to obtain a white g-C3N4 nanosheet.
2.3 La–TiO2/g-C3N4 hybrid preparation
First, 0.6 g of g-C3N4 nanosheet powder was added to 30 mL of
anhydrous ethanol and ultrasonically dispersed for 2 h. Then,
3.0 mL of Ti(OC4H9)4 was added to 20 mL of absolute ethanol to
obtain a pale yellow transparent solution. La(NO)3$6H2O was
added stoichiometrically to dissolve the mixture completely into
a transparent mixed solution and stirred for 30 min. The g-C3N4
nanosheet dispersion was added to the Ti(OC4H9)4 mixed solu-
tion and stirred for 30 min. Next, 1.5 mL of HF was pipetted
dropwise into the resulting mixed solution and stirred for 15 min.
The solution was then transferred to a 100 mL Teon-lined high-
value kettle and maintained at 160 ꢀC for 20 h. It was subse-
quently cooled to room temperature, and the powder was washed
several times with absolute ethanol and deionized water. The
To demonstrate the formation of the LaxTiCN sample het-
erostructure, scanning electron microscopy (SEM) and trans-
mission electron microscopy (TEM) analyses were conducted.
ꢀ
resulting sample was placed in a vacuum oven at 60 C for 4 h.
The atomic ratios of La to TiO2 were 0.5%, 1%, and 2%. For
comparison, the same method was used to prepare pure TiO2.
2.4 Photocatalytic activity evaluation
The photocatalytic activity of La (x%)–TiO2/g-C3N4 (LaxTiCN) in
the degradation of RhB in aqueous solution was tested under
visible light irradiation. A 500 W Xe lamp was employed as the
light source, and the visible wavelength was adjusted using
a 420 nm lter. For the experiments, 50 mg of LaxTiCN and
50 mL of RhB solution (5 mg Lꢁ1) were added to the tube
reactor. The suspension was magnetically stirred in the dark for
Fig. 1 XRD patterns of TiO2, g-C3N4, TiCN, and LaxTiCN composites
30 min prior to each assay so that it reached adsorption with various La contents.
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Table 1 The lattice parameters and the crystal size of the samples
Samples
a ¼ b (nm)
c (nm)
Crystal size (nm)
g-C3N4
TiO2
TiCN
0.6412
0.3786
0.3796
0.3782
0.2448
0.9526
0.9512
0.9508
6.72
8.57
8.52
7.66
La1TiCN
Fig. 2a is the SEM diagram of pure TiO2. The prepared TiO2
exhibits a three-dimensional, ower-like structure, which was
formed by the staggered accumulation of 500 nm lamellae.
Fig. 2b presents the TEM and SEM diagrams of g-C3N4 that were
obtained aer secondary thermal spalling, displaying a struc-
ture with numerous pores. The size of the observed g-C3N4
nanosheet is about 100 nm, enabling the g-C3N4 to cover the
TiO2 lamellae. Fig. 2c and d are SEM diagrams of the La1TiCN
composite material prepared by using the one-step sol-
vothermal method. All of the sample particles exhibit three-
dimensional, ower-like structures of the same size.
Compared with pure TiO2, the lamella surface of the ower-like
structure of the La1TiCN is coarser and the accumulation
between lamellae is denser, which is probably due to the exis-
tence of the g-C3N4 nanosheet, which provided more attach-
ment points for the formation of TiO2 nanoparticles and
enhanced the aggregation effect of the TiO2 nanoparticles
during self-assembly. Fig. 2f is a TEM diagram of the La1TiCN
composite material, from which it is clear that g-C3N4 nano-
sheet covers the surface of the TiO2 lamellae. The element
mapping in Fig. 3 further proves the existence of elemental Ti,
O, C, N, and La. It is evident that the Ti and O distribution
diagrams can outline the basic shape of the three-dimensional,
Fig. 3 (a) TEM image of the La1TiCN sample. Corresponding energy-
dispersive X-ray spectroscopy elemental mapping images of (b) Ti, (c)
O, (d) C, (e) N, (f) La, and (g) EDX plots.
ower-like structure; the distributions of C and N are basically
consistent with the material structure in Fig. 3d and e; and La is
sparsely and evenly dispersed over the particle surface. The EDX
spectrum conrms the presence of C, N, O, Ti, and La elements
in La1TiCN and the percentage of La is 1 wt% in Fig. 3g.
The amount of g-C3N4 in the sample was determined by TG
analysis. Fig. 4 shows a comparison of TG curves for g-C3N4,
TiCN, and La1TiCN samples. Pure g-C3N4 has a large weight loss
in the range of 500–700 ꢀC. While TiCN and La1TiCN lose weight
rapidly in the same temperature range, which can be attributed
to the combustion of g-C3N4. Therefore, the actual amount of
g-C3N4 in the TiCN and La1TiCN4 samples were about
12.77 wt% estimating from the weight loss.
To determine the structural parameters of the synthesized
samples, the N2 adsorption–desorption isotherms were
measured in Fig. 5. The specic surface area of pure g-C3N4 is
126.5 m2 gꢁ1, which mainly depends on the porous structure of
g-C3N4. Compared with pure TiO2 (46.53 m2 gꢁ1), the composite
TiCN (62.75 m2 gꢁ1), which is mainly produced by the
Fig. 2 SEM images of (a) TiO2, (b) the g-C3N4 nanosheet, and (c) and
(d) La1TiCN. (e) and (f) TEM images of La1TiCN.
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Fig. 4 TGA curves of g-C3N4, TiCN, and LaTiCN.
deposition of the porous g-C3N4 structure onto the TiO2 surface,
provides a larger surface area. The surface area of La1TiCN is
72.77 m2 gꢁ1, indicating that La3+ can increase the surface area
of the catalyst and provide more reactive centres. Fig. 4b pres-
ents the pore size distribution curves of the samples. The pure
g-C3N4 pore size is 3.4 nm, while the TiO2 pore size is 5–12 nm.
The main pore size of t La1TiCN presents a layered void
composed of 3.8 nm (small pores) and 7.0 nm (larger meso-
pores). The existence of the layered void structure is benecial
to multi-light scattering/reection, which greatly enhances the
excitation.26 In addition, a hierarchical porosity composed of
mesopores facilitates fast mass transport, resulting in good
electrode performance.27 The similar effects from mesopores
and a hierarchical porosity of the as-prepared LaxTiCN increase Fig. 5 (a) N2 adsorption–desorption isotherms of TiO2, g-C3N4, and
La1TiCN composites with various La contents and (b) pore size distri-
butions of the corresponding samples.
the photocatalytic activity.
The compositions and combination modes of the TiO2, g-
C3N4, TiCN, and La1TiCN samples were analysed by performing
Fourier transform-infrared (FT-IR) spectroscopy. The results are
shown in Fig. 6. For pure TiO2, a strong absorption peak is
evident at 500–700 cmꢁ1, corresponding to the stretching
vibrations of the Ti–O and Ti–O–Ti bands, and the two strong
peaks at 3350–3550 cmꢁ1 and 1645 cmꢁ1 are attributable to the
water and hydroxy absorbed by the TiO2 surface. In the pure g-
C3N4, the absorption peaks at 1239 cmꢁ1
, ,
1317 cmꢁ1
1407 cmꢁ1, and 1571 cmꢁ1 are attributable to the stretching
vibrations of the C–N hybridized by sp3; the peak at 1637 cmꢁ1
is attributable to the stretching vibrations of C]N hybridized
by sp2; and the peak at 808 cmꢁ1 corresponds to the out-of-
plane bending vibrations of the 3-S-triazine units. For the
TiCN and LaxTiCN composite samples, the main characteristic
absorption peaks of TiO2 and g-C3N4 are observable, indicating
that the composite heterojunction structure of g-C3N4/TiO2
formed.28 This nding is consistent with the X-ray diffraction
(XRD) and X-ray photoelectron spectroscopy (XPS) results.
Notably, compared with the patterns of the characteristic peaks
of the pure g-C3N4 nanosheet, those of the g-C3N4 in the
LaxTiCN composite sample are sharper, because the LaxTiCN
composite sample had smaller g-C3N4 lamellae than the pure
Fig. 6 FT-IR spectroscopy patterns of TiO2, g-C3N4, TiCN, and Lax-
TiCN composites with various La contents.
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g-C3N4 nanosheet, causing the atoms in the g-C3N4 neigh- ower-like structure in a hydrotherm through self-assembly.
bouring 3-S-triazine ring to separate from each other and During the formation of the samples, the generation of small
weakening the interactions among the atoms. Therefore, the g-C3N4 through cutting during the formation of the TiO2 crystal
atoms in g-C3N4 in the LaxTiCN samples were under less con- and through the formation of smaller g-C3N4 particles was the
straining force, so the vibrations were stronger.29
key to the formation of the heterojunction structure.30
Based on the analysis of the above experimental results, the
The chemical compositions of the prepared LaxTiCN
formation mechanism of the LaxTiCN heterojunction catalyst samples and the chemical states of all of the elements were
was determined and is depicted in Fig. 7. First, the Ti precursor analysed by performing XPS. Fig. 8a shows the full XPS results
micromolecule inltrated the gaps among the g-C3N4 lamellae. for the La1TiCN sample, where the spectral peaks of C 1s, N 1s,
Then, the TiO2 crystal grew on the lamella surface of g-C3N4 and Ti 2p, O 1s, and La 3d are easily observable, indicating that the
formed a sheet structure. The g-C3N4 was divided into increas- sample was mainly composed of C, N, Ti, O, and La. Notably,
ingly smaller lamellae, and La3+ was deposited on the crystal a weak F 1s peak is evident at 684.3 eV, which is probably
surface. Finally, the obtained composite lamellae formed a La attributable to the uorination caused by the introduction of
(x%)–TiCN heterojunction catalyst with a three-dimensional, HF into the preparation process.31
Fig. 7 Schematics illustrating LaxTiCN formation.
Fig. 8 (a) Overview of the XPS results and individual high-resolution XPS results for (b) Ti 2p, (c) O 1s, (d) C 1s, (e) N 1s, and (f) La 3d of as-prepared
La1TiCN.
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Fig. 8b presents the XPS results for Ti 2p, exhibiting two
peaks at 459.4 eV and 465.1 eV, which respectively correspond
to the Ti 2p3/2 and Ti 2p1/2 orbitals. Fig. 8c shows the XPS
results for O 1s. The peak at 530.3 eV corresponds to Ti–O in
the TiO2 lattice, and that at 532.1 eV is attributable to the
hydroxy or adsorbed water molecules on the sample surface.
Fig. 8d presents the XPS results for C 1s, where there are two
characteristic peaks at 284.8 eV and 288.3 eV, which are
attributable to the C–C bands hybridized by sp3 in g-C3N4 and
the N–C]N bands hybridized by sp2, respectively. Three peaks
can be tted from the XPS results for N 1s in Fig. 8e: C]N–C at
398.8 eV, N–(C)3 at 399.6 eV, and C–N–H at 401.1 eV. In
addition, no signicant Ti–C (C 1s 283.0 eV)32 or Ti–N (N 1s
399.3 eV)33,34 signal is observable, and the binding energy of Ti
2p3/2 does not shi lower,34 indicating that atomic C and N
were not incorporated into the TiO2 lattice. An additional
explanation is that the g-C3N4 particles in the compound were
only deposited on the surface of the TiO2, forming a hetero-
junction structure.
Fig. 8f presents the XPS results for La 3d, showing peaks at
837.1 eV and 854.3 eV, which respectively correspond to La 3d5/2
and La 3d3/2, as well as satellite peaks at 841.5 eV and 857.6 eV,
which were respectively split by La 3d5/2 and La 3d3/2. According
to the literature, the characteristic peaks of La2O3 are located at
834.9 eV and 851.8 eV,35 indicating that La3+ in the LaxTiCN
sample and elemental O with stronger electronegativity formed
Ti–O–La, and the binding energy shied higher.36 These nd-
ings agree with the XRD results.
UV-vis diffuse reectance spectrometry was then conducted
to measure the absorbances of the samples. Fig. 9a shows the
UV-vis diffuse reection spectra of the g-C3N4, TiO2, TiCN, and
LaxTiCN composite catalysts. The absorption edge wavelengths
of the pure TiO2 sample and g-C3N4 are about 390 nm and
460 nm, respectively. Compared with the absorption edge of the
pure TiO2 particles, that of the TiCN composite sample is
shied towards longer wavelengths and displays strong
absorption in the 380–430 nm range, because the interface
matching effect between TiO2 and g-C3N4 changed the optical
properties of TiO2.37 When doped with La3+, La0.5TiCN and
La1TiCN display stronger visible light absorption in the 380–
450 nm range. La1TiCN exhibits the best visible light absorp-
tion, while that of La0.5TiCN is slightly weaker, probably
because the La3+ concentration was too low and could not
provide more electronic row traps.38 Meanwhile, for the La2-
TiCN sample, the UV and visible light absorption are both
signicantly lowered, because an excessively high La3+ concen-
tration can reduce the light transmittance of the catalysts and
light absorption efficiency.39
The band-gap energies of the samples were estimated by
performing Kubelka–Munk conversion, and the results are
shown in Fig. 8b. The band-gap energies of pure TiO2, g-C3N4,
TiCN, and La1TiCN are respectively 3.00 eV, 2.50 eV, 2.78 eV,
and 2.45 eV. The La1TiCN sample has a lower band-gap
energy, probably because the introduction of La3+ led to the
formation of a new impurity energy level in the TiO2 band
gap, narrowing its band gap and extending its light absorp-
tion range.40
Fig. 9 (a) UV-vis diffuse reflection spectra and (b) plots of (ahv)2 versus
energy hv for the catalysts.
3.2 Photocatalytic activity and photodegradation
mechanism
Under visible light irradiation, the photocatalytic efficiencies of
the various samples were determined based on the RhB
degradation, with the results shown in Fig. 10a. The photo-
catalytic activities of the various samples were further compared
by using the equation ln(C0/C) ¼ kt, where C0 is the initial RhB
concentration, C is the instantaneous RhB concentration at
time t, and k is the kinetic constant.
The rst-order kinetic model of RhB degradation was ob-
tained, with the results presented in Fig. 10b and the calculated
k values given in Fig. 10c. Compared with the photocatalytic
efficiencies of pure TiO2 and g-C3N4, those of all of the LaxTiCN
composite samples are increasingly enhanced, mainly because
the large and dense hetero interface formed between TiO2 and
g-C3N4 could speed up the efficient electron–hole shiing. In
addition, along with continuously increasing La3+, the photo-
catalytic efficiencies of the La0.5TiCN and La1TiCN samples are
increasingly enhanced. The photocatalytic efficiency of La1TiCN
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Fig. 10 (a) Photocatalytic degradation of RhB under visible light irradiation, (b) linear transforms – ln(C/C0) of the kinetic curves of RhB
degradation, (c) apparent pseudo-first-order rate constant kapp with different catalysts, (d) recycling of the La1TiCN in the removal of RhB dyes,
and (e) scavenger test results obtained using La1TiCN.
is 95.2%, and the kinetic constant is 0.0325 minꢁ1, which is 2.1 band gap, c is the absolute electronegativity of the semi-
and 10 times those of pure TiO2 and g-C3N4, respectively, and conductor, and Ee is the energy of free electrons based on the
1.63 times that of LaxTiCN. Incorporating La3+ can rstly hydrogen scale and has a value of 4.50 eV.41 For TiO2 and
provide more electron traps and improve the electron–hole g-C3N4, c is 5.81 and 4.72, respectively.
separation efficiency in La1TiCN. In addition, incorporating
The VB and CB edge electric potentials of TiO2 and g-C3N4
La3+ can increase the specic surface area of TiCN, with the were calculated. The VB and CB edge potentials of TiO2 were
strongest photocatalytic effect. Notably, the photocatalytic effect determined to be +2.83 eV and ꢁ0.17 eV, respectively, and those
of La2TiCN is reduced, because excessive La3+ can serve as the of g-C3N4 were found to be +1.47 eV and ꢁ1.03 eV, respectively.
recombination centre of electron–holes, reducing the photo-
Under visible light irradiation, the electrons from the VB of
catalytic activity of La2TiCN. For evaluating the recyclability of TiO2 and g-C3N4 were respectively excitated into the CB and
the ower-like LaxTiCN heterojunction composites, cycling le positively charged holes in the VB. Since the CB of g-C3N4
experiments were performed by washing the composite with (ꢁ1.03 eV) has a potential lower than that of TiO2 (ꢁ0.17 eV), the
ethanol for several times. As shown in Fig. 10d, the photo- excitated electrons on the g-C3N4 surface can easily shi to the
catalytic efficiency of RhB decreases from 95.2 to 88.3% for TiO2 surface through the hetero interface.42 Finally, these
La1TiCN aer ve cycles. This fact demonstrates that the
obtained ower-like LaxTiCN heterojunction composites have
good stability under visible-light irradiation and are less photo-
corroded during the photocatalytic oxidation, which can be
applied as a new option for dye wastewater treatment.
To determine the reactive species during a light-catalysed
reaction, it is possible to eliminate holes h+, superoxide radi-
cals $O2ꢁ, and hydroxyl radicals $OH by adding NaHCO3, BQ,
and TBA. The obtained results are shown in Fig. 9d. It is evident
that adding NaHCO3 and BQ signicantly inhibited the light
degradation of RhB in the La1TiCN sample, indicating that
under visible light irradiation, O2ꢁ and h+ are the main active
substances in RhB photolysis. The equations EVB ¼ c ꢁ Ee +
0.5BG and ECB ¼ EVB ꢁ BG were then employed to calculate the
Fig. 11 Proposed visible light photodegradation mechanism of the
VB and CB potentials, EVB and ECB, respectively, where BG is the La1TiCN hybrid photocatalyst.
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electrons can be captured by the La3+ with free orbitals.43
The electrons accumulated on the La3+ surface can easily
be captured by the O2 dissolved in solution to generate super-
oxide $O2ꢁ, and the generated $O2ꢁ can subsequently be used to
degrade RhB. Similarly, the holes in the VB of TiO2 can easily be
transferred to the VB of g-C3N4. Compared with $OH/OHꢁ
(1.99 eV vs. NHE) and $OH/H2O (2.68 eV vs. NHE),11 the reduc-
tion potential of the VB of g-C3N4 is even lower (1.65 eV vs.
NHE). Therefore, the holes in the VB of g-C3N4 can be used to
degrade RhB directly. Fig. 11 illustrates the LaxTiCN hetero-
junction degradation mechanism. The close interface formed
by TiO2 and g-C3N4 is a green channel for electron passage,
which is benecial to increase the transfer rate of g-C3N4
photoelectrons. The proper amount of La is introduced, the
interface barrier of La and TiO2 becomes higher and the space
charge region is narrowed in the system. The generated large
electric eld promotes the light-induced electron orientation of
g-C3N4 to the interface between La and TiO2, and the electron–
hole recombination is effectively prevented in this process.
Thus, during the photocatalytic reaction, h+ and $O2ꢁ are the
most important active species, which is consistent with the
results of the active component trapping experiment. In addi-
tion, the hetero interface formed between TiO2 and g-C3N4 can
facilitate charge transfer among the particles, and the incor-
poration of La3+ can help enhance the electron–hole pair
separation efficiency in such a three-way catalytic system and
further promote the photocatalytic efficiency.
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LaxTiCN heterojunction composites were fabricated success-
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