Solar Energy Materials & Solar Cells 137 (2015) 175–184

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Solar Energy Materials & Solar Cells
journal homepage: www.elsevier.com/locate/solmat

Z-scheme SnO2 À x/g-C3N4 composite as an efficient photocatalyst
for dye degradation and photocatalytic CO2 reduction
Yiming He a,n, Lihong Zhang a, Maohong Fan c, Xiaoxing Wang a, Mikel L. Walbridge c,
Qingyan Nong a, Ying Wu b,n, Leihong Zhao b
a

Department of Materials Physics, Zhejiang Normal University, Jinhua 321004, China
Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China
c
School of Energy Resources, University of Wyoming, Laramie, WY 82071, United States
b

art ic l e i nf o

a b s t r a c t

Article history:
Received 17 September 2014
Received in revised form
29 January 2015
Accepted 30 January 2015
Available online 3 March 2015

Highly efficient SnO2 À x/g-C3N4 composite photocatalysts were synthesized using simple calcination of gC3N4 and 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 min À 1, which was 8.8 times higher than that of g-C3N4. The SnO2 À x/g-C3N4
composite also showed high photocatalytic activity for CO2 reduction 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 SnO2 À x/g-C3N4. Results indicated
that the introduction of SnO2 À x on g-C3N4 increased its surface area and enhanced light absorption
performance. More importantly, a hetero-junction structure was formed between SnO2 À x and 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.

Keywords:
Photocatalysis
SnO2 À x
g-C3N4
Z-scheme
Visible light

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 reducibility 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 efficiency, which can be partly attributed to its low surface area. Hence,

n

Corresponding authors. Tel.: þ 86 579 83792294; fax: þ86 579 83714946.
E-mail addresses: (Y. He), (Y. Wu).

/>0927-0248/& 2015 Elsevier B.V. All rights reserved.

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-C3N4 photocatalyst is hardly competent for efficient 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-C3N4 and another semiconductor. g-C3N4-based composite 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/gC3N4[15], CdS/g-C3N4[16], AgX (X¼ Cl, Br, I)/g-C3N4 [17,18], MoO3/gC3N4 [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-C3N4 and the semiconductor. Two mechanisms are usually
applied to explain the synergetic effect. The first mechanism is the
double-charge transfer mechanism [10–18], in which the photogenerated electrons in the conduction band (CB) of g-C3N4 are 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


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Y. He et al. / Solar Energy Materials & Solar Cells 137 (2015) 175–184

photocatalytic efficiency is enhanced. The other mechanism is the
direct Z-scheme-type mechanism [19–21], in which the photogenerated 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-C3N4 and 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-C3N4 composite degraded MO 10.4 times faster than g-C3N4
under visible light. The promotion effect of MoO3 is 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 investigations are necessary to develop this type of photocatalysts further.
In this paper, an efficient Z-scheme type photocatalyst, SnO2 À x/
g-C3N4 composite is presented. Sn2 þ -doped SnO2 (SnO2 À x), which

was prepared by heating Sn6O4(OH)4 in N2, was chosen as the
doping semiconductor because of its low CB position and capability of harvesting visible light [23]. The decoration of SnO2 À x
remarkably promoted the photocatalytic activity of g-C3N4 in
rhodamine B (RhB) degradation and CO2 photoreduction. Investigation of the structure, surface area, and optical property of the
composite showed that the relatively high photoactivity of
SnO2 À x/g-C3N4 composite could be ascribed to a direct Z-scheme
mechanism. The Z-scheme mechanism of the SnO2 À x/g-C3N4
photocatalyst was demonstrated and explained for the first time.

2. Experimental section

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 nm4 λ 4420 nm). Other filters (λ 4320 nm, λ 4360 nm,
λ 4480 nm and λ 4580 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 SnO2À x/gC3N4 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 CO2 reduction was carried out in a stainlesssteel 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
CO2 reduction 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
80 1C. 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.

2.1. Catalysts preparation
2.4. Characterizations
Melamine (C3H6N6, 499%), tin dichloride dihydrate (SnCl2 Á 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-C3N4 powders were prepared by directly calcining melamine in a muffle furnace at 520 1C for 4 h. Pure SnO2À x was prepared
by heating Sn6O4(OH)4 at 400 1C in N2 for 2 h. Sn6O4(OH)4 was
prepared by a deposition method. In a typical synthesis run, 6.768 g
of SnCl2 Á 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 was filtered, and washed many

times by water and ethanol to remove Cl À and K þ . Yellow Sn6O4(OH)4
was obtained in a powder form after drying in oven at 60 1C for 12 h
(Fig. S1).
The SnO2À x/g-C3N4 composites were prepared according to the
following procedure. A given amount of Sn6O4(OH)4 and g-C3N4 were
mixed and ground in an agate mortar for 20 min. Then, the mixture
was calcined at 400 1C in N2 for 2 h to obtain the SnO2À x/g-C3N4
catalyst. By this way, the SnO2À x/g-C3N4 (SC) composites with the
SnO2À x concentration 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 SnO2À x
was determined by thermogravimetry (TG) analysis (Fig. S2).

TG analysis (Netzsch STA449) was carried out in a flow of air
(10 mL/min) at a heating rate of 10 1C/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 employing Cu Kα radiation (40 kV/40 mA). A field emission scanning electron
microscope (LEO-1530) and a JEM-2010F transmission electron microscope 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 cm À 1. 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 integrating 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 photocurrent (PC) responses measurements were performed by using a
CHI 660B electrochemical workstation with a standard threeelectrode 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 Â 5 cm2) were cleaned successively by


Y. He et al. / Solar Energy Materials & Solar Cells 137 (2015) 175–184

177

acetone, 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 Â 0.8 cm2. Finally, the coated ITO
glass was dried at 50 1C 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 À 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 SnO2 À x/g-C3N4 composites
The structure of the synthesized SnO2 À x/g-C3N4 composites
was characterized by XRD and FT-IR. Fig. 1a shows the powder
XRD patterns of g-C3N4, SnO2 À x, and SnO2 À x/g-C3N4 with different
SnO2 À x concentrations. Pure g-C3N4 has two distinct peaks at 27.41

and 13.11, which can be indexed to the (002) and (100) diffraction
planes [26]. Pure SnO2 À x exhibits 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 Sn2 þ may be very
low and does not change the crystal structure of SnO2, which is
consistent with Long's result [23]. For the SnO2 À x/g-C3N4 hybrids,
the XRD patterns display a combination of the two sets of
diffraction data for both g-C3N4 and SnO2 À x. With the increase
in SnO2 À x content, the peaks of g-C3N4 weaken. No other phase is
detected, which indicates that the SnO2 À x/g-C3N4 hybrids are
composed of g-C3N4 and SnO2 À x; similar result is obtained by
FT-IR. Fig. 1b shows that the SnO2 À x/g-C3N4 consists of two sets of
characteristic vibration peaks. The IR peak at 567 cm À 1 can be
ascribed to the characteristic peak of SnO2 À x, while the peaks in
the range of 1245–1574 cm À 1 can 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 À x and the SnO2 À x/g-C3N4
photocatalyst were investigated by SEM and TEM. In Fig. 2a and b,
a stacked layer structure is clearly observed in the g-C3N4 sample,
which is consistent with previous reports [15,19]. The SnO2 À 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 À x nanoparticles (Fig. 2e). The size of SnO2 À x
in the composite is similar to that of the pristine SnO2 À x. The TEM
image provides a more evident observation about the two components (Fig. 2f). The darker part with spherical shape should be
SnO2 À x and the lighter part is g-C3N4, which further demonstrates
the well dispersion of SnO2 À x on g-C3N4. An inserted highresolution TEM (HRTEM) image shows the microstructure of the

SnO2 À x/g-C3N4 composite. 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 in Fig. 1a. The lattice fringe is difficult to observe in
g-C3N4. However, the SnO2 À x nanoparticles are evidently anchored
on the g-C3N4 surface. Some chemical bonds may be formed
between SnO2 À x and 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-C3N4 and

Fig. 1. XRD patterns (a) and FT-IR spectra (b) of SnO2 À x/g-C3N4 composites with
different SnO2 À x concentrations.

SnO2 À x and promotes the separation of photogenerated electron–
hole pairs. Meanwhile, the HRTEM image also suggests that the
SnO2 À x/g-C3N4 hybrids in structure are heterogeneous rather than
a physical mixture of two separate phases of SnO2 À x and g-C3N4.
The close interaction between SnO2 À x and g-C3N4 in the composite 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 SnO2 À x
and g-C3N4 (42.2 wt% SC-PM). Compared with pure g-C3N4, sharp
weight loss occurs at a lower temperature for SnO2 À x/g-C3N4,
which can be attributed to the catalytic role of SnO2 À x [10,13]. The
amount of catalyst and the contact between SnO2 À x and 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 SnO2 À x concentration, the difference in their sharp weight
losses is still evident. The catalytic oxidation of g-C3N4 in 42.2 wt%
SC composite is faster than that in 42.2 wt% SC-PM, indicating the
tight contact between SnO2 À x and g-C3N4 in SnO2 À x/g-C3N4

composite. This result is consistent with the TEM analysis.
Fig. 4 shows the XPS spectra of the SnO2 À x/g-C3N4 composites.
The survey scan XPS spectra provide the C 1s and N 1s peaks for gC3N4 and 42.2 wt% SC, as well as the Sn 3p, 3d, and O 1s peaks for
SnO2 À x 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 spectra of C 1s are shown in Fig. 4b. SnO2 À x shows one C 1s peak at
284.6 eV as a result of its external carbon contamination [28]. In


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Y. He et al. / Solar Energy Materials & Solar Cells 137 (2015) 175–184

Fig. 2. SEM and TEM images of g-C3N4 (a, b), SnO2 À x (c, d), and 42.2 wt% SC (e, f) photocatalysts.

Fig. 3. TG profiles of g-C3N4, 42.2 wt% SC and 42.2 wt% SC-PM.

the case of g-C3N4 and 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 existence 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 SnO2 À x/g-C3N4 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 À x sample. The Sn 3d5/2 and Sn 3d3/2
peaks of Sn2 þ located at 486.3 and 494.7 eV, respectively, whereas
those peaks centered at 486.9 and 495.3 eV could be assigned to
Sn4 þ [29]. The result in Fig. 4c suggests that the existence of some
Sn2 þ in the SnO2 À x sample. Some Sn2 þ cations are not oxidized to

Sn4 þ during the calcination process in nitrogen atmosphere.
When the SnO2 À x sample is calcined in air at 600 1C for 2 h, the
Sn 3d5/2 and Sn 3d3/2 peaks shift to 487.0 and 495.4 eV, respectively (Fig. S5), which further proves the existence of Sn2 þ in the
SnO2 À x sample. For the 42.2 wt% SC sample, the Sn 3d XPS peak
displays a negative shift compared with that of SnO2 À x; 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 Sn2 þ oxidation. Meanwhile, combined with the slight shift in the C 1s and N 1s spectra, the result
in Fig. 3c represents the interactions between SnO2 À x and 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 SnO2 À x/gC3N4 composite is not a physical mixture, which is consistent with
the TEM analysis. Fig. 4d shows the VB X-ray photoelectron spectra
of g-C3N4 and SnO2 À x. The VB edge of g-C3N4 is 1.51 eV, which is
close to the reported values [10]. The value for the SnO2 À x sample


Y. He et al. / Solar Energy Materials & Solar Cells 137 (2015) 175–184

179

Fig. 4. XPS spectra of SnO2 À x, g-C3N4 and 42.2 wt% SC composite, (a) survey spectra, (b) C 1s, (c) Sn 3d, (d) VB XPS of g-C3N4 and SnO2 À x.

is 2.70 eV, which is negative to that of SnO2 (EVB ¼3.48 eV) [32].
This result indicates that the doped Sn2 þ 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 photoactivity [33,34]. In the case of SnO2 À x/g-C3N4, the enhanced
separation efficiency of electron–hole pairs should be observed

considering that the interactions between g-C3N4 and SnO2 À x has
been proven. Therefore, PL experiment was performed to verify
the aforementioned hypothesis. Fig. 5 shows the PL spectra of gC3N4, 42.2 wt% SC, and the physical mixture 42.2 wt% SC-PM. Pure
g-C3N4 has 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-C3N4 and enhanced
separation efficiency of charges would result in this change
[35,36]. Hence, a physical mixture of 42.2 wt% SC-PM was characterized as a reference sample. The result suggests that the
emission band of the physical mixture is weaker than that of gC3N4, but stronger than that of 42.2 wt% SC. This condition
confirms that the synthesized SnO2 À x/g-C3N4 has higher separation efficiency of electron–hole pairs than g-C3N4.

Fig. 5. Photoluminescence spectra of pure g-C3N4, 42.2 wt% SC composite, and
42.2 wt% SC-PM.

The EIS and PC analyses were conducted to confirm the high
efficiency of SnO2 À x/g-C3N4 hybrid in hindering the recombination
of electron–hole pairs. The EIS spectra of SnO2 À x, g-C3N4, and
SnO2 À x/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-C3N4 or SnO2 À x. Given that the arc radius on the EIS spectra


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Y. He et al. / Solar Energy Materials & Solar Cells 137 (2015) 175–184

Fig. 6. EIS (a) and transient photocurrent responses of pure g-C3N4, SnO2 À x and

42.2 wt% SC composite (b).

reflects the reaction rate at the surface of an electrode [37,38], the
data in Fig. 6a suggest the more effective separation of photogenerated electron–hole pairs and a faster interfacial charge
transfer on SnO2 À x/g-C3N4 hybrid under this condition. Fig. 6b
displays the photocurrent transient responses for SnO2 À x, g-C3N4
and 42.2 wt% SC electrodes. Fast and uniform photocurrent
responses are evidently observed for each switch-on and switchoff event in both electrodes. The photocurrent of the SnO2 À x/gC3N4 electrode is approximately 4 and 20 times higher than those
of the SnO2 À x and g-C3N4 electrodes, respectively. This result is
consistent with the EIS and PL analyses; and clearly indicates that
the introduction of SnO2 À x into g-C3N4 can effectively enhance the
separation efficiency of photogenerated electron–hole pairs
[39,40].
The optical properties of SnO2À x/g-C3N4 samples were probed by
UV–vis diffuse reflectance spectroscopy (Fig. 7). The doping of Sn2 þ
on SnO2 generates an impurity energy level in the VB and narrows
the band gap [23]. Hence, SnO2À x can 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
SnO2À x sample calcined in air for 2 h was also characterized by DRS.
The result shown in Fig. S6 indicates that the band gap of the sample
remarkably increases after calcination, which proves the contribution 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 SnO2À x/g-C3N4 samples display an absorption
edge similar to that of g-C3N4, indicating their ability to respond to
visible light. Meanwhile, a noticeable correlation between the
SnO2À x content and the UV–vis spectral change is observed. The

Fig. 7. UV–vis spectra of SnO2 À x/g-C3N4 (a) composites and estimated band gaps of
g-C3N4 and SnO2 À x (b).


absorption in the visible region increases with SnO2À x contents of
the SnO2À x/g-C3N4 samples. These results may have been caused by
the interactions between SnO2À x 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 SnO2 À x/g-C3N4 hybrids are listed in
Table 1, as well as that of SnO2 À x and g-C3N4 for comparison. The
BET surface area of g-C3N4 is 13 m2/g, which is slightly higher than
that of SnO2 À x (9 m2/g). Comparing with SnO2 À x or g-C3N4, the
SnO2 À x/g-C3N4 composites 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 SnO2 À x. In another word, some
reactions might have occurred between g-C3N4 and the precursor
of SnO2 À x during calcination, which is consistent with the aforementioned hypothesis on the interaction between g-C3N4 and
SnO2 À x. However, no regularity between the SnO2 À x contents 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 À x/g-C3N4 composites.
The photocatalytic activity of the as-prepared SnO2 À x/g-C3N4
hybrids was evaluated by RhB degradation under visible-light
irradiation (Fig. 8a). SnO2 À x 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


Y. He et al. / Solar Energy Materials & Solar Cells 137 (2015) 175–184

equation: ln(Ct/C0)¼ À 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 SnO2 À x shows weak ability
in RhB degradation, while pure g-C3N4 exhibits certain photoactivity with a reaction rate constant of 0.01 min À 1. Compared with
the g-C3N4 sample, the SnO2 À x/g-C3N4 hybrids display markedly
higher photocatalytic activity because of the increased separation efficiency of electron–hole pairs. The photocatalytic activity is
enhanced gradually with increased SnO2 À x content 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 min À 1 (Fig. 8b), which is

Table 1
Specific surface area of g-C3N4, SnO2 À x, and SnO2 À x/g-C3N4 composites.
Catalysts

S/m2 g À 1

g-C3N4
SnO2 À x
5.4 wt% SC
15.5 wt% SC
29.6 wt%SC
42.2 wt% SC
55.6 wt%SC
42.2 wt% SC-PM

13
9
25

23
43
41
34
11

Fig. 8. Photocatalytic activities of SnO2 À x/g-C3N4 composites on photodegradation
of RhB under visible-light irradiation (λ 4420 nm) (a) and the corresponding
kinetic studies (b).

181

8.8 times higher than that of pure g-C3N4. However, further
increase in the SnO2 À x content in the composites leads to the
decrease in photocatalytic activity.
The stability of the optimized SnO2 À x/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 SnO2 À x/g-C3N4 can also be
proven by XRD analysis (Fig. S7). The XRD pattern of the used
SnO2 À x/g-C3N4 sample reveals that no change have occurred
observed after the photocatalytic reaction. The results in Figs. 9a
and S4 suggest that the SnO2 À x/g-C3N4 photocatalyst can be
reused completely for wastewater treatment. In addition to high
stability, the SnO2 À x/g-C3N4 hybrid also exhibits the feasibility for
the degradation of various organics. Fig. 9b shows the photocatalytic activity of the 42.2 wt% SC sample for photodegradation of
RhB, MO, MB, and phenol under visible light irradiation. The
SnO2 À x/g-C3N4 hybrid 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 SnO2 À x/g-C3N4
composite can still be seen as an efficient photocatalyst.
To demonstrate the high photocatalytic activity of SnO2 À x/gC3N4, the prepared composite samples were evaluated using the
reaction of photocatalytic CO2 reduction into fuels that is known to
be a challenging but promising application for sustainable energy
resources [42–44]. The test results are shown in Fig. 10. The blank

Fig. 9. Cycling runs of 42.2 wt% SC composite (a) and its photocatalytic activity for
different organics (b) under visible light irradiation (λ 4420 nm).


182

Y. He et al. / Solar Energy Materials & Solar Cells 137 (2015) 175–184

Fig. 10. Photocatalytic activities of SnO2 À x/g-C3N4 composites on photocatalytic
CO2 reduction under simulated sunlight irradiation.

Fig. 11. Possible schemes for electron–hole separation and transport at the visiblelight-driven SnO2 À x/g-C3N4 composite interface.

test indicates that the reduced products could be ignored in the
absence of either a photocatalyst or simulated sunlight irradiation.
À1 À1
Pure g-C3N4 shows a CO2 reduction rate of 5:32 μmol h
gcat ,
which is slightly higher than that of P25. The detected products
are CO, CH3OH, and CH4. For P25, only CO and CH4 are observed
which is due to the low conduction band position of P25 and the

easy formation of CO and CH4 products [45]. No reduced carbon
product is observed in the presence of SnO2 À x as a result of its low
CB band potential. However, the decoration of SnO2 À x on g-C3N4
can effectively promote the catalytic performance for CO2 photoreduction. With increased the SnO2 À x concentration, the photocatalytic activities of SnO2 À x/g-C3N4 composites increase gradually
and then decrease. The highest photocatalytic performance is
obtained with the use of 42.2 wt% SC sample. The CO2 reduction
À1 À1
rate reaches 22:7 μmol h
gcat , which is 4.3 and 5 times higher
than those of g-C3N4 and P25, respectively. The decoration of
SnO2 À x on g-C3N4 generates an efficient photocatalyst for both CO2
photoreduction and dye photodegradation.

hetero-junction structure to suppress the recombination of electron–hole pairs, as proved by the PL, EIS and photocurrent
analyses. However, the route of charge transfer remains controversial because both the double-charge-transfer and Z-scheme
mechanism can promote the separation of electron and holes. For
the first mechanism, the photogenerated electrons on the g-C3N4
surfaces would transfer to SnO2 À x because of the difference in CB
edge potentials, whereas the holes in SnO2 À x would move to the
VB of g-C3N4. Thus, the electrons and holes are separated and
accumulated on the surface of SnO2 À x and g-C3N4, respectively.
However, the enriched electrons on the SnO2 À x cannot reduce CO2
to fuel because of the low CB potential. If SnO2 À x/g-C3N4 follows a
double-charge-transfer mechanism, the decoration of SnO2 À x
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 SnO2 À x/
g-C3N4 hybrids. In Fig. 11, the photogenerated electrons from the
SnO2 À x 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 photoactivity of SnO2 À 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 À 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 of O2À )
[24,25] and KI (quencher of H þ 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 O2À and H þ 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 À x is more positive than EO2 =O À ( À0.046 V) and the electrons
2
on SnO2 À x cannot reduce O2 to O2À species[46], the active
trapping experiments indicate that the photoexcited electrons in
SnO2 À x/g-C3N4 hybrids 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

3.3. Possible photocatalytic mechanism in the SnO2 À x/g-C3N4 system
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 SnO2 À x/gC3N4 hybrids, the introduction of SnO2 À x promotes 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 SnO2 À x/g-C3N4 hybrids and
photoactivities can be observed. The SnO2 À x/g-C3N4 sample with
the highest surface area does not exhibit the highest photocatalytic activity. The adsorption experiment in the dark also verifies
that the RhB adsorption ability of the SnO2 À x/g-C3N4 photocatalyst
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 SnO2 À x/g-C3N4.
Therefore, the high activity of SnO2 À x/g-C3N4 may have been
caused by the excellent separation efficiency of electron–hole
pairs. The VB edges of SnO2 À x and g-C3N4 are determined to be
2.70 and 1.51 eV, respectively via the VB XPS experiment. Using
the equation of ECB ¼EVB À Eg, the CB edge potentials of the two
semiconductors can be obtained. From Fig. 11, the CB potentials of
g-C3N4 and SnO2 À x are À 1.19 and 0.20 eV, respectively. The two
semiconductors have suitable band potentials and can form the


Y. He et al. / Solar Energy Materials & Solar Cells 137 (2015) 175–184

183

because excessive coupling of SnO2 À x leads to the shielding of the
active site on g-C3N4 surfaces, similar to the results obtained by
Wang et al. [48–50]; they found that co-exposure of both semiconductors on the surface is necessary to enhance photocatalytic
activity in the hetero-junction system.


4. Conclusion

Fig. 12. Photodegradation of RhB over 42.2 wt%SC photocatalyst with different
quenchers (λ 4420 nm).

Sn2 þ -doped SnO2 was hybridized with g-C3N4 to generate an
efficient photocatalyst for dye photodegradation and photocatalytic CO2 reduction. The experimental data indicate that SnO2 À x
introduction leads to the formation of SnO2 À x–g-C3N4 heterojunction, which hinders the recombination of electron–hole pairs and
results in enhanced photoactivity. Meanwhile, the reactive species
trapping experiment verifies that the SnO2 À x/g-C3N4 composite
follows a direct Z-scheme mechanism. This study might provide a
promising approach to address the low photoactivity of pristine gC3N4 for water purification and CO2 reduction.

Acknowledgments
This work was financially supported by Natural Science Foundation of Zhejiang Province in China (LY14B030002).

Appendix A. Supporting information
Supplementary data associated with this article can be found in
the online version at />References
Fig. 13. Photocatalytic activities of g-C3N4 and 42.2 wt% SC in RhB degradation
under light with different wavelength.

on the Z-scheme mechanism. Sasaki et al. found that the photocatalytic 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 photoactivity of SnO2 À x/g-C3N4 and g-C3N4 in RhB degradation was
tested by irradiation at different wavelengths. The result indicates
that the photocatalytic activity of SnO2 À x/g-C3N4 is much higher
than that of g-C3N4 when the wavelengths of the cut-off filter are

320, 360, and 420 nm (Fig. 13). However, when the wavelength is
480 nm, both the photoactivities of g-C3N4 and SnO2 À x/g-C3N4
significantly decrease. Since all incoming photons with wavelengths lower than 480 nm are stopped during the experiment,
resulting the excitation of SnO2 À x but not g-C3N4, the decreased
activity for pure g-C3N4 is reasonable. However, for the SnO2 À x/gC3N4 sample, the result in Fig. 13 indicates that the present
photocatalysis system (SnO2 À x/g-C3N4) works through a direct Zscheme mechanism. The photoexcitation of both semiconductors
is required to highlight the promotion effect of SnO2 À x. 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 SnO2 À x can greatly enhance
the photocatalytic efficiency, the concentration of SnO2 À x plays a
critical role. The increase in the SnO2 À x content can increase the
interfaces between SnO2 À x and g-C3N4, which favors the formation of heterojunction structures and the separation of electron–
hole pairs. As a result, the photocatalytic activity of SnO2 À x/g-C3N4
is enhanced. However, when the SnO2 À x concentration is higher
than 42.2 wt%, a lower photocatalytic activity is observed, possibly

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