Ultrathin 2D/2D Ti3C2Tx/semiconductor dual-functional photocatalysts for simultaneous imine production and H2evolution

Article abstract of DOI:10.1039/d1ta03573h

Ultrathin 2D/2D Ti₃C₂T_x/semiconductor (CdS and Bi₂MoO₆) dual-functional photocatalysts have been constructed for the oxidative coupling of benzylamines to imines combined with H₂generation under visible light irradiation (λ≥420 nm). The optimal 2D/2D Ti₃C₂T_x/CdS sample exhibits high photocatalytic performance toward H₂evolution (219.7 μmol g^?1h^?1) and imine production (155.8 μmol g^?1h^?1), which is 5 times and 6 times higher than that of pure CdS, respectively.In situirradiated XPS and photoelectrochemical characterizations reveal that the enhanced photoactivity over Ti₃C₂T_x/semiconductor heterostructures can be attributed to the facilitated charge separation from the semiconductors to the Ti₃C₂T_xcocatalyst. A possible reaction mechanism is proposed based onin situFTIR spectroscopy of benzylamine adsorption and imine product desorption and reaction intermediate detection usingin situESR. This work provides a systematic strategy to construct ultrathin 2D/2D Ti₃C₂T_x/semiconductor heterojunctions for photocatalytic synthesis of high value-added products combined with H₂generation.

Full text of DOI:10.1039/d1ta03573h

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Ultrathin 2D/2D Ti3C2Tx/semiconductor dual-
functional photocatalysts for simultaneous imine
Cite this: DOI: 10.1039/d1ta03573h
production and H2 evolution†
c
c
Hao Wang,a Peng Hu,a Jie Zhou,a Maarten B. J. Roeffaers,
Bo Weng,
*
a
ab
*
*
Yongqing Wang
and Hongbing Ji
Ultrathin 2D/2D Ti3C2Tx/semiconductor (CdS and Bi2MoO6) dual-functional photocatalysts have been
constructed for the oxidative coupling of benzylamines to imines combined with H2 generation under
visible light irradiation (l $420 nm). The optimal 2D/2D Ti3C2Tx/CdS sample exhibits high photocatalytic
performance toward H2 evolution (219.7 mmol gꢀ1
h h
ꢀ1) and imine production (155.8 mmol gꢀ1 ꢀ1), which
is times and times higher than that of pure CdS, respectively. In situ irradiated XPS and
5
6
photoelectrochemical characterizations reveal that the enhanced photoactivity over Ti3C2Tx/
semiconductor heterostructures can be attributed to the facilitated charge separation from the
semiconductors to the Ti3C2Tx cocatalyst. A possible reaction mechanism is proposed based on in situ
FTIR spectroscopy of benzylamine adsorption and imine product desorption and reaction intermediate
detection using in situ ESR. This work provides a systematic strategy to construct ultrathin 2D/2D
Ti3C2Tx/semiconductor heterojunctions for photocatalytic synthesis of high value-added products
combined with H2 generation.
Received 28th April 2021
Accepted 13th July 2021
DOI: 10.1039/d1ta03573h
rsc.li/materials-a
photocatalysts merge the selective amine oxidation to produce
imines with proton photoreduction to generate H2 in a closed
Introduction
redox cycle to simultaneously achieve two value-added prod-
ucts. However, photocatalysts capable of performing these two
coupled reactions are still limited.13–15
Imines, as biologically active nitrogen-containing organic
compounds, are essential building blocks for the synthesis of
various ne chemicals and pharmaceuticals.1–3 Generally, the
formation of imines requires harsh reaction conditions, such as
high temperature or high oxygen pressure, thus leading to
operational complexity and signicant energy consumption.4–6
An alternative strategy to produce imines is using semi-
conductor photocatalysts such as TiO2, CdS, BiOCl and atmo-
spheric pressure dioxygen as the oxidant for the direct oxidation
of amines.7–9 Unfortunately, dioxygen derived radicals (e.g.,
superoxide radicals) with strong oxidation ability lead to poor
product selectivity and hamper reactivity control.10 Moreover, in
the presence of dioxygen, the protons dehydrogenated from the
amine coupling reaction typically go to waste due to the
formation of water. Converting these protons to produce H2 is
a valuable approach to boost the overall reaction from an
economic perspective.11,12 Ideally, efficient dual-functional
Ultrathin two-dimensional (2D) semiconductors with thick-
ness below 5 nm have attracted a great deal of research interest
in photocatalytic research due to their unique and excellent
physicochemical properties.16–19 For instance, the typical visible-
light-driven semiconductors, e.g., CdS and Bi2MoO6, with an
ultrathin 2D structure exhibit higher photocatalytic efficiency
than their bulk counterparts due to the facilitated charge
separation efficiency and enhanced reactant adsorption
capacity.20–23 Despite these advantages, single ultrathin 2D
semiconductor catalysts still suffer from insufficient redox
potentials, unavoidable charge carrier recombination and
limited reactant adsorption capability.24–26 To address these
issues, hybridizing cocatalysts with 2D semiconductors is
believed to efficiently promote the transfer of charge carriers
and then increase the photocatalytic activity. In particular, 2D/
2D nanostructures with face-to-face contact possess larger
contact area than the 0D/2D, 1D/2D hybrids with only point-to-
point or line-to-face contact.27,28 This leads to effective charge
separation due to large intimate contact interfaces, and then
outstanding photocatalytic performance.
aFine Chemical Industry Research Institute, School of Chemistry, Sun Yat-sen
University, Guangzhou, 510275, P. R. China. E-mail: wangyq223@mail.sysu.edu.cn;
jihb@mail.sysu.edu.cn
bSchool of Chemical Engineering, Guangdong University of Petrochemical Technology,
Maoming, 525000, P. R. China
ccMACS, Department of Microbial and Molecular Systems, KU Leuven, Celestijnenlaan
200F, 3001, Leuven, Belgium. E-mail: bo.weng@kuleuven.be
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ta03573h
Among various cocatalysts, ultrathin 2D noble-metal-free
transition metal carbon/nitride Mxenes (e.g., Ti3C2Tx) have
received widespread attention in the eld of photocatalysis due
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to their wide light absorption capacity, abundant surface ultrathin 2D/2D hybrid materials of TSx and TMx was claried
bridging groups and unsaturated active metal centers.29–31 by AFM. As for the pure semiconductor nanosheets, the average
Moreover, the ultrathin 2D Mxenes have proven to be an ideal thickness of CdS (Fig. S3, ESI†) and Bi2MoO6 (Fig. S4a and b,
support to in situ construct 2D/2D heterostructured photo- ESI†) is 2.2 nm and 1.2 nm, respectively. Fig. 1a and b show that
catalysts.32–34 For instance, Huang et al. reported the photo- the nanosheet with 2.26 nm thickness corresponds to the
catalytic overall water splitting with ultrathin 2D/2D Ti3C2/ Ti3C2Tx nanosheet while the 2.08 nm corresponds to the
BiVO4 nanosheets with enhanced performance compared to thickness of the CdS sample, which suggests the ultrathin 2D/
pure BiVO4 due to the improved charge transfer towards the 2D structure of the TS1.5 sample. Additionally, the thick-
cocatalyst Ti3C2.32 Similarly, ultrathin 2D/2D Ti3C2/g-C3N4 and nesses of Bi2MoO6 and Ti3C2Tx nanosheets in the TM1.5
Ti3C2/Ni-MOF heterojunctions have also been reported for effi- composite are 0.9 nm and 1.9 nm, respectively, (Fig. S4c and d,
cient CO2 reduction and MB degradation, respectively.33,34 ESI†), further conrming the systematic strategy for fabricating
Despite the various applications, the construction of ultrathin ultrathin 2D/2D Ti3C2Tx/semiconductor heterojunctions.
2D/2D semiconductor/Mxene composites for dual-functional
photocatalytic organic transformation and H2 generation has image in Fig. 1c reveals that the CdS nanosheets present as
not yet been reported. highly dispersed, large-size, ower-like sheets, while the TS1.5
Field-emission scanning electron microscope (FE-SEM)
Herein, we report the fabrication of ultrathin 2D/2D Ti3C2Tx/ sample comprises assemblies of curled, wrinkled, and aggre-
semiconductor (CdS and Bi2MoO6) dual-functional photo- gated sheets (Fig. 1d). The elemental mapping measurements
catalysts for the simultaneous oxidative coupling of benzyl- show that Cd, S, Ti, O and C elements are uniformly distributed
amines to imines and H2 generation under visible light in the TS1.5 composite (Fig. 1e), indicating the successful
irradiation (l $420 nm). The H2 and imine yields of the optimal hybridization of CdS with Ti3C2Tx nanosheets. Similar struc-
2D/2D 1.5%Ti3C2Tx/CdS (TS1.5) sample are 219.7 mmol gꢀ1 hꢀ1 tures are also observed in the TM1.5 sample with good distri-
and 155.8 mmol gꢀ1 hꢀ1, respectively, which are 5 times and 6 bution of different elements (Fig. S5, ESI†). Aberration-
times higher than that of pure CdS. Additionally, the intro- corrected scanning transmission electron microscopy (AC-
duction of 2D Ti3C2Tx could also signicantly enhance the STEM) was employed to investigate the morphology and
photocatalytic performance of blank Bi2MoO6, and the optimal microstructure of the as-prepared samples. As shown in
2% Ti3C2Tx/Bi2MoO6 composite (75.2 mmol gꢀ1 hꢀ1 and 63.8 Fig. S6,† the nanosheets of CdS show a 2D nanosheet structure
mmol gꢀ1 hꢀ1) produces 4 times and 3 times more H2 and imine and the edges of the nanosheets are slightly curled, indicating
than bare Bi2MoO6. The enhanced photoactivity over the the ultra-thin properties, which is consistent with the results of
Ti3C2Tx/semiconductor heterostructures is attributed to the
facilitated charge separation from the semiconductors to the
Ti3C2Tx cocatalyst, as proved by in situ irradiated XPS and
photoelectrochemical analysis. Moreover, in situ FTIR spec-
troscopy shows that the Ti3C2Tx/CdS catalyst exhibits strong
adsorption toward benzylamine reactants and easy desorption
for imine products, thus leading to a improved photoactivity as
compared to bare CdS. Additionally, the reaction intermediates
have been detected by in situ ESR and a possible reaction
mechanism for photocatalytic benzylamine coupling combined
with H2 generation is proposed.
Results and discussion
The synthesis route of the ultrathin 2D/2D Ti3C2Tx/CdS (TSx,
where x represents the addition amount of Ti3C2Tx) and
Ti3C2Tx/Bi2MoO6 (TMx, where x represents the addition amount
of Ti3C2Tx) composites is illustrated in Scheme S1 (ESI†). First,
ultrathin Ti3C2Tx nanosheets were prepared by etching the
metal Al layer from the precursor Ti3AlC2 in LiF and HCl solu-
tions. The thickness of Ti3C2Tx nanosheets is around 2.1 nm as
determined by atomic force microscopy (AFM, Fig. S1 (ESI†)),
which conrms the ultrathin structure of Ti3C2Tx nanosheets.
Zeta potential measurement in Fig. S2 (ESI†) suggests the
Fig. 1 AFM image and height cutaway view of TS1.5 (a and b). SEM
images of CdS (c), TS1.5 (d) and the corresponding element mappings
of TS1.5 (e). Aberration-corrected STEM images of TS1.5 (f and g), the
inset in g is the corresponding crystal structure of CdS and Ti3C2Tix.
negatively charged surface of Ti3C2Tx nanosheets. Then, the
positive cations (such as Cd2+ and Bi3+) can be effectively
adsorbed on the Ti3C2Tx surface via the electrostatic self-
assembly process and react with corresponding anions to
The colours of green, purple, yellow, blue and red correspond to the
form Ti3C2Tx/semiconductor heterostructures. The thickness of atoms of S, Cd, C, Ti and O.
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SEM and AFM. As for the sample of TS1.5 (Fig. 1f), the two layers
of ultrathin nanosheets with transparent and smooth features
are closely packed and form obvious grain boundaries between
the nanosheets. This indicates that the ultrathin CdS nano-
sheets are uniformly adhered to the surface of Ti3C2Tx, sug-
gesting the formation of an ultrathin 2D/2D heterojunction
structure. Furthermore, the HR-STEM image in Fig. 1g exhibits
the clear contact interface between CdS and Ti3C2Tx nano-
sheets. The lattice fringes with a width of 0.22 nm could be
attributed to the crystal (0012) plane of Ti3C2Tx.35 The (111)
crystal plane of CdS with a lattice spacing of 0.34 nm is also
observed.21,36 Notably, the close interfacial contact in the TS1.5
sample is benecial for the interfacial charge transfer between
CdS and Ti3C2Tx nanosheets. The aberration-corrected STEM
images of Bi2MoO6 and TM1.5 in Fig. S7a and b (ESI†) show that
the ultrathin 2D Bi2MoO6 nanosheets are closely adhered on
Ti3C2Tx nanosheets, forming an ultrathin 2D/2D Ti3C2Tx/
Bi2MoO6 heterojunction with a clear interface region. The
lattice spacings of 0.274 nm and 0.275 nm match well with the
(200) and (002) planes of the Bi2MoO6 sample while the Ti3C2Tx
nanosheet exposes the (0012) crystal plane with a lattice spacing
of 0.22 nm (Fig. S7c, ESI†).37,38 These results indicate that
semiconductor nanosheets are successfully formed on Ti3C2Tx
nanosheets with intimate interfacial contact, featuring an
ultrathin 2D/2D heterojunction structure.
The structure and crystal phase of these samples were
characterized by powder X-ray diffraction (XRD). The precursor
Ti3AlC2 shows a strong (002) diffraction peak at 9.7ꢁ, as shown
in Fig. S8a (ESI†). Compared to Ti3AlC2, the (002) peak of
Ti3C2Tx is broader and shis toward a lower angle, indicating
the ultrathin layer structure.38 The XRD results in Fig. S8b (ESI†)
show that the characteristic diffraction peaks of TSx composites
are located at 25.3ꢁ, 26.5ꢁ, 28.6ꢁ, 36.9ꢁ, 43.3ꢁ, 48.2ꢁ, and 52.1ꢁ,
which is attributed to the hexagonal wurtzite-structure of CdS
with exposed (100), (002), (101), (102), (110), (103), and (112)
crystal planes, respectively.21,39 However, the signals of Ti3C2Tx
are absent in the spectra, which could be attributed to the low
addition amount of Ti3C2Tx. Signicantly, with the increase of
the Ti3C2Tx content in TSx composites, all the hybrid materials
show reduced peak intensities and broader peaks compared to
pure CdS. This can be explained by the inhibited growth of CdS
nanosheets in the Ti3C2Tx support, which leads to the smaller
grain size and thinner thickness, and then the lower and
broader XRD peaks. Similar results are also observed for the
TMx composites (Fig. S8c, ESI†).40
Fig. 2 (a) Raman spectra of TS1.5 and CdS; (b) Raman spectra of TM1.5
and Bi2MoO6; high-resolution XPS spectra of the core levels of Cd 3d
(c), S 2p (d) over the CdS and TS20 samples and Bi 3f (e) and Mo 3d (f)
over the Bi2MoO6 and TM1.5 samples.
absorption properties of Ti3C2Tx/semiconductors were investi-
gated by UV-vis diffuse reectance spectroscopy (DRS). As
illustrated in Fig. S9 (ESI†), the light absorption ngerprints of
TSx and TMx composites are analogous with those of pure CdS
and Bi2MoO6, corresponding to the inherent band gap
absorption of CdS and Bi2MoO6, respectively. Moreover, with
the increase of the addition amount of Ti3C2Tx, the absorption
edges of the composites remain constant, suggesting the
unchanged band gap of these samples. Notably, with the
introduction of small amounts of Ti3C2Tx, the visible light
absorption intensities of all TSx and TMx samples are
enhanced, which is attributed to the full spectrum absorption of
black Ti3C2Tx. This is further conrmed by the darker color of
the samples (Fig. S9, ESI†).
X-ray photoelectron spectroscopy (XPS) was used to explore
the electron density of Ti3C2Tix/semiconductor composites.
Similar survey spectra for CdS and TS1.5 are observed in
Fig. S10† (ESI). Notably, due to the low addition amount of
Ti3C2Tx (1.5%) and compact encapsulation of Ti3C2Tx with CdS
nanosheets, which shields the Ti signal, the elements belonging
to Ti3C2Tx in the TS1.5 composite cannot be observed obviously.
Therefore, the TS20 sample with a high content (20%) of
Ti3C2Tx was prepared for XPS analysis. Fig. 2c shows that the Cd
3d core levels of CdS are tted with two components located at
404.3 and 411.0 eV, which are assigned to Cd 3d5/2 and Cd 3d3/2
bonds, respectively. Interestingly, in the TS20 sample, these
Raman spectroscopy was further employed to elucidate the
structural complexity of these materials and illustrate the
interaction behaviour between semiconductors and Ti3C2Tx. As
shown in Fig. 2a, the two main peaks located at 302 and
601 cmꢀ1 are attributed to the longitudinal optical (LO) mode
and the overtones of CdS, respectively.21 Notably, a shi of the
Raman peak towards a lower wavenumber for the TS1.5
composite is observed, which suggests a strong interaction
between Ti3C2Tx and CdS. Similarly, the Raman peaks of TM1.5
shi to a lower wavenumber compared to those of their pure
species, which proves the existence of strong interaction in
Ti3C2Tx/Bi2MoO6 heterostructures (Fig. 2b).41 The photo-
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peaks shi to higher binding energies (411.3 and 404.5 eV) as and 155.8 mmol gꢀ1 hꢀ1, respectively, which are 5 times and 6
compared with those of CdS.21,36,39 Correspondingly, the times higher than those of pure CdS. Moreover, the selectivity of
binding energies for the S 2p spectrum in CdS are lower than imine is nearly 100% over the TS1.5 sample (Fig. S13, ESI†).
those of the TS20 composite (Fig. 2d). Moreover, the XPS spec- However, when the addition amount of Ti3C2Tx reaches 2% and
trum of Ti 2p in the TS20 sample (Fig. S11, ESI†) exhibits 5% in the composites (i.e., TS2 and TS5), their photocatalytic
a negative shi as compared with that reported for Ti3C2Tx in efficiencies are obviously decreased, indicating that high
the literature.42 The shis could be attributed to the variation of Ti3C2Tx contents have a negative effect on the performance of
the chemical state of the elements in hybrid composites, in the composites. This is because the high weight content of
which the electron can transfer from CdS to Ti3C2Tx, thus Ti3C2Tx may block the light to reduce the light absorption of
leading to a change of the electron cloud density over different CdS and reduce the light intensity through the inside of the
elements. These results suggest the strong interaction between reaction solution, thus leading to the photoactivity decrease.
semiconductor CdS and the Ti3C2Tx cocatalyst, which is Concurrently, the photocatalytic activities of a series of TMx
consistent with the results of Raman spectroscopy. The inter- composites were also evaluated under the same reaction
action between the semiconductor and Ti3C2Tx can be further conditions. The results in Fig. S14a (ESI†) show that the
validated in the TM1.5 composites. As shown in Fig. 2e and f, hybridization of Ti3C2Tx can signicantly enhance the yields of
the binding energies of both Bi 3f and Mo 3d over the TM1.5 H2 and imine of Bi2MoO6. The photocatalytic H2 and imine
composite are larger than those of the Bi2MoO6 sample while formation rate over the optimal TM2 sample is 75.2 mmol gꢀ1
the Ti 2p core-level in TM1.5 exhibit a negative shi trend hꢀ1 and 63.8 mmol gꢀ1 hꢀ1, respectively, which is approximately
(Fig. S12, ESI†).27 This is caused by the change in the electron 4-fold and 3-fold higher than that of bare Bi2MoO6. The stability
cloud density of each component, indicating the strong inter- of TS1.5 and TM2 samples was evaluated by continuous pho-
action between Bi2MoO6 and Ti3C2Tx.
tocatalytic reactions. As shown in Fig. 3b and S14b,† no
The performance of TSx and TMx composites was evaluated apparent photoactivity decrease is observed during the ve
by photocatalytic benzylamine oxidation to produce imine successive recycle tests for photocatalytic coupling of benzyl-
along with simultaneous H2 evolution in an argon atmosphere amine oxidation with H2 evolution, which suggests the tolerable
under simulated visible light irradiation (l $420 nm). Control recyclability of TS1.5 and TM2 composites. The XRD and
experiments were executed to eliminate interference factors. As aberration-corrected STEM patterns of the TS1.5 sample before
shown in Table S1,† H2 and imine are not detected in the and aer the reaction remain unchanged, further clarifying the
absence of light and/or photocatalyst, indicating that the reac- high stability of TS1.5 composites (Fig. S15 and S16 ESI†).
tion is driven by a photocatalytic process. As shown in Fig. 3a,
The insight and interpretation of the photocatalytic mecha-
pure CdS shows relatively low photocatalytic activity with the H2 nism were elaborated through in situ Fourier Transform
and imine production rate of 48.2 mmol gꢀ1 hꢀ1 and 28.1 mmol Infrared Spectroscopy (In situ FTIR), in situ photoelectron
gꢀ1 hꢀ1, respectively. With the addition of a small amount of spectroscopy and in situ Electron Spin Resonance (In situ ESR)
Ti3C2Tx (e.g., 0.5%), the photocatalytic H2 production and imine technologies. The results of in situ infrared deuterium water
production rates increase to 100.5 mmol gꢀ1 hꢀ1 and 78.9 mmol adsorption are shown in Fig. 4a and b. The main peaks located
gꢀ1 hꢀ1, respectively, which suggests that the introduction of at 1194 and 2763–2400 cmꢀ1 are associated with the dD–O and
Ti3C2Tx as the cocatalyst could promote the photocatalytic nD–O vibration peaks, repesctively.43,44 These results indicate that
reaction process. The photoactivity of TSx composites gradually D2O molecules can be adsorbed on the surface of the catalyst to
improves with the increasing Ti3C2Tx amount and the optimal form TS1.5/D–O–D surface coordination species. The adsor-
weight content is demonstrated to be 1.5%, suggesting that bed D2O molecules will be activated and reduced by photo-
a synergistic interaction between CdS and Ti3C2Tx is required to generated electrons to produce D2 under visible light irradiation
optimally enhance the photoactivity of CdS. Specically, the H2 (Fig. S17, ESI†). Furthermore, the infrared peak corresponding
and imine yields over the TS1.5 sample are 219.7 mmol gꢀ1 hꢀ1 to the adsorption of D2O on CdS is obviously weaker than that of
TS1.5, which indicates that the TS1.5 sample with multifunc-
tional sites on the sur-/interface is more conducive to the
adsorption of D2O molecules than CdS, thus leading to
enhanced photocatalytic performance. Additionally, Zhang and
co-workers recently used deuterated benzylamine to conrm
that the produced H2 product also comes from the deuterated
amine.39 Therefore, in our reaction system, both the benzyl-
amine and water could serve as proton sources for forming H2
fuel.
In situ FTIR adsorption of pure benzylamine and imine was
also performed to investigate the adsorption (desorption)
behaviors and capabilities on the surface of CdS and TS1.5
Fig. 3 (a) Photocatalytic activity of different samples for benzylamine
oxidation and H2 generation under visible light irradiation (l $420 nm);
(b) recycling tests over the TS1.5 composite for photocatalytic ben-
photocatalysts. The conspicuous peaks detected at 1250–1340
and 1590–1650 cmꢀ1 can be assigned to C–N bond stretching
zylamine oxidation integrated with H2 production (each cycle is 4 h).
vibration and N–H bond stretching vibration peaks of the
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