Study of the enhanced visible-light-sensitive photocatalytic activity of Cr2O3-loaded titanate nanosheets for Cr(VI) degradation and H2 generation

Article abstract of DOI:10.1039/c7cy00644f

Visible-light-sensitive titanate nanosheets (TNSs) were synthesized with Cr₂O₃ successfully loaded and well dispersed on Cr(III)-TNSs by a one-step hydrothermal method for enhanced photocatalytic activity. As in our previous reports,

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Study on Enhanced Visible-Light-Sensitive Photocatalytic Activity
of Cr2O3-Loaded Titanate Nanosheets for Cr(VI) degradation and
H2 Generation
Received 00th January 20xx,
Accepted 00th January 20xx
Junqian Ding,a Julan Ming,b Dingze Lu,a Wenhui Wu,a Min Liu,b Xiaona Zhao,a Chunhe Li,a Minchen
Yang,a and Pengfei Fang*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
The visible-light-sensitive titanate nanosheets were synthesized with Cr2O3 successfully loaded and well dispersed on
(Cr(III)-TNSs) by one-step hydrothermal method for enhanced photocatalytic activity. As our previous reports, the as-
prepared samples obviously exhibited porous flake-like structure with large specific surface area (300-400 m2/g).
Accompanied with the hydrothermal growth of TNSs, loaded Cr2O3 not only influenced the layered structure of TNSs, but
also brought about remarkable enhancement on the visible-light response and the separation of photogenerated carriers.
The XPS fitting results indicated that Cr ions mainly existed in trivalent forms and caused the change in binding states
toward Ti and O. Based on the evaluation of photocatalytic activity, it was found that Cr(III)-TNSs obtained an significantly
improvement for RhB and K2Cr2O7 (Cr(VI)) degradation under visible-light irradiation, which was attributed to the retarding
of charge recombination and the effective electron transfer from Cr2O3 to TNSs. The better photocatalytic activity was
obtained with an optimal content (0.5 at.%) of Cr3+, and the degradation rate (Kapp) was 2.9-fold and 4.1-fold, respectively,
as compared to TNSs. The samples were also evaluated by the photocatalytic H2 generation with the photodeposition of Pt
nanoparticles as co-catalyst. The results showed that the visible-light H2 production rate of Pt0.25/Cr(III)-TNSs (0.25 wt.% of
Pt) was observably enhanced under the efficient sacrificial agent/water system, giving a relatively high H2 generation rate
of 473 μmol h-1 g-1 at 0.5 at.% of Cr3+ content. The cyclic tests demonstrated the nice stability and recycling performance of
samples. The alternative mechanisms for the visible-light-sensitive photocatalytic activity were also proposed.
become a severe problem due to its toxicity, stubbornness and
mutagenic nature, which can cause serious harm to human
health such as carcinogenic disease and organic damage.6–8 X.
1. Introduction
TiO2-based nanomaterials are well-known as an efficient
photocatalyst owing to their great advantages containing low
cost, strong oxidizing power, physicochemical stability and
environmental friendliness.1,2 It has been commonly used in
the researching fields of photocatalysis and solar energy
conversion, especially in the removal of toxic pollutants from
wastewater containing reactive dyestuff and heavy metal
substances (e.g. Cr(VI), Pb(II), Hg(II) and As(III)) and the
development of green power (e.g. hydrogen energy) in the
case of global environment deterioration and energy crisis,
which have been widely studied through lots of effort.3–5 As a
typical researching object among these pollutants, dichromate
(Cr(VI)) is highly water soluble in aquatic environment and has
Yuan et al. has reported the related work on the photocatalytic
reduction of Cr(VI).9–11 By using the functionalized titanium
metal-organic frameworks and decorated graphene
composites as highly efficient photocatalysts, the visible-light
assisted degradation of Cr(VI) as well as dye molecules was
successfully achieved. When it comes to the photocatalytically
splitting of water into H2, many methods have been proposed
and the deposition of noble metals as cocatalysts and the
designing of composite semiconductors are commonly
regarded as the effective and feasible ways for H2
production.5,12
However, it’s difficult to utilize visible light due to its wide
band gap of about 3.2 eV and the recombination of photo-
induced electron and hole pairs is too fast in traditional TiO2
materials.13 For this reason, much work has been done to
improve its photocatalytic performance. Several ways,
including design of nanostructure and morphology,14 coupling
TiO2 with sensitizing organic dyes or narrow band gap
semiconductors,15,16 doping nonmetallic or metallic
19,20
elements,17,18 and the modifications on the surface for TiO2
have been widely studied and paid to extend its
photoresponse and improve its photoactivity. Although great
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enhancement has been made in visible-light response, TiO2, as well as revealing the key effect of surface
improving the charge separation efficiency of TiO2 still remains modification, which simultaneously promoted the absorption
challenging. The fast recombination of photo-generated in visible-light region and charge separation. What’s more, as
electrons and holes gives rise to the quenching of most of the the bimetallic compounds co-existing on the supported
excited charge carriers before they reach the surface and semiconductor have been studied to improve photocatalytic
participate in the photocatalytic reaction. Based on the recent performance and facilitate water splitting successfully,33–36 it’s
researches, it’s noteworthy that some approaches of loading spontaneously expected that the dispersion of a noble metal
metal oxides or hydroxides on the surface of TiO2 physically or (Pt, Rh, Au) as cocatalyst on the basis of MOx-loaded TiO2
chemically by introducing rare-earth or transition metal ions system (M = transition metal) would show a favorable
(Mn, Fe, Cr, Co, V, etc.) were regarded as the effective means performance in H2 production by the synergistic effects of the
for photocatalyst responding to visible light. These candidates photosensitizer (MOx) and cocatalyst.
are very promising for practical applications, especially for the
Herein, we reported a TiO2-based titanate nanosheets
photocatalytic degradation of organic or inorganic pollutants (TNSs) loaded by trivalent chromium oxides (Cr2O3), which was
and the development of green energy under visible light prepared by a facile one-step hydrothermal treament in the
irradiation.3,18,19 In contrast to
a
single component alkaline environment. It had shown that the obtained TNSs, as
photocatalyst, the building of an interfacial heterostructure two-dimensional (2D) nanosheets , possessed a large specific
between supported semiconductor oxides and loaded metal surface area, which was expected that more reactive sites
oxide/hydroxide nanoparticles has shown a high separation were exposed and the introduced metal ions could spread
efficiency of photogenerated charges and extensive light easily and well disperse on the surface. Hence, more photo-
absorption by the approach of directly interface charge generated electrons could fast transfer and reach the surface
transfer (IFCT).21,23 According to some reports, the interfacial easily to involve in the photocatalytical reactions. In this work,
charge transfer efficiency is greatly dependent on the size of we determined that the effects of the introduced Cr3+ ions and
loaded particles.13,19,24 Hence, it’s of great importance to different amount of that on the visible-light response,
develop the ways of loading appropriate-sized and highly- separation of photo-generated carriers for Cr2O3-loaded TNSs
dispersed metal oxide nanoparticles on TiO2 photocatalyst for (Cr(III)-TNSs) by different characterizations. The enhancement
enhanced charge separation efficiency.
on the photocatalytic redox activity by the degradation of
Among these 3d transitional metal ions (Cr, Mn, Fe, Co, V, Rhodamine B (RhB) and Cr(VI) in K2Cr2O7 solution under visible
etc.), chromium (Cr) appears to be a promising choose for TiO2 light irradiation was demonstrated. Furthermore, it was also
owing to the unique properties for the supported chromium studied that the obtained TNSs photocatalysts combining
oxide species, including chemical states and coordination Cr2O3 with Pt exhibited a synergistic effect for initiating the
environment.25,26 There have been some reports clarifying the hydrogen evolution activated by visible light. The Pt/Cr(III)-
roles of these surface Cr oxide species during the redox TNSs in various proportions of Cr2O3 were fabricated by a
reaction, particularly the decomposition of organic simple photodeposition of Pt nanoparticles on the obtained
substrates.22,27 However, few researches are reported for sample surface. As Pt could serve as electron sink and active
supporting the degradation of pollutants and solar energy sites for proton reduction to H2,37 under the sacrificial
conversion under visible light irradiation. In addition, for agent/water system for this study, the Pt/Cr(III)-TNSs
chromium-modified materials, most of the work was supposed photocatalysts exhibited significantly enhancement on the
to incorporate chromium ions into the lattice using different efficiency of photocatalytic H2 generation, which was due to
methods, such as hydrothermal/solvothermal methods, the cooperation between the IFCT from photoactivated Cr2O3
chemical/physical vapor deposition, and ion implantation.28–30 and the electron sink of Pt. As mentioned above, it was
In view of many reports regarding the substitution of metal demonstrated
that
Cr(III)/TNSs
system
functioned
ions for lattice sites, most of the photocatalysts are sensitive photocatalytically and exhibited excellent photocatalytic redox
to visible light due to the localized narrow band newly-formed activity for its heterostructure formed at the interface. And it
in their forbidden band, which originates from the d-orbitals of might find potential applications in water purification fields
metal ions dopant.31,32 While the mechanism for obtained and efficient utilization and conversion of solar energy.
visible-light response by loading metal ions on the support
surface is entirely different from that of doped materials. Irie
et al.13,22 reported that the d-orbitals of grafted metal oxide
species existed discretely on the TiO2 surface, and it was
proposed that the visible-light catalytic activity was arisen
from directly interface charge transfer (IFCT) from d-orbital
levels of metal ions to the conduction band (CB) of titania, as it
was also demonstrated by the work group of Nolan.24 This
modified effect on the photocatalytic activity of TiO2-based
materials has been reported and exemplified combining DFT
simulations and detailed experiments in their work. It had
delivered a new coupling of molecule-scale metal oxides with
2. Experimental
2.1 Materials
P25 (TiO2, 70% anatase, 30% rutile) was the product of
Degussa Co., Ltd. The NaOH, HCl, ethyl alcohol (C2H5OH),
acetone (C3H6O), phosphoric acid (H3PO4), 1, 5–
Diphenylcarbohydrazide
(C13H14N4O),
Cr(NO3)3·9H2O,
Rhodamine B (RhB), potassium dichromate (K2Cr2O7),
isopropanol (IPA), tert–butyl alcohol (TBA), triethanolamine
(TEOA), 1, 4–benzoquinone (BQ) and potassium persulfate
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(K2S2O8) were obtained from Sinopharm Group Co., Ltd, and with an excitation wavelength of 448 nm. UV-Vis diffuse
chloroplatinic acid (H2PtCl6) was provided by Sigma-Aldrich reflectance spectroscopy (UV-vis DRS) of the samples was
Co., LLC. All of the reagents were analytically pure and utilized recorded on a Shimadzu UV-2550 spectrometer equipped with
without further purification. Deionized water was used in all a BaSO4-coated integrating sphere, using BaSO4 as reference.
experiments.
X-ray photoelectron spectroscopy (XPS, Thermo Electron VG
Multilab2000) was performed to analyze the component and
chemical states of samples. The binding energies were
2.2 Preparation of photocatalysts
Cr2O3-loaded TNSs (Cr(III)-TNSs) with various contents were referenced to the C 1s peak at 284.6 eV. The spectra were
obtained by hydrothermal method. 0.8 g P25 was dispersed in analyzed by the XPS Peak software. The fluorescence emission
50 mL of 10
M NaOH solution with vigorous stirring. spectra (FL) were examined by a Hitachi FL 4600 instrument to
Precalculated content of chromium nitrate solution was added determine the separation efficiency of photo-generated
slowly into the suspension. Under vigorous stirring for 40 min, electron and hole pairs.
the mixture was then transferred into 100 mL Teflon-lined
2.5 Photoelectrochemical measurements of photocatalysts
autoclave and treated at 130 °C for 3 h. After the reaction
ended, the autoclave was immediately cooled down to room The photocurrent of samples was measured on an
temperature using cold water. The resultant white powders electrochemical workstation (CHI-660C Instruments, Chenhua,
were then rinsed with deionized water until neutral condition China) with a standard three-electrode system using the
was obtained, followed by pickling with proton exchanging in sample-coated electrode as the working electrode with an
200 mL HCl solution (0.1 mol/L). Finally, the powders were active area of ca. 1.5 cm2, a Pt wire as the counter electrode,
washed and collected with deionized water for several times and a calomel electrode as a reference electrode. A 300 W Xe
and then dried at 70 °C for 12 h. The obtained samples were lamp was used as light source and a cutoff filter was utilized to
denoted as x-Cr-TNSs, where x (x = 0.1, 0.25, 0.5, 0.75, 1.0, provide visible light (λ > 420 nm), and 0.1 M Na2SO4 aqueous
1.5% and 2.0%) represented Cr(NO3)3 concentration in the solution was served as the electrolyte. The working electrode
precursor solution. For comparison, the bare titanate was prepared as following steps: 0.01 g of the sample was
nanosheets (TNSs) sample was synthesized in the same dispersed in 1 mL ethanol and sonicated to make a slurry. After
process without adding chromium source and the pure Cr2O3 that, the slurry was coated onto a 2 cm × 1 cm indium tin oxide
sample was also prepared by a similar procedure as mentioned film-coated glass (ITO glass) by the spin-coating method. Next,
above without adding P25.
the electrode was dried in an oven. All working electrodes to
be measured had a similar film thickness.
2.3 Photodeposition with Pt on photocatalysts
2.6 Photocatalytic performance of photocatalysts
The Pt0.25/Cr(III)-TNSs and Pt0.25/TNSs samples were prepared
by the impregnation of above-prepared Cr(III)-TNSs and bare 2.6.1 Evaluation of RhB solution
TNSs powders (0.5 g) in 100 mL isopropanol aqueous solution
The photocatalytic activities of samples were assessed by the
(10 vol.%), mixing with 625 μL of H2PtCl6 aqueous solution (2
g/L) to load 0.25 wt.% of Pt (weight ratio of Pt:TNSs) on the
surfaces of samples. Before irradiation, the suspensions were
sonicated in an ultrasonic bath and degassed in a vacuum
system for 30 min. Then the reaction mixture was irradiated by
UV illumination (300 W Xe lamp) for 3 h under vigorous
stirring. After that, the precipitates were collected by
centrifuge and repeatedly washed with deionized water and
ethanol for several times. The resultants were finally dried at
65 °C for 12 h.
oxidative degradation of RhB solution (20 ppm). The light
source was a 300 W Xe lamp with a cut-off filter (λ > 420 nm).
The distance between the light source and the solution was 15
cm. Prior to the reaction, 100 mg of sample was put into 100
mL RhB aqueous solution (20 ppm). The reaction apparatus
was placed in water bath to maintain at room temperature.
The solution was under stirring in darkness for 1 h to achieve
the
adsorption-desorption
equilibrium
before
the
photocatalytic reaction, and then was irradiated under light
source. The reaction was continually processed for 40 min.
Every 5 min, 1.6 mL solution was collected and centrifuged to
obtain the supernatant liquor at 12000 r/min for 3 min. Then
0.8 mL of supernatant liquor was mixed with 2.4 mL of
deionized water and tested to determine the concentration of
RhB in the solution by the UV-Vis spectrometer. Blank
experiment was conducted without any photocatalyst to verify
the photo-stability of RhB molecules. The stability and
reusability of samples was also evaluated by repeating
photocatalytic experiment, and five consecutive cycles were
tested, each lasting for 70 min.
2.4 Characterizations of photocatalysts
The morphology and compositions of the as-prepared samples
was characterized using scanning electron microscope (SEM)
(SIRION, FEI, Netherlands) and high-resolution transmission
electron microscopy (HRTEM) taken on JEM–2010 FEM. The
BET specific surface area was measured by N2 adsorption-
desorption isotherms at 77 K using the instrument (JW–
BK122W, China). The X-ray diffraction (XRD) patterns were
recorded on a X-ray diffractometer (Bruker D8 advance) using
Cu Kα radiation. The result was obtained with a step of 0.02° in
the range (2θ) from 20° to 80° for the determination of phase 2.6.2 Evaluation of K2Cr2O7 (Cr(VI)) solution
composition of the samples. The Raman spectra were
collected using a Horiba Lab RAM HR Raman spectrometer
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The photocatalytic reduction of aqueous Cr(VI) to Cr(III) was suggested that the raw TiO2 powders reacted with NaOH
performed in a 200 mL quartz reactor containing 100 mg solution during the hydrothermal treatment and TiO2
photocatalyst and 100 mL of Cr(VI) aqueous solution at 20 nanoparticles were stripped into thin lamellar fragments. With
ppm. The initial pH value of reactant solution was adjusted to the exfoliation process developed, large amounts of Ti–O
3.0 by adding HCl. The solution was stirred in darkness for 1 h bonds were broken to form a highly disordered phase
to reach the adsorption-desorption equilibrium, and then containing titanate nanocrystals. Then, the thin lamellar
irradiated for 120 min under a 300 W Xe lamp with a cut-off fragments jointed with each other and made larger planar
filter (λ > 420 nm). For every 20 min, 2 mL of solution was nanosheets.40,41 As seen in Fig. 1 (e-f), the obtained
collected and centrifuged for 3 min to obtain the supernatant Cr(III)-TNSs did not possess well-developed lattice fringes and
liquor at 12000 r/min for 3 min. The final testing sample was show other characteristic crystal clearly, indicating a very weak
obtained by 1 mL of supernatant liquor mixing with 2.2 mL of crystallinity. Only some low crystalline phases corresponding
deionized water. The Cr(VI) concentration of the testing to chromium oxides could be found from the images of
solution was analyzed colorimetrically at 540 nm using the 2.0%Cr-TNSs. Some of them were overlapped with the H2Ti3O7
diphenylcarbazide method (DPC) by UV-Vis spectrometer. phases. It was observed that these small quantity of fringes
Blank experiment was carried out without any photocatalyst to have spacings of 0.167 and 0.267 nm, which were in
verify the stability of Cr(VI). The stability and reusability of agreement with the spacings of the (116) and (104) planes of
samples was also evaluated by repeating photocatalytic Cr2O3, respectively,42 suggesting that the introduced Cr3+ ions
experiment, and five consecutive cycles were tested, each were dispersed on the supporting TNSs surface in Cr2O3 forms
lasting for 120 min.
with poor crystallinity due to the low proportion of Cr:TiO2 and
low temperature of treatment. The energy dispersive
spectrometer (EDS) was measured to confirm the element
2.6.3 Evaluation of H2 evolution
The photocatalytic H2 evolution was carried out in a 500 mL compositions of samples. Besides Ti and O elements, Cr
top-irradiation quartz reaction vessel connected to an inclosed element was also observed from the EDS spectra (inset of Fig.
vitreous gas recirculation system. A 300 W Xe lamp was used 1 (d)), proving the existence of Cr element.
as the light source and a cutoff filter was applied to generate
visible light (λ > 420 nm). Typically, H2 production was and pore structure of samples were measured by N2
performed by dispersing 30 mg of catalyst in 50 mL aqueous adsorption-desorption curves. Figure. displayed the
The effects of the Cr3+ ions on the specific surface area (SBET
)
2
solution of triethanolamine (TEOA) (10 vol.%) under magnetic adsorption- desorption isotherms and pore size distribution
stirring. Prior to the photoreaction, the system was plots for bare TNSs and x-Cr-TNSs (x = 0.1, 0.5%, 1.5%, 2.0%). It
vacuumized for 30 min to remove air and the dissolved oxygen was clearly seen that all the samples exhibited a type-IV
completely. The reaction temperature was maintained at 6 °C isotherm, indicating the existence of mesopores (2-50 nm)
through the cooling water circulation system and the according to
accumulated amount of evolved gases was in situ monitored
every 30 min interval using the gas chromatography equipped
with a thermal conductive detector (TCD) (N2 as the carrier
gas, and 5 Å molecular sieve column).
3. Results and discussion
3.1 Structural characterization and morphology of photocatalysts
In this work, Cr2O3-loaded titanate nanosheets (Cr(III)-TNSs)
heterostructural composites were synthesized by the facile
hydrothermal strategy. To clarify the morphologies and
microstructures of the as-synthesized samples, the SEM and
high-resolution TEM (HRTEM) images of P25, bare TNSs and
2.0%Cr-TNSs were shown in Fig. 1. Compared with the granular
structure of raw P25 materials, it was clearly observed that the
samples displayed a flake-like and porous structure in Fig. 1 (b-
d). Raw P25 nanoparticles with high crystallinity could hardly
be found in the SEM or HRTEM images (Fig. 1), revealing that
most of the P25 nanoparticles had already transformed into
titanate nanosheets during hydrothermal process at 130 °C for
3 h, which was in accordance with our previous reports.38,39
From Fig. 1 (e), it displayed the lattice fringes, which showed
that the interplanar distance was ca. 0.54 nm and was
corresponding to the (201) plane of H2Ti3O7. Some researches
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the process might somewhat be influenced by the Cr3+ brought
into the reaction system. On account of their high octahedral
stabilization energy, the Cr3+ ions would be expected to occupy
the octahedral sites.44 Thus, it was assumed that a portion of
the introduced Cr3+ ions had a chance to replace Ti4+ or occupy
in the octahedral sites of TiO6 and stabilize at the interface
between the titanate layers and solution by forming Ti–O–Cr
bonds. Notably, the hysteresis hoops observed in Cr(III)-TNSs
showed a lower absorption than bare TNSs at higher range
(0.80 < P/P0 < 0.99), indicating the decrease of the stacking
pores and the piled nanosheets in Cr(III)-TNSs. Consequently,
the junctions between the titanate layers were probably
changed, which would promote the interlaminar connection
and inhibit the exfoliation of the single nanosheets from the
lamellar titanate, leading to the decrease in SBET of Cr(III)-TNSs.
The result was also confirmed by the distribution curves of
pore diameter and the pore volumes (Table 1). Meanwhile, all
the samples exhibited an uniform average pore size with
similar distribution (estimated using the adsorption branch of
the isotherm). These results illustrated that the introduction of
Cr3+ ions did not significantly change the textural properties of
TNSs. Such open mesoporous architecture with large surface
area and connected pore-system played an vital role for its
being able to improve the transport of reactants and products.
The XRD patterns of bare TNSs and Cr(III)-TNSs were shown
in Figure. 3. All the samples showed the observed diffraction
patterns attributed to anatase-rutile mixed phase of TiO2
(JCPDS No. 65-5714 and JCPDS No. 65-1119). The broadened
shapes and relatively weak intensities of characteristic peaks
revealed the poor crystallinity after treatment. During the
hydrothermal process, the crystalline phase of P25 precusor
was broken and subsequently a highly disorder structure was
formed. From the XRD results, only relatively weak
characteristic peaks of titania were observed, implying that
few P25 powders were residual and most of them were
transformed to titanate nanosheets (TNSs) with poor
crystallinity. Notably, the titanate nanosheets
Fig. 1. SEM images of (a) P25, (b-c) TNSs, (d) 2.0%Cr-TNSs; EDS spectra (inset) of (c)
TNSs, (d) 2.0%Cr-TNSs; (e-f) HRTEM images of 2.0%Cr-TNSs.
the Brunauer-Deming-Deming-Teller (BDDT) classification. The
corresponding hysteresis loops were type H3 at a relative
pressure (P/P0) range of 0.6-1.0, being representative for slit-
shape pores.43 The hysteresis loops was derived from the
aggregation of flake-like titanate fragments. As seen from
Table 1, the SBET of bare TNSs was 375.6 m2/g using the
Brunauer-Emmett-Teller (BET) method. Compared to the bare
TNSs, the Cr(III)-TNSs samples possessed smaller surface area
(SBET), which decreased from 351.88 to 229.6 m2/g with the
incremental Cr3+ content. Under the hydrothermal treatment,
Ti–O–Na bonds in the alkali-treated specimen formed and then
the proton
Fig. 2. The N2 adsorption/desorption isotherms and the corresponding pore size
distribution curves of x-Cr-TNSs (x = 0%, 0.1%, 0.5%, 1.5% and 2.0%).
Table 1 The textual properties and structural parameters of samples
Sample
Sa
DbPore (nm)
VcPore
(cm3/g)
1.05
VdFWHM (cm-1)
BET
(m2/g)
375.60
351.88
311.18
282.12
291.98
259.74
273.25
229.60
TNSs
10.81
9.38
16.37
16.09
15.21
14.91
14.48
14.19
14.04
13.61
0.1%Cr-TNSs
0.25%Cr-TNSs
0.5%Cr-TNSs
0.75%Cr-TNSs
1.0%Cr-TNSs
1.5%Cr-TNSs
2.0%Cr-TNSs
0.85
9.42
0.76
10.67
11.12
10.70
10.06
9.92
0.80
0.85
0.72
0.71
0.59
a
b
c
The specific surface area
The average pore diameter
The total pore volume
d
The FWHM of Raman peak (the predominant mode in anatase phase)
exchange of Na+ by H+ took place during the rinsing and acid
pickling, thus resulting in flake-like particles, which were
believed to be the layered titanate structures and each layer
was connected by the edge-sharing octahedral TiO6.41
However,
Fig. 3. XRD patterns of P25, TNSs and the spectra (inset) of x-Cr-TNSs (x = 0.0%, 0.1%,
0.25%, 0.5%, 0.75%, 1.0%, 1.5% and 2.0%).
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were exfoliated under low reaction temperature (130 °C)
before the transformation of layered titanate from nanosheet
to nanotube. Comparing the positions and intensities of the
diffraction peaks in all cases, it could be found that different
concentrations of Cr3+ showed negligible influences on the
phase structure of TNSs. Furthermore, no other peaks
corresponding to chromium oxide or hydroxide species were
found in Figure. 3 due to their low content and relatively weak
crystallization. Thus, it was inferred that these Cr3+ ions were
in amorphous oxide form and uniformly dispersed on the
surface of TNSs.
To further investigate the surface structure of Cr(III)-TNSs,
Raman spectra were tested. As shown in Figure. 4, all samples
displayed similar peak positions. The predominant peaks
identified at 146 cm-1 (Eg), broad bands around 400 cm-1 (Eg),
510 cm-1 (Eg) and 640 cm-1 (Eg) were corresponding to vibration
modes of anatase phase, while a broad band around 448 cm-1
(Eg) was assigned to rutile phase.45 The overlapped vibration
peaks of 456 cm-1 and the Raman peaks at 268 cm-1 were
attributed to the broken symmetry in the bent TiO6 layers and
the presence of the 2D lepidocrocite-type TiO6 octahedral
layers, respectively, suggesting the presence of titanate.1
As can be seen from the spectrum, with increasing amounts
of Cr3+ from 0% to 2.0%, obvious enhancement and decreased
FWHM could be observed at 146 cm-1 as the dominating
vibration mode of anatase structure. The result indicated that
the introduction of Cr3+ had a non-negligible impact on the Ti–
Fig. 4. Raman spectra of x-Cr-TNSs (x = 0.0%, 0.1%, 0.25%, 0.5%, 0.75%, 1.0%, 1.5% and
2.0%).
The surface chemical compositions and bonding states of x-
Cr-TNSs (x = 0.5% and 2.0%) samples were systematically
investigated by XPS analysis and compared with those of bare
TNSs. The fully scanned spectra (Fig. 5 (a)) for Cr(III)-TNSs
showed that Cr element existed in the samples besides Ti, O
and C elements, while the C element should be assigned to
impurities from air and the C 1s peak around 284.6 eV was
used for calibration. The XPS results with the areas
corresponding to the binding energies for the Ti 2p, O 1s and
Cr 2p regions were analyzed. From Fig. 5 (c), the Ti 2p core
level spectra for bare TNSs displayed two spin-orbit splitting
peaks observed at ~458.5 eV and ~464.2 eV, which were
assigned to Ti4+ 2p3/2 and Ti4+ 2p1/2, respectively, while for
2.0%Cr-TNSs (or 0.5%Cr-TNSs), the asymmetric Ti 2p spectra
showed similar peak positions of Ti4+ compared with those for
bare TNSs and could be fitted into two components of Ti3+
(457.5 eV and 463.1 eV) and Ti4+ (458.5 eV and 464.1 eV). It
could be observed that the loading of Cr2O3 caused the
generation or distinct increase of Ti3+. Since XPS was a
measurement technique mainly for surface characterization, it
could be deduced that most of Ti3+ might distribute in the
surface layers of TNSs. And Ti3+ was regarded as the favorable
state for the photocatalytical performance.48 During the
hydrothermal treatment, the reduction of Ti4+ probably
happened in the delamination and growth process of titanate
nanosheets, accompanied with the loading of Cr3+ and the
formation of Cr2O3, implying that the local chemical states of
external titanium ions were slightly influenced by the Cr3+ ions
attached to the surface layers of TNSs. As shown in Fig. 5 (c),
the broad and asymmetric O 1s signals for both bare TNSs and
Cr(III)-TNSs were fitted into one peak centered at ~530 eV
assigned to O2- ion of Ti–O bonds within TNSs, and a shoulder
located at ~531 eV, illustrating the presence of plentiful
surface hydroxyl groups or chemisorbed H2O molecules.49
When trivalent chromium species hybridized with superficial
reactive
O
lattice of TNSs. Considering the results from N2
adsorption/desorption measurement and XRD analysis, it was
deduced that the newly-formed Cr2O3 were loaded on the
surface of TNSs by Ti–O–Cr bonds during the hydrothermal
treatment, inducing a possible change in polarization of the Ti–
O lattice, leading to the enhancement of Raman peaks.46 From
the main peaks 147 cm−1 (anatase phase), it was also proved
by the slightly shift of vibrational frequency with the
attachment of Cr2O3 to the TNSs surface by Ti–O–Cr bonds.35,47
For the samples with 0.1-2.0 mol.% of Cr3+, Raman spectra
didn’t show any peak ascribed to Cr–O bending or stretching
modes owing to the much smaller quantity and size of Cr2O3
than those of TNSs, indicating that the loaded Cr2O3 was well-
dispersed on the surface. The results were also consistent with
electron microscope and XRD analysis.
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Fig. 6. UV-Vis diffuse reflectance spectra of reference Cr2O3, as-prepared bare TNSs,
and Cr2O3-loaded TNSs with various molar ratio of Cr3+
Fig. 5. XPS spectra for (a) survey spectrum; (b) Ti 2p region; (c) O 1s region; (d) Cr 2p
.
region of TNSs, 0.5%Cr-TNSs and 2.0%Cr-TNSs.
nm and one absorption shoulder at ~450 nm, as well as the
onset of absorption at ~400 nm. As a reference, the similar
trend of Cr2O3 absorption was also consistent with the
previous study.26 Besides the intrinsic band of Cr2O3, the broad
absorption band in the range of 600~800 nm was assigned to
sites of TNSs, the O 1s core level spectra for Cr(III)-TNSs
exhibited two new peaks at ~529 eV (Cr–O) and ~530.5 eV (Ti–
O–Cr), suggesting that the Cr3+ ions mainly existed on the
surface as oxides by the attachment of Ti–O–Cr bonds to the
interface between TNSs and chromium oxides.50 Fig. 5 (d)
showed that the XPS spectra of Cr 2p regions for Cr(III)-TNSs
(0.5%Cr-TNSs and 2.0%Cr-TNSs). Compared to individual Cr2O3
as reference, the Cr(III)-TNSs samples exhibited a similar trend
for Cr 2p core level spectra. For the Cr(III)-TNSs samples, the
dominant peaks at around 577 and 586.5 eV should be
ascribed to Cr 2p3/2 and Cr 2p1/2, respectively, indicating the
presence of Cr3+ loaded as Cr2O3 structure.51 In addition to a
slight amount of hexavalent chromium ions (~580.3 eV) found
in Cr(III)-TNSs by curve fitting in the Cr 2p spectra, a wide
shoulder peak located at lower binding energies (~578.6 eV)
was clearly observed, suggesting that a trace of chromium ions
existed with one or more valence states which were higher
than the valence of Cr3+. Thus, it was concluded that some Cr3+
might mostly transform into higher valence and remain excited
states by releasing the photo-induced electrons through the
Ti–O–Cr bonds. Through this interfacial charge transfer
approach, the change in the electronic states of Cr3+
consequently caused the broadening and a shift to the higher
binding energy of Cr3+ 2p signal peaks. As Cr2O3 and CrO2 were
both regarded as the thermodynamically stable oxides, the
peak of Cr 2p3/2 observed at ~575 eV should be assigned to a
tetravalent chromium ion in CrO2 chemical environment.52
4
the A2g→4T2g d–d transition of Cr3+ in an octahedral
surroundings. For individual Cr2O3, the large absorption peak
4
at ~450 nm could be assigned to A2g→4T1g of Cr3+ in an
octahedral environment.22 Note that, as for all the Cr(III)-TNSs
samples, the newly-formed dominant peaks were exhibited by
Cr2O3 except for the absorption from interband transition of
TiO2. And with increased loading amount of Cr2O3, the
absorption intensity at these new regions enhanced and the
initial absorption edge at ~480 nm was gradually bathochromic
shift to ~540 nm (inset) for the absorption of Cr2O3 aroused by
the charge transfer from 3d sub-band of Cr2O3 to the
conduction band (CB) of TNSs. The formation and tailing of
new adsorption shoulder demonstrated that the excited
electrons of Cr3+ were transferred to the CB of TiO2 via the
approach of direct IFCT (Interface Charge Transfer) during the
photocatalytic reaction. The newly-formed and extended
absorbance of samples in the visible region suggested that the
visible-light photoactivity of Cr(III)-TNSs could be remarkably
improved.
Figure.
7
showed the fluorescence (FL) emission
spectroscopy of bare TNSs and x-Cr-TNSs samples (x = 0.1%,
0.25%, 0.5%, 0.75%, 1.0%, 1.5%, 2.0%). It could be observed
that the intensities of FL emission peaks for Cr(III)-TNSs
samples were lower than that for bare TNSs (Fig. 7 (a)). In
addition, with the increase in loading amount of Cr2O3, the FL
emission intensity initially declined, and then increased instead
when reaching the minimum value for 0.5%Cr-TNSs (Fig. 7 (b)).
It was reported that the FL emission spectrum was the result
from the recombination of photo-generated electron-hole
pairs, the lower FL emission intensity indicated a lower
recombination rate 53. Therefore, the decline in FL intensities
for Cr(III)-TNSs samples was attributed to the decreased
number of recombination of electron-hole pairs, indicating the
enhanced separation efficiency for photo-induced carriers. The
3.2 Optical response and photochemical properties of bare
TNSs and Cr2O3-loaded TNSs samples
As the optical absorption properties were very important to
photocatalytical activity, bare TNSs and Cr(III)-TNSs were
tested by the diffuse reflectance UV-Vis spectroscopy. As can
be seen from the Fig. 6, bare TNSs showed only the absorption
in UV region with a cutoff wavelength at ~400 nm which
corresponded to the interband transition of TiO2. As for Cr(III)-
TNSs, the spectra exhibited the absorption from 620 nm to 800
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Cr2O3-loaded TNSs led to the effective quenching of the mg/L) aqueous solutions, respectively, as shown in Fig. 9. The
photoluminescence with different efficiencies. Consequently, blank tests without any catalysts showed that RhB and Cr(VI)
it was deduced that the Cr2O3 loaded on TNSs surface with were relatively stable and the photolysis process was
appropriate amount would act as traps to capture the photo- negligible. It could be clearly observed that the degradation
induced electrons, and thus enhanced the charge separation efficiencies of TNSs for RhB and Cr(VI) were both greatly
efficiency, which suggested the enhancement of photocatalytic enhanced after loading with Cr2O3. When the loading amount
activity; whereas excess loading Cr2O3 would result in of Cr3+ was relatively low (≤ 0.5 at.%), the degradation rate
agglomeration and serve as recombination centers during the continually enhanced with the increase of loading amount, and
reactions, leading to the decrease of photocatalytic efficiency.
0.5%Cr-TNSs showed the best photocatalytic efficiency and
activity among these samples. However, with further increase
in the Cr3+ content, the degradation efficiency decreased
instead. For RhB degradation, the reaction curves and reaction
rate histograms corresponding to the first-order kinetic
equation (ln(Ct/C0) = −Kappt) were directly observed from Fig. 9
(a) and Fig. 9 (c), respectively, where Kapp was the apparent
rate constant, C0 was the initial concentration of solution, t
was the reaction time, and Ct was the concentration of
solution at the reaction time of t. The visible-light degradation
rate (Kapp) of 0.5%Cr-TNSs was 2.9-fold as compared to bare
TNSs.
Fig. 7. (a) Fluorescence spectra of TNSs and Cr2O3-loaded TNSs using an exicitation of
300 nm; (b) fluorescence intensity at 452 nm and 468 nm as a function of loaded
amount.
Moreover, the photocatalytic reduction of toxic Cr(VI) in
K2Cr2O7 aqueous solution was also investigated as typical
heavy metal pollutants under visible light irradiation. It was
well known that the pH value of reaction medium also play a
crucial role in the photo-reduction of Cr(VI). The effect of the
solution pH on the photocatalytic reduction of Cr(VI) was
investigated for 0.5%Cr-TNSs sample and the photocatalytic
reduction efficiency and reaction rate at different pH values
under visible light were shown in Fig. S4 (a-b). It could be
observed that the lower pH could significantly improve
photocatalytic reduction of Cr(VI), and the higher reaction rate
reached 19.89×10-3 min-1 at pH = 1, while the reaction rate was
only 0.82×10-3 min-1 at pH = 9. Thus, the higher efficiency in
Cr(VI) degradation would be expected at lower pH due to the
existence of abundant H+.3 Here the photocatalytic activities of
various Cr2O3-loaded TNSs were evaluated by controlling the
initial pH = 3 in the solution. From Fig. 9 (a) and Fig. 9 (c),
similarly, the reaction curves and reaction rate histograms for
Cr(VI) degradation were conform to the first-order kinetic
equation (ln(Ct/C0) = −Kappt). Under visible light irradiation, the
photo-degradation rate (Kapp) of 0.5%Cr-TNSs was 4.1-fold as
compared to bare TNSs. Thus, for both the photocatalytic
oxidation of RhB and the reduction of Cr(VI), it was confirmed
that appropriate loading amount of Cr2O3 definitely enhanced
the photocatalytic activity of TNSs a lot, whereas when the
excess amount was introduced, at the least, the photo-
degradation rate declined, which was unfavourable for the
enhancement of photocatalytical
The photocurrent transient measurement method was used
to further examine the photoelectrochemical properties of the
samples and the results were shown in Figure 8. According to
the photocatalytic evaluation and PL results, it was found that
the promotion effects of Cr2O3 on the photo-activity of TNSs
were quite evident. Steady-state photocurrent responses of
Cr(III)-TNSs exhibited higher current density than that of bare
TNSs (Fig. 8 (a)), implying that the enhanced visible-light-
harvesting abilities and higher charge transfer efficiency in
Cr(III)-TNSs.54 When the introduced content of Cr3+ was 0.5
at.%, the current density was improved dramatically, which
was in agreement with the high photocatalytic degradation
rates of the 0.5%Cr-TNSs for RhB and Cr(VI). When a contrast
was made between the Cr2O3-loaded TNSs and mechanical
mixed Cr2O3&TNSs samples, the Cr2O3-loaded TNSs samples
displayed a much higher current density (Fig. 8 (b)), suggesting
that the loaded Cr2O3 was well dispersed on the TNSs with
superior interfacial contact.
Fig. 8. Surface photocurrent curves for the photoelectric behavior of (a) TNSs
and Cr2O3-loaded TNSs; (b) Cr2O3-loaded TNSs and Cr2O3-TNSs mixed samples
(the Cr:TiO2 ratios of 0.5% and 1.0% were chosen).
3.3 Evaluation of visible-light photocatalytic activities
3.3.1 Photocatalytic degradation of RhB and Cr(VI)
The visible light photocatalytic activities of all samples were
evaluated by the degradation of RhB (20 mg/L) and Cr(VI) (20
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shown in Fig. S3 (a-b), as compared to the pristine sample, it
could be clearly observed that the sample remained almost
the same structure even after the five cyclic photocatalytic
tests, and the influence on the structure of the sample was
negligible during the reaction. In addition, confirmed by the
fitting results (Fig. S3 (c-d)) and the elemental analysis (Table
S1) from XPS spectra, the chemical states and contents of Cr
and Ti almost retained constant after the photocatalytic
reactions by turns, demonstrating that the as-prepared Cr2O3-
loaded TNSs photocatalyst possessed excellent photocatalytic
stability and reusability under visible
Fig. 10. Cyclic performance testing for RhB solution (a); Cr(VI) solution (b) in the
Cr2O3-loaded TNSs system. The recycle stability of photocatalytic efficiency for
RhB solution and Cr(VI) solution was tested using the 0.5%Cr-TNSs sample.
light irradiation.
Fig. 9. Visible-light photocatalytic performance evaluation of TNSs and Cr2O3-loaded
TNSs: photocatalytic degradation of (a) RhB and (b) Cr(VI); reaction kinetics (ln (C/C0) =
−Kappt) of (c) RhB and (d) Cr(VI); reaction rate constant (Kapp) values of (e) RhB and (f)
3.3.2 Photocatalytic H2 generation
Cr(VI) under visible-light irradiation. The initial concentrations of RhB and Cr(VI) were It was commonly recognized that the nanosize metal as
both 20 ppm. The initial pH of RhB and Cr(VI) solution were 7.0 and 3.0, respectively.
cocatalyst, such as Pt, Au and Pd, promoted the catalytic
efficiency due to the build-up Mott-Schottky barrier, which
performance. It could be ascribed to the agglomeration of
Cr2O3, leading to the decrease of interfacial charge transfer
efficiency. The photocatalytic performance was just consistent
with the result from the FL emission and photocurrent spectra.
Furthermore, for Cr(III)-TNSs system, the formed
heterostructure of binary oxides was much more favorable for
the degradation of pollutants and the enhancement on visible-
light catalytic activity as compared to single semiconductor
oxides. Meanwhile, as shown in Fig. 10, the photo-degradation
of RhB and Cr(VI) were both monitored over five cycles by
using 0.5%Cr-TNSs sample for evaluating the photocatalytic
stability under the same conditions. Since regeneration and
reuse of photocatalyst played an important role on its practical
application, the sample after the adsorption-photocatalysis
process was treated with desorption and regeneration for the
reuse of next cycle test during the every cycle of Cr(VI)
reduction. In a series of processes of adsorption (Ads),
desorption (Des) and regeneration (Reg), Fig. S1 and S2
showed the further characterization of different-stage samples
in the whole cycles by XRD, Raman and XPS analysis. And the
results exhibited the good stability of recovered sample. The
sample reused for the cycle test of RhB degradation was also
treated by the regeneration step. The photodegradation rates
almost retained constantly and didn’t exhibit any significant
decline during the five consecutive cycles. Even after the cyclic
performance tests for the degradations of RhB and Cr(VI), the
Ti and Cr 2p XPS spectra, XRD patterns and Raman spectra of
the testing sample were also investigated, respectively. As
faciliated the charge transport and had the lower over-
potential for hydrogen-production reaction kinetics.36,55 Here
the as-prepared TNSs were employed as the supporting
material for Pt loading by photodeposition in this work.
Since Pt was known as the most effective cocatalyst in
achieving water splitting, the effect of Pt depositing on TNSs
was investigated. Fig. 11 (a-b) exhibited the photocatalytic
hydrogen evolution test over TNSs and Cr(III)-TNSs with Pt
(0.25 wt.%) via in-situ photodeposition. As expected, when
0.25 wt.% of Pt was loaded on the bare titanate nanosheets,
the activity of the metal/nanosheets for photocatalytic
hydrogen evolution showed an unnegligible enhancement.
Compared with the Pt0.25/TNSs, the samples with the identical
content of Pt (Pt0.25/x-Cr-TNSs, x = 0.1%, 0.25%, 0.5%, 0.75%,
and 1.0%) exhibited a remarkable enhancement on the H2
generation rate, and Pt0.25/0.5%Cr-TNSs showed the relative
high performance, giving the H2 evolution rate of ~473 μmol h-
1
g-1. When the loading content of Cr2O3 further increased, the
H2 production efficiencies of the samples (Pt0.25/x-Cr-TNSs, x =
1.5% and 2.0%) decreased instead. Nevertheless, the Pt-
deposited Cr(III)-TNSs
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Fig. 11. Visible-light photocatalytic H2 evolution of platinized TNSs and Cr2O3-loaded
TNSs: (a) accumulative amount of H2 evolution; and (b) H2 evolution rate of Pt0.25/TNSs
and Pt0.25/x-Cr-TNSs (x = 0.1%, 0.25%, 0.5%, 0.75%, 1.0%, 1.5% and 2.0%); (c) Cyclic
performance tests for H2 evolution were conducted using Pt0.25/0.5%Cr-TNSs sample.
with an appropriate proportion of Cr2O3 still could bring about
a significant enhancement on the H2 generation rate compared
with the Pt0.25/TNSs, which implied that the optical response of
Cr2O3 in the TNSs must play a crucial role in the photocatalytic
H2 production. It was demonstrates that the loaded Cr2O3 was
a good photosensitizer with a slight amount of Pt deposited as
the co-catalyst for the design of high-performance
photocatalysts. In addition, the photocatalytic stability of
Pt0.25/0.5%Cr-TNSs was also demonstrated for H2 evolution
(Fig. 11 (c)). In each cycle, the system was deaerated by
evacuation before the next test to make sure there was no
residual gas, and the process was same as the photocatalytic
H2 evolution mentioned above. In the first two run, there was
a slight decline in the total H2 production, which was 40.4
μmol and 39.1 μmol, respectively, a little less than that of
original materials (42.6 μmol). However, even after five cycles,
the high photocatalytic performance was mostly retained at
around 38.5 μmol in the total H2 production, demonstrating a
good stablility and reusablity. As the catalyst was repeatedly
used for cyclic test, the decrease of H2 evolution rate might be
mainly due to the consumption of sacrificial reagents and the
influence of intermediate products during the reaction.
Compared with some catalysts using organic and
organometallic molecules as photosensitizers, the Pt/Cr-TNSs
resulted from the loading of Cr2O3 as photosensitizer was
superior in terms of stability, and the nice reusability of
Fig. 12. Capture experiments by introducing different trapping agents in the
photocatalytic degradation process of RhB (a); and Cr(VI) (b) in the Cr2O3-loaded TNSs
system. The capture experiments were performed using the 0.5%Cr-TNSs sample.
scavengers for Cr(VI) degradation, respectively.56,57 Fig. 12
displayed the results of the capture experiments for 0.5%Cr-
TNSs sample. It was clearly observed that the
photodegradation of RhB was almost not affected by the
presence of TBA, suggesting that OH· was not the main active
specie. While with the addition of BQ and TEOA, respectively,
the degradation efficiencies of RhB both decreased obviously.
And it further displayed that the photocatalysis process of RhB
was most greatly suppressed in the case of TEOA as a
-
scavenger. This revealed that ·O2 and h+ were both the
dominant active species in the photocatalytic reaction,
determining photoactivity for the visible-light-sensitive TiO2-
based titanate nanosheets with loaded Cr2O3 in this work. For
the reduction of Cr(VI), the addition of hole scavenger (TEOA)
could significantly enhance the photocatalytic efficiency of
0.5%Cr-TNSs sample, while the degradation rate evidently
decreased with the addition of electron scavenger (K2S2O8).
Since the presence of TEOA as a hole scavenger could retard
the recombination of photogenerated carriers, and K2S2O8 as
an electron scavenger could consume the photogenerated
electrons, which inhibited the photocatalytic reaction, the
results confirmed that the photogenerated electrons were
dominated in the photocatalytic reduction of Cr(VI).
platinized Cr(III)-TNSs (0.25 wt.% Pt) indicated
attachment of Pt nanoparticles to the TNSs surface.
a firm
4. Discussion on the mechanism for the
enhanced photocatalytic activity of Cr2O3-
loaded TNSs
4.1 Photocatalytic degradation of RhB and Cr(VI)
In order to get insights into the photocatalytic mechanism of
heterostructural Cr(III)/TNSs system, capture experiments
were conducted by introducing different trapping agents to
investigate the reactive radicals involved in the reaction
process of RhB and Cr(VI) degradation, respectively. The
detailed experiments were the same as the photo-degradation
experiments mentioned above. In the experiments, tert-butyl
alcohol (TBA), triethanolamine (TEOA) and 1, 4-benzoquinone
(BQ) were employed as the hydroxyl radical (OH·), hole (h+)
To explain the enhancement of the visible-light catalytical
activity and better understand the mechanism, the band
structures of Cr2O3 and TNSs were investigated by the UV-vis
absorption and XPS valence band spectra (Fig. 13), and a
possible energy level diagram for the charge separation and
transfer of Cr2O3/TNSs heterostructure in the degradation of
-
and superoxide radicals (·O2 ) scavengers for RhB degradation,
respectively. And triethanolamine (TEOA) and potassium
persulfate (K2S2O8) were used as hole (h+) and electron (e-)
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Fig. 13. (a) Tauc plots of as-prepared bare TNSs, x-Cr-TNSs (x = 0.5% and 2.0%) and
(inset) Cr2O3 for the estimation of optical band gap. The intercepts of the linear part
well fit to the experimental data (α = 0) gave the values of the fundamental band gap
and the transition band gap; (b) XPS valence-band spectra recorded for the as-prepared
bare TNSs and Cr2O3. The intercepts of the linear part well fit to the experimental data
gave the valence band positions.
generated carriers. Hence, in the present of Cr2O3/TNSs
system, the charge separation and photocatalytic activity were
remarkably enhanced, which was consistent with the PL
spectra and photocurrent results.
Under visible light irradiation, these photo-excited electrons
transferred from 3d-orbital energy level of Cr2O3 nanoparticles
into the CB of TNSs, and then the redox of CrIII-CrIV occurred
[Eqs. (1)-(2)]. Combined with the results of trapping
experiments, as the CrIV excited state with highly active holes
(h+) generated by IFCT contributed to oxidation, it was deemed
that the more negative the redox potential of the reductant,
the more active for its oxidation. Thus, the newly-formed CrIV
possessing strong oxidative ability were supposed to oxidize
substrates and return to the initial and stable ground state CrIII.
Simultaneously, by the approach to direct IFCT, these activated
electrons gathering in the CB of TNSs were captured by O2
molecules, or involved in the fast reduction reaction with
substrates directly [Eqs. (3)]. The majority of them were
RhB and Cr(VI) was schematically illustrated in Scheme 1 (a).
The intrinsic band gap energy for TNSs (n-type semiconductor)
was found to be 3.17 eV from the absorption edge of TNSs
(Fig. 13 (a)). The valence band edge measured from the XPS
spectra for TNSs was positioned at about 2.98 eV (Fig. 13 (b))
and the conduction band edge of TNSs was consequently
determined at −0.19 eV. While Cr2O3 was a p-type extrinsic
semiconductor at low temperature,58 and the predominant d-d
transition band gap was valued at ~2.35 eV except for the
intrinsic band of Cr2O3. According to the XPS valence band
spectra,the EVB level for Cr2O3 standed at ~2.33 eV (vs. NHE),
which was in good agreement with that as reported.58 While
the potential of the CrIII/CrIV redox couple (E0) was reported to
be 2.1 eV.22 Thus, the energy gap (Eg) between the TNSs ECB
and the Cr2O3 E0 was 2.29 eV, corresponding to the light
energy of wavelength (λ = 540 nm) by the equation: λ =
1240/Eg. And it was in good agreement with the result of
photocurrent action on the significant enhancement under
visible light with wavelength above 420 nm (≤ 540 nm). This
indicated that the high energy electrons of Cr2O3 could be
induced at wavelength shorter than 540 nm and then
transfered thermodynamically to the CB of TNSs. Hence, the
visible-light ranged from 420 to 540 nm wavelength was
beneficial to the directly interfacial transfer of high energy
electrons from the d-orbital energy level (E0) of Cr2O3 to the CB
of TNSs. Interestingly, the similar effect of surface modification
on the photocatalytic activity of TiO2-based materials had been
reported and exemplified with the combination of theoretical
simulations and detailed experiments by the group of M.
Nolan.24 This report had delivered new coupling of molecular
scale metal oxides and TiO2, as well as revealing the key effect
of surface modification aroused by the formation of new
interfacial Metal−O−Ti bonds, e.g., band gap modification and
charge carrier localization, which would promote both the
visible-light response and charge separation. The new routes
of charge transfer would be generated by the dispersed 3d
orbitals of Cr2O3 forming new bonds (Ti−O−Cr) to the surface
of TNSs in the status of heterostructure, resulting in strong
electronic coupling. In general, it could be deduced that when
the heterostructure was formed between TNSs and Cr2O3, an
electric field tended to form at the interface. In the equilibrium
state of the hetero-structural interface, negative charges
appeared on the side of p-type Cr2O3 while positive ones
aggregated on the n-type TNSs caused by the formed
interfacial electric field. Thus, it could be deduced that the
photo-induced high energy electrons produced from the 3d
sub-band of Cr2O3 could transfer thermodynamically to the CB
of TNSs through interface electric field. However, for single
Cr2O3, these visible-light excited electrons in the CrIV excited
state were easily relaxed to the ground state (CrIII) in extremely
short period, leading to the high recombination rate for photo-
-
directly reduced to form ·O2 radical anions through one-
electron reduction; only few O2 molecules accepted more
electrons and then, •OH radicals generated during a series of
reactions [Eqs. (4)-(6)]. Subsequently, these reactive species
with strong oxidative ability participated in the degradation of
pollutants (e.g. RhB) [Eqs. (7)-(9)]. In general, the
-
photogenerated holes (h+) and superoxide radicals (·O2 ), as
highly reactive species, both played a significant role in the
photocatalytic oxidation of dye molecules, while the
photogenerated electrons (e-) participated in the reduction of
heavy metal pollutants (e.g. Cr(VI)). In addition to the fast
reaction with absorbed substrates, the slight holes left in the
motivated Cr2O3 nanoparticles might oxidize OH- groups to
create ·OH radicals as well [Eqs. (10)]. Notably, the tetravalent
chromium ions (Cr4+) in the composites also played a role
during the photocatalytic process though the content of Cr4+
was extremely low. When involving in the photocatalytic
reactions, it was deemed that these concomitant Cr4+ could
assist the oxidizing reaction with adsorbed substrates during
the photocatalytic process. Via directly reacting with pollutants
(e.g. RhB) or oxidizing the surface hydroxyl groups (OH-) into
hydroxyl radicals (·OH), the Cr4+ could accept electrons and
then was reduced to Cr3+, thus separately accelerating the
oxidation process and the generation of reactive species,
which was definitely beneficial to the reactions during the
photocatalytic process.
In the oxidation of absorbed organic substrates (RhB
molecules) over bare TNSs under visible light irradiation, the
self-photosensitized oxidation of surface-adsorbed RhB
molecules possibly happened and injected electrons into the
CB of TNSs.53 Since the DRS result of bare TNSs showed no
response in the visible-light region, its visible-light catalytical
ability was majorly from the self-sensitive decomposition of
RhB molecules. When it comes to Cr2O3-loaded TNSs, the
excited RhB molecules would inject electrons into the CB of
TNSs [Eqs. (11)-(12)] or directly involve in the oxidative
reactions [Eqs. (7)-(9)]. Then these electrons in the CB of TNSs
could participate in the photoreaction processes mentioned
above as well [Eqs. (3)-(6)]. Thus, the generation of superoxide
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-
anion (·O2 ) and photogenerated holes (h+) were accelerated adsorbed oxygen (Pt−Oads).59 In the case of adding isopropanol
via interfacial charge transfer (IFCT) during the degradation to the suspension, the large amount of unsaturate surface
process and beneficial for photocatalytic degradation caused atoms on the photoreduced Pt might partially bond with
60
by their strong oxidizability.
oxygen atoms, forming Pt−Oads
.
However, the 4f7/2 peaks
Cr2O3/TNSs
(1)
+
hν (Vis)
→
CrIII*(e−
+
h+)/TNSs assigned to Pt0 and Pt−Oads were located at higher bingding
energies than those of the Pt foil (ca. 70.2 and 71.5 eV,
(e−) respectively) as reported. This shift was attributed to the
decrease in the polarization energy with smaller size of the Pt
CrIII*
(2)
(e−
+
h+)/TNSs
→
CrIV(h+)/TNSs
O2ads + e− → ·O2
(3)
or H2CrO4 + 6H+ + 3e− → Cr3+ + 4H2O particle compared to that of bulk Pt and the loaded Pt species
on TNSs were easier to become somewhat electron-
−
ads
2e−
(4)
+
O2ads
+
2H2O
→
H2O2
+
2OH− deficient.37 And it should be noted that the broadened peaks
for Pt species were possibly the result from the non-uniformly
(5) distribution of Pt paticles and the formation of Pt
OH− agglomerates on the TNSs in this work.61
−
H2O2 + ·O2 ads → ·OHads + OH− + O2
H2O2
(6)
+
e−
→
·OHads
+
As shown in Fig. 15, to further investigate the photo-
Pollutants (e.g. RhB) + h+ (CrIV) → Degradaꢀon products + CrIII absorbance properties, the UV-vis diffuse reflectance spectra
(7)
(DRS) of bare TNSs and Cr(III)-TNSs before and after
Degradaꢀon products photodeposition of Pt were analyzed. Compared with bare
TNSs,
−
ads
Pollutants (e.g. RhB)
+
+
·O2
→
→
(8)
Pollutants (e.g. RhB)
(9)
OH− ads+ h+ (CrIV)→ ·OHads + CrIII
·OHads
Degradaꢀon products
Table 2 Pt 4f data and their valence states obtained from the XPS analysis spectra for
Pt0.25/TNSs and Pt0.25/x-Cr-TNSs (x
photodeposition with Pt
= 0.5% and 2.0%) at the 0.25 wt.% level of
(10)
RhB + hν (Vis) → RhB* + e−
BEa (eV)
Ptδ/Ptc (%)
Pt/Tib
(%)
Sample
(11)
RhB* + e− + TNSs → RhB* + TNSs(e−)
Pt0
Pt-Oads
72.41
72.53
72.45
Pt0
Pt-Oads
26.83
27.40
27.27
TNSs
70.74
70.81
70.78
0.96
0.93
0.97
73.17
72.60
72.73
(12)
0.5%Cr-TNSs
2.0%Cr-TNSs
4.2 Photocatalytic H2 evolution
a
b
c
The binding energies of Pt 4f7/2
With noble metal Pt photodeposited on the surface of
samples, participating in the photocatalytic water splitting, the
synergistic effect between Cr2O3 and Pt was also considerable
in the process of H2 evolution under visible-light irradiation for
Cr(III)-TNSs.
The XPS peak area ratios for Pt 4f relative to that for Ti 2p
The XPS peak area ratios for Pt0 and Pt-Oads 4f relative to that for Pt 4f
except for the transition band and characteristic absorption
band in Cr(III)-TNSs, the optical absorption of Pt0.25/TNSs and
Pt0.25/Cr(III)-TNSs in the visible light region both increased,
owing to the localized surface plasmon resonance (LSPR) of Pt
nanoparticles on the pore-wall of TNSs.55,59 In addition, from
Fig. 15 (a), the results for both Pt0.25/0.5%Cr-TNSs and
Pt0.25/2.0%Cr-TNSs showed the broad absorption from 620 nm
to 800 nm and the absorption shoulder at ~450 nm, which was
in accordance with the DRS results for Cr(III)-TNSs except for a
slight decrease in absorbance. According to Fig. 15 (b), the
transient photocurrent of the Pt-deposited samples were
improved obviously. Photocurrent responses of Cr(III)-TNSs
showed higher current densities than that of bare TNSs,
suggesting the higher charge separation efficiency for
Pt/Cr(III)-TNSs. When the introduced content of Cr3+ was 0.5
at.%, the current density was readily improved, which was
consistent with the high photocatalytic activity of H2 evolution
for Pt0.25/0.5%Cr-TNSs sample.
Fig. 14. XPS analysis spectra of Pt 4f for (a) Pt0.25/TNSs; (b) Pt0.25/0.5%Cr-TNSs; (c)
Pt0.25/2.0%Cr-TNSs. The Pt0.25/TNSs, Pt0.25/0.5%Cr-TNSs and Pt0.25/2.0%Cr-TNSs
were obtained by photodepositing Pt with the same weight ratio (Pt:TNSs) at
0.25 wt.%.
Fig. 14 showed the XPS analyses for Pt/TNSs and Pt/x-Cr-
TNSs (x = 0.5% and 2.0%). The Pt:TNSs weight ratio for all
samples was theoretically at 0.25 wt.%. From the fitting
spectra of the samples, no appreciable difference in peak
positions was identified for the Pt 4f spectra. And for these
samples, the Pt 4f spectra showed a similar trend and the
proportions of Pt:Ti and Pt0 (Pt-Oads):Pt were calculated by
their peak areas in Table 2. The peaks at ~70.8 and ~74.4 eV
were attributed to 4f7/2 and 4f5/2 electrons of Pt0 (metallic Pt),
respectively. In addition, the 4f7/2 peak, observed at ~72.4 eV,
had a lower BE than that associated with PtIIO species (ca. 73.4
eV). Thus the two minor peaks could be assigned to Pt with an
These results proved, when Pt nanoparticles were loaded on
the surface of TNSs as cocatalyst, the LSPR effect accelerated
the separation of photo-generated carriers and extended the
range of photoabsorption.55,59 Moreover, upon visible light
excitation, the surface 3d sub-band electrons on Cr2O3 tended
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to be excited to high-energy level and transfer into the CB of 2H+ + 2e-
ARTICLE
→
H2
TNSs through the Cr(III)/TNSs interfaces for the surface (16)
protonic
TEOA + h+ (CrIV)
→
TEOA+ + CrIII
reduction reaction [Eqs. (13)-(14)]. The remaining holes in (17)
Cr2O3 could be occupied by electrons in the presence of
appropriate electron donors, such as TEOA in the current study
5. Conclusions
for H2 production. Subsequently, the sacrificial reagent reacted
with photo-generated holes (h+) to promote the separation of
electrons-hole pairs and more electrons (e-) enriched in the CB
of TNSs could react with H+ deriving from the ionization of H2O
to produce H2 through metallic Pt which served as active sites
[Eqs. (15)-(17)]. However, trace H2 evolved was observed
during the photocatalytic tests for Cr(III)-TNSs samples with
the absence of Pt, due to the large H2 evolution over-potential
on TNSs surface and the fast backward reaction of H2 with
dissolved O2 in the solution. As TiO2-based materials, the result
The visible-light-sensitive titanate nanosheets with loaded
Cr2O3 was synthesized via a simple one-step hydrothermal
method for improving the visible-light photocatalytic activity.
These chromium oxide species were well-dispersed on the
surface of TNSs and preferably proposed to exist as Cr2O3-like
structure with low crystallinity by forming Ti−O−Cr bonds. The
as-prepared samples showed obviously flake-like structure,
possessing high specific surface area (300-400 m2/g).
Accompanied with the loading of Cr2O3, the layered structure
of nanosheets was influenced during the hydrothermal growth
of TNSs. And the newly-formed heterostructure (Ti−O−Cr) at
the interface was demonstrated to expand the visible-light
absorption region for TNSs and achieve the efficient separation
and transport of photo-induced carriers via interfacial charge
transfer (IFCT). Based on the comparison of degradation
efficiencies, it turned out that the Cr2O3-loaded TNSs (Cr(III)-
Fig. 15. (a) UV-Vis diffuse reflectance spectra of as–prepared bare TNSs and
Cr2O3-loaded TNSs before (inset) and after photodepositing Pt (Pt0.25/TNSs,
Pt0.25/x-Cr-TNSs (x = 0.5% and 2.0%)); (b) Surface photocurrent curves for the
photoelectric behavior of Pt0.25/TNSs and Pt0.25/x-Cr-TNSs (x = 0.5%, 1.0% and
2.0%).
was common in most of the studies.55,62 Therefore, an
alternative mechanism should be attributed to the IFCT at the
Cr(III)/TNSs interface, which could promote the electron-hole
separation of Cr2O3 under visible light irradiation (Scheme 1
(b)). This would allow a longer lifetime for photo-induced
electrons to diffuse toward metallic Pt, which served as
electron-sink and active sites for proton reduction to H2 by
lowering the over-potential under sacrificial agent/water
system in the present study,55 and favorably suppressed the H2
back-oxidation derived from its weak adsorption and
motivation ability to O2 and H2 molecules. In brief, the overall
high photocatalytic activity of Pt/Cr2O3 decorated TNSs in H2
production can be ascribed to the synergy between the visible-
light sensitization of Cr2O3 and the electron-sink effect of Pt
nanoparticles. And furthermore, compared to traditional bulk
TiO2 materials, the unique structure of titanate nanosheets
with large surface area could strengthen the light harvesting
ability and increase the contact between the Pt/Cr(III)-TNSs
and the reagent, which might be in favor of solar-to-fuel
conversion.
Scheme 1. Schematic diagram for the charge separation and transfer of the
photocatalysts of Cr2O3-loaded TNSs in the process of degradation of RhB and Cr(VI) (a);
and in the process of H2 evolution (b) under visible-light irradiation.
Cr(III)/TNSs
(13)
+
hν (Vis)
→
CrIII* (e−
+
h+)/TNSs
CrIII* (e− + h+)/TNSs
→
CrIV (h+)/TNSs (e−)
TNSs) with the appropriate content exhibited an significantly
improved photocatalytic activity for oxidation of RhB and
reduction of Cr(VI) under visible light irradiation, which could
be attributed to the retarding of the charge recombination and
the effective electron transfer from Cr2O3 to TNSs. In addition,
Pt
(14)
TNSs (e−)
(15)
→
TNSs + Pt (e-)
Pt
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(photosensitizer) and metallic Pt (electron sink) that readily
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Cr(III)-TNSs was confirmed and the alternative mechanism for the
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Acknowledgements
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This work was financially supported by the National Basic
Research Program of China (973 Program, Nos.
2009CB939704, and 2009CB939705), and the Large-scale
Instrument and Equipment Sharing Foundation of Wuhan
University (Nos. LF20150681 & 20150683), and Science and
Technology Projects of the State Grid Zhejiang Electric Power
Corporation (No. 5211DS15002H).
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Cr2O3-loaded titanate nanosheets with large surface area shows excellent visible–light
catalytic activity for Cr(VI) degradation and H2 generation.