Hybrid catalysts of molybdovanadophosphoric acid and g-C3N4with tunable bandgaps

Article abstract of DOI:10.1039/d0dt02138e

The integration of semiconductors and polyoxometalates provides promising benefits for the rational tuning of hybrid materials' electronic band structures; however, the intrinsic influence of certain hybridization approaches on the resulting bandgaps of their complexes has seldom been noted. Herein, graphitic carbon nitride and a series of phosphovanadomolybdates (H3+xPMo12-xVxO40, x = 0-3) have been complexed through electrostatic charge attraction, and their optical and electronic properties are fully explored to investigate the effect of minor variations of the polyoxometalate structures on the hybrid bandgaps and electronic structures. The conduction band edge of the hybrids increases along with the expansion of the number of vanadium centers in the phosphovanadomolybdate, providing potential guidance for the design of hybrid catalysts. This journal is

Full text of DOI:10.1039/d0dt02138e

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Hybrid Catalysts of Molybdovanadophosphoric Acid and g-C3N4
with Tunable Bandgaps
Linkun Cai,ab Jie Hu,ab Mu Li ab and Panchao Yin *ab
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The integrating of semi-conductors and polyoxometalates provides conductive nano-materials (SCNMs) for the hybridity of their
promising benefit for the rational tuning of hybrid materials’ bandgaps because of the broad availability of SCNMs with
electronic band structures; however, seldom has noticed the various electronic structures.6-13 Specifically, POM-capped NPs
intrinsic discipline of certain hybridization approaches to the demonstrate excellent electro- and photo-catalytic
resulted bandgaps of their complexes. Herein, graphitic carbon performances for their tunable bandgaps and robusticity.13
nitride and a series of phosphovanadomolybdates (H3+xPMo12- POMs are able to bind onto the surface of carbon-based low
xVxO40, x = 0~3) have been complexed through electrostatic charge dimensional nanomaterials effectively for highly efficient anode
attraction, respectively and their optical and electronic properties materials for lithium ion battery and water splitting catalysis.14-
are fully explored for effect of minor variation of polyoxometalate 18 Rational design of these hybrid complexes with suitable band
structures to the hybrids’ bandgaps and electronic structures. The structures is critical for novel electrochemical and catalytic
19
conduction band edge of the hybrids increases along with the performance.10,
However, the quantitative prediction of
expanding of the number of vanadium centers in the electronic band structures of the hybrids is still beyond our
phosphovanadomolybdate, providing potential guidance to the capacity and the further exploration of bandgap shifting of the
design of hybrid catalysts.
hybrids in response to minor structural altering of POMs should
be beneficial.
Polyoxometalates (POMs) are a group of nano-scaled metal
oxide clusters composing of up to hundreds of metal centers
with well-defined structures and excellent redox and super
Meanwhile, due to their ultra-small sizes (from sub-nm to 6 nm),
POMs usually are expected to own higher catalytic activity
compared to conventional metal oxide nanoparticles and solid
substrate.4 However, the high solubilities of POMs in reaction
media impose great difficulty in catalytic materials recycling.
Therefore, POMs are immobilized in various matrices, e.g. NPs,
porous materials, polymer network and 2D nano-materials
while the introduction of strong attraction among POMs and
matrices ensures the long term stability of the catalysts.20-24 The
strong association of POMs on catalytically active nanomaterials
like graphitic carbon nitride (g-C3N4, CN) provides a feasible way
2
acidic properties.1, They mimic the surface structures of
colloids and bulks of metal oxides, and thus serve as perfect
4
model systems for the exploration catalytic mechanisms.3,
POMs’ compositions are dominated by the coordination
polyhedral of early transition metals and thus, their electronic
band structures and bandgaps are locked, limiting POMs’
general applications in catalysis and electronic-active
materials.5 The introduction of hetero-atoms and/or organic
ligands is able to shift bandgaps of POMs; nevertheless, the
availability of effective bonding modes with hetero-atom and
organic ligands limit the rational tuning of POMs’ electronic
band structures.6-8 Therefore, POMs are integrated with semi-
a. South China Advanced Institute for Soft Matter Science and Technology, South
China University of Technology, Guangzhou 510641, P. R. China.
E-mail: yinpc@scut.edu.cn
b. State Key Laboratory of Luminescent Materials and Devices, South China
University of Technology, Guangzhou 510641, P. R. China.
E-mail: yinpc@scut.edu.cn
†
Electronic Supplementary Information (ESI) available: Details of materials,
synthesis, IR, XPS, DRS, TEM, CV, EIS, GC and catalysis results. See
DOI: 10.1039/x0xx00000x
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for simultaneously improving POMs’ catalytic activity and of interconnected small pieces of the nanosheets (Fig. S2†),
20, 25
recyclability.18,
Herein, we report the complexation of which is attributed to the successful protonation of CN.
phosphovanadomolybdates26-28 (H3+xPMo12-xVxO40, x = 0~3) and Moreover, the small pieces of nanosheets are observed in
g-C3N4 (CN) via electrostatic interaction and the investigation of aggregated rather than the discrete structure for CN, which
the intrinsic influence of POMs structures (numbers of could be attributed to electrostatic attraction among POMs and
vanadium centers) to the electronic band structures of the CN. Small and wide-angle X-ray scattering (SAXS/WAXS)
POMs@g-C3N4 hybrids (Vx-CN) via typical electrochemical and measurements indicate that, the PMo12-xVxO40 molecules were
optical spectroscopic studies. Meanwhile, excellent catalyst finely dispersed on the CN support. The pattern of unmodified
recovery and reclamation have been confirmed using the hybrid protonated CN (pCN) pronounces well-resolved characteristic
catalysts in the oxidation of sulfur containing compounds under scattering peaks in q range of 0.4 Å-1 ~ 2 Å-1, attributing to the
mild condition.
typical graphitic-like structure. Interestingly, in addition to the
The POMs we applied to support on CN here are the same intrinsic scattering peaks of CN nanosheets, the scattering
phosphovanadomolybdate compounds, H3+xPVxMo12-xO40 curves of Vx-CN (Fig. 2c) all exhibit 2 oscillation peaks at 0.55 Å-
described as essentially redox oxidants of high potential that in 1 and 1.31 Å-1, respectively, which can be ascribed as unique
turn could easily be re-oxidized by molecular oxygen. The strong form factor of Keggin type POMs in aqueous solution. This
charge interaction between protonated C3N4 (pCN) nanosheets suggests that the molecules of PMo12-xVxO40 are finely dispersed
and phosphovanadomolybdates leads to their aggregation and into CN in an amorphous-like state, similarly to how it is
the formation of large colloids in the reaction solutions. The decentralized in solutions. Dispersing in molecular level, the
main vibration bands of the hybrids from FT-IR results agree hydrolytic stability of POMs can be greatly enhanced, and the
well with that of pristine CN and pure POMs, suggesting the catalytic function of POMs may be combined with the porosity
completeness of CN and POM structure during the synthetic and the sorption properties of CN.
reactions (Fig. S1†). X-ray photoelectron spectra (XPS) reveal Meanwhile, nitrogen adsorption-desorption isotherms
that H3+xPMo12-xVxO40 are successfully incorporated into pCN via measurements are used to provide surface analysis of the
electrostatic interaction (Fig. S4†). V3-CN shows obvious hybrids. Both pCN and Vx-CN hybrids exhibit the characteristic
increased fraction of C-C units (284.6 eV) compared to pure pCN type V isotherms, in which pCN has a H3 hysteresis loop while
and features of N elements (N-(C)2@398.5 eV and N-(C)3@400.0 that of Vx-CN is with H1 type characteristic of spherical
eV). In comparison to H6PMo9V3O40, the existence of Mo6+ aggregates, indicating that the nanocomposites exhibit a
specie and a higher peak area ratio (2.17>1.15) of Mo=O (530.6 relatively more homogeneous distribution than pCN, further
eV) and Mo-O-O-Mo/V (531.8 eV) in V3-CN is confirmed by Mo confirming the uniform dispersion of POMs wrapped in the
3d and O 1s spectrum. Meanwhile, TGA tests (Fig. S3†) show structure of pCN. Based on the Barrett-Joyner-Halenda (BJH)
that V0-CN and V2-CN share the same decomposition
temperature about 520
14.56 % as final residual near 800
º
C, leaving an amount of 13.03 % and
C, respectively. It is thus
º
confirmed that the loading amounts of POMs in different types
of Vx-CN samples are nearly the same, ensuring the credibility
of this work.
Extended microscopical and scattering experiments have been
carried out to probe structure characteristics of the hybrids.
Sheet-like structures can be observed in the TEM
measurements of all CN and Vx-CN samples, confirming the
remaining of CN nano-structures after protonation and the
introduction of PMo12-xVx onto their surface. Vx-CN is composed
2 | J. Name., 2012, 00, 1-3
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desorption branches, the average pore sizes of pCN, V0-CN and to Fig. S8 and S9†, there are some oxidation featuVrieews aArpticpleeOanrleinde
V2-CN are found to be 6.63 nm, 10.58 nm, 13.70 nm (Fig. 2a, b), in the range from 0.6 V to 1.0 V, which cDaOnI:b1e0.1a0s3s9ig/Dn0eDdTt0o21t3h8eE
respectively. The immobilization of POMs expands the pore oxidation processes occurred on the surface of CN.
structure and increase the surface roughness of pCN, leading to Electrochemical impendence spectra (EIS) are also conducted to
the further increase of the average pore sizes. Correspondingly, all the samples. As shown in the Nyquist plot in Fig. S7†, the arc
the peak centered at 30 nm from the PSD peak (Fig. 2b) has radius of all the Vx-CN samples obviously far greater than that
drastically increase after the incorporation of POMs into pCN. of pCN, further confirming that the binding of PMo12-xVx and CN
As a result, V0-CN and V2-CN presented relatively larger surface could effectively weaken the charge/carrier recombination
area (78.43 m2/g and 106.67 m2/g) than bulk pCN (13.61 m2/g), because it is commonly believed that the smaller radius of
which allows the hybrids to contact with the substrates more Nyquist plot has, the smaller charge transfer resistance it
easily in catalysis. The light absorption property of the hybrids presents. In a word, the heterojunction interface between these
(Fig. 3a) and various starting PMo12-xVx clusters (Fig S6†) are two components can provide more efficient charge carrier
studied using UV-vis diffuse reflectance spectra (DRS). transport.
Compared to pCN, PMo12-xVx species obviously have sufficiently The valence band potentials of Vx-CN and pCN pasted on ITO
lower band edges and better adsorption capability. Hence, the electrode has been estimated by electrochemical Mott-
modification of different PMo12-xVx onto CN has resulted in red- Schottky measurements32 at various frequencies (1.0 kHz,
shifting and excellent enhancement of the absorption into the 1.5kHz, 2.0 kHz) and presented in Fig. 4a, b and Fig. S10†. The
visible region (Fig. 3a) owing to the existence of Mo or V positive slopes of the MS plots suggest that Vx-CN and pCN are
addenda atoms.29, 30 To determine the exact optical band-gap both n-type semiconductors. By extrapolating the linear portion
energies of all the hybrid materials, Tauc plots31 are employed of the curve to intercept of the abscissa, the conduction band
to the DRS corresponding to the absorbance based on Tauc potential had been estimated (referenced to the NHE). With the
formula31:
obtained data, combined the band gap values obtained from
Tauc plots, we are allowed to calculate the corresponding
valence band potential of the samples. The band structure33 of
Vx-CN nanocomposites and pCN can thus be obtained. As shown
in Fig. 4c, much lower VB levels of Vx-CN have been observed,
with positive shifts from 2.28 V to 2.71 V, 2.46 V, 2.45 V, 2.19 V,
respectively. Interestingly, the VB edges and CB edges of Vx-CN
follow the same trend of downshifting alongside with the
increase number of Vanadium atoms of the immobilized PMo12-
xVx. The introduction of PMo12-xVx as electron reservoirs can
effectively deepen the VB edge of CN so as to heighten its
photooxidation capability. Due to the low CB level of CN
calcined from urea precursor, the lower CB levels of Vx-CN
cannot meet the need of enabling hydrogen production from
water. However, it is expected that the band structure can be
tuned by using CN synthesized from other precursors that have
higher CB level or increasing the number of Vanadium atoms in
Vx-CN units. To sum up, it is possible to engineer the appropriate
band structure of the nanocomposites with decreased electron-
trap recombination for improved photocatalytic activity. The
combination of CN and H3+xPMo12-xVxO40 has shown great
enhanced light harvesting ability and narrower band gaps
according to experimental results and has revealed certain
(훼ℎ휈)1/2 = 퐴(ℎ휐 − 퐸푔)
For CN based materials which obey the typical indirect allowed
transition mechanism, the value of n is set to be 2. The
estimated band-gap values (Eg) were obtained by extrapolating
a linear fitting to the hν axis intercept in the Tauc plot (Fig. 3b),
i.e. the plot of (αhν)0.5 vs incident light energy (hν) in eV. As
shown Fig. 3b, the HOMO-LUMO gap of Vx-CN (x=0~3, 2.66 eV,
2.49 eV, 2.69 eV, 2.44 eV) is obviously narrower than that of
pCN (2.78 eV), suggesting the capability of PMo12-xVx in
enhancing the light harvesting ability and restraining the
radiation recombination of photogenerated carriers of pCN.
Electrochemical tests are further applied to investigate redox
properties of the catalyst. Cyclic voltammetry (CV) tests are
carried out on the samples at the same condition. The
voltammograms of four different kinds of PMo12-xVxO40 solution
show that there are at least 3 to 4 electron distinctive reduction
peaks occurred in the range from -0.5 V to 0.6 V, while the
reduction peaks of PMo12-xVx in Vx-CN appeared in similar
features. However, the intensity of oxidation and reduction
peaks in the CVs of Vx-CN samples are more reversible than that
of H3+xPMo12-xVx solution, implying that Vx-CN can act as good
containers of electron and a better reusable catalyst. According
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DOI: 10.1039/D0DT02138E
There are no conflicts to declare.
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The work is supported by the National Natural Science
Foundation of China (Nos. U1832220, 21961142018, and
51873067), the Natural Science Foundation of Guangdong
Province (No. 2018A030313503), and the Fundamental
Research Funds for the Central Universities (No. 2018JQ04).
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Received 00th January 20xx,
Accepted 00th January 20xx
Hybrid Catalysts of Molybdovanadophosphoric Acid and g-C3N4
with Tunable Bandgaps
DOI: 10.1039/x0xx00000x
Linkun Cai, Jie Hu, Mu Li and Panchao Yin *
Nanocomposite of Phosphovanadomolybdates (H3+xPMo12-xVxO40, x = 0~3) and g-C3N4 can absorb nearly the full light spectrum,
and its energy band structure are tuned by the number of vanadium atoms in H3+xPMo12-xVxO40 in certain discipline.
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