Capped and surface clean zinc sulfide nan oparticles synthesized in [603744]

Elsevier Editorial System(tm) for Colloids
and Surfaces A: Physicochemical and Engineering Aspects
Manuscript Draft

Manuscript Number:

Title: Capped and surface clean zinc sulfide nan oparticles synthesized in
the presence of Triton X -100

Article Type: Research Paper

Keywords: Capped ZnS nanoparticles; uncapped ZnS nanoparticles; Triton X –
100; azo dye solution bleaching; photocatalytic activity.

Corresponding Author: Dr. Anca Dumbrava, Ph.D.

Corresponding Author's Institution: Ovidius University of Constanta

First Author: Anca Dumbrava, Ph.D.

Order of Authors: Anca Dumbrava, Ph.D.; Daniela Berger, Ph.D.; Gabriel
Prodan; Florin Moscalu, Ph.D.; Aurel Diacon, Ph.D.

Abstract: Herein we report the synthesis of ZnS nanopowders capped with
Triton X -100, using two different synthesis routes, which were
characterized by X -ray diffraction, transmission electron microscopy,
FTIR spectroscopy, UV -visible spectroscopy, photolum inescence
spectroscopy, and surface area measurements. We also report a method to
remove the capping agent, which changed the ZnS photocatalytic properties
tested in the bleaching of an azo dye solution. All the synthesized
nanopowders have great photocata lytic activity, and also a high specific
surface area, depending on the synthesis route. Thus, using Triton X -100
capped ZnS nanopowder with specific surface area of 191 m2/g, we
performed in only 60 min the almost complete photobleaching of the azo
dye solution, meaning a photocatalytic activity of 97.79%, and further a
photocatalytic activity of 99.75% was achieved in 120 min. Based on the
photocatalytic results, the obtained functionalized zinc sulfide samples
can be considered suitable catalysts for a g reen, very efficient and
quick strategy for photobleaching of organic pollutants.

Suggested Reviewers: A. S. Aybek
[anonimizat]

Y. P. Zhang
[anonimizat]

V. A. Gevorgyan
[anonimizat]

P. Praus
[anonimizat]

C. Wang
[anonimizat] .cn

Dear Editor,
Please find enclosed our manuscript entitled “Capped and surface clean zinc sulfide
nanoparticles synthesized in the presence of Triton X -100 “, by Anca Dumbrava, Daniela
Berger, Gabriel Prodan, Florin Moscalu and Aurel Diacon, submitted for publication in
“Colloids and surfaces A – physicochemical and engineering aspects ”.
We bring to your attention a study about the effect and importance of surfactant , together with
the synthesis method , in the synthesis of ZnS nanopowders. Thus, we succeeded the
increasing of ZnS nanopow der specific surface area for more than 9 times by using Triton X –
100 as surfactant and trituration as s ynthesis technique. The ZnS nanopowders have a
remarkable high catalytic activity, the effect being almost instantaneous ( 93% after 15 min ),
bleaching an azo dye solution in less than one hour. The novelty of our study comes from the
using of Triton X -100 in the synthesis of ZnS nanopowders, the synthesis method and, as a
consequence, the great variation of ZnS properties . Thus, we used in the synthesis of ZnS
nanopowders a combination between experimental technique and surfactant which led to
remarkable properties of ZnS nanopowders. T o our knowledge, it is the first report about the
synthesis of Triton -X-capped, and also uncapped ZnS nanopowders , by this experimental
route. The photocatalytical properties of ZnS nanopowders recommended them as green
catalysts for wastewater treatment.
The manuscript is brought to your attention with the belief that it s atisfies the selection criteria
of your journal and with the hope that it will be considered for publication.
The present manuscript reports new, original work that is not being submitted to another
journal. The present work is not cons idered for publication elsewhere in any other form. Any
part of the present work is not published elsewhere. All the co -authors are aware of this
submission and contributed to this work.
Thank you for your attention to this letter.
Sincerely,
Anca Dumbrava
Assoc. Prof. ,
Department of Chemistry and Chemical Engineering ,
Ovidius University of Constanta , Romania
*Cover Letter

*Graphical Abstract (for review)

Highlights
 Triton X -100 was used as surfactant in ZnS nanopowders synthesis by two methods
 Capped and uncapped ZnS nanoparticles were obtained
 ZnS nanopowders with high specific surface area were synthesised
 A very high capacity for bleaching an azo dye solution was evidenced
 Some of ZnS powders properties were crucially modified by the synthesis technique
*Highlights (for review)

Capped and surface clean zinc sulfide nanoparticles synthesized in the presence of
Triton X -100

Anca DUMBRAVA*, 1, Daniela BERGER2, Gabriel PRODAN3, Florin MOSCALU4, Aurel DIACON5

1 Department of Chemistry and Chemical Engineering, Ovidius University of Constanta, 124 Mamaia Blvd.,
Constanta 900527, Romania
2 University Politehnica of Bucharest, Department of Inorganic Chemistry, Physical Chemistry and
Electrochemistry, Polizu Street 1 -7, Bucharest 011061, Romania
3 Electron Microscopy Laboratory, Ovidius University of Constanta, 124 Mamaia Blvd., Constanta 900527,
Romania
4 Department of Physics, Ovidius University of Constanta, 124 Mamaia Blvd., Constanta 900527, Romania
5 University Politehnica of Bucharest, Depar tment of Bioresources and Polymer Science, Polizu Street 1 -7,
Bucharest 011061, Romania

Abstract . Herein we report the synthesis of ZnS nanopowders capped with Triton X -100, using two different
synthesis routes , which were characterized by X-ray diffraction, transmission electron microscopy, FTIR
spectroscopy, UV –visible spectroscopy , photoluminescence spectroscopy , and surface area measurements . We
also report a method to remove the capping agent , which chang ed the ZnS photocatalytic proper ties tested in the
bleaching of an azo dye solution . All the synthesized nanopowders have great photocatalytic activity, and also a
high specific surface area , depending on the synthesis route. Thus, using Triton X -100 capped ZnS nanopowder
with specific surface area of 191 m2/g, we performed in only 60 min the almost complete photobleaching of the
azo dye solution , meaning a photo catalytic activity of 97.79%, and further a photocatalytic activity of 99.75%
was achieved in 120 min. Based on the photocatalytic results, the obtained functionalized zinc sulfide samples
can be considered suitable catalyst s for a green, very efficient and quick strategy for photobleaching of organic
pollutants.

* Corresponding author: e -mail – adumbrava@univ -ovidius.ro ; tel/fax – 0040241606490 *Manuscript

Keywords: capped ZnS nanoparticles; uncapped ZnS nan oparticles; Triton X -100; azo dye solution bleaching;
photocatalytic activity.

1. Introduction
Metal sulfides, mainly as nan oparticles, have applications in a variety of devices, such as solar cells, light –
emitting diodes, sensors, thermoelectric devices, lithium -ion batteries, fuel cells and nonvolatile memory devices
[1]. The synthesis of metal sulfide colloidal nanoparticles typically consists in a chemical reaction between a
metal salt and a sulfide ion precursor, in the presence of capping agents, in order to stabilize the high energy
surface of the nanoparticles and protect them from aggregation. The c olloidal nanoparticles are synthesized and
stabilized in solution by using of organic molecules or polymers which could bind on the particle surface ; the
linked molecules are often denoted as either surfactants, ligands, or capping agents in the literature [2].
Among metal sulfides, zinc sulfide has focused strong interest in many areas of research, being
extensively studied, especially as nanomaterial [ 3]. However, new aspects are continuously evidenced and ZnS is
still of interest for researchers. As an important II –VI semiconducting material, ZnS has wide band -gap energy
(3.7 eV ) and a large exciton energy (≈ 40 meV) [1]. Actually, zinc sulfide has band -gap energy in the range of
3.6 – 3.9 eV , depending on its structure and morphology. Various shapes of ZnS , like sphere s, rod s, tube s and
wires, were successfully prepared ; the shape and also the size can be varied from bulk particle to nano crystal,
depending on synthetic route. ZnS structural properties can be easily tailored through chemical modifications
[3].
Nanostructured ZnS has versatile potential applications in optoelectronic devices , due to its excellent
properties of luminescence and photochemistry ( e.g. flat-panel displays, injection lasers, ultraviolet light –
emitting diodes, thin film electroluminescent, in solar energy power, etc.) [ 1, 3].
ZnS is also used as semiconductor photocatalyst in green synthesis of organic compounds (like
substituted tetrazoles [ 4], xanthene and its derivatives [ 5], etc.) and in removing of toxic or organic water
pollutants, owing to the highly negative reduction potentials of excited electrons and the rapid ge neration of
electron –hole pairs ( e.g. photocatalytic degradation of reactive black 5 azo dye [ 6], Ponceau S and crystal violet
dyes [ 7] or other pollutants) [ 1]. Zinc sulfide possesses high negative reduction potential of excited electrons,
due to its high er conduction band position in an aqueous solution , as compared to other extensively studied
photocatalysts. ZnS nanoparticles exhibit superior photocatalytic activity because of the increased

surface/volume ratio with enhanced redox potential as compare d to their bulk counterpart and trapped holes
arising from surface defects [7].
The preparation routes for zinc sulfide are one -pot synthesis, sol -gel technique , hydrothermal method ,
solid state reaction , etc. [3, 8]. For semiconductors nanoparticles synthesis , various surfactants can be employed
in order to form a monolayer on the nano particles surface (e.g. carboxylates , sodium citrate, oleic acid , etc.) and
synthetic pol ymers (polyethylene glycol , Triton X -100, polyvinyl alcohol , etc.), whose nature may strong ly
influence the nanoparticles properties [9]. The capping agents play a versatile role in colloidal synthe sis of
nanoparticles other than stabilizers. For instance, capping agents often act as ligands for metal ions form ing
coordination compounds and thereby affect their reduction kinetics. Functional groups in capping agents interact
with unsaturated surface atoms through dynamic adsorption and desorption. Their binding a ffinities to the
surface are dependent on distinct atom geometries of differen t facets and as a result, the particles shape could be
modified in the capping agents presence . In many cases, capping agents act as a physical barrier to restrict the
free access of reagents to catalytically active sites on the particle surface . The so -called “surface -clean ”
nanoparticles generally are not truly naked but they are free of long-chain organic compounds. Surface -clean
nanoparticles are stabilized by small molecules , which are easily displaced by reactants during catalytic
reactions. These small molecules including small adsorbates that are intentionally added for exchange of long –
chain organics, solvent molecules, solute ions, and even gases from the nanoparticle growth or storage
surroundings [2].
Having in view the importance of surfactants in the properties of ZnS nano particles , herein we report
the synthesis of capped and surface clean ZnS nanoparticles in order to optimize the photocatalytic properties,
evidencing the surfactant importance (Triton X -100) in nanoparticles structure and properties. All synthesized
ZnS samples exhibited catalytic properties in the discoloration of Congo red solution .
The azo dyes are used in large quantities in textile industry and wastewater s from this contain high
amount of dyestuff . The textile dyes often have aromatic structure and do not degrade easily under natural
conditions because of their highly photostabil ity and very small amount of dye in water (< 1 ppm for some dyes)
are visible and undesirable [7].
Pristine and functionalized ZnS nanoparticles can be used in the organic pollutants photo catalytic
degradation from wastewaters, and we also proposed a method for capping agents removal, obtaining surface
free ZnS. We demonstrated the variation of ZnS properties by removing of capping agent.
2. Experimental

2.1. Materials and methods
The high purity reagents were obtained from Sigma -Aldrich (zinc acetate, Zn(CH 3COO) 2∙2H 2O; Triton X -100,
TX; ammonia aqueous solution, 25%; Congo red) and Merck (thioacetamide, TAA), being used as received
without further purification. The Congo red (CR, C.I. Direct Red 28, M.W. = 696.67 g mol−1, C32H24N6O6S2Na2)
is the disodium salt of 3, 3′ -([1,1′ -biphenyl] -4,4′-diyl)bis(4 -aminonaphthal ene-1-sulfonic acid).
2.2. Synthesis of ZnS nanopowders
For the synthesis of zinc sulfide sample denoted ZnS 1-TX, Zn(CH 3COO) 2∙2H 2O (2.19 g, 10 mmol) and TAA
(0.75 g, 10 mmol) were dissolved in 50 mL of water and 2 mL of Triton X -100 were added. Further, 25% NH 3
aqueous solution was added dropwise, under continuous stirring, till pH = 6. The reaction mixture was heated
under magnetic stirring at 80 °C for 4 h. The white powder was filtered off, washed with water and dried. For the
synthesis by trituration technique of the sample s labeled ZnS 2-TX and ZnS 3-TX, Zn(CH 3COO) 2∙2H 2O (2.19 g,
10 mmol) was dissolved in 2 mL Triton X -100 and then TAA (0.75 g, 10 mmol) was added. The reaction
mixture was triturated without adding water until the complete dissolution of the reagents. The resulted mixture
was then intermittently triturated and left together for 3 days (ZnS 2-TX), respective for 30 days (ZnS 3-TX). A
homogenous white paste was obtained in time. For the removal of the unreacted compounds and byproducts, t he
paste was mixed with water, stirred for 30 min and the white powder was filtered off, washed with water and
dried.
Apart from this, ZnS powder was also prepared by a typical chemical precipitation method, under
similar conditions with Zn S 1-TX, to compare its properties and to highlight the influence of TX. Thus,
Zn(CH 3COO) 2∙2H 2O (2.19 g, 10 mmol) and TAA (0.75 g, 10 mmol) were dissolved in 50 mL of water and then
25% NH 3 aqueous solution was added dropwise, under continuous stirring, till pH = 6. The reaction mixture was
heated under magnetic stirring at 80 °C for 4 h. The white powder was filtered off, washed with water and dried.
2.3. Characterization of ZnS nanopowders
The synthesized ZnS powders were investigated by X -ray diffraction (XRD) p erformed on a Rigaku Miniflex 2
diffractometer with Ni filtered CuKα radiation, in the range of 2 , 10-70°, scan rate of 2°/min and a step of
0.02°. The TEM investigation of ZnS powders was performed on a Philips CM 120 ST transmission electron
microscope operated at 100 kV, with 2 Ǻ resolution. The UV –visible diffuse reflectance spectra of ZnS powders
were recorded in the range of 220 –850 nm, on a Jasco V 550 spectrophotometer w ith an integrating sphere,
using MgO as the reference. The FT IR spectra were recorded on Bruker Tensor 27 spectrometer using KBr
pellets technique, in the wave number range of 400 – 4000 cm−1. The photoluminescence spectra were recorded

on a Jasco FP -6500 spectrofluorometer . The specific surface area values were determined using Brunauer –
Emmett -Teller (BET) by measuring the adsorbed volume of nitrogen in the relative pressure range of 0.01 -0.25
in seven points. The measurements were performed on Quantachrom e Autosorb iQ 2 pore size analyzer after the
samples were outgassed at 60°C for 17 h.
2.4. Catalytic properties of ZnS nanopowders
The photocatalytic properties of ZnS samples were tested in the degradation of CR azo dye. Before irradiation,
dye solutions were stirred in the dark for 15 min after the addition of the catalyst, to reach the adsorption –
desorption equilibrium. The experiments were performed in covered Pyrex reactors of 250 mL volume in the
same time with two samples, which were exposed to the am bient light using a 45 W halogen lamp (Sky Glory,
China) as simulated visible light source. The lamp spectrum was measured with a HR4000CG -UV-NIR high –
resolution spectrometer, from Ocean Optics, confirming the matching with the solar spectrum. The halogen lamp
was kept perpendicularly to the surface of solution, and the distance between the UV –vis source and the reactor
containing the reaction mixture was fixed at 15 cm. The suspension was magnetically stirred (500 rpm) and
exposed to light. The suspension was sampled periodically and the samples were centrifuged at 5000 rpm for 10
min and then filtered off, to completely remove the catalyst particles. Control experiment s were repeated without
photocatalyst in the CR solution (“no catalyst”), in the same con ditions .
The CR degradation was monitored by UV –vis spectroscopy. The UV –vis absorption spectra of CR
solutions were recorded in the range of 200 –900 nm, on a Jasco V 550 spectrophotometer. The absorption peaks
corresponding to CR appeared at 497 nm, 347 nm and 237 nm , as it was also reported by other researchers [e.g.
10]. All the record ed spectra were analyzed using spectra m anager software and plotted against the irradiation
time.
The dye degradation was estimated by Ct/C0 ratio, where Ct and C0 are the concentrations of CR at
certain time, t, and initial concentration, respectively. The catalyst efficiency was determined by the degradation
efficiency ( R) or photoc atalytic activity ( PA) [11, 12 ], which can be calculated as [13]:

(1)
where A0, At are the absorbance value for CR solutions when the reaction time is 0 and t, respectively (based on
Lambert – Beer law).
3. Results and discussion

The influence of capping agents in the synthesis of colloidal metal sulfides nanoparticles was studied . According
to some authors, ligands are necessary to stabilize nanoparticles during synthesis but, once the particles have
been deposited on a substrate , the presence of the ligands is detrimental for catalytic activity [ 14].
The methods for ligand removal involved thermal and oxidative treatments, which can affect the size or
morphology of the particl es, in turn altering their catalytic activity. Another procedure for removal of capping
agents, for freshly prepared colloidal nanoparticles, consists in a repeated washing by a large amount of solvent
and collected by centrifugation [ 14].
In our study, we used a very efficient surfactant, Triton X -100 (4-(1,1,3,3 -tetramethylbutyl)phenyl –
polyethylene glycol, t-octylphenoxypolyethoxyethanol, polyethylene glycol t-octylphenyl ether), a non -ionic
surfactant from the alkyl phenyl polyethoxylate (PEO) class , composed of a PEO hydrophilic chain a nd a
hydrophobic aromatic group [15]. The influence of either synthesis time or solvent was investigated .
3.1. Characterization of ZnS nanopowders
3.1.1. X -ray diffraction
The crystalline phase was identified by X -ray diffraction technique. The XRD patterns (Fig. 1) showed that ZnS
samples crystallized in the zinc sulfide (sphalerite) with cubic symmetry (F -43M space group) (JCPDS No. 05 –
0566).
The crystallite size values f or zinc sulfide powders were determined from the XRD data using Rigaku
PDXL software based on Scherrer’s equation from (1 1 1) diffraction peak . All values of crystallite size are
similar and very s mall, ranging from 2. 6 nm (ZnS 1-TX) to 2.7 nm ( ZnS 2-TX and ZnS 3-TX). For comparison,
a value of 2. 5 nm was determined for the pristine ZnS sample .
3.1.2. Transmission electron microscopy
The ZnS nanopowders morphology was investigated by TEM. For ZnS 1-TX, the TEM images (Fi g. 2.a)
evidenced the tendency for agglomerates formation of primary fine ZnS nanoparticles, with the size in the range
of 50 – 230 nm and a n average diameter of 128 nm. The TEM images for ZnS 2-TX (Fig. 2.b) show interesting
aspects. The powder has apparently an amorphous surface, but at higher magnification very small crystallites are
visible. The TEM images for ZnS 3-TX powder (Fig. 2.c) revealed the aggregation of primary nano particles
forming agglomerates with dimension of micrometer s (an average diameter around 29 µ m). For comparison, the
TEM image for pristine ZnS was also presented (Fig. 2.d) . Its TEM investigation evidenced the agglomerates
formation of ZnS nanoparticles, with the size varying from 30 nm and 300 nm and a n average diameter of 110

nm. Thus, using the trituration technique, in absence of water (ZnS 2-TX), the particles are better dispersed and
the effect of capping agent being more evident . A similar result was reported for CdS nanoparticles [16].
Irrespective of the agglomeration tendency , the particle s are spherical with similar size , the difference in
powder morphology being only the particles agglomeration. Varying the precursor for sulfide ion and the
synthesis conditions, the shape of ZnS particles can be changed ; e.g. R. Lu et al. [17] obtained ZnS nanotubes in
aqueous medium, using CS 2, an insoluble in water compound, as sulfur source and TX as surfactant .
3.1.3. FT -IR spectroscopy
The presence of TX as capping agent can be evidenced by FTIR spectroscopy. The most interesting observation
is the absence of any band in the IR spectrum of ZnS 3-TX. In the FTIR spectra of both ZnS 1-TX and ZnS 2-
TX samples (Fig. 3) were identified two bands assigned to residual acetate groups at around 1550 cm-1 (asCOO)
and 1400 cm-1 (sCOO) [ 18]; the bands also exist in the pristine ZnS spectrum. T he presence of residual acetate
in ZnS powders was report ed previously by other authors ; in the capped ZnS, the se groups can be coordinated to
zinc ions or can react with organic polym er [19]. The skeletal vibrations of aromatic rings can be detected in the
same domain as acetate (1600 – 1400 cm-1) and, most probably, are overlapped by stronger acetate bands. The
presence of TX can be correlated with 1022 cm-1 and 934 cm-1 vibrations ( ZnS 1-TX, ZnS 2-TX). In this range,
the vibrations can be assigned to C –O (stretching vibration , often doublet). In all spectra, a broad ab sorption
band in the range of 3400 -3000 cm-1 demonstrated the presence of -OH groups associated by hydrogen bonding .
The broad band around 3400 cm-1 is probably overlapped with bands assigned to C ar-H stretching (3080 – 3030
cm-1, often numerous bands), especially in ZnS 2-TX spectrum [18, 20 ].
3.1.4. UV-Vis spectroscopy
The optical properties of the ZnS nanopowders were studied by the UV -visible diffuse reflectance spectroscopy.
The UV -vis absorption spectra are shown in Fig. 4.
The UV –vis absorption spectra of ZnS 1-TX and ZnS 2-TX samples (Fig. 4) are very similar, having a
maximum at 302 nm. The electronic spect rum of ZnS 3-TX powder is a little different in the 350 – 450 nm
domain, but the position o f absorption maximum is quite similar (309 nm). For comparison, the spectrum of
pristine ZnS (prepared as described above) contains an intense absorption band at 303 nm, while the spectrum of
a commercial ZnS powder (ZnS powder, < 10 µm, Aldrich) contains an intense absorption band at 323 nm.
3.1.5. Band gap energies
For a semiconductor like ZnS, the band gap value is important for its electrical and optical properties in some
applications ( e.g. photovoltaic devices or photocatalysis) . It is important to know how the optical properties,

such as energy band gap, vary with the synthesis parameters [ 21]. The zinc sulfide belongs to type -II
semiconductors and the optical band gap (Eg) can be estimated using the Tauc relation [ 22]:
  (2)
where α is the absorption coef ficient of the ZnS at a certain value of wavelength λ, h is Planck's constant, C is the
proportionality constant, ν is the frequency of light, and n = ½ (for direct transition mode materials). The
absorption coef ficient is evaluated using the relation:

(3)
where k is a c onstant, Rmax is the maximum re flectance and Rmin is the minimum re flectance. The consideration
of equations (2) and (3) gives Eq. (4 ):
  (4)
where C' is a constant. From Eq. (4 ), a Tauc plot can be drawn of ( αhν)2 versus hν. The point of the extrapolation
of the linear part that meets the abscissa will give the value of the band gap energy of the material [22, 23 ].
The band gap values (Fig. 5) can be correlated with structural characteristics, like crystallite size or
particles aggregation . The relationship between the band gap and particle dimension, and thus the size effect on
the electronic properties of semiconductors , was previously observed for some other materials [ 24, 25 ]. For the
studied ZnS nanopowders, the determined Eg values ar e comparable, as we expected based on the crystallites
size. A higher value for ZnS 2-TX is also correlated with smaller dimension of particles and small agglomerates .
The band gap energy value for a commercial ZnS sample (ZnS powder, < 10 µm, Aldrich) determined by the
same method was 3.75 eV.
3.1.6. Photoluminescence spectroscopy
The photoluminiscent properties of Zn S nanopowders , investigated by the photoluminescence (PL)
spectroscopy , main ly depend on interfacial processes, which occur at the boundary between the particles and the
surrounding medium. Therefore, t hese properties are strongly influence by the synthesis method and the nature
of coating agent [ 26, 27 ]. So the PL spectra can be correlated with the particles dimension an d photocatalytic
activity. The ZnS nanopowders were excited by laser irradiation at a wavelength of λ = 320 nm and the resulting
luminescence spectra are shown in Fig. 6.
It is known that two emissions (namely excitonic and trapped photoluminescence) are o bserved from
semiconductor nanomaterials. In principle, the excitonic emission is sharp and found near the absorption edge,
while the trapped emission is broad and found at longer wavelength [28]. For the studied samples, the blue

emission s in PL spectra were sharp , placed bellow 335 nm ; these band s are normally found close to those shown
in UV –vis diffused reflectance spectra (the absorption bands at about 300 – 310 nm, band gap energies 3.80 –
4.00 eV). In each spectrum, the second band is broad and splitted in three other ba nds, which are partially
overlapp ed. All ZnS samples contain a maximum of broad band centered at about 425 – 430 nm , and an other one
at about 465 nm can be observed in the spectra of ZnS 1-TX, ZnS 3-TX, and ZnS . The differences between
spectra consist in the relative intensity of the se bands and the wavelength for the absorption bands ; thus, a blue
shift of 5 and 7 n m was observed for ZnS 1-TX and ZnS 2-TX, respectively compared to ZnS . For ZnS 2-TX, a
shoulder can be observed near 460 nm. The ZnS 3-TX spectrum is rather like the spectrum of ZnS than with the
other ZnS TX.
The most intense band at about 425 nm ( i.e. 423 nm) was found for ZnS 2-TX; the highest intensity can
be explained by a large surface /volume ratio [ 28], which was indeed demonstrated by specific surface area
measurements . It was evidenced that the emission intensity is increases with the particles size decrease [29]. The
emission probably arises from the radiative recombination because of the surface defect s of ZnS [ 28], resulted as
surface modification by the capping agents .
As it is known, the higher intensity of PL spectrum means the lower separation rate of photo -induced
charge carriers and, possibly, the lower photocatalytic activity [ 30]. This behavior, which is not consistent with
the photocatalytic properties of ZnS nanopowders described below , suggests the importance of adsorption
especially for ZnS 2-TX and ZnS 1-TX in comparison with pristine ZnS, for which the photo degradation of
Congo red seems to be the main catalytic process.
3.1.7. The specific surface area
The specific surface area (SSA) was evaluated by BET method. Both ZnS 2-TX and ZnS 3-TX, obtained by
trituration techniques in non -aqueous solvent, have high SSAs, unusually for ZnS powders. The higher value of
SSA (191 m2/g) was obtained for ZnS 2-TX, a characteristic which can be correlated with the intensity of PL
spectrum and with cataly tic properties, explained by a high adsorption capacity . The SSA for ZnS 3-TX (169
m2/g) is closely to the value determined for ZnS 2-TX, highlighting the influence of synthesis method in the
specific surface area of powders. By changing the synthesis technique, ZnS powder with low SSA ( 21 m2/g for
ZnS 1-TX) was obtained. In conclusion, the synthesis in non -aqueous medium by trituration be ing a very
convenient procedure to obtain ZnS nanopowders with very high surface area.
3.2. Photocatalytic properties

The catalytic activity of capped nanoparticles involves many aspects and its study is very interesting, because the
capping agents may p lay a versatile role in colloidal synthesis of nanoparticles, different from stabilizers [ 2]. A
comparison between the catalytic properties of capped and surface clean ZnS may evidence the role of capping
agent in the catalytic process.
The catalytic properties of synthesized ZnS samples were demonstrated in the photocatalytic
degradation of an azo dye ( Congo red ). The semiconductor assisted photocatalytic degradation of dyes, which is
an advanced oxidation process , has emerged as an impor tant destructive technology that offers complete
mineralization of most of the organic pollutants [ 7]. So, we study the CR degradation not only to demonstrate the
photo catalytic properties of synthesized ZnS samples , but also as a n application of these pow ders in the
environmental protection.
By monitoring the absorbance at 497 nm wavelength (the azo bond degradation) as a function of
irradiation time, t he UV –Vis absorbance spectra indicated the disappearance of CR by breaking up the azo bond .
The CR concen tration was computed from the absorbance values for the CR solutions and the bleaching of the
solutions was estimated by the Ct/C0 ratio (Fig. 7).
The catalytic activity for all ZnS nanopowders obtained using TX as surfactant was superior to those of
pristine ZnS. It deserves to be mentioned the huge catalytic activity of ZnS 2-TX nanopowder. Thus, after 60
min the following values for PA can be mentioned: ZnS 1-TX – 79.34% and 80.30% (light); Z nS 2-TX – 97.18%
and 97.79% (light); ZnS 3-TX – 76.65% and 80.51% (light); ZnS – 27.32% and 31.43% (light). The
concentration of “no catalyst” samples remains almost constant in time ( a variation of 3.06% , respective 3.07% ,
light, after 120 min).
Therefore, the ZnS 1-TX, ZnS 2-TX and ZnS 3-TX samples have very high photocatalytic activity,
much higher than pristine ZnS and their photocatalytic activity is almost no dependent (for ZnS 1-TX and ZnS
2-TX) on the samples illumination , while that of pristi ne ZnS and, to a lesser degree, of ZnS 3-TX, increase d
with the illumination of sample. Having in view the short time of disc oloring process (a solution of CR with ZnS
2-TX sample is almost colorless in less than 60 min), the main catalytic process is probably an adsorption of CR
on the ZnS surface in the case of ZnS TX, respective a chemical degradation for pristine ZnS sample . The huge
capacity to decolorize the CR solution , noticed for the ZnS 2-TX sample , can be explained by a high capacity of
CR adsorption, as demonstrated the great specific surface area and t he PL spectrum (a large surface to volume
ratio).

We recorded the UV -Vis DRS spectra for ZnS powders recovered after the catalytic process (Fig. 8). A
strong band around 500 nm may be assigned to CR adsorbed onto ZnS catalyst, for all samples – ZnS 1-TX (508
nm), ZnS 2-TX (506 nm) and ZnS 3-TX (516 nm). The band assigned to CR is moved to higher wavelengths,
especially for ZnS 3-TX. For comparison, the absorption band assigned to CR wa s registered at 526 nm for
pristine ZnS powder ( without TX ). The more pronounced change of color for ZnS 3-TX sample is an evidence
for chemical decomposition of CR and/or for coordination of adsorbed CR to Zn(II) ions, which are coordinative
unsaturated and more available , in the absence of capping molecules . The band shift to higher wavelengths is
due to the decreasing of pH onto the ZnS surface and/or in the CR solution during the decomposition, knowing
that the CR has the property to change its color from red to blue by acidity increasing. Thus, it was demonstrated
[31] the oxidation of CR to CO 2, NO 3-, SO 42-, H+ and H 2O as final products, which may decrease the pH of
solution [ 16]. This observation confirms the degradation of CR, simultaneously with the adsorption onto catalyst
surface. In a qualitative estimation, the adsorption process seems to be dominant, and the pH decreasing was
weaker, especially for ZnS 2-TX, which have th e higher catalytic activity. A higher catalytic
photodecomposition activity can b e correlated with a greater pH de creasing for the control sample, ZnS , and to a
lesser extent, for ZnS 3-TX (the surface clean catalysts , but also a very high SSA ).
Thus, the red color of the catalyst after the catalytic process is another reason which determined us to
consider the adsorption of CR onto ZnS surface as an essential aspect for the discoloration of CR solution. The
importance of adsorption in the catalyt ic process is also suggested by the insignificant dependence of catalytic
activity by the illumination of reaction medium.
The trituration of precursors mixture and surfactant in absence of water, for a relative short time, can be
considered the most effi cient method for obtaining ZnS powder with very high catalytic activity. The
applicability of this experimental technique in the synthesis of powders with very good catalytic properties was
verified for other systems too [ 16].
Congo red is a benzidine -based anionic disazo dye, both CR and benzidine being toxic to many
organisms and suspected carcinogen and mutagen. In many countries they are excluded because of health
concerns, but CR is still widely used in several countries. CR is highly resistant to the biodegradation and its
removal from wastewaters is an important environmental problem [ 32]. The benzidine may be one of the CR
degradation products, so the simple breakdown of the azo bonds (the chromophores) is not enough for the
environmental safety [ 33]. In our study, the UV -vis spectra demonstrated that the CR solution is not only
bleached, but the intensity of band assigned to aromatic rings in the electronic spectr a also decreased in time

(Fig. 9 ). However, the ratio between the intensity of the bands assigned to azo group, respective to aromatic
rings decreased in time, a feature which may prove a faster breakdown of azo bond in comparison with the
degradation of aromatic rings.
The capping agents ensure the homodispersity of nanoparticles in solution during the synthesis and also
during the catalytic processes in which the semiconductor may be involved, which is beneficial for further
processing and application; the catalytic properties of semiconductor nanopowders intrinsically derive from the
inorga nic cores, being determined by their size, shape, composition, etc. [ 2]. In our experiments the ZnS TX
nanopowders were homogeneously dispersed in CR solution, but the y were not evidenced that the capping agent
act as a barrier for active sites. O n the contrary, higher catalytic activity was observed for ZnS 2-TX nanopowder
proving that the synthesis technique promote d a high photocatalytic activity .
Thus, using the Triton X -100, we obtained ZnS nanoparticles with high specific surface area , especi ally
by trituration technique in absence of water. The photocatalytic activity , estimated from PL spectra (the higher
intensity of PL spectrum, the lower photocatalytic activity ) is low, but the high capacity of absorption
compensa ted this characteristic. The surfactant remov al increase the capacity of ZnS nanopowder to decompose
the dye .
4. Conclusions
We obtained ZnS nanopowders with high specific surface area by one pot synthesis using thioacetamide as
sulfide ion source and Triton X -100 as surfactant . The syntheses were performed in aqueous solutions , respective
in Triton X-100 as solvent , through two techniques and varying the reaction time. A surprising result was the
obtaining of surface clean ZnS in Triton X -100 after a longer reaction time. The p hotocatalytic activity of
synthesized ZnS nanoparticles was demonstrated in the discoloration of CR aqueous solution. All nanopowders
exhibited superior photocatalytic activity than those obtained in the absence of Triton X -100, but ZnS powder
obtained by trituration of a mixture of precursors and Triton X -100 in absence of water for a shorter time , with a
great SSA of 191 m2/g, has much higher catalytic activity in the CR bleaching, the dye solution being almost
colorless after less than an hour. The surface clean ZnS nanopowder has also a high SSA and furthermore a
higher capacity to decompos e the dye compared to the other ZnS nanopowders prepared in the presence of TX –
100.
References
[1] C.H. Lai, M.Y. Lu, L.J. Chen, Metal sulfide nanostructures: synthes is, properties and applications in energy
conversion and storage, J. Mater. Chem. 22 (2012) 19 -30. Doi: 10.1039/c1jm13879k

[2] Z. Niu, Y. Li, Removal and utilization of capping agents in nanocatalysis, Chem. Mater. 26 (2014) 72 -83.
Doi: 10.1021/cm4022479
[3] S. Ummartyotin, Y. Infahsaeng, A comprehensive review on ZnS: From synthesis to an approach on solar
cell, Renew. Sust. Energ. Rev. 55 (2016) 17 –24. Doi: 10.1016/j.rser.2015.10.120
[4] H. Naeimi, F. Kiani, M. Moradian, ZnS nanoparticles as an efficient and reusab le heterogeneous catalyst for
synthesis of 1 -substituted -1Ht etrazoles under solvent -free conditions, J. Nanopart. Res. 16 (2014) 2590.
Doi: 10.1007/s11051 -014-2590 -0
[5] K.V.V. Satyanarayana, M. Ravi Chandra, P. Atchuta Ramaiah, Y.L.N. Murty, E.N. Pandit, S. V.N. Pammi,
A novel reusable and efficient nano -ZnS catalyst for green synthesis of xanthenes and its derivatives under
solvent free conditions, Inorg. Chem. Front. 1 (2014) 306 –310. Doi: 10.1039/c3qi00016h
[6] E.K. Goharshadi, M. Hadadian, M. Karimi, H. Azizi -Toupkanloo, Photocatalytic degradation of reactive
black 5 azo dye by zinc sulfide quantum dots prepared by a sonochemical method, Mat. Sci. Semicon. Proc.
16 (2013) 1109 –1116. Doi: 10.1016/j.mssp.2013.03.005
[7] J. Kaur, M. Sharma, O.P. Pandey, Synthesis, ch aracterization, photocatalytic and reusability studies of
capped ZnS nanoparticles, Bull. Mater. Sci. 37 (2014) 931 –940. Doi: 10.1007/s12034 -014-0028 -z
[8] A. Dumbrava, C. Badea, G. Prodan, I. Popovici, V. Ciupina, Zinc sulfide fine particles obtained at low
temperature , Chalcogenide Lett. 6 (2009) 437 -443.
[9] P.I.P. Soares, A.M.R. Alves, L.C.J. Pereira, J.T. Coutinho, I.M.M. Ferreira, C.M.M. Novo, J.P.M.R. Borges,
Effects of surfactants on the magnetic properties of iron oxide colloids, J. Colloid Interf. Sci. 4 19 (2014) 46 –
51. Doi: 10.1016/j.jcis.2013.12.045
[10] M. Movahedi, A.R. Mahjoub, S. Janitabar -Darzi, Photodegradation of Congo red in aqueous solution on
ZnO as an alternative catalyst to TiO 2. J. Iran. Chem. Soc. 6 (2009) 570 -577.
[11] L. Ren, Y. Li, J. Hou, X. Zha o, C. Pan, ACS Appl. Mater. Inter. 6 (2014) 1608 -1615.
Doi:10.1021/am404457u
[12] T. Leshuk, R. Parviz, P. Everett, H. Krishnakumar, R.A. Varin, F. Gu, Photocatalytic activity of
hydrogenated TiO 2, ACS Appl. Mater. Inter. 5 (2013) 1892 – 1895. Doi: 10.1021/am3 02903n
[13] N. Soltani, E. Saion, W.M.M Yunus, M. Navasery, G. Bahmanrokh, M. Erfani, M.R. Zare, E. Gharibshahi,
Photocatalytic degradation of methylene blue under visible light using PVP -capped ZnS and CdS
nanoparticles, Sol. Energy 97 (2013) 147 -154. Doi:10.1 016/j.solener.2013.08.023

[14] J.A. Lopez -Sanchez, N. Dimitratos, C. Hammond, G.L. Brett, L. Kesavan, S. White, P. Miedziak, R.
Tiruvalam, R.L. Jenkins, A.F. Carley, D. Knight, C.J. Kiely, G.J. Hutchings, Facile removal of stabilizer –
ligands from supported go ld nanoparticles, Nat. Chem. 3 (2011) 551 -556. Doi: 10.1038/nchem.1066.
[15] J. Saien, Z. Ojaghloo, A.R. Soleymani, M.H. Rasoulifard, Homogeneous and heterogeneous AOPs for rapid
degradation of Triton X -100 in aqueous media via UV light, nano titania hydrogen p eroxide and potassium
persulfate, Chem. Eng. J. 167 (2011) 172 –182. Doi: 10.1016/j.cej.2010.12.017
[16] A. Dumbrava, G. Prodan, D. Berger, M. Bica, Properties of PEG -capped CdS nanopowders synthesized
under very mild conditions, Powder Technol. 270 (2015) 197 -204. Doi: 10.1016/j.powtec.2014.10.012
[17] R. Lo, C. Cao, H. Zhai, H. Zhu, Synthesis and characterization of semiconductor zinc sulfide nanotubes by
soft-template method, Chinese Sci. Bull. 49 (2004) 101005 -1008.
[18] E. Pretsch, P. Bühlmann, M. Badertscher, Structure Determination of Organic Compounds, fourth edition,
pp. 289, 314 -315, Springer -Verlag Berlin Heidelberg 2009
[19] A. Kharazmi, N. Faraji, R.M. Hussin, E. Saion, W.M.M. Yunus, K. Behzad, Structural, optical, opto –
thermal and thermal properties of ZnS -PVA nanofluids synthesized through a radiolytic approach, Beilstein
J. Nanotechnol. 6 (2015) 529 -536. Doi:10.3762/bjnano.6.55
[20] J. Coates, Interpretation of Infrared Spectra. A Practical Approach, in Encyclopedia of Analytical
Chemistry. R. A. Meyers (Ed.), J ohn Wiley & Sons, Chichester, 2000, pp.10815 –10837.
[21] N.S. Das, P.K. Ghosh, M.K. Mitra, K.K. Chattopadhyay. Effect of film thickness on the energy band gap of
nanocrystalline CdS thin films analyzed by spectroscopic ellipsometry. Physica E 42 (2010) 2097 –2102.
Doi: 10.1016/j.physe.2010.03.035
[22] J. Tauc, R. Grigorovici, A. Vancu, Optical properties and electronic structure of amorphous g ermanium,
Phys. Status Solidi B 15 (1966) 627 -637. D oi: 10.1002/pssb.19660150224
[23] R. Rusdi, A.A. Rahman, N. S. Mohamed, N. Kamarudin, N. Kamarulzaman, Preparation and band gap
energies of ZnO nanotubes, nanorods and spherical nanostructures, Powder Technol. 210 (2011) 18 -22.
[24] P.E. Lippens, M. Lannoo, Calculation of the band gap for small CdS and ZnS crystallites, Physical Rev. B
39 (1989) 10935 -10942.
[25] A. Dumbrava, G. Prodan, F. Moscalu, Investigations on the influence of surfactant in morphology and
optical properties of zinc oxide nanopowders for dye -sensitized solar cells applications, Mat. Sci. Semicon.
Proc. 1 6 (2013) 1095 –1104. Doi: 10.1016/j.mssp.2013.03.007

[26] V. Smyntya, B. Semenenko, V. Skobeeva, N. Malushin, Photoactivation of luminescence in CdS
nanocrystals, Beilstein J. Nanotechnol. 5 (2014) 355 –359. Doi: 10.3762/bjnano.5.40
[27] A. Dumbrava, D. Berger, G. Pro dan, F. Moscalu, A. Diacon. Facile synthesis, characterization and
application of functionalized cadmium sulfide nanopowders, Mater. Chem. Phys. 173 (2016) 70 -77. Doi:
10.1016/j.matchemphys.2016.01.040
[28] S. Kakarndee, S. Juabrum, S. Nanan, Low temperature sy nthesis, characterization and photoluminescence
study of plate -like ZnS, Mater. Lett. 164 (2016) 198 –201. Doi:10.1016/j.matlet.2015.10.154
[29] E. Mosquera, N. Carvajal, Low temperature synthesis and blue photoluminescence of ZnS submicron
particles, Mater. Let t. 129 (2014) 8 –11. Doi:10.1016/j.matlet.2014.05.037
[30] J. Liqiang, Q. Yichun, W. Baiqi, L. Shudan, J. Baojiang, Y. Libin, F. Wei, Fu Honggang, S. Jiazhong,
Review of photoluminescence performance of nano -sized semiconductor materials and its relationships wi th
photocatalytic activity, Sol. Energ Mat. Sol. Cells 90 (2006) 1773 -1787. Doi: 10.1016/j.solmat.2005.11.007.
[31] H. Lachheb, E. Puzenat, A. Houas, M. Ksibi, E. Elaloui, C. Guillard, J.M. Herrmann, Photocatalytic
degradation of various types of dyes (Alizarin S, Crocein Orange G, Methyl Red, Congo Red, Methylene
Blue) in water by UV -irradiated titania, Appl. Catal. B: Environ. 39 (2002) 75 –90. Doi: 10.1016/S0926 –
3373(02)00078 -4
[32] S. Dafare, P.S. Deshpande, R.S. Bhavsar, Photocatalytic degradation of Congo red dy e on combustion
synthesized Fe 2O3, Indian J. Chem. Technol. 20 (2013) 406 – 410.
[33] M. Ișik, D.T. Sponza, Effect of oxygen on decolorization of azo dyes by Escherichia coli and Pseudomonas
sp. and fate of aromatic amines, Process Biochem. 38 (2003) 1183 -1192. Doi: 10.1016/S0032 –
9592(02)00282 -0

Figures caption

Figure 1. The XRD patterns of ZnS nanoparticles.
Figure 2. The TEM images for samples ZnS 1-TX (a), ZnS 2-TX (b), ZnS 3-TX (c), and ZnS (d).
Figure 3. The IR spectra of ZnS samples.
Figure 4. The UV –vis absorption spectra of ZnS nanopowders.
Figure 5. Tauc plots for ZnS samples.
Figure 6. The PL spectra of ZnS samples, using an excitation wavelength of 320 nm.
Figure 7. The photocatalytic degradation curves of CR over the ZnS 1-TX, ZnS 2-TX, ZnS 3-TX, and ZnS powders.
Figure 8. The UV –vis absorption spectra of ZnS powders after the catalytic process.
Figure 9. The photocatalytic degradation curves of CR over ZnS 3-TX (L = light).

Figures caption

Figure 1
Click here to download high resolution image

Figure 2-a-1
Click here to download high resolution image

Figure 2-a-2
Click here to download high resolution image

Figure 2-b-1
Click here to download high resolution image

Figure 2-b-2
Click here to download high resolution image

Figure 2-b-3
Click here to download high resolution image

Figure 2-c-1
Click here to download high resolution image

Figure 2-c-2
Click here to download high resolution image

Figure 2-d-1
Click here to download high resolution image

Figure 2-d-2
Click here to download high resolution image

Figure 3
Click here to download high resolution image

Figure 4
Click here to download high resolution image

Figure 5
Click here to download high resolution image

Figure 6
Click here to download high resolution image

Figure 7
Click here to download high resolution image

Figure 7 legend
Click here to download high resolution image

Figure 8
Click here to download high resolution image

Figure 9
Click here to download high resolution image