High efficient photocatalytic selective oxidation of benzyl alcohol to benzaldehyde by solvothermal-synthesized ZnIn2S4 microspheres under visible light irradiation

Article abstract of DOI:10.1016/j.jssc.2013.07.015

Hexagonal ZnIn₂S₄ samples have been synthesized by a solvothermal method. Their properties have been determined by X-ray diffraction, ultraviolet-visible-light diffuse reflectance spectra, field emission scanning electron microscopy,

Full text of DOI:10.1016/j.jssc.2013.07.015

Journal of Solid State Chemistry 205 (2013) 134–141
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
High efficient photocatalytic selective oxidation of benzyl alcohol
to benzaldehyde by solvothermal-synthesized ZnIn2S4 microspheres
under visible light irradiation
a
a
Zhixin Chen a,b, , Jingjing Xu , Zhuyun Ren , Yunhui He a,b, Guangcan Xiao a,b
n
a State Key Laboratory Breeding Base of Photocatalysis, College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350002, P.R. China
b Instrumental Measurement & Analysis Center, Fuzhou University, Fuzhou 350002, P.R. China
a r t i c l e i n f o
a b s t r a c t
Article history:
Hexagonal ZnIn2S4 samples have been synthesized by a solvothermal method. Their properties have been
determined by X-ray diffraction, ultraviolet–visible-light diffuse reflectance spectra, field emission
scanning electron microscopy, nitrogen adsorption–desorption and X-ray photoelectron spectra. These
results demonstrate that ethanol solvent has significant influence on the morphology, optical and
electronic nature for such marigold-like ZnIn2S4 microspheres. The visible light photocatalytic activities
of the ZnIn2S4 have been evaluated by selective oxidation of benzyl alcohol to benzaldehyde using
molecular oxygen as oxidant. The results show that 100% conversion along with 499% selectivity are
reached over ZnIn2S4 prepared in ethanol solvent under visible light irradiation (λ4420 nm) of 2 h, but
only 58% conversion and 57% yield are reached over ZnIn2S4 prepared in aqueous solvent. A possible
mechanism of the high photocatalytic activity for selective oxidation of benzyl alcohol over ZnIn2S4 is
proposed and discussed.
Received 20 April 2013
Received in revised form
5 July 2013
Accepted 14 July 2013
Available online 22 July 2013
Keywords:
ZnIn2S4
Solvothermal
Photocatalyst
Selective oxidation
Marigold-like
& 2013 Elsevier Inc. All rights reserved.
1. Introduction
In the last few years, most of the reports on photocatalysis
regarding selective oxidation of alcohols are focused on TiO2 or
Selective oxidation of alcohols to carbonyls is a fundamental
and significant transformation for the synthesis of fine chemicals,
because carbonyl compounds such as ketone and aldehyde deri-
vatives are widely utilized in the confectionary, fragrance and
pharmaceutical industries [1–6]. However, conventional organic
synthesis for the oxidation of alcohols into corresponding alde-
hydes or ketones not only involves environmentally, toxic, corro-
sive stoichiometric oxidants (such as chromate, hypochlorite or
permanganate etc.) and harsh condition (such as high temperature
and pressure), but also produces a large quantity of hazardous
wastes [7–10]. Therefore, increasing efforts have been made to
develop some clean processes to solve these problems [5–13].
Introducing the utility of sunlight into selective oxidation of
organic chemistry, i.e. photocatalytic selective oxidation, is a green
technique for organic synthesis. The required mild conditions and
the possibility to decrease the undesired environmental pollutions
highlight its potential as a promising route for organic synthesis.
Thus, enormous attention has been paid to the photocatalytic
selective oxidation of alcohols [1–3,12–15].
TiO2-based materials [2–4,15–18]. For instance, Zhao et al. have
synthesized an ingenious coupled system consisting of dye-
sensitized TiO2 and 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO)
for visible-light-induced aerobic oxidation of alcohols to corre-
sponding aldehydes, for benzyl alcohol, the 80% conversion is
reached under visible light irradiation of 18 h [3]. Xu et al. have
synthesized the nanocomposites of TiO2-5%GR which exhibits
visible light photocatalytic performance toward selective oxidation
of benzyl alcohol to benzaldehyde, yet, only 30% conversion is
reached under the irradiation of 4 h [18]. The similar phenomenon
is also observed in nontitania-based photocatalysts [19,20], for
example, the coenocytic Pd@CdS synthesized by Xu et al. which
exhibits enhanced photocatalytic activity toward selective oxida-
tion of benzyl alcohol to benzaldehyde as compared to blank-CdS,
under visible light irradiation of 4 h, the conversion for benzyl
alcohol and the yield for benzaldehyde are about 31% and 30% over
the coenocytic Pd@CdS nanocomposite, respectively [19]. Overall,
the utility of sunlight and the transformation efficiency of alcohols
to corresponding aldehydes for these photocatalysts are relative
low. In order to improve the efficiency of this photocatalytic
process, one of the challenges as well as opportunities faced by
the researchers is to develop other novel visible-light-driven
photocatalysts used for selective organic transformation.
n
Corresponding author at: State Key Laboratory Breeding Base of Photocatalysis,
College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou
350002, P.R. China. Fax: +86 591 8789 2447.
ZnIn2S4, an important semiconducting material of ternary
chalcogenides, has been extensively studied as a potential eco-
E-mail address: czx@fzu.edu.cn (Z. Chen).
0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jssc.2013.07.015

Z. Chen et al. / Journal of Solid State Chemistry 205 (2013) 134–141
135
friendly photocatalyst, because it has a band gap corresponding
to the visible light absorption with considerable chemical stability
[21–36]. However, a literature survey leads us to the finding
that most of the reports of ZnIn2S4 are focused on water splitting
[22–29] and nonselective degradation of volatile organic com-
pounds [30–36]. It still remains unclear whether ZnIn2S4 can be
applied to photocatalytic selective organic transformations.
Herein, we synthesized ZnIn2S4 samples via a facile solvothermal
method and utilized them for photocatalytic selective oxidation of
benzyl alcohol under ambient conditions. To the best of our
knowledge, it is the first time to apply ZnIn2S4 prepared in ethanol
solvent for photocatalytic selective oxidation. Furthermore, the
ZnIn2S4 microspheres show remarkably enhanced photoactivity
compared with ZnIn2S4 sample prepared in aqueous solvent under
the same conditions. A possible mechanism of the high photo-
catalytic activity is proposed and discussed.
(Thermo Fisher Scientific) at 3.0 ꢁ 10ꢂ10 mbar with monochro-
matic Al Kα radiation (E¼1486.2 eV). Photoelectrochemical mea-
surements were performed in a homemade three electrode quartz
cells with a PAR VMP3Multi potentiotat apparatus. Pt plate was
used as the counter, and Ag/AgCl electrode used as the reference
electrodes, while the working electrode was prepared on fluoride–
tin oxide (FTO) conductor glass. The sample powder (10 mg) was
ultrasonicated in 0.5 mL of anhydrous ethanol to disperse it evenly
to get slurry. The slurry was spread onto a FTO glass whose side
part was previously protected using Scotch tape. The working
electrode was dried overnight under ambient conditions. A copper
wire was connected to the side part of the working electrode using
a conductive tape. Uncoated parts of the electrode were isolated
with epoxy resin. The electrolyte was 0.2 M of aqueous Na2SO4
solution without additive. The visible light irradiation source was a
300 W Xe arc lamp system equipped with a UV cutoff filter
(λ4420 nm).
2. Experimental section
2.4. Photocatalytic activity
2.1. Materials
The photocatalytic selective oxidation of benzyl alcohol was
performed as follows. A mixture of benzyl alcohol (BA, 0.1 mmol)
and 8 mg of catalyst was dissolved in the solvent of benzotri-
fluoride (BTF, 1.5 mL), which was saturated with pure molecular
oxygen. The above mixture was transferred into a 10 mL Pyrex
glass bottle filled with molecular oxygen at a pressure of 0.1 MPa
and stirred for half an hour to make the catalyst blend evenly in
the solution. The suspensions were irradiated by a 300 W Xe arc
lamp (PLS-SXE 300, Beijing Trusttech Co. Ltd.) with a UV-CUT filter
to cut off light of wavelength o420 nm. After the reaction, the
mixture was centrifuged at 12,000 rmp for 20 min to completely
remove the catalyst particles. The remaining solution was ana-
lyzed with an Aglient Gas Chromatograph (GC-7820). The catalytic
activity with higher concentration of benzyl alcohol (0.2 mmol and
0.3 mmol) over ZIS-EtOH was labeled as ZIS-EtOH-2BA and ZIS-
EtOH-3BA, respectively. Conversion of benzyl alcohol, yield of
benzaldehyde, and selectivity for benzaldehyde were defined as
follows:
Zinc chloride (ZnCl2), indium chloride (InCl3 ꢀ 4H2O), thiaceta-
mide (TAA), ethanol were obtained from Sinopharm Chemical
Reagent Co., Ltd. (Shanghai, China). All materials were used as
received without further purification. Deionized water that was
used in the synthesis was obtained from local sources.
2.2. Photocatalyst preparation
In the typical reaction, ZnCl2 (1 mmol) and InCl3 ꢀ 4H2O
(2 mmol) were dispersed in 27 ml of ethanol by stirring 0.5 h to
obtain the homogeneous dispersion. Then, excessive TAA (8 mmol)
was added into the above mixture solution. The mixture solution
was aged with vigorous stirring for 0.5 h. Then, it was transferred
to a 100 ml Teflon-lined stainless steel autoclave and maintained
at 433 K for 24 h. The resulting product was cooled at room
temperature and recovered by filtration, washed with deionized
water and absolute ethanol several times. The final sample was
fully dried at 333 K in a vacuum for characterization and phtoca-
talytic reaction. For comparison, the sample was synthesized using
the similar approach except that aqueous instead of ethanol was
used as the solvent. In this paper, ZnIn2S4 prepared in aqueous-,
ethanol-mediated conditions were labeled as ZIS-H2O, ZIS-EtOH,
respectively.
Conversion (%)¼[(C0ꢂCbenzyl alcohol)/C0] ꢁ 100
Yield (%)¼Cbenzaldehyde/C0 ꢁ 100
Selectivity (%)¼[Cbenzaldehyde/(C0ꢂCbenzyl alcohol)] ꢁ 100
where C0 is the initial concentration of benzyl alcohol and Cbenzyl
and Cbenzaldehyde are the concentration of the substrate
alcohol
benzyl alcohol and the corresponding benzaldehyde, respectively,
at a certain time after the photocatalytic reaction.
2.3. Characterization
Crystal phase properties of the samples were analyzed with a
Bruker D8 Advance X-ray diffractometer (XRD) using Ni-filtered Cu
Kα radiation at 40 kV and 40 mA in the 2θ range from 151 to 801
with a scan rate of 0.021 per second. The optical properties of the
samples were characterized by UV–vis diffuse reflectance spectro-
scopy (DRS) using a UV–vis spectrophotometer (Cary500, Varian
Co.), in which BaSO4 was used as the internal reflectance standard.
The morphology of the samples was determined by a field
emission scanning electron microscopy (FESEM) on a FEI Nova
NANOSEM 230 instrument. Nitrogen adsorption–desorption
isotherms and the Brunauer–Emmett–Teller (BET) surface areas
were collected at 77 K on a Micrometritics ASAP2020 analyzer.
The photoluminescence spectra (PL) for solid samples were
investigated on an Edinburgh FL/FS900 spectrophotometer. The
irradiation source (λ4420 nm) was a 300 W Xe arc lamp system,
which was the light source for our photocatalytic selective oxida-
tion of alcohols. X-ray photoelectron spectroscopy (XPS) analysis
was conducted on a ESCALAB 250 photoelectron spectroscope
3. Results and discussion
3.1. Properties of ZnIn2S4 photocatalyst
Fig. 1 shows the XRD patterns of the ZIS-H2O and ZIS-EtOH
samples, the peaks at 2θ values of 21.6, 27.7, 39.8, 47.8, 52.5, 55.6
and 75.6 can be indexed to (006), (102), (108), (112), (1012), (202)
and (213) crystal lanes of hexagonal phase of ZnIn2S4 (JCPDS no.
65-2023), respectively. No other impurities, such as binary
sulfides, oxides or organic compounds related to reactants, are
detected. As displayed in Fig. 1, XRD patterns of ZIS-EtOH and ZIS-
H2O show almost the same profiles, although their reaction
solvents are different.
To further confirm the composition and chemical state of
ZnIn2S4 samples prepared in different solvents, XPS measurements
are carried out. As shown in Fig. 2, all peaks are calibrated using
C1s (284.6 eV) as the reference. A survey spectrum shown in

136
Z. Chen et al. / Journal of Solid State Chemistry 205 (2013) 134–141
Fig. 2a indicates the presence of Zn, In, S, C and O in which C and O
may come from the absorbed gaseous molecules. High-resolution
spectra of the samples are also characterized for Zn2p, In3d and
S2p to determine the valency state and atomic ratio. As shown in
Fig. 2b–d, the data are consistent with the values of literatures
[35–38]. Meanwhile, it is clearly seen that the binding energies of
Zn2p, In3d and S2p for ZIS-EtOH and ZIS-H2O samples are nearly
the same not only in the full survey spectrum but also in the high-
resolution spectra. Overall, XRD and XPS results indicate that the
pure hexagonal ZnIn2S4 could be easily obtained under the current
synthetic conditions.
FESEM is taken to directly analyze the morphology of the
samples. As shown in Fig. 3a, with regard to ZIS-H2O, there are
few marigold-like microspheres consisting of nanosheets, most
nanosheets on the surface of microspheres are seriously collapsed,
which is consistent with our previous report [34]. On the contrary,
when ethanol is used as solvent during the synthesis of ZnIn2S4
(Fig. 3b), the morphology of as-formed ZnIn2S4 microspheres is
significantly different from the ZIS-H2O sample, the marigold-like
spherical superstructures are obtained with the average diameter
around 8 mm (in the inset of Fig. 3b) and the microspheres are
composed of numerous nanosheets. The FESEM result indicates
that the solvent has a significant influence on the morphology of
samples, that is, ethanol can improve the formation of such novel
marigold-like microspheres of ZnIn2S4 under analogous synthetic
conditions as compared to aqueous.
DRS are used to determine the optical properties of the
samples. It can be seen in Fig. 4a that ZnIn2S4 samples exhibit a
steep absorption edge in the visible range, which suggests that the
relevant band gap is due to the intrinsic transition of the
nanomaterials rather than transition from impurity levels. As
indicated in Fig. 4a, a qualitative shift to shorter wavelength is
observed in the absorption edge of ZIS-EtOH microspheres,
namely, the extent of visible light which can be used is enlarged
for ZIS-EtOH compared with ZIS-H2O, thus, it is anticipated that
more visible light can be used by ZIS-EtOH to photoexcite
electron-hole charge carriers. A plot obtained via the transforma-
tion based on the Kubelka–Munk function versus the energy of
light is shown in the inset of Fig. 4a. The estimated band gap
values of the samples are 2.67 and 2.56 eV, corresponding to ZIS-
EtOH, ZIS-H2O samples, respectively. This result indicates that the
synthesis solvent has a significant effect on the optical property of
light absorption for the ZnIn2S4 samples.
ZIS-EtOH
The surface area and porosity of ZIS-H2O and ZIS-EtOH are
investigated. As shown in Fig. 4b, the nitrogen adsorption–deso-
rption isotherms of two samples exhibit type IV isotherm with
typical hysteresis loops characteristic of mesoporous structure
[39,40], and the pore size distribution plots (inset) indicate that
the pore diameter of samples is 2–4 nm. It is well known that the
typical hysteresis loop is observed with aggregates of plate like
particles giving rise to slit-shaped pores [41]. That is, both of the
two samples are all consisting of a large number of ZnIn2S4
ZIS-H2O
JCPDS No. 65-2023
20
30
40
50
60
70
80
2 Theta (degree)
Fig. 1. XRD patterns of ZIS-H2O and ZIS-EtOH.
Survey
Zn2p
ZIS-EtOH
ZIS-EtOH
ZIS-H2O
ZIS-H2O
0
200
400
600
800
1000 1200
1020 1025 1030 1035 1040 1045 1050 1055
Binding Energy (eV)
Binding Energy (eV)
In3d
S2p
ZIS-EtOH
ZIS-EtOH
ZIS-H2O
ZIS-H2O
445
450
455
460
158 160 162 164 166 168 170 172
Binding Energy (eV)
Binding Energy (eV)
Fig. 2. XPS spectra of ZIS-H2O and ZIS-EtOH: (a) survey XPS spectrum and (b–d) high-resolution spectra of Zn2p, In3d and S2p.

Z. Chen et al. / Journal of Solid State Chemistry 205 (2013) 134–141
137
Fig. 3. FESEM images of the as-prepared samples: (a) ZIS-H2O and (b) ZIS-EtOH.
90
60
30
0
ZIS-H O
120
ZIS-EtOH
80
40
0
ZIS-EtOH
ZIS-H O
hv(eV)
200
300
400
500
600
700
800
0.0
0.2
0.4
0.6
0.8
1.0
Wavelength /nm
Relative Pressure (P/P0)
Fig. 4. DRS spectra (a) and N2 adsorption-desorption isotherms (b) of ZIS-H2O and ZIS-EtOH.
nanosheets, which is consistent with the FESEM result. The BET
surface area of ZIS-EtOH and ZIS-H2O is measured to be ca.
63 m2 gꢂ1 and 98 m2 gꢂ1, respectively. According to our previous
study, the surface area have an important influence on the
photocatalytic activity, namely, larger surface area can endow
the ZnIn2S4 sample with better activity toward visible light
photocatalytic degradation of dyes [34]. The photocatalytic activity
of the ZnIn2S4 samples for selective oxidation has been of interest
to us. Herein, we use selective oxidation of benzyl alcohol as a
probe reaction to study photocatalytic activity of the sample under
ambient conditions.
shown in Fig. 5d), similarly, the benzyl alcohol is almost trans-
formed to benzaldehyde after irradiation of 10 h. Namely, the high
concentration benzyl alcohol can also be selective oxidation to
benzaldehyde over constant quantity of catalyst (8mg) as long as
extend the time of irradiation. Importantly, the conversion and
yield is much higher than many other photocatalysts. For example,
Zhang et al. report that CdS-5%GR nanocomposite exhibits only
about 42% conversion and 41% yield for selective oxidation of benzyl
alcohol to benzaldehyde under same condition except the reaction
time is 4 h [41]. As well, core–shell Pd@CeO2 nanocomposite has been
designed by Zhang et al., with visible light irradiation for 20 h; the
complete 100% selectivity for the target product of benzaldehyde can
be obtained over the multi-Pd@CeO2 shell nanocomposite along with
a ca. 28% yield [42]. These results indicate that the ZIS-EtOH sample
can serve as a promising photocatalyst for selective oxidation of
benzyl alcohol to corresponding benzaldehyde.
It is known that the photocorrosion or photodissolution of
catalysts might occur on the photocatalyst surface in the photo-
catalytic reaction. Herein, the ZIS-EtOH photocatalysts before and
after selective oxidation of benzyl alcohol are compared by XRD
and XPS investigation. Fig. 6a shows the XRD patterns of ZIS-EtOH
before and after reaction. The position and ratio of peaks are
nearly the same and no new peaks are created. Furthermore, XPS
is applied to test the surface chemical states of the photocatalysts.
Fig. 6b indicates that the main peaks are owned by elements Zn, In
and S. The binding energies of Zn2p, In3d and S2p before and after
reaction are nearly the same. Furthermore, the four runs lifetime
test has been done and the results are shown in Fig. 7. During the
experiments repeated four times, the selectivity is nearly constant
(100%) for each run of recycled test, and there is no significant loss
of the conversion and the yield during four successive recycling
tests for selective oxidation of benzyl alcohol to benzaldehyde over
ZIS-EtOH under visible light irradiation. These results all demon-
strate that the as-prepared ZIS-EtOH sample is a relatively stable
3.2. Photocatalytic activity and stability
Fig. 5 lists the photocatalytic performance of selective oxidation
of benzyl alcohol over ZIS-EtOH and ZIS-H2O under visible light
irradiation. As can be clearly seen in Fig. 5a and b, ZIS-EtOH
exhibits higher photoactivity than ZIS-H2O in the same photo-
catalytic reaction system. The conversion of benzyl alcohol
increases gradually along the evolution of the reaction. The yield
of corresponding benzaldehyde also rises with the reaction time
progressing. After irradiation for 2 h, 100% conversion of benzyl
alcohol and 99% yield of benzaldehyde are reached over the
ZIS-EtOH, but only 58% conversion and 57% yield are reached over
ZIS-H2O. In addition, the selectivity to benzaldehyde is 499% over
both ZIS-EtOH and ZIS-H2O. Furthermore, the catalytic activity
with higher concentration of benzyl alcohol and the constant
quantity of catalyst has been performed. As shown in Fig. 5c, the
conversion of benzyl alcohol and the yield of benzaldehyde
increase gradually along the evolution of the reaction when the
benzyl alcohol concentration increases to two times (0.2 mmol),
both the conversion of benzyl alcohol and the yield of benzalde-
hyde are nearly 100% after irradiation of 5 h. When the benzyl
alcohol concentration increases to three times (0.3 mmol, as

138
Z. Chen et al. / Journal of Solid State Chemistry 205 (2013) 134–141
Fig. 5. Time-online photocatalytic selective oxidation of benzyl alcohol (BA) to benzaldehyde under visible light irradiation (λ4420 nm) over (a) ZIS-EtOH/ 0.1 mmol BA,
(b) ZIS-H2O/ 0.1 mmol BA, (c) ZIS-EtOH/ 0.2 mmol BA and (d) ZIS-EtOH/ 0.3 mmol BA.
after
after
before
before
20
30
40
50
60
70
80
0
200
400
600
800
1000 1200
2 Theta (degree)
Binding Energy (eV)
Fig. 6. XRD (a) and XPS (b) patterns of ZIS-EtOH before and after photocatalytic reaction.
Conversion
Yield
Selectivity
120
100
80
60
40
20
0
1
2
3
4
Recycled Times
Fig. 8. Remaining fraction of benzyl alcohol after the dark adsorption–desorption
equilibrium achieved over ZnIn2S4 samples.
Fig. 7. Cycling photocatalytic selective oxidation of benzyl alcohol to benzaldehyde
over ZIS-EtOH under visible light irradiation (λ4420 nm) for 2 h.
3.3. Discussion of mechanism
visible-light-driven photocatalyst for the selective oxidation of
benzyl alcohol in the reaction medium of BTF solvent under
ambient conditions.
To study the influence of synthesis solvents on the photocata-
lytic activity over the ZnIn2S4 samples, many efforts have been

Z. Chen et al. / Journal of Solid State Chemistry 205 (2013) 134–141
139
paid to reveal the essence of the higher photocatalytic activity for
ZIS-EtOH. To begin with, adsorption experiments in the dark for
benzyl alcohol have been performed. As seen in Fig. 8, there is no
obvious difference in terms of adsorptivity between ZIS-EtOH and
ZIS-H2O, and both of them exhibit almost similar adsorptivity for
benzyl alcohol in the dark. The BET surface area and dark
adsorption results strongly suggest that the enhanced photocata-
lytic activity of ZIS-EtOH compared with ZIS-H2O cannot be
attributed to the change in surface area [41].
To further understand the significant role of ethanol solvent on
enhancing the photocatalytic activity, photoelectronchemical
experiments are performed. Fig. 9a shows the photocurrent
transient response for ZIS-EtOH and ZIS-H2O electrodes under
visible light irradiation. It is easy to observe that the significant
photocurrent decay appears with the increased switch-on and -off
cycles, indicating that the obvious recombination process of
photogenerated electron–hole pairs is occurring. Furthermore, it
should be noted that the ethanol solvent in the synthesis
procedure is beneficial for improving the lifetime of photogener-
ated charge carriers, which can be reflected by the higher photo-
current density of ZIS-EtOH than ZIS-H2O. Moreover, the
photocurrent is quite stable, i.e., no obvious photocurrent decay
is observed. This suggests that the lifetime of photogenerated
charge carriers is able to be remarkably boosted by the solvother-
mal strategy and the transport of photogenerated electrons is
markedly effective. Therefore, the inhibition degree of photogen-
erated electron–hole pairs' recombination is better than ZIS-H2O.
This result is also strongly evidenced by the following results of PL
spectra.
[43–45], which results from emitted photons with the recombina-
tion of electron–hole pairs after a photocatalyst is irradiated. As
displayed in Fig. 9b, the PL intensity obtained over ZIS-EtOH is
much weaker than that of ZIS-H2O. This might result from the
suppressed recombination of photoexcited electrons and holes via
band-to-band emission transition, and this phenomenon might be
beneficial for the photocatalytic reaction [23]. The result of the PL
further verifies that the ZIS-EtOH can be beneficial to the separa-
tion of electron–hole pairs under irradiation, which is benefit to
photocatalytic activity toward selective oxidation of benzyl alcohol
to benzaldehyde under visible light irradiation [14,23,41].
In addition, the electrochemical impedance spectroscopy (EIS)
Nyquist plots and Mott–Schottky plots have been collected to
further determine the advantage of ZIS-EtOH over ZIS-H2O and
ascertain the detail of energy band. As shown in Fig. 10a, the
Nyquist plots of ZIS-EtOH and ZIS-H2O electrode materials cycled
in 0.2 M Na2SO4 electrolyte solution both show semicycles at high
frequencies. In electrochemical spectra, the high-frequency arc
corresponds to the charge transfer limiting process and can be
attributed to the double-layer capacitance in parallel with the
charge transfer resistance at the contact interface between elec-
trode and electrolyte solution [46,47]. It can be clearly seen from
Fig. 10a that the decrease of the arc when use the ZIS-EtOH as the
electrode material in comparison to ZIS-H2O, which means ZIS-
EtOH can facilitate the interfacial charge transfer [41]. Fig. 10b
exhibits the Mott–Schottky plots for ZIS-EtOH and ZIS-H2O sam-
ples. The positive slope of the Cꢂ2–E plots suggests the expected
n-type semiconductor of ZnIn2S4 in the nanomaterials [48]. The
flatband potentials of ZIS-EtOH and ZIS-H2O obtained by extra-
polation of the Mott–Schottky plots approximately equals ꢂ0.41 V
and -0.82 V respectively [49], which are more negative than the
The PL spectra are often employed to study surface processes
involving the electron–hole fate of the semiconductor TiO2
ZIS-EtOH
ZIS-H2O
0.6
OFF
2
ZIS-H O
ON
0.4
0.2
0.0
ZIS-EtOH
20 40 60 80 100 120 140 160 180 200 220
Irradiation Time /s
400
500
600
700
Wavelength (nm)
Fig. 9. Transient photocurrent response in 0.2 M Na2SO4 aqueous solution (pH¼6.8) without bias versus Ag/AgCl (a) and PL spectra (b) of ZIS-EtOH and ZIS-H2O.
2.4x1011
2.0x1011
1.6x1011
1.2x1011
8.0x1010
ZIS-H2O
ZIS-H2O
20
15
10
5
ZIS-EtOH
ZIS-EtOH
4.0x1010
0.0
-0.82eV
-0.8
-0.41eV
-0.4
0
0
2
4
6
8
10
12
0.0
0.4
0.8
5
Potential/V vs. Ag/AgCl
′′
Z (×10 ohm)
Fig. 10. Nyquist impedance plots (a) and Mott–Schottky plots (b) for the ZIS-EtOH and ZIS-H2O samples in 0.2 M Na2SO4 aqueous solution (pH¼6.8).

140
Z. Chen et al. / Journal of Solid State Chemistry 205 (2013) 134–141
for the utilization of semiconducting materials of ternary chalco-
genides as visible light photocatalyst for selective organic
transformation.
Acknowledgments
This work is financially supported by the Natural Science
Foundation of Fujian, China (2011J05024).
References
[1] S. Higashimoto, N. Suetsugu, M. Azuma, H. Ohue, Y. Sakata, J. Catal. 274 (2010)
76–83.
[2] M. Zhang, Q. Wang, C. Chen, L. Zang, W. Ma, J. Zhao, Angew. Chem. Int. Ed. 48
(2009) 6081–6084.
[3] M. Zhang, C. Chen, W. Ma, J. Zhao, Angew. Chem. Int. Ed. 47 (2008) 9730–9733.
[4] Y. Shiraishi, T. Hirai, J. Photochem. Photobiol. C: Photochem. Rev. 9 (2008)
157–170.
[5] A. Maldotti, A. Molinari, R. Amadelli, Chem. Rev. 102 (2002) 3811–3836.
[6] G. Palmisano, E. Garcia-Lopez, G. Marci, V. Loddo, S. Yurdakal, V. Augugliaro,
L. Palmisano, Chem. Commun. 46 (2010) 7074–7089.
Fig. 11. Illustration of the proposed reaction mechanism for selective oxidation of
benzyl alcohol to benzaldehyde over the ZIS-EtOH under the visible light
irradiation.
ꢂ
standard reduction potential of O2/O2 (ꢂ0.15 V vs NHE) [3].
Therefore, they are thermodynamically permissible for the trans-
formation of photogenerated electrons to the absorbed O2 for
produce the superoxide radicals ( ꢀ O2ꢂ).
[7] D.I. Enache, J.K. Edwards, P. Landon, B. Solsona-Espriu, A.F. Carley, A.A. Herzing,
M. Watanabe, C.J. Kiely, D.W. Knight, G.J. Hutchings, Science 311 (2006)
362–365.
Based on the above discussion, a tentative reaction mechanism
for photocatalytic selective oxidation of benzyl alcohol to benzal-
dehyde over ZIS-EtOH is proposed, as illustrated in Fig. 11. Under
visible light irradiation, the electron is excited from valence band
(VB) of ZnIn2S4 microspheres to its conduction band (CB), leaving
the hole in the VB. The photoinduced electrons transfer to the
surface of ZnIn2S4, and molecular oxygen in the reaction system
can be activated by accepting the photogenerated electrons, for
example, the formation of superoxide radicals ( ꢀ O2ꢂ) [20]. Then,
the substrate benzyl alcohol adsorbed on the surface of ZnIn2S4
can be oxidized by activated oxygen ( ꢀ O2ꢂ) and holes (h+) to give
the corresponding benzaldehyde. For ZIS-EtOH, its higher photo-
catalytic activity compared with ZIS-H2O may be due to more
visible light absorption and the prolonged lifetime of photogener-
ated electron–hole pairs.
[8] R.A. Sheldon, I.W.C.E. Arends, G.-J. ten Brink, A. Dijksman, Acc. Chem. Res. 35
(2002) 774–781.
[9] G.J.t. Brink, I.W.C.E. Arends, R.A. Sheldon, Science 287 (2000) 1636–1639.
[10] G. Palmisano, V. Augugliaro, M. Pagliaro, L. Palmisano, Chem. Commun. (2007)
3425–3437.
[11] M.A. Fox, M.T. Dulay, Chem. Rev. 93 (1993) 341–357.
[12] S. Yurdakal, G. Palmisano, V. Loddo, V. Augugliaro, L. Palmisano, J. Am. Chem.
Soc. 130 (2008) 1568–1569.
[13] L.L. Chng, N. Erathodiyil, J.Y. Ying, Acc. Chem. Res., 2013, http://dx.doi.org/10.
1021/ar300197s.
[14] Y. Zhang, N. Zhang, Z.R. Tang, Y.J. Xu, Phys. Chem. Chem. Phys. 14 (2012)
9167–9175.
[15] Q. Wang, M. Zhang, C. Chen, W. Ma, J. Zhao, Angew. Chem. Int. Ed. 49 (2010)
7976–7979.
[16] S. Higashimoto, N. Kitao, N. Yoshida, T. Sakura, M. Azuma, H. Ohue, Y. Sakata, J.
Catal. 266 (2009) 279–285.
[17] G. Palmisano, S. Yurdakal, V. Augugliaro, V. Loddo, L. Palmisano, Adv. Synth.
Catal. 349 (2007) 964–970.
[18] Y. Zhang, Z.R. Tang, X. Fu, Y.J. Xu, ACS Nano 5 (2011) 7426–7435.
[19] N. Zhang, S. Liu, X. Fu, Y.J. Xu, J. Mater. Chem. 22 (2012) 5042–5052.
[20] N. Zhang, X. Fu, Y.J. Xu, J. Mater. Chem. 21 (2011) 8152–8158.
[21] X. Gou, F. Cheng, Y. Shi, L. Zhang, S. Peng, J. Chen, P. Shen, J. Am. Chem. Soc. 128
(2006) 7222–7229.
4. Conclusions
[22] Z.D. Xu, Y.X. Li, S.Q. Peng, G.X. Lu, S.B. Li, RSC Adv. 2 (2012) 3458–3466.
[23] S. Shen, L. Zhao, X. Guan, L. Guo, J. Phys. Chem. Solids 73 (2012) 79–83.
[24] B. Chai, T.Y. Peng, P. Zeng, X.H. Zhang, Dalton Trans. 41 (2012) 1179–1186.
[25] Y. Li, J. Wang, S. Peng, G. Lu, S. Li, Int. J. Hydrog. Energy 35 (2010) 7116–7126.
[26] S. Shen, L. Zhao, Z. Zhou, L. Guo, J. Phys. Chem. C 112 (2008) 16148–16155.
[27] M.T. Li, J.Z. Su, L.J. Guo, J. Int, Hydrog. Energy 33 (2008) 2891–2896.
In summary, we have synthesized the ternary chalcogenides
ZnIn2S4 photocatalysts with hexagonal crystal phase and
marigold-like microspheres morphology via
a facile one-pot
approach in ethanol-mediated condition. The results demonstrate
that ZIS-EtOH exhibits a higher visible light photocatalytic activity
than ZIS-H2O toward selective oxidation of benzyl alcohol to
benzaldehyde using oxygen as oxidant under ambient conditions.
It is found that ethanol solvent has a significant effect on the
properties of the samples, including morphology, optical, electro-
nic nature and photocatalytic activity as compared with aqueous
solvent. The improved photocatalytic activity of ZIS-EtOH samples
can be ascribed to synergetic effect of visible light absorption and
the prolonged lifetime of photogenerated electron–hole pairs.
Importantly, our work is the first time to utilized ZnIn2S4
microspheres synthesized by solvothermal method for selective
organic transformation. The results suggest that the as-obtained
ZIS-EtOH microspheres are able to serve as a promising visible-
light-driven photocatalyst for selective oxidation of benzyl alcohol
to corresponding benzaldehyde under ambient conditions. Of
course, we will study whether ZIS-EtOH can exhibit well
photocatalytic activity for others alcohols such as p-methybenzyl
alcohol, p-methoxbenzyl alcohol, p-nitrobenzyl alcohol, cinnamyl
alcohol etc., detailed investigation is still undergoing in our
laboratory. It is hoped that our current work could widen
the application of ZnIn2S4 and open promising prospects
[28] B. Chai, T. Peng, P. Zeng, X. Zhang, X. Liu, J. Phys. Chem.
C 115 (2011)
6149–6155.
[29] Z. Lei, W. You, M. Liu, G. Zhou, T. Takata, M. Hara, K. Domen, C. Li, Chem.
Commun. (2003) 2142–2143.
[30] S. Peng, Z. Peining, S.G. Mhaisalkar, S. Ramakrishna, J. Phys. Chem. C 116 (2012)
13849–13857.
[31] Z. Chen, D. Li, G. Xiao, Y. He, Y.-J. Xu, J. Solid State Chem. 186 (2012) 247–254.
[32] Y. Chen, S. Hu, W. Liu, X. Chen, L. Wu, X. Wang, P. Liu, Z. Li, Dalton Trans. 40
(2011) 2607–2613.
[33] F. Fang, L. Chen, Y.-B. Chen, L.-M. Wu, J. Phys. Chem. C 114 (2010) 2393–2397.
[34] Z. Chen, D. Li, W. Zhang, Y. Shao, T. Chen, M. Sun, X. Fu, J. Phys. Chem. C 113
(2009) 4433–4440.
[35] Z. Chen, D. Li, W. Zhang, C. Chen, W. Li, M. Sun, Y. He, X. Fu, Inorg. Chem. 47
(2008) 9766–9772.
[36] X. Hu, J.C. Yu, J. Gong, Q. Li, Cryst. Growth Des. 7 (2007) 2444–2448.
[37] W. Cai, Y. Zhao, J. Hu, J. Zhong, W. Xiang, J. Mater. Sci. Technol. 27 (2011)
559–562.
[38] W.J. Fan, Z.F. Zhou, W.B. Xu, Z.F. Shi, F.M. Ren, H.H. Ma, S.W. Huang, J. Int,
Hydrog. Energy 35 (2010) 6525–6530.
[39] D.V.P. Bavykin, V.N. Lapkin, A.A. Walsh, F. C, J. Mater. Chem. 14 (2004)
3370–3377.
[40] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, 2nd ed.,
Academic Press, London, 1982.
[41] N. Zhang, Y. Zhang, X. Pan, X. Fu, S. Liu, Y.J. Xu, J. Phys. Chem. C 115 (2011)
23501–23511.
[42] N. Zhang, S. Liu, X. Fu, Y.J. Xu, J. Phys. Chem. C 115 (2011) 22901–22909.
[43] Y.T. Liang, B.K. Vijayan, K.A. Gray, M.C. Hersam, Nano Lett. 11 (2011)
2865–2870.

Z. Chen et al. / Journal of Solid State Chemistry 205 (2013) 134–141
141
[44] Y.J. Xu, Y. Zhuang, X. Fu, J. Phys. Chem. C 114 (2010) 2669–2676.
[45] Y. Yao, G. Li, S. Ciston, R.M. Lueptow, K.A. Gray, Environ. Sci. Technol. 42 (2008)
4952–4957.
[46] T. Lu, Y. Zhang, H. Li, L. Pan, Y. Li, Z. Sun, Electrochim. Acta 55 (2010)
4170–4173.
[47] H.L. Guo, X.-F. Wang, Q.-Y. Qian, F.-B. Wang, X.-H. Xia, ACS Nano 3 (2009)
2653–2659.
[48] Y.J. Chen, R.K. Huang, D.Q. Chen, Y.S. Wang, W.J. Liu, X.N. Li, Z.H. Li, ACS Appl.,
Mater. Interfaces 4 (2012) 2273–2279.
[49] D.S. Kong, Langmuir 24 (2008) 5324–5331.