Vol. 36, No. 7
Journal of Semiconductors
July 2015
Photocatalytic degradation of RhB and TNT and photocatalytic water splitting with
CZTS microparticles
S. S. ShindeŽ
Hanyang University, Ansan 305340, Korea
Abstract: Cu2 ZnSnS4 (CZTS) is a main candidate material for solar energy conversion through both photovoltaics
and photocatalysis based on environmentally friendly elements and with a direct band gap of 1.5 eV. We report
the synthesis of quasi Cu2 ZnSnS4 microparticles with unprecedented narrow size distributions. The structural,
morphological and core level analysis has been carried out by XRD, SEM and XPS techniques. These microparticles
have shown excellent photocatalytic activity toward degradation of Rhodamine B dye (RhB) and TNT under visible
light. The extent of mineralization has been analyzed by COD and TOC values. Photocatalytic water splitting for
H2 generation has also been reported.
Key words: Rhodamine B; TNT; CZTS; photocatalysis; H2 production
DOI: 10.1088/1674-4926/36/7/073003
EEACC: 2520
ation from water under visible light irradiation has been reported.
1. Introduction
Current functional nanomaterials must meet numerous
very demanding properties that cannot be realized with a
unique compound. The use of heterostructured materials generally requires a wide range of applications. In multiphase compounds, interface reactions and physical properties will affect
the active performance. An efficient photocatalytic system requires an intimate interface between two phases, light absorption and co-catalyst.
Solar energy conversion is a particularly interesting application requiring development of high performance, environmentally friendly and inexpensive heterostructured materialsŒ1 . While several semiconductors have been proposed to
harvest sunlightŒ2 Cu2 ZnSnS4 (CZTS) have outstanding optoelectronic properties, with a direct band gap energy of 1.5 eV
and a composition based on elements that abound in the Earth’s
crust. Such an environmentally friendly and low-cost material
has been demonstrated for large potential photodegradation of
pollutants.
The CZTS particles can be prepared by several techniques
such as hydrothermal, colloidal solution, chemical bath deposition (CBD), successive ionic layer adsorption and reaction
(SILAR), sol–gel, solvothermal, sputtering, co-evaporation, EB evaporation, sulfurization, co-precipitation, pulsed laser deposition (PLD), and spray pyrolysisŒ3 12 . However, due to the
difficulties in tuning the composition, phase, size, and shape
of such complex materials, the preparation of CZTS-based
nanoparticles has not yet been achieved.
In this study, we reported synthesis of CZTS microparticles and its structural, morphological and core level analysis by spectroscopic techniques. These micro-particles were
tested for photodegradation of pollutants in solution Rhodamine B and TNT. The extent of mineralization is also studied by measuring chemical oxygen demands (CODs) and total
organic carbons (TOCs). Also photocatalytic hydrogen gener-
2. Experimental
Highly dispersed CZTS microcrystals were prepared by
the colloidal reaction of copper, tin, and zinc precursors with
tert-dodecylmercaptan and dodecanethiol in the presence of
oleylamine (OLA). In brief, 5 mM of CuCl2 2H2 O, 4 mM of
ZnO and 1.5 mM of SnCl4 5H2 O were dissolved in tetrahydrofuran (THF). Then, 20 mM of OLA and 20 ml of distilled
1-octadecene were added to the reaction mixture. The solution was heated to 175 ıC under nitrogen flow and kept for
1 h to remove low boiling point impurities and water. After
purging, the mixture was cooled to 100 ıC, and 40 mM of
tert-dodecylmercaptan and 3 mM of dodecanethiol were added.
The solution was then heated to 250 ıC for 1 h. The obtained
CZTS micro-particles were thoroughly purified by multiple
precipitation and re-dispersion using 2-propanol and chloroform. Finally, these CZTS particles were dissolved in THF to
remove poorly soluble unreacted metal complexes and large
Zn-reach particles and then by centrifugation, the final product
was collected and stored for later use. Then this final product
is annealed for 200 ıC.
The structural properties of the as-deposited and annealed
CZTS particles were studied using high resolution X-ray
diffraction (XRD) with Cu K˛ radiation of 1.54056 Å (X’pert
PRO, Philips, Eindhoven, Netherlands). The surface morphology of particles was observed by using FESEM (field emission scanning electron microscopy, Model: JSM-6701F, UK).
The chemical composition and valence states of constituent elements were analyzed by an X-ray photoelectron spectroscope
(XPS, VG Multilab 2000, Thermo VG Scientific, UK) with
monochromatic Mg-K˛ (1253.6 eV) radiation source. The carbon 1 s line corresponding to 285 eV was used for the calibration of the binding energies in the spectrometer. The concentration of an element was calculated from the integrated intensity
† Corresponding author. Email: [email protected]
Received 21 November 2014, revised manuscript received 28 January 2015
073003-1
© 2015 Chinese Institute of Electronics
J. Semicond. 2015, 36(7)
S. S. Shinde
Figure 1. X-ray diffraction patterns of as-grown and annealed CZTS
particles.
of core level peak and its sensitivity factor. The composition ratio was also determined from a ratio of atomic concentrations.
To evaluate the photodegradation of Rhodamine B (RhB)
and TNT (Trinitrotoluene) under visible light, 10 mg CZTS
particles were suspended in 50 mL of 10 ppm aqueous solution
of RhB and TNT. The solution was stirred and kept in the dark
overnight to achieve the equilibrium. Then the suspension was
illuminated with a 300 W Xe lamp. The concentration change
RhB and TNT was monitored by measuring the UV–vis absorption at regular intervals. The peak absorbance at 552 and
232 nm was used to determine the concentration of RhB and
TNT respectively. The photocatalytic activity was analyzed by
the time profiles of C0 =C , where C is the concentration of
RhB and TNT at the irradiation time and C0 the concentration
just after the absorption equilibrium before irradiation, respectively.
For the photocatalytic H2 generation experiments, 10 mg
of powder was dispersed in 50 mL of deionized water, with
0.1 M Na2 S and NaSO3 as hole scavengers. The whole reaction was carried out in a glass reactor. Before the solution
was irradiated, the reactor was thoroughly purged with nitrogen to remove all oxygen in the headspace of the reactor and
dissolved oxygen in water. A 300 W Xe lamp was used to irradiate the sample. The reaction product was monitored by periodical sampling the gas phase from the glass chamber using
a gas tight syringe and analyzing it by a gas chromatograph
(Varian GC-450) fitted with a thermal conductivity detector to
detect H2 , O2 and N2 and a flame ionization detector to detect
hydrocarbons. Photo-stability was tested by analyzing the photocatalytic activity of the samples on three consecutive runs by
washing photocatalyst.
3. Results and discussion
3.1. Structural analysis
Figure 1 shows the XRD patterns of as deposited and annealed CZTS particles. It is seen that the crystallinity enhances
with annealing the as grown particles. The annealed CZTS particles are polycrystalline and the matching of standard and observed ‘d ’ values (ASTM data card No. 26-0575) confirms the
Kesterite (tetragonal) crystal structure. It shows well resolved
peaks corresponding to strongest characteristic of the (112) orientation. The other peaks are attributed to the (200), (220) and
(312) planes of CZTS. The annealed CZTS particles show an
increase in the peak intensity for all planes due to enhancement
in the crystallinity as compared to as-grown particles. The grain
size is increases due to annealing treatment.
The SEM images of CZTS particles prepared for stoichiometric concentration is shown in Figure 2. It is observed that,
due to re-crystallization with annealing, resultant grain size is
increased, compared to as-synthesized particles as evident in
XRD.
Figure 3 shows X-ray photoelectron spectroscopy (XPS)
core level peaks of each element in the as-grown and annealed
CZTS particles. It is seen that, the spectral shapes of Cu 2p, Zn
2p, Sn 2d, and S 2p peaks are not change due to annealing. The
pronounced splitting of the Cu 2p and Zn 2p spectral line into
the 2p1=2 and 2p3=2 core levels is observed. The binding energy
around 952.7 and 932.8 eV can be ascribed to the Cu 2p1=2 and
Cu 2p3=2 core levels transition of Cu (I) atoms. The doublet
lines of Zn corresponding to 2p3=2 and 2p1=2 are observed at
1022.5 and 1045.6 eV, respectively. The Zn 2p3=2 line has been
shifted by EZn D 0:9 eV from the reported average binding
energy position of 1021.6 eV for elemental zincŒ13 . The sign of
chemical shifts indicate electron transfer during bonding process leading to a net change of the charges. The charge transfer
can be estimated using a simple electrostatic modelŒ14 according to which the atoms emitting photoelectrons are treated as
a thick uniform conducting charge shell of valance electrons
lying between radii and surrounding inner core levels. Figure
3(c) shows the XPS spectra of Sn 3d core levels of CZTS particles. The peaks of binding energy at 486.5 and 494.9 eV are
the transition of Sn 3d electrons which are in good agreement
with the energies reported for SnŒ15 . The binding energy of Sn
3d core level shift towards higher binding energy region due to
formation of constituent phases which is the influence of annealingŒ16 . Analysis of sulfur core states show the slight doublet of S 2p3=2 and S 2p1=2 core levels around binding energy
of 161.7 and 162.8 eV. In comparison with spectra, the binding
energy of Zn 2p and Sn 3d is slightly shifted towards higher
binding energy from their as-grown particles. The amount of
chemical shifts of Zn 2p and Sn 3d is 0.18 and 0.3 eV, respectively. On the other hand, the shifts of Cu 2p and S 2p are very
less than the limit of resolution. The chemical shifts can be explained a sulfurization of Zn and Sn. The difference between
the electron negativities of Zn and S is larger than that of Sn
and SŒ17 . Therefore, Zn atoms are attracted strongly toward S
atoms as compared with Sn atoms. The change in full width at
half maximum (FWHM) of each peaks due to annealing confirms the variation in the crystallinity. A small amount of S–O
peaks is existed in S 2p spectra of the as-grown particles. The
major component located at 168.9 eV is assigned to S–O bonds
in SO24 speciesŒ17 . Sulfur atoms bond with oxygen after the
growth because the S–O peaks are observed only on the surface
of particles.
3.2. Photocatalysis of RhB and TNT
To evaluate the photocatalytic potential of CZTS particles,
the degradation under visible light of aqueous Rhodamine B
and TNT is tested as model systems. In typical measurement
073003-2
J. Semicond. 2015, 36(7)
S. S. Shinde
Figure 2. Typical FESEM images of chemically prepared as-grown and annealed (at 200 ıC) CZTS particles.
Figure 3. Typical XPS core level peaks of chemically prepared as-grown and annealed (at 200 ıC) CZTS particles.
we used 10 mg CZTS particles in 50 mL of 10 ppm aqueous solution of RhB and TNT. The solution is stirred and equilibrated
for overnight and then illuminated by xenon lamp of 300 W.
The RhB & TNT concentration is monitored by measuring optical extinction at regular intervals. Figures 4 and 5 show the
plot of extinction spectra against time and plot of C =C0 versus
time and ln(C0 =C ) versus time for RhB and TNT respectively.
As the reaction time increases, the concentration of both system decreases and it is plotted to calculate the degradation kinetics. The photo-degradation is fitted with pseudo-first-order
reaction kineticsŒ18 ,
ln
C0
D kt;
C
where k is the apparent first order rate constant. The rate constants for RhB and TNT are 0.04 and 0.088 min 1 . The photodegradation of RhB and TNT in the presence of CZTS is relatively slow but degrades up to 98% and 86% within 6 h. In
order to intangible from these external parameters and to make
comparison of experimental data obtained under various conditions possible, it is useful to defineŒ19 ,
(1)
073003-3
k 0 D kV;
(2)
k 00 D
(3)
p D k 000
k0
;
A
kVF
D
;
iph
(4)
J. Semicond. 2015, 36(7)
S. S. Shinde
Figure 4. Plot of (a) extinction spectra and (b) degradation kinetics
versus reaction time for RhB.
where F is Faraday’s constant (96500 mol 1 /, V the volume,
A is the area of electrode, p or k 000 the rate constant or kinetic
parameter, which is independent of total photocurrent (iph /, and
total volume of solution. The p reflects the efficiency of oxidative degradation of the solute (when extinction is used) or
of the amount of oxidizable atoms in the solution (in the case
of COD). The values of the parameters k 0 , k 00 and p or k 000 are
found to be 2.3 10 2 cm3 /s, 2.68 10 4 cm/s and 149.6 M 1
and 2.7 10 2 cm3 /s, 3.68 10 4 cm/s and 167.6 M 1 for
RhB and TNT respectively.
Reusability of CZTS photocatalyst for the degradation of
RhB and TNT is evaluated. The solution resulting from the
photocatalytic degradation of the organic species is filtered,
washed and the photocatalyst is dried. The dried catalyst samples are used for the degradation of organic species under similar conditions. The filtrate is subjected to AAS analysis to
assess the loss of any elemental ions in solutions but it did
not show any sign of deterioration of CZTS photocatalyst.
The photocatalyst CZTS could be used repeatedly without any
treatment for further catalytic activity up to 150 h.
Apart from extinction study, the extent of mineralization
is analyzed by measuring COD and TOC as a function of time
for RhB and TNT. Figure 6 compares the information obtained
from chemical oxygen demand (COD) and total organic carbon (TOC) measurements. COD study as a function of illumination time give the concentration of oxidizable matter left in
the solution. It is concluded that the suppression of electron-
Figure 5. Plot of (a) extinction spectra and (b) degradation kinetics
versus reaction time for TNT.
Figure 6. Extent of mineralization by COD and TOC versus reaction
time for RhB and TNT.
hole recombination and the generation of more OH radicals in
samples play important roles in the enhanced rate of photomineralization. The COD and TOC decreases from 51 to 0.15
& 74 to 12 and 28 to 0.08 & 42 to 7 mg/L respectively. The
observed decay constants indicate the destruction of the main
elements of RhB and TNT. Measurements of TOC at the beginning and at the end of degradation experiments show that
the decay rate of the extinction at 552 and 232 nm, ext552 ,
ext232 and the TOC are directly correlated, with d(ext552 / / dt
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J. Semicond. 2015, 36(7)
S. S. Shinde
Figure 7. Hydrogen evolution rate of CZTS particles with reaction
time with recycle runs for five times.
d(TOC) / dt and with d(ext232 / / dt d(TOC) / dt respectively. In excess oxygen, photocatalytic reduction is less frequently encountered than the oxidation, presumably because
the reducing power of photo-generated electrons is significantly lower than the oxidizing power of photo-holes and also
most reducible substrates do not compete kinetically with oxygen in the trapping of conduction band electronsŒ20 .
We also examined the activity of CZTS towards photocatalytic hydrogen evolution from water splitting. The H2 production rate increased with reaction time and seen that H2 evolution is about 100 mol/h. The photo-stability of the catalyst was tested by measuring the H2 evolution during consecutive runs as shown in Figure 7. After each run, the photocatalyst was recovered by centrifugation and re-dispersed in a new
deionized water solution containing fresh hole scavengers. No
catalyst deactivation was observed after five cycles. Actually,
the activity of the CZTS catalyst increased slightly with time,
which may be explained by the decomposition of residual organic molecules at the particle surface.
4. Conclusions
We successfully synthesized the CZTS microparticles by
colloidal reaction. The enhanced photocatalytic activity of
CZTS particles for RhB and TNT (98% and 86%) under visible light is due to enriching its electron concentration and promoting a chemical reaction. The photocatalytic H2 evolution is
about 100 mol/h.
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