The synthesized composite exhibited excellent photocatalytic performance for rhodamine B RhB degradation under visible light irradiation.. More importantly, a hetero-junction structure w
Trang 1Z-scheme SnO 2 x /g-C 3 N 4 composite as an ef ficient photocatalyst
Yiming Hea,n, Lihong Zhanga, Maohong Fanc, Xiaoxing Wanga, Mikel L Walbridgec,
a
Department of Materials Physics, Zhejiang Normal University, Jinhua 321004, China
b
Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China
c
School of Energy Resources, University of Wyoming, Laramie, WY 82071, United States
a r t i c l e i n f o
Article history:
Received 17 September 2014
Received in revised form
29 January 2015
Accepted 30 January 2015
Available online 3 March 2015
Keywords:
Photocatalysis
SnO 2x
g-C 3 N 4
Z-scheme
Visible light
a b s t r a c t Highly efficient SnO2x/g-C3N4composite photocatalysts were synthesized using simple calcination of
g-C3N4and Sn6O4(OH)4 The synthesized composite exhibited excellent photocatalytic performance for rhodamine B (RhB) degradation under visible light irradiation The optimal RhB degradation rate of the composite was 0.088 min1, which was 8.8 times higher than that of g-C3N4 The SnO2x/g-C3N4 composite also showed high photocatalytic activity for CO2reduction and photodegradation of other organic compounds Various techniques including Brunauer–Emmett–Teller method (BET), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PL) and an electrochemical method were applied to determine the origin of the enhanced photoactivity of SnO2x/g-C3N4 Results indicated that the introduction of SnO2xon g-C3N4increased its surface area and enhanced light absorption performance More importantly, a hetero-junction structure was formed between SnO2xand g-C3N4, which efficiently promoted the separation of electron–hole pairs by a direct Z-scheme mechanism to enhance the photocatalytic activity This study might represent an important step for the conversion of solar energy using cost-efficient materials
& 2015 Elsevier B.V All rights reserved
1 Introduction
Photocatalysis has attracted remarkable interest because it offers a
sustainable pathway to drive chemical reactions such as degradation
of organic pollutants, water splitting, and carbon fixation [1–3] A
significantly efficient, stable, and inexpensive photocatalyst that can
harvest visible light is considered the key factor for the economical
application of photocatalysis Therefore, development of an efficient
visible-light-driven photocatalyst has been extensively investigated
Although various novel visible-light-responsive materials, such as
CaBi2O4, BiVO4, Ag3PO4, etc.[4–8], have been reported, only a few of
these materials have attracted much interest Graphitic carbon nitride
(g-C3N4) is an outstanding photocatalyst because of its high
reduci-bility and visible-light adsorption [8] In addition, g-C3N4 is also
inexpensive because it is a metal-free semiconductor and can be
synthesized by simple heating of urea or melamine at 500–600 1C
However, pure g-C3N4 exhibits non-satisfactory photocatalytic ef
fi-ciency, which can be partly attributed to its low surface area Hence,
fabricating nanostructured g-C3N4 to increase the surface area has been suggested to enhance photocatalytic activity[9] However, the promotion effect of this approach is limited Increasing studies have shown that pure g-C3N4photocatalyst is hardly competent for ef fi-cient organic pollutant degradation or solar fuel generation because of the disadvantageous rapid charge recombination More and more researchers pay attentions on multi-component photocatalysts that comprise of g-C3N4and another semiconductor g-C3N4-based com-posite photocatalysts have become a hot topic in photocatalysis
Up to date, numerous of g-C3N4-based composites, such as LnVO4
(Ln¼Sm, Dy, Bi, La)/g-C3N4 [10–13], TaON/g-C3N4 [14], Ag3VO4
/g-C3N4[15], CdS/g-C3N4[16], AgX (X¼Cl, Br, I)/g-C3N4 [17,18], MoO3
/g-C3N4 [19], S-TiO2/g-C3N4 [20] and BiOCl/g-C3N4 [21], have been reported The composite photocatalysts present much higher activity than pure g-C3N4, which is mainly attributed to the coupling effect between g-C3N4and the semiconductor Two mechanisms are usually applied to explain the synergetic effect Thefirst mechanism is the double-charge transfer mechanism[10–18], in which the photogener-ated electrons in the conduction band (CB) of g-C3N4are injected into the CB of another semiconductor Meanwhile, the photogenerated holes from the semiconductor transport to the valence band (VB) of g-C3N4 As a result, the electrons and holes are separated, and the
Contents lists available atScienceDirect
journal homepage:www.elsevier.com/locate/solmat
Solar Energy Materials & Solar Cells
http://dx.doi.org/10.1016/j.solmat.2015.01.037
0927-0248/& 2015 Elsevier B.V All rights reserved.
n Corresponding authors Tel.: þ86 579 83792294; fax: þ86 579 83714946.
E-mail addresses: hym@zjnu.cn (Y He), ying-wu@zjnu.cn (Y Wu).
Trang 2photocatalytic efficiency is enhanced The other mechanism is the
direct Z-scheme-type mechanism[19–21], in which the
photogener-ated electrons in the CB of the coupled semiconductor are injected into
the VB and annihilate the holes of g-C3N4 This process also facilitates
electron–hole separation, and suppresses charge recombination,
thereby improving the photocatalytic activity Meanwhile, given the
strong reducibility and oxidability of the electrons on g-C3N4and holes
on the coupled semiconductor, Z-scheme composites usually present
high photocatalytic activity For example, Katsumata et al synthesized
an Ag3PO4/g-C3N4 composite and applied it in the photocatalytic
oxidation of methyl orange (MO)[22] Compared with pure Ag3PO4,
only one-third of irradiation time was needed for the Z-scheme hybrid
to completely degrade MO solution Moreover, we fabricated an
efficient Z-scheme photocatalyst MoO3/g-C3N4 [19]; the prepared
MoO3/g-C3N4composite degraded MO 10.4 times faster than g-C3N4
under visible light The promotion effect of MoO3is nearly the best of
the reported dopants Hence, this Z-scheme-type composite shows
great potential as an efficient photocatalyst for conversion of solar
energy to chemical energy However, to the best of our knowledge,
only a few studies have focused on this photocatalyst and the number
of efficient Z-scheme photocatalysts is still limited Additional
inves-tigations are necessary to develop this type of photocatalysts further
In this paper, an efficient Z-scheme type photocatalyst, SnO2 x/
g-C3N4composite is presented Sn2þ-doped SnO2(SnO2x), which
was prepared by heating Sn6O4(OH)4 in N2, was chosen as the
doping semiconductor because of its low CB position and
cap-ability of harvesting visible light[23] The decoration of SnO2x
remarkably promoted the photocatalytic activity of g-C3N4 in
rhodamine B (RhB) degradation and CO2photoreduction
Investi-gation of the structure, surface area, and optical property of the
composite showed that the relatively high photoactivity of
SnO2x/g-C3N4composite could be ascribed to a direct Z-scheme
mechanism The Z-scheme mechanism of the SnO2 x/g-C3N4
photocatalyst was demonstrated and explained for thefirst time
2 Experimental section
2.1 Catalysts preparation
Melamine (C3H6N6,499%), tin dichloride dihydrate (SnCl2 2H2O,
498%), potassium hydroxide (KOH, 485%), ethanol (499.7%) were
purchased from Sinopharm Chemical Reagent Corp., PR China P25
(TiO2, Degussa) was purchased from Beijing Entrepreneur Corp., China
All these reagents were used without further purification
Pure g-C3N4powders were prepared by directly calcining
mela-mine in a muffle furnace at 520 1C for 4 h Pure SnO2xwas prepared
by heating Sn6O4(OH)4 at 4001C in N2 for 2 h Sn6O4(OH)4 was
prepared by a deposition method In a typical synthesis run, 6.768 g
of SnCl2 2H2O was dissolved in a mixture solvent of 50 mL H2O and
20 mL ethanol to obtain solution A 3.93 g of KOH was dissolved in
30 mL H2O to obtain solution B Then, solution B was added dropwise
into solution A under stirring to generate white precipitate After
stirring for two hours, the precipitate wasfiltered, and washed many
times by water and ethanol to remove Cland Kþ Yellow Sn6O4(OH)4
was obtained in a powder form after drying in oven at 601C for 12 h
(Fig S1)
The SnO2x/g-C3N4 composites were prepared according to the
following procedure A given amount of Sn6O4(OH)4and g-C3N4were
mixed and ground in an agate mortar for 20 min Then, the mixture
was calcined at 4001C in N2 for 2 h to obtain the SnO2x/g-C3N4
catalyst By this way, the SnO2x/g-C3N4 (SC) composites with the
SnO2xconcentration of 5.4 wt%, 17.5 wt%, 29.6 wt%, 42.2 wt%, 55.6 wt
% were prepared and denoted as 5.4 wt%SC, 17.5 wt%SC, 29.6 wt%SC,
42.2 wt%SC, 55.6 wt%SC, respectively The concentration of SnO2x
was determined by thermogravimetry (TG) analysis (Fig S2)
2.2 Photodegradation of RhB The photocatalytic degradation of RhB was carried out in an outer irradiation-type photoreactor Typically, 100 mL of RhB solution with
an initial concentration of 10 mg/L and 0.1 g of photocatalyst were added to a 250 mL Pyrex glass cell The RhB solution containing the photocatalyst powder was magnetically stirred before and during photocatalytic reaction The visible light source for photocatalysis was
a spherical Xe lamp (350 W) equipped with a UV cut and an IR cut filters (800 nm4λ4420 nm) Other filters (λ4320 nm,λ4360 nm,
λ4480 nm andλ4580 nm) were also used to cut off the light with different wavelengths Prior to irradiation, the suspension was agitated for an hour to ensure adsorption/desorption equilibrium at room temperature At regular intervals, samples were withdrawn and centrifuged to remove photocatalyst for analysis The concentration
of aqueous RhB was determined by measuring its absorbance of the solution at 554 nm using a UV–vis spectrophotometer The RhB degradation was calculated by Lambert–Beer equation In addition to RhB, MO, methyl blue (MB) and phenol were also used as the simulative pollutants to investigate the photoactivity of SnO2x
/g-C3N4 composite The procedures of the scavenging experiments of reactive oxygen species were similar to that of the photodegradation experiment The detailed process was described elsewhere[24,25] 2.3 Photocatalytic reduction of CO2
The photocatalytic CO2reduction was carried out in a stainless-steel reactor with a quartz window on the top of the reactor (Fig S3)
A 500 W Xe lamp was used as the light source In the photocatalytic
CO2reduction reaction system, 20 mg of solid catalyst was placed on a
Teflon catalyst holder in the upper region of the reactor 4 mL water was pre-injected into the bottom of the reactor Prior to the light irradiation, the above system was thoroughly purged by CO2 to remove air in the reactor During reaction, the pressure of CO2 was kept to be 0.3 MPa and the photoreaction temperature was kept at
801C After light irradiation for 4 h, the gas product was analyzed by a gas chromatograph (GC-950) with a FID and a TCD detector Only the products of CO, CH4, and CH3OH were detected
2.4 Characterizations
TG analysis (Netzsch STA449) was carried out in a flow of air (10 mL/min) at a heating rate of 101C/min The specific surface areas were measured on Autosorb-1 (Quantachrome Instruments) by the BET method The powder X-ray diffraction (XRD, Philips PW3040/60) was used to record the diffraction patterns of photocatalysts employ-ing Cu Kαradiation (40 kV/40 mA) Afield emission scanning electron microscope (LEO-1530) and a JEM-2010F transmission electron micro-scope were employed to observe the morphology of the catalysts The FT-IR spectra of the catalysts were recorded on Nicolet NEXUS670 with a resolution of 4 cm1 The XPS measurements were performed with a Quantum 2000 Scanning ESCA Microprobe instrument using AlKα The C 1s signal was set to a position of 284.6 eV The UV–vis diffuse reflectance spectra (DRS) of catalysts were recorded on a UV– vis spectrometer (PerkinElmer Lambda900) equipped with an inte-grating sphere The PL spectra were collected on FLS-920 spectrometer (Edinburgh Instrument), using a Xe lamp (excitation at 365 nm) as light source
The electrochemical impedance spectroscopy (EIS) and photo-current (PC) responses measurements were performed by using a CHI 660B electrochemical workstation with a standard three-electrode cell at room temperature The prepared sample, Ag/AgCl (saturated KCl), and a Pt wire were used as the working electrode, the reference electrode, and the counter electrode, respectively The working electrodes were prepared as follows Indium tin oxide (ITO) glass pieces (1.5 5 cm2) were cleaned successively by
Trang 3acetone, boiling NaOH (0.1 mol/L), deionized water, and dried in an
air stream Then, 0.018 g sample and 0.002 g polyvinylidene
fluoride was mixed and ground for three minutes After adding
of three drops of 1-Methyl-2-pyrrolidinone, the mixture was
ultrasonicated for 20 min to obtain a suspension, which was then
coated onto the ITO glass substrate The coated area on the ITO
glass was controlled to be 0.8 0.8 cm2 Finally, the coated ITO
glass was dried at 501C to obtain the working electrode The EIS
experiment was performed in aqueous 0.1 M Na2SO4 solution in
the dark The potential was varied between 0 and 1 V (vs Ag/
AgCl) with an AC amplitude of 10 mV and frequencies in the 200–
4000 Hz range For PC measurement, a 350 W Xe arc lamp served
as the light source and Na2SO4(0.5 M) aqueous solution was used
as the electrolyte
3 Results and discussion
3.1 Characterizations of SnO2x/g-C3N4composites
The structure of the synthesized SnO2 x/g-C3N4 composites
was characterized by XRD and FT-IR Fig 1a shows the powder
XRD patterns of g-C3N4, SnO2x, and SnO2x/g-C3N4with different
SnO2xconcentrations Pure g-C3N4has two distinct peaks at 27.41
and 13.11, which can be indexed to the (002) and (100) diffraction
planes[26] Pure SnO2xexhibits several strong peaks at 26.61,
33.91, 38.01 and 51.81, which matches well with the standard
diffraction data for the tetragonal phase of SnO2(PDF 41-1445)
This result indicates that the content of doped Sn2þ may be very
low and does not change the crystal structure of SnO2, which is
consistent with Long's result[23] For the SnO2x/g-C3N4hybrids,
the XRD patterns display a combination of the two sets of
diffraction data for both g-C3N4 and SnO2x With the increase
in SnO2 xcontent, the peaks of g-C3N4weaken No other phase is
detected, which indicates that the SnO2x/g-C3N4 hybrids are
composed of g-C3N4 and SnO2x; similar result is obtained by
FT-IR.Fig 1b shows that the SnO2x/g-C3N4consists of two sets of
characteristic vibration peaks The IR peak at 567 cm1 can be
ascribed to the characteristic peak of SnO2x, while the peaks in
the range of 1245–1574 cm1can be assigned to the characteristic
vibration peaks of C–N heterocyclics in g-C3N4[27] This result is in
excellent agreement with the XRD analysis
The morphologies of g-C3N4, SnO2 xand the SnO2x/g-C3N4
photocatalyst were investigated by SEM and TEM InFig 2a and b,
a stacked layer structure is clearly observed in the g-C3N4sample,
which is consistent with previous reports [15,19] The SnO2 x
sample displays a nanospherical shape with an average diameter
of 50 nm (Fig 2c and d) In the SEM micrograph of the
representative composite (42.2 wt% SC), g-C3N4 sheets are found
to be covered by SnO2 xnanoparticles (Fig 2e) The size of SnO2 x
in the composite is similar to that of the pristine SnO2 x The TEM
image provides a more evident observation about the two
com-ponents (Fig 2f) The darker part with spherical shape should be
SnO2xand the lighter part is g-C3N4, which further demonstrates
the well dispersion of SnO2x on g-C3N4 An inserted
high-resolution TEM (HRTEM) image shows the microstructure of the
SnO2x/g-C3N4composite Two clear lattice fringes are observed in
the HRTEM image of 42.2 wt% SC The interplanar spacings are
approximately 0.3476 and 0.2740 nm, which are very close to the
(110) and (101) planes of SnO2, respectively, in accordance with
the XRD result inFig 1a The lattice fringe is difficult to observe in
g-C3N4 However, the SnO2xnanoparticles are evidently anchored
on the g-C3N4 surface Some chemical bonds may be formed
between SnO2xand g-C3N4, leading to a close interface between
the two semiconductors in the as-prepared composite This tight
coupling is favorable for the charge transfer between g-CN and
SnO2xand promotes the separation of photogenerated electron– hole pairs Meanwhile, the HRTEM image also suggests that the SnO2x/g-C3N4hybrids in structure are heterogeneous rather than
a physical mixture of two separate phases of SnO2xand g-C3N4 The close interaction between SnO2 xand g-C3N4in the compo-site can also be observed via TG analysis Fig 3 shows the TG profiles of g-C3N4, 42.2 wt% SC, and the physical mixture of SnO2x and g-C3N4(42.2 wt% SC-PM) Compared with pure g-C3N4, sharp weight loss occurs at a lower temperature for SnO2x/g-C3N4, which can be attributed to the catalytic role of SnO2x[10,13] The amount of catalyst and the contact between SnO2 xand g-C3N4
are two important factors that influence the catalytic oxidation of g-C3N4 Although 42.2 wt% SC and 42.2 wt% SC-PM have nearly the same SnO2xconcentration, the difference in their sharp weight losses is still evident The catalytic oxidation of g-C3N4in 42.2 wt%
SC composite is faster than that in 42.2 wt% SC-PM, indicating the tight contact between SnO2x and g-C3N4 in SnO2x/g-C3N4
composite This result is consistent with the TEM analysis
Fig 4shows the XPS spectra of the SnO2x/g-C3N4composites The survey scan XPS spectra provide the C 1s and N 1s peaks for
g-C3N4and 42.2 wt% SC, as well as the Sn 3p, 3d, and O 1s peaks for SnO2x and 42.2 wt% SC These results are consistent with the chemical composition of the photocatalyst, as proven by the XRD and FT-IR analyses The high-resolution X-ray photoelectron spec-tra of C 1s are shown inFig 4b SnO2xshows one C 1s peak at 284.6 eV as a result of its external carbon contamination[28] In
Fig 1 XRD patterns (a) and FT-IR spectra (b) of SnO 2x /g-C 3 N 4 composites with different SnO 2x concentrations.
Trang 4the case of g-C3N4and 42.2 wt% SC composite, another C 1s peak is
found, which corresponds to the carbon atoms bonded with three
N neighbors in its chemical bone structure, suggesting the
exis-tence of g-C3N4[28] Notably, the C 1s binding energy of 42.2 wt%
SC is slightly higher than that of pure g-C3N4, which is similar to
the N 1s XPS peak of the SnO /g-C N composite (Fig S4).Fig 4c
displays the Sn 3d high-resolution XPS peak Two signals at binding energies of 486.7 eV (Sn 3d5/2) and 495.1 eV (Sn 3d3/2) are observed for the SnO2 xsample The Sn 3d5/2and Sn 3d3/2
peaks of Sn2þlocated at 486.3 and 494.7 eV, respectively, whereas those peaks centered at 486.9 and 495.3 eV could be assigned to
Sn4þ[29] The result inFig 4c suggests that the existence of some
Sn2þin the SnO2xsample Some Sn2 þcations are not oxidized to
Sn4þ during the calcination process in nitrogen atmosphere When the SnO2xsample is calcined in air at 6001C for 2 h, the
Sn 3d5/2and Sn 3d3/2peaks shift to 487.0 and 495.4 eV, respec-tively (Fig S5), which further proves the existence of Sn2 þin the SnO2 xsample For the 42.2 wt% SC sample, the Sn 3d XPS peak displays a negative shift compared with that of SnO2x; the binding energies of Sn 3d5/2 and 3d3/2 move to 486.5 and 494.9 eV, respectively Clearly, the coupled g-C3N4 shows its contribution in hindering the Sn2þ oxidation Meanwhile, com-bined with the slight shift in the C 1s and N 1s spectra, the result
inFig 3c represents the interactions between SnO2xand g-C3N4 [30,31], which may be via the chemical bonds of Sn–O–N or Sn–O–
C The XPS result demonstrates that the synthesized SnO2x
/g-C3N4composite is not a physical mixture, which is consistent with the TEM analysis.Fig 4d shows the VB X-ray photoelectron spectra
of g-C3N4and SnO2x The VB edge of g-C3N4is 1.51 eV, which is close to the reported values[10] The value for the SnO sample
Fig 2 SEM and TEM images of g-C 3 N 4 (a, b), SnO 2x (c, d), and 42.2 wt% SC (e, f) photocatalysts.
Fig 3 TG profiles of g-C 3 N 4 , 42.2 wt% SC and 42.2 wt% SC-PM.
Trang 5is 2.70 eV, which is negative to that of SnO2(EVB¼3.48 eV)[32].
This result indicates that the doped Sn2þ generates an impurity
energy level in the VB and elevates the VB edge[23]
The interactions between the components are important for
the formation of a hetero-junction structure in the composite
photocatalysts, and this structure contributes to the separation of
electron–hole pairs and subsequently results in their high
photo-activity [33,34] In the case of SnO2 x/g-C3N4, the enhanced
separation efficiency of electron–hole pairs should be observed
considering that the interactions between g-C3N4and SnO2xhas
been proven Therefore, PL experiment was performed to verify
the aforementioned hypothesis.Fig 5shows the PL spectra of
g-C3N4, 42.2 wt% SC, and the physical mixture 42.2 wt% SC-PM Pure
g-C3N4has a strong emission band at 460 nm, which is attributed
to the recombination process of self-trapped excitations[35] The
PL spectrum of 42.2 wt% SC is similar to that of pure g-C3N4, which
indicates that the emission band originates from the incorporate
g-C3N4 Meanwhile, the emission peak is much lower than that of
g-C3N4 In general, the decreased content of g-C3N4and enhanced
separation efficiency of charges would result in this change
[35,36] Hence, a physical mixture of 42.2 wt% SC-PM was
char-acterized as a reference sample The result suggests that the
emission band of the physical mixture is weaker than that of
g-C3N4, but stronger than that of 42.2 wt% SC This condition
confirms that the synthesized SnO2x/g-C3N4has higher
separa-tion efficiency of electron–hole pairs than g-C N
The EIS and PC analyses were conducted to confirm the high
efficiency of SnO2x/g-C3N4hybrid in hindering the recombination
of electron–hole pairs The EIS spectra of SnO2x, g-C3N4, and SnO2x/g-C3N4 composite are shown in Fig 6a The arc radius
of the EIS Nyquist plot of the 42.2 wt% SC is smaller than that of g-C N or SnO Given that the arc radius on the EIS spectra
Fig 4 XPS spectra of SnO 2x , g-C 3 N 4 and 42.2 wt% SC composite, (a) survey spectra, (b) C 1s, (c) Sn 3d, (d) VB XPS of g-C 3 N 4 and SnO 2x
Fig 5 Photoluminescence spectra of pure g-C 3 N 4 , 42.2 wt% SC composite, and 42.2 wt% SC-PM.
Trang 6reflects the reaction rate at the surface of an electrode[37,38], the
data in Fig 6a suggest the more effective separation of
photo-generated electron–hole pairs and a faster interfacial charge
transfer on SnO2x/g-C3N4 hybrid under this condition Fig 6b
displays the photocurrent transient responses for SnO2 x, g-C3N4
and 42.2 wt% SC electrodes Fast and uniform photocurrent
responses are evidently observed for each on and
switch-off event in both electrodes The photocurrent of the SnO2x
/g-C3N4electrode is approximately 4 and 20 times higher than those
of the SnO2 xand g-C3N4 electrodes, respectively This result is
consistent with the EIS and PL analyses; and clearly indicates that
the introduction of SnO2xinto g-C3N4can effectively enhance the
separation efficiency of photogenerated electron–hole pairs
[39,40]
The optical properties of SnO2x/g-C3N4samples were probed by
UV–vis diffuse reflectance spectroscopy (Fig 7) The doping of Sn2þ
on SnO2generates an impurity energy level in the VB and narrows
the band gap[23] Hence, SnO2xcan absorb visible light, and its
band gap energy is determined to be 2.50 eV by the K–M equation,
which is much smaller than that of SnO2[41] For comparison, the
SnO2xsample calcined in air for 2 h was also characterized by DRS
The result shown inFig S6indicates that the band gap of the sample
remarkably increases after calcination, which proves the
contribu-tion of Sn2 þ, as supported by the XPS results Pure g-C3N4 can
absorb light with wavelength lower than 460 nm and has a band
gap of 2.70 eV The SnO2x/g-C3N4samples display an absorption
edge similar to that of g-C3N4, indicating their ability to respond to
visible light Meanwhile, a noticeable correlation between the
SnO content and the UV–vis spectral change is observed The
absorption in the visible region increases with SnO2xcontents of the SnO2x/g-C3N4samples These results may have been caused by the interactions between SnO2x and g-C3N4 (via the formed chemical bonds), which results in modifications of the fundamental process of formation of electron/hole pair during irradiation[37] The BET surface areas of SnO2x/g-C3N4 hybrids are listed in
Table 1, as well as that of SnO2xand g-C3N4for comparison The BET surface area of g-C3N4is 13 m2/g, which is slightly higher than that of SnO2x(9 m2/g) Comparing with SnO2xor g-C3N4, the SnO2x/g-C3N4composites exhibit much higher BET values Given that the BET value of 42.2 wt% SC-PM is only 11 m2/g, the high surface area of the composites indicates that some changes occur
on the incorporate g-C3N4 or SnO2x In another word, some reactions might have occurred between g-C3N4and the precursor
of SnO2xduring calcination, which is consistent with the afore-mentioned hypothesis on the interaction between g-C3N4 and SnO2 x However, no regularity between the SnO2xcontents and BET values is observed The 29.6 wt% SC sample shows the highest specific surface area of 43 m2
/g
3.2 Photocatalytic activities of the SnO2 x/g-C3N4composites The photocatalytic activity of the as-prepared SnO2x/g-C3N4
hybrids was evaluated by RhB degradation under visible-light irradiation (Fig 8a) SnO2x and g-C3N4 samples are used for comparison Fig 8b shows the plots of ln(Ct/C0) vs irradiation time The reaction rate constants k are calculated by the kinetics
Fig 6 EIS (a) and transient photocurrent responses of pure g-C 3 N 4 , SnO 2x and
42.2 wt% SC composite (b).
Fig 7 UV–vis spectra of SnO 2x /g-C 3 N 4 (a) composites and estimated band gaps of g-C 3 N 4 and SnO2x(b).
Trang 7equation: ln(Ct/C0)¼ kt where k is the pseudo-first-order rate
constant, C0 is the RhB concentration after adsorption, and Ct
represents the concentration at reaction time t As shown in
Fig 8a, the self-degradation of RhB can be negligible in the
absence of a photocatalyst The pristine SnO2xshows weak ability
in RhB degradation, while pure g-C3N4 exhibits certain
photoac-tivity with a reaction rate constant of 0.01 min1 Compared with
the g-C3N4sample, the SnO2 x/g-C3N4 hybrids display markedly
higher photocatalytic activity because of the increased
separa-tion efficiency of electron–hole pairs The photocatalytic activity is
enhanced gradually with increased SnO2xcontent from 5.4 wt%
to 42.2 wt% The 42.2 wt% SC sample presents the highest
efficiency for RhB degradation under visible light irradiation The
k value is determined to be 0.088 min1 (Fig 8b), which is
8.8 times higher than that of pure g-C3N4 However, further increase in the SnO2x content in the composites leads to the decrease in photocatalytic activity
The stability of the optimized SnO2x/g-C3N4 composite (42.2 wt%SC) was investigated by a 10-run cycling test under the same condition For each run, the photocatalyst was recycled, cleaned, and dried The photodegradation efficiency of 42.2 wt%SC shows no apparent decrease after the 10 reuse cycles, indicating its stability (Fig 9a) The stability of SnO2x/g-C3N4 can also be proven by XRD analysis (Fig S7) The XRD pattern of the used SnO2x/g-C3N4 sample reveals that no change have occurred observed after the photocatalytic reaction The results inFigs 9a and S4 suggest that the SnO2 x/g-C3N4 photocatalyst can be reused completely for wastewater treatment In addition to high stability, the SnO2 x/g-C3N4hybrid also exhibits the feasibility for the degradation of various organics.Fig 9b shows the photocata-lytic activity of the 42.2 wt% SC sample for photodegradation of RhB, MO, MB, and phenol under visible light irradiation The SnO2x/g-C3N4hybrid exhibits high degradation efficiency for all three dyes For phenol, only 40% of the initial content is degraded under visible light irradiation for 90 min However, considering the high concentration of phenol (50 mg/L), the SnO2x/g-C3N4
composite can still be seen as an efficient photocatalyst
To demonstrate the high photocatalytic activity of SnO2x
/g-C3N4, the prepared composite samples were evaluated using the reaction of photocatalytic CO2reduction into fuels that is known to
be a challenging but promising application for sustainable energy resources[42–44] The test results are shown inFig 10 The blank
Table 1
Specific surface area of g-C 3 N 4 , SnO 2x , and SnO 2x /g-C 3 N 4 composites.
Fig 8 Photocatalytic activities of SnO 2x /g-C 3 N 4 composites on photodegradation
of RhB under visible-light irradiation (λ4420 nm) (a) and the corresponding
kinetic studies (b).
Fig 9 Cycling runs of 42.2 wt% SC composite (a) and its photocatalytic activity for different organics (b) under visible light irradiation (λ4420 nm).
Trang 8test indicates that the reduced products could be ignored in the
absence of either a photocatalyst or simulated sunlight irradiation
Pure g-C3N4 shows a CO2 reduction rate of 5:32μmol h 1gcat 1,
which is slightly higher than that of P25 The detected products
are CO, CH3OH, and CH4 For P25, only CO and CH4are observed
which is due to the low conduction band position of P25 and the
easy formation of CO and CH4products[45] No reduced carbon
product is observed in the presence of SnO2xas a result of its low
CB band potential However, the decoration of SnO2xon g-C3N4
can effectively promote the catalytic performance for CO2
photo-reduction With increased the SnO2x concentration, the
photo-catalytic activities of SnO2 x/g-C3N4composites increase gradually
and then decrease The highest photocatalytic performance is
obtained with the use of 42.2 wt% SC sample The CO2reduction
rate reaches 22:7μmol h 1g 1
cat, which is 4.3 and 5 times higher than those of g-C3N4 and P25, respectively The decoration of
SnO2xon g-C3N4generates an efficient photocatalyst for both CO2
photoreduction and dye photodegradation
3.3 Possible photocatalytic mechanism in the SnO2x/g-C3N4system
The surface area, light absorption ability, and separation
efficiency of electron–hole pairs are closely correlated with the
catalytic performance of a photocatalyst In the case of SnO2x
/g-C3N4 hybrids, the introduction of SnO2 xpromotes the surface
area of g-C3N4, which is beneficial for dye adsorption and the
subsequent photocatalytic reaction However, no regularity
between the BET surface areas of SnO2x/g-C3N4 hybrids and
photoactivities can be observed The SnO2 x/g-C3N4sample with
the highest surface area does not exhibit the highest
photocata-lytic activity The adsorption experiment in the dark also verifies
that the RhB adsorption ability of the SnO2x/g-C3N4photocatalyst
shows certain consistency with the BET surface area (Fig S8), but
not in agreement with the photocatalytic activity This result
indicates that the specific surface area and the light absorption
capability (as shown by DRS analysis), are not the dominant
factors affecting the photocatalytic activity of SnO2x/g-C3N4
Therefore, the high activity of SnO2x/g-C3N4 may have been
caused by the excellent separation efficiency of electron–hole
pairs The VB edges of SnO2 xand g-C3N4 are determined to be
2.70 and 1.51 eV, respectively via the VB XPS experiment Using
the equation of ECB¼EVBEg, the CB edge potentials of the two
semiconductors can be obtained FromFig 11, the CB potentials of
g-C3N4and SnO2xare 1.19 and 0.20 eV, respectively The two
semiconductors have suitable band potentials and can form the
hetero-junction structure to suppress the recombination of elec-tron–hole pairs, as proved by the PL, EIS and photocurrent analyses However, the route of charge transfer remains contro-versial because both the double-charge-transfer and Z-scheme mechanism can promote the separation of electron and holes For thefirst mechanism, the photogenerated electrons on the g-C3N4
surfaces would transfer to SnO2xbecause of the difference in CB edge potentials, whereas the holes in SnO2 xwould move to the
VB of g-C3N4 Thus, the electrons and holes are separated and accumulated on the surface of SnO2x and g-C3N4, respectively However, the enriched electrons on the SnO2xcannot reduce CO2
to fuel because of the low CB potential If SnO2 x/g-C3N4follows a double-charge-transfer mechanism, the decoration of SnO2x would not promote the photocatalytic CO2 reduction of g-C3N4 This result is inconsistent with the photocatalytic experiment Therefore, a Z-scheme mechanism is more suitable for the SnO2x/ g-C3N4hybrids InFig 11, the photogenerated electrons from the SnO2x semiconductor recombine with photogenerated holes from the g-C3N4 This process can also markedly improve the photogenerated electron–hole pair separation and retain the electrons on the CB of g-C3N4, which results in the high photo-activity of SnO2 x/g-C3N4 composites in the photocatalytic CO2
reduction to fuels under simulated sunlight irradiation
A series of radicals trapping experiments were performed using benzoquinone (BQ), KI, and isopropanol (IPA) scavengers to further prove the direct Z-scheme mechanism of SnO2 x/g-C3N4.Fig 12
shows the photocatalytic activity of 42.2 wt% SC in the presence of these quenchers The inset is the corresponding kinetic constants
of 42.2 wt% SC and g-C3N4 The addition of BQ (quencher ofO
2)
[24,25] and KI (quencher of Hþ and dOH) [24,25] results in a significant suppression of the degradation rate, whereas IPA (quencher of dOH) [24,25] has nearly no effect on the RhB degradation in the presence of 42.2 wt% SC catalyst This result indicates that the O2 and Hþ are the main reactive species during the photocatalytic oxidation of RhB A similar result is also obtained on g-C3N4 Considering that the CB edge potential of SnO2 xis more positive than EO 2 =O
2 (0.046 V) and the electrons
on SnO2x cannot reduce O2 to O
2 species[46], the active trapping experiments indicate that the photoexcited electrons in SnO2 x/g-C3N4hybrids accumulate on the CB of g-C3N4 This result demonstrates that the direct Z-scheme mechanism works in the composite
In addition to the scavenging experiments of the reactive species, the photocatalytic activity of composite photocatalyst under different light sources can also provide useful information
Fig 10 Photocatalytic activities of SnO 2x /g-C 3 N 4 composites on photocatalytic
CO 2 reduction under simulated sunlight irradiation.
Fig 11 Possible schemes for electron–hole separation and transport at the visible-light-driven SnO 2x /g-C 3 N 4 composite interface.
Trang 9on the Z-scheme mechanism Sasaki et al found that the
photo-catalytic activity of a Z-scheme composite was dominated by the
absorption of the semiconductor with a wider band gap[47] This
rule was also applied by Kondo et al to verify the Z-scheme
mechanism of S-TiO2/g-C3N4[20] In the current study, the
photo-activity of SnO2x/g-C3N4 and g-C3N4 in RhB degradation was
tested by irradiation at different wavelengths The result indicates
that the photocatalytic activity of SnO2 x/g-C3N4 is much higher
than that of g-C3N4when the wavelengths of the cut-offfilter are
320, 360, and 420 nm (Fig 13) However, when the wavelength is
480 nm, both the photoactivities of g-C3N4 and SnO2x/g-C3N4
significantly decrease Since all incoming photons with
wave-lengths lower than 480 nm are stopped during the experiment,
resulting the excitation of SnO2xbut not g-C3N4, the decreased
activity for pure g-C3N4is reasonable However, for the SnO2x
/g-C3N4 sample, the result in Fig 13 indicates that the present
photocatalysis system (SnO2 x/g-C3N4) works through a direct
Z-scheme mechanism The photoexcitation of both semiconductors
is required to highlight the promotion effect of SnO2x Otherwise,
the invalidation of the Z-scheme mechanism would lead to a
significant decrease in photocatalytic activity of the composite
Meanwhile, although the coupling of SnO2xcan greatly enhance
the photocatalytic efficiency, the concentration of SnO2xplays a
critical role The increase in the SnO2xcontent can increase the
interfaces between SnO2xand g-C3N4, which favors the
forma-tion of heterojuncforma-tion structures and the separaforma-tion of electron–
hole pairs As a result, the photocatalytic activity of SnO2x/g-C3N4
is enhanced However, when the SnO2xconcentration is higher
than 42.2 wt%, a lower photocatalytic activity is observed, possibly
because excessive coupling of SnO2 xleads to the shielding of the active site on g-C3N4surfaces, similar to the results obtained by Wang et al.[48–50]; they found that co-exposure of both semi-conductors on the surface is necessary to enhance photocatalytic activity in the hetero-junction system
4 Conclusion
Sn2 þ-doped SnO2 was hybridized with g-C3N4to generate an
efficient photocatalyst for dye photodegradation and photocataly-tic CO2 reduction The experimental data indicate that SnO2x
introduction leads to the formation of SnO2x–g-C3N4 heterojunc-tion, which hinders the recombination of electron–hole pairs and results in enhanced photoactivity Meanwhile, the reactive species trapping experiment verifies that the SnO2x/g-C3N4 composite follows a direct Z-scheme mechanism This study might provide a promising approach to address the low photoactivity of pristine
g-C3N4for water purification and CO2reduction
Acknowledgments This work wasfinancially supported by Natural Science Foun-dation of Zhejiang Province in China (LY14B030002)
Appendix A Supporting information Supplementary data associated with this article can be found in the online version athttp://dx.doi.org/10.1016/j.solmat.2015.01.037 References
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