Smart Bifunctional Sb2Se3 Nanorods for Integrated Water Purification: Insoluble Liquid Separation and Photoelectrochemical Degradation

Article abstract of DOI:10.1002/cssc.202000438

Antimony selenide (Sb₂Se₃) nanostructures enable bifunctional water purification by a single membrane through i) physical separation of water-insoluble oil and ii) photoelectrocatalytic degradation of water-soluble organic compounds. Sb₂Se₃ nanorods with exposed surfaces of {h 0 0} and {h 0 l} planes exhibit superhydrophobicity (water contact angle of ≈159°) owing to extremely low surface energy of those dangling-bond-free van der Waals planes. Based on crystallographic understanding, superhydrophobic Sb₂Se₃ nanorods were produced on a mesh-type substrate for utilization as a membrane for physical water/oil separation. Sb₂Se₃ exhibited an optimal photocathodic response with p-type electrical conductivity under visible light along the longitudinal crystal direction. This indicated that the nanorods could be used as photoelectrocatalytic material for chemical water purification. A smart membrane with Sb₂Se₃ nanostructures was proposed as a candidate for integrated water purification that can simultaneously accomplish water/oil separation and photoelectrocatalytic degradation of organic compounds in wastewater. Linear sweep voltammetry measurements of the Sb₂Se₃-membrane showed cathodic photocurrent generation (up to approximately 10 mA cm^?2 at 0 V vs. reversible hydrogen electrode), which was enough to reduce O₂ to an oxygen radical (O₂^.?) for degradation of methyl orange. Consequently, solar-driven integrated water purification was demonstrated for the first time by using a single material with a dual function of superhydrophobicity and photoactivity.

Full text of DOI:10.1002/cssc.202000438

Accepted Article
Title: Smart Bifunctional Sb2Se3 Nanorods for Integrated Water
Purification: Insoluble Fluid Separation and Photoelectrochemical
Water-soluble-organic Degradation
Authors: Young Been Kim, Sung Woon Cho, Nishad Gopal
Deshpande, Sung Hyeon Jung, Dong Su Kim, Ki Ju Park,
Han-Ki Kim, and Hyung Koun Cho
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the VoR from the journal website shown below when it is published
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To be cited as: ChemSusChem 10.1002/cssc.202000438
Link to VoR: http://dx.doi.org/10.1002/cssc.202000438
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10.1002/cssc.202000438
ChemSusChem
FULL PAPER
Smart Bifunctional Sb2Se3 Nanorods for Integrated Water
Purification: Insoluble Fluid Separation and Photoelectrochemical
Water-soluble-organic Degradation
Young Been Kim,†[a] Sung Woon Cho,†[a][b] Nishad G. Deshpande,[a] Sung Hyeon Jung,[a] Dong Su
Kim, [a] Ki Ju Park,[c] Han-Ki Kim,[a] and Hyung Koun Cho*[a]
Abstract: Antimony selenide (Sb2Se3) nanostructures enable
bifunctional water purifications using a single membrane: i) physical
separation of water-insoluble oil and ii) photoelectrocatalytic
degradation of water-soluble organic. Sb2Se3 nanorods with exposed
surfaces of (h00) and (h0l) planes exhibit superhydrophobicity (water
contact angle of ~159°) due to extremely low surface energy of those
dangling-bond-free van der Waals planes. Based on crystallographic
understanding, superhydrophobic Sb2Se3 nanorods can be produced
on mesh-type substrates for the utilization as a membrane for physical
water-oil separation. Next, Sb2Se3 exhibits optimal photocathodic
response with p-type electrical conductivity under visible-light along
the longitudinal crystal direction. This indicates the nanorods can be
used as photoelectrocatalytic material for chemical water purification.
In this regard, a smart membrane with Sb2Se3 nanostructures was
proposed as an incomparable candidate for the integrated water
purification, which can accomplish simultaneously water–oil
separation as well as photoelectrocatalytic organic degradation in
waste water. Linear sweep voltammetry measurement from Sb2Se3-
membrane shows cathodic photocurrent generation (up to
approximately 10 mA/cm2 at 0 V vs. RHE) which is enough to reduce
organics (e.g., industrial dyes). The increased rate of such
incidents, in addition to sewage of dyes in water, has become a
major environmental concern that requires careful preventive
measures and treatment solutions so as to avoid devastating
effects. Generally, water purification is achieved by independent
sequential processes, which involve “physical” removal of the
water-insoluble oils (water–oil separation) followed by “chemical”
degradation of water-soluble organics in water, respectively.[1-6]
The water-insoluble oil separation has primarily been carried out
through physical approaches such as gravity, centrifugation,
ultrasonic and adsorption. At first, the water–oil separation using
membrane
filter
method
specifically
utilizes
the
hydrophilic/hydrophobic characteristics of mesh-type membranes
by promoting functional surfaces with selective water–oil
permeability.[3,4] In general, mostly two separation methods : i) oil-
removing type separating oil from water via mesh membranes
having superhydrophobic surfaces, ii) water-removing type
separating water from oil via filters having superhydrophilicity.[5]
Therefore, surface wettability of membrane is a crucial and
prerequisite property controlled by nanostructuring the surfaces
or engineering the surface bonding states (OH or CH bonds).[5-7]
Among all the materials, organic materials are initiatively used
because their surfaces have C–H bonding chains contributing to
superhydrophobicity. However, they have an intrinsically short-
term capability of water–oil separation performance because of
their poor thermal, chemical, and mechanical stabilities. In this
regard, inorganic oxide materials have alternatively been
considered through additional process such as metal-
anodization-induced nano-texturing.[8-12]
The most effective approach for complete water purification is
conceptually based on the simultaneous realization of “physical”
filtration (water-oil separation) and “chemical” degradation using
a single bi-functional membrane. However, few studies have
reported on this, and the previously studied materials had
exhibited individually independent purification functions. Artificial
degradation of water-soluble organics can be performed through
-
O2 into oxygen radical (·O2 ) for degradation of methyl-orange.
Consequently, the solar-driven integrated water purification is first
demonstrated from
a
single material with bi-function of
superhydrophocity and photoactivity.
Introduction
Water pollution is ceaselessly caused by the accidental and
negligent influx (e.g., industrial oil leakage and marine oil spill
accidents) of water-insoluble oily components and water-soluble
[a]
Y. B. Kim, S. W. Cho, N. G. Deshpande, S. H. Jung, D. S. Kim, H.-
K. Kim, Prof. H. K. Cho
School of Advanced Materials Science and Engineering,
Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon,
Gyeonggi-do, 16419 (Republic of Korea)
E-mail: chohk@skku.edu
chemical
reactions
such
as
the
photocatalytic
or
photoelectrochemical purification using photoactive materials
under light irradiation. Interestingly, even though an optimal
photoactive material with hydrophobicity can realize the
integrated water purification, a bifunctional single membrane
capable of performing both water–oil separation and
photodegradation has been rarely developed for the inorganic
materials. Though several inorganic materials such as TiO2,[13]
ZnO,[14] CaCO3,[15] zeolites,[16] silica,[17] Cu(OH)2,[18] and graphene
oxides[19] achieve the photocatalytic or photoelectrocatalytic
[b]
[c]
[†]
S. W. Cho
Department of Printed Electronics Engineering, Sunchon National
University, Sunchon, Jeonnam, 57922, Republic of Korea
K. J. Park
Solar Flex, 128-133, Wolsan-ro, Eumbong-myen, Asan,
Chungcheongnam-do, 31416 (Republic of Korea)
These authors contributed equally to this work
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cssc.202000438
ChemSusChem
FULL PAPER
activities, these photoactive oxides have required additional
process, such as polymeric surfactant-decorated treatment or
nanostructuring, in order to attain surface hydrophobicity due to
their intrinsically hydrophilic surface properties. The above
approaches also involve loading various complex chemical
substances on the substrate or utilizing processes under harsh
conditions. Furthermore, most of the photoactive metal oxides
have larger band-gaps, hence, photoactivity can be limited under
visible-light region. In addition, even though some powder-type
materials can exhibit high photocatalytic performance and
hydrophobic property due to their large surface area, these
powders cannot be suitably loaded on mesh-type substrate with
without dangling bonds, and each plane is weakly linked by van
der Waals interaction. This suggests that the degree of dangling
bonds on the surface can be dependent on the crystal planes
exposed to the surface due to the dramatically anisotropic crystal
structure of Sb2Se3. Based on crystallography, Scheme 1b
visualize the theoretical concept of distinguished surface
wettability between vertically/horizontally aligned Sb2Se3 crystals.
Typically, the Sb2Se3 crystals were preferentially grown along the
<0k0> directions with covalent bonding. At first, the surface of
vertically aligned Sb2Se3 crystals is expected to predominantly
show hydrophilicity because of the exposed {0k0} planes with the
numerous dangling bonds on the top/bottom ends of the crystal
optimal durability, resulting in the limitation on long-term operation. structure. On the contrary, the horizontally aligned Sb2Se3 crystals
In view of the above, a versatile material, binary antimony
selenide (Sb2Se3), which has an unusual crystal structure of the
dangling bond-free one-dimensional (1D) zig-zag ribbon frame
and a high light-absorption property with narrow band gap (1.0-
1.3 eV) was suggested. It contains earth-abundant elements and
is simple to prepare due to its simple binary stoichiometric
property.[20,21] The Sb2Se3 has recently been studied as a p-type
photoabsorber of photovoltaic and photoelectrochemical
researches, and the Sb2Se3 nanostructures with anisotropic
crystallographic/electrical properties have been readily achieved
due to preferred crystal growth orientation. However, the results
of the Sb2Se3 based on environmental concerns such as water
purification have rarely been reported. Presumably, it is expected
that the anisotropic crystal structure can induce intrinsic and
permanent hydrophobicity in appropriately oriented crystal
direction. Based on the above expectation, Sb2Se3 nanostructure
was proposed as the optimal material with bi-function
(photoactivity and hydrophobicity) for integrated water purification
and we fabricated a purification system possessing both water–
with the exposure of the {h00} or {h0l} planes having no dangling
bonds, can produce a superhydrophobic surface without any
surface treatment. The cohesive energy between water
molecules and Sb2Se3 surface makes water droplets circular,
owing to weak adhesive forces on the dangling-bond-free {h00}
or {h0l} planes. On the other hand, the {0k0} planes exhibit
general hydrophilicity with enough wettability because the
adhesive force is stronger than the surface tension of the water
droplets. Therefore, it indicates that the high-purity 1D structure
of Sb2Se3 crystals with no secondary phases can exhibit both
hydrophobicity and hydrophilicity depending on the exposure of
specific crystal-planes, and its nanostructures with hydrophobicity
can facilitate probably the selective wettability for water-oil
separation.
oil
separation
(superhydrophobicity)
and
solar-driven
photoelectrocatalytic organic degradation.
Results and Discussion
Superhydrophobic Sb2Se3–membrane for physical water-oil
separation
As shown in Scheme 1a, the typical nanostructured materials
can exhibit hydrophobic characteristic due to Cassie-Baxter effect,
which was induced by air-cavity, even if the materials in flat-form
show hydrophilicity. However, these hydrophobicity are not
related to intrinsic material properties; thus, a complex fabrication
process is needed, and the surface properties are not sustainable.
The Sb2Se3 has a one-dimensional (1D) ribbon-like orthorhombic
crystal structure in which individual Sb and Se atoms are linked in
a zig-zag manner by strong covalent bonds along the [010]
direction (JCPDS, 75-1462), and the 1D ribbon-like structures are
parallel-stacked by weak van der Waals forces, as shown in
Scheme 1b. From the ribbon-like structure, the {0k0} planes
preferentially generate a surface containing numerous dangling
bonds by breaking the ribbon-like Sb2Se3 molecules covalently
bonded between Sb and Se atoms. However, the {h0l} and {h00}
planes normal to the {0k0} planes intrinsically have a surface
Scheme 1. Schematics of surface wettability depending on morphology and
surface crystallographic bonds: (a) conventional approach for surface wettability
control via nanostructuring resulting in Cassie-Baxter effect. (b) Crystallographic
surface properties of Sb2Se3 exposing different crystal planes with/without
dangling bonds. (i) Hydrophilic surface generated by exposing the {0k0} planes
and (ii) hydrophobic surface generated by exposing the {h0l} and {h00} planes
by breaking the covalent bonds and weak van der Waals interactions,
respectively, of the one-dimensional ribbon-like crystal structure of Sb2Se3.
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10.1002/cssc.202000438
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Figure 2. The relationship between Sb2Se3 nanorod-orientation and the
exposed crystal plane on surface. (a) X-ray diffraction (XRD) results of the
Sb2Se3 nanorods coated on O2-plasma-treated glass and polyimide (PI). (b)
Transmission electron microscopy (TEM) results: (i) low-magnitude image of a
crystalline Sb2Se3 nanorod; (ii) and (iii) diffraction patterns (DPs) measured with
different zone axes (AZs) with rotation angle of 63.4˚.
Figure 1. Strategic thermal transfer process and morphological analysis of each
Sb2Se3 nanostructures with different morphologies. (a) Schematic of thermal
process consisting of source and targets (mesh, polyimide, glass) resulting in
various inclination of nanorods. Schematics and SEM images of (b) randomly
oriented Sb2Se3 nanorods as the source for transfer process, and (c) vertical
Sb2Se3 nanorods transferred on ionic surface and (d) horizontal Sb2Se3
nanorods transferred on benign surface without ionic surface. O2-plasma-
treated and polyimide were utilized as the substrates with/without ionic surface,
respectively.
polyimide-coated glasses were selectively used as the substrates
with/without ionic surfaces, respectively. Accordingly, O2-plasma-
treated glasses having abundant hydroxyl group (OH), allowed
the preferential growth of the Sb2Se3 {0k0} planes with polar
dangling bonds along the vertical direction (Figure 1c). In contrast,
the Sb2Se3 nanorods were transferred on the polyimide surface,
and their products revealed a high density of nanorods to lie
horizontally on the substrate (Figure 1d), as identified in the
results of SEM, becuase van der Waals interactions of {h0l} or
{h00} planes could preferentially be adhesive onto non-ionic
surfaces with CH bonds.
To verify the relationship between the orientation of the
nanorods and the specific exposure of crystallographic planes, X-
ray diffraction (XRD) measurements were utilized as shown in
Figure 2a (JCPDS, 75-1462). A (020) XRD peak was observed in
the vertical Sb2Se3 nanorods indicating that the surfaces with
hydrophilic {0k0} planes are susceptible to exposure, whereas the
exposure of the hydrophobic Sb2Se3 surfaces on the polyimide is
certainly distinguished from the presence of various diffraction
planes normal to the {0k0}. From microstructure transmission
electron microscopy (TEM) image of a single Sb2Se3 nanorod
(Figure 2b), a mirror-like atomically flat side-facet was observed,
which was due to the presence of the dangling-bond-free side
walls. The anisotropic preferential growth direction is the [010]
revealing strong covalent bonds (Sb–Se). Thus, the top/bottom
ends of the synthesized Sb2Se3 nanorods are intrinsically
composed of numerous dangling bonds and expected to have a
high surface energy. In contrast, from the diffraction patterns, the
side facets of the nanorods are dangling-bond-free {h00} and {h0l}
planes with a low surface energy.
The water-wettability of Sb2Se3 nanorods with different
orientations was analyzed through contact angle measurements
(Figure 3). The contact angle measurements were performed on
three samples with different nanorods alignments (different
inclination distribution). Figure 1 shows two samples with
extremely opposite cases of vertical and horizontal alignments.
An additional sample consisting of the nanorods, close to random
distribution (Figure 1b), with medium inclination was compared.
The total surface energies were calculated from contact angles
with an inverse relation of wettability from conventional dispersive
(diiodomethane) and polar (water) solutions. As theoretically
suggested in Scheme 1b, the vertical Sb2Se3 nanorods exhibit
largely low contact angles (diiodomethane = 14.33° and water
droplet = 30.45°) of < 90° as shown in Figure 3a, indicating
favorable wetting of the surface due to the exposed {0k0} crystal
To prove the theoretical expectation of the surface wettability
depending on the exposed crystal plane, the Sb2Se3
nanostructures were fabricated using a modified process based
on previously reported methods, which involved electrodeposition
(Sb-Se precursors) and thermal annealing process (selenization
of precursors) on Mo/glass substrate. The pre-synthesized
Sb2Se3 nanostructures on Mo substrate were used as a source of
this study’s artificial thermal-transfer process, which was
strategically designed to maintain the nanostructure of Sb2Se3
even after deposition on the target substrates. Figure 1a shows
the thermal transfer process, where the source was thermally
evaporated and transferred on below target substrates. Since
Sb2Se3 is the only stable stoichiometric crystal phase in the Sb-
Se system, the transferred materials are pure Sb2Se3, unlike
conventional chalcogenide compounds with various crystal
phases.[6] Thus, the Sb2Se3 can exist in a single phase without
secondary products and be simply processed at relatively low
temperatures. Additionally, the Sb2Se3 has high vapor pressure at
low vacuum and low temperature. These properties help in readily
transferring Sb2Se3 nanostructures at relatively low temperatures
on various targets, such as flat-type or mesh-type substrates
while maintaining the intact crystal phase and morphology without
any issues related to the presence of secondary phases. Figure
1b shows the results of scanning electron microscopy (SEM) of
Sb2Se3 synthesized on Mo substrate, which has randomly
oriented nanostructures. To investigate the surface dependency
of wettability on nanostructure alignment, the Sb2Se3
nanostructures were transferred to glass and polyimide
substrates (targets), respectively, via the thermal-transfer process
(Figures 1c and 1d). In the transfer process, the crystal orientation
of Sb2Se3 nuclei on target surfaces influenced the overall growth
direction of nanostructures due to its anisotropic crystal growth
along preferential [010] direction connected by strong covalent
bonds (Sb-Se). In this respect, the surface chemical bonding of
the target substrates could effectively contribute to the initial
growth direction of Sb2Se3 nanorod. Thus, O2-plasma-treated and
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10.1002/cssc.202000438
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end of the nanorods) and the hydrophobic side-walls of nanorods.
Nevertheless, these alignments of Sb2Se3 sample still
demonstrates excellent hydrophobic character indicating that the
partial exposure of the side walls in the Sb2Se3 nanorods
maintains weak adhesive forces and induces low surface energy.
This confirms that
a mesh-type membrane loaded with
randomly aligned Sb2Se3 nanorods can be used for water-oil
separation. It is worth noting that applying Sb2Se3 nanorods to
curved or round surfaces would inevitably induce a random
alignment of the nanorods (Figure 4a). Also, it is expected that a
round support coated with Sb2Se3 exhibits superhydrophobicity
due to the partial exposure of dangling-bond-free sidewalls of
nanorods. In this study, platinum (Pt) mesh, as a substrate having
permeable mesh-shape, was utilized to keep chemical and
thermal stability/durability without any additional side reactions.
The pre-synthesized Sb2Se3 nanostructures were thermally
transferred onto these membranes. A SEM image of the Sb2Se3
coated on mesh substrate exhibited randomly inclined nanorods,
and its water contact test revealed excellent superhydrophobicity
(Figure 4b). Additionally, XRD result (Figure 4c) also
demonstrated that the randomly oriented crystal-planes were
exposed toward the surface, which is almost similar to the result
on the Sb2Se3 nanorods on the polyimide of Figure 2a.
Consequentially, it was found that most of sidewalls of nanorods
would be contacted with liquid during water-oil separation process,
resulting in the superhydrophobicity on water droplet test.
Figure 3. The hydrophobicity depending on the inclination of Sb2Se3 nanorods
affecting exposure state of van der Waals plane in nanorods. Water contact
angle (CA) results of various Sb2Se3 with different slope-degree of nanorods
observed by a contact angle meter and conceptual illustration. (a) The
hydrophilic property of vertical structure due to exposure of dangling bonds, and
(b and c) superhydrophobic properties of randomly and horizontally oriented
Sb2Se3 nanorods, respectively. (d) Dynamic images of rebounding water droplet
onto superhydrophobic Sb2Se3 revealing ultralow surface energy.
planes at the end of nanorods. On the other hand, it is expected
that the horizontally aligned Sb2Se3 nanorods with the exposure
of the {h00} or {h0l} surfaces considerably minimizes the water
contact, and Figure 3c shows the hydrophobicity (water contact
angle = 154.91°) of horizontal nanorods. The water contact angle
above 150° has been particularly referred to superhydrophobicity,
which occasionally cause the underestimation of contact angle
values due to the remarkable rebound of the water droplets. A
dynamic water bounce phenomenon has been observed for only
a
few surface-treated or functionalized superhydrophobic
materials.[22] In this regard, this study’s horizontally aligned
Sb2Se3 nanorods exhibit the remarkably rebound phenomenon,
as confirmed by dynamic images (Figure 3d), during the water-
drop test on superhydrophobic Sb2Se3 nanorods implying that
real contact angle is above 154.91°, where water droplets quickly
flow into the edge side of sample. Based on the calculations with
measured contact angles of water and diiodomethane (Figure S1),
the hydrophilic and superhydrophobic Sb2Se3 samples exhibited
a surface energy of 69.82 and 28.94 mN/m, respectively.[23]
Various studies reporting inorganic materials [two-dimensional
(2D) structures] with no surface treatments have demonstrated
superhydrophobic surfaces. In particular, such 2D sheets, e.g.,
graphene (34–37 mN/m), MoS2 (34–42 mN/m), and WS2 (38–40
mN/m), have ideally no dangling bonds on the covalently bonded
surfaces, and their surface-energy is also low.[24-26] However, the
1D ribbon-like- Sb2Se3 structure has a competitively lower
surface-energy than 2D materials if the exposed surfaces
appropriately consist of the ideal {h0l} or {h00} planes with no
dangling bonds. For the case of the randomly oriented nanorods
(Figure 1b), it exhibit considerable hydrophobicity (water contact
angle ~ 150°), even though the exposed surfaces do not perfectly
correspond to the {h00} or {h0l} planes. Here, the water droplets
simultaneously contacted with both the hydrophilic surface (the
Figure 4. Conceptual model and morphological analyses of randomly oriented
Sb2Se3 nanorods on round-type substrate. (a) The illustration of Sb2Se3
nanostructures on round-type substrate resulting in random-exposure of
hydrophilic/hydrophobic parts in nanorods and hydrophobicity. (b) SEM images
of Sb2Se3 coated on Pt mesh with random orientations. The insets show the
superhydrophobicity of Sb2Se3 coated membranes and the optical microscopy
(OM) image exhibiting the dense-deposition. (c) XRD result reveals the random
orientations of Sb2Se3 nanostructure resulting in inevitable exposure of
hydrophobic parts in nanorods.
To investigate the validity of utilizing the Sb2Se3-coated mesh
(Sb2Se3-membrane or Sb2Se3-filter) as the water/oil separation
membrane, the selective permeability of the Sb2Se3-membrane
was explored by repeatable drop tests of water and hexane liquids.
For visual distinction, the water and hexane solutions were dyed
with water-soluble methyl-orange (MO) and hexane-soluble oil-
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10.1002/cssc.202000438
ChemSusChem
FULL PAPER
decomposition of the water-soluble organics using the
photoelectrochemical reactivity of the Sb2Se3-membrane
fabricated in this study. It indicated that the photodegradation of
floating wastewater on Sb2Se3–membrane could be performed
sequentially after separating the oil.[20,21,27] Therefore, the Sb2Se3
can form a new cornerstone for fabricating a smart membrane that
can
sequentially
separate
water–oil
mixtures
and
photoelectrochemically degrade organic pollutants.
To explore the possibility of bifunctional performance of
Sb2Se3-membrane, photoelectrochemical analysis of Sb2Se3 was
carried out using an identical membrane. The Sb2Se3 can drive
photoelectrocatalytic degradation reactions of organics in
electrolyte due to their appropriate band gap and high absorption
efficiency in the visible region; however, there are no reports on
water purifications using the Sb2Se3 photoelectrode. To evaluate
the theoretical suitability of Sb2Se3 for photodegradation, an
energy band structure was established from the electrochemical
and optical analyses. As shown in Figure 6a, Mott–Schottky (M–
S) analysis was performed with the voltage sweep at a fixed
frequency to determine the flat-band potential and carrier
concentration in acetonitrile solution, which was used as an
electrolyte due to the rich chemistry of Sb and Se in water.[28] The
M–S plot indicates a negative slope of d(C-2)/dV confirming the p-
type conductivity of the Sb2Se3 with holes as the majority carriers
and the flat-band potential of 0.32 V versus reversible hydrogen
electrode (vs. RHE) by extrapolating linear fit of the 1/C2 plot,
following the M–S equation below (eq. 1):
Figure 5. The selective permeability of Sb2Se3-membrane with
polar/dispersive fluids. (a) Water (polar) and (b) hexane (dispersive) drop tests
show the superhydrophobic/oleophilic characteristics of Sb2Se3-membrane
due to specific surface wettability capable to confine water and transmit oil in
mixture selectively.
blue, respectively. Hexane was preferentially selected as an
insoluble oil in the water–oil separation test, and MO has been
1
2
푘푇
푞
generally utilized as
a water-soluble organic pollutant for
=
2 (퐸 − 퐸푓푏
푞휀휀 푁 퐴
0 퐷
−
)
(1)
2
퐶
photoelectrocatalytic degradation analysis. An orange-water
droplet rebounded from the Sb2Se3-membrane, as shown in
Figure 5a, which is similar to the observation for the
superhydrophobic Sb2Se3 with horizontal orientation on flat-
substrate (Figure 3d), indicating that the Sb2Se3-membrane has
intrinsically superhydrophobicity due to the surfaces of {h0l} or
{h00} planes having no dangling bonds. However, as shown in
Figure 5b, a blue-hexane droplet passes promptly through the
superhydrophobic Sb2Se3 filter. Eventually, the superhydrophobic
Sb2Se3 filter makes it possible to physically separate water and oil
present in a mixture by selectively permeating only oil. In contrast,
both the orange-water (water containing MO) and blue-hexane
(hexane dyed with oil-blue) solutions freely pass through the
pristine mesh without coating Sb2Se3, indicating that it cannot
individually separate the water and oil solutions (Figure S2) and
proving the superior function of Sb2Se3-membrane.
where C is the capacitance of the Sb2Se3 interface with the
electrolyte, 휀 is the dielectric constant, 휀0 is vacuum permittivity, A
is area contacted with electrolyte, ND is carrier density, T is
absolute temperature, E is applied potential, q is electron charge,
and Efb is the flat-band potential.[29] Additionally, the energy band
gap was measured using UV–visible spectroscopy. Figure 6b
shows the plot of the absorption coefficient revealing the energy
band gap of 1.33 eV, measured by UV–Vis spectrometer:
퐵
(ℎ푣 − 퐸푔)1/2
(2)
(
)
=
α ℎ푣
ℎ푣
The valence band maximum (VBM) was estimated by
extrapolating linear fit of forwarding current at positive potential in
cyclic voltammetry without light illumination, as shown in Figure
6c.[30] From the results in Figures 6b and 6c, the VBM and
conduction band minimum (CBM) were located at 0.9 V and -0.4
V respectively, vs. RHE due to the intersection of linear
extrapolation and the measured energy band gap of 1.33 eV. The
calculated energy band diagram (Figure 6d) and the electrical
parameters of the Sb2Se3 photoabsorber demonstrate the optimal
charge transport property and space charge width for the
photoelectrochemical applications. A drift-assisted transport in
the space charge region at lower potentials was evident from the
analysis, which enhances the collection efficiency of the
photogenerated carriers beneficial for the photodegradation
Photoelectrochemical degradation of methyl-orange using
Sb2Se3 membrane
Conventional studies on water-oil separation have only focused
on materials or surface treatments for membranes with selective
permeability, and the chemical decomposition of water-soluble
organics in filtered water was independently performed as a
separate field of research. The binary Sb2Se3 has been studied
as the p-type photo-absorbing material with narrow band gap in
photoelectrochemical application due to optimal photoactivity i.e.,
the multifunctional operation of integrated water purification with
a
single material could be realized by achieving the
process.[31-35]
Figure
6e
shows
the
schematic
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10.1002/cssc.202000438
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photoelectrocatalytic purification process of polluted water
containing methyl orange (after the physical separation of water–
oil), where a cell consists of a Sb2Se3 as the working electrode
and counter electrode (normally Pt). The photoelectrons
generated from the photoabsorber participate in reduction
reactions with water-soluble oxygen and produce active ionic
was applied to suppress the water splitting reaction for hydrogen
generation. The solution containing MO was stably
photodegraded with time, and each samples purified for 5, 15, 30,
and 60 minutes respectively, collected for evaluating the degree
of transparency, which were visually distinguished from the
orange colour of non-purified reference solution. To evaluate
purification status with quantitative analysis, the samples were
compared by the associated peak intensity in absorbance
measurement.[37] In the results of absorbance analysis, the peak
contributed by MO was detected at the centre wavelength of 464
nm, and their peak intensities in the purified samples decreased
consistently with increasing purification time (Figure 7c). As
shown in Figure 7d, the MO dye solution became transparent after
light irradiation for only 30 min, indicating the superior
performance of the Sb2Se3-membrane on photo-degradation of
MO dye. The following equation was used to calculate the
degradation rate of the MO:
-
oxygen species (·O2 ), which can spontaneously degrade
pollutant organic molecules.[23]
(
) × 100 %
(3)
퐶 −퐶
0
푡
(
)
=
퐷푒ꢀ푟푎푑푎푡ꢁ표푛 푟푎푡푒 %
퐶
0
where C0 is the initial absorbance of MO, and Ct is the absorbance
of MO after ‘t’ minutes. Figure 7c demonstrated that Sb2Se3
membrane was photocatalytically active in the visible-range with
the irradiation, and almost 96% of MO can be degraded in 60 min.
This indicated that Sb2Se3 membrane exhibits higher
photocatalytic degradation efficiency.
Figure 6. Electrochemical analyses of Sb2Se3-membrane for evaluating the
suitability for photodegradation. (a) Mott-Schottky plot of Sb2Se3 measured in
0.1-M ferrocene-acetonitrile solution. (b) Energy band gap measured from UV-
visible spectroscopy. (c) Cyclic voltammetry in 1-M Na2SO4 electrolyte and dark
state. (d) Energy band structure of Sb2Se3 at equilibrium state which is
established from (a)-(c) results. (e) Schematic reaction process of
photoelectrocatalytic organic degradation driven by the electrons generated in
Sb2Se3 with surface energy band bending under light illumination; the
conductive electrode makes it possible to supply external potential into Sb2Se3
as a driving force of O2 reduction (oxygen radical generation) and direct organic-
degradation with photocarriers.
To investigate the photoelectrocatalytic performance of Sb2Se3,
linear sweep voltammetry (LSV) measurements were performed
in an electrolyte containing MO, sodium sulfate (Na2SO4), and
hydrogen peroxide (H2O2), and it showed remarkable cathodic
photocurrent density of 15 mA cm-2 at -0.65 V (-0.16 V versus
reversible hydrogen electrode at pH 5), under 1-sun light
illumination, as shown in Figure 7a. The results indicated that
photoelectrocatalytic water purification using Sb2Se3 membrane
-
is enough to reduce O2 into oxygen radical (·O2 ) for
photodegradation of MO with cathodic charge injection into
electrolyte at low voltage. The reduction reactions producing
-
oxygen radicals (e- + O2 → ·O2 ) are vigorously observed under
Figure 7. Photoelectrochemical performance of the Sb2Se3-membrane. (a)
Linear sweep voltammetry (LSV) and (b) chronoamperometry result of Sb2Se3-
membrane measured at an applied voltage of -0.4 V in an electrolyte containing
methyl-orange and H2O2 under chopped AM 1.5 light illumination using a three-
electrode system. The cathodic photocurrent was sufficient to generate oxygen
radical species and direct organic degradation reactions. (c) Absorbance results
and (d) photoimages of solutions purified with times via photoelectrochemical
process. Methyl-orange solution exhibits a strong absorbance peak at a
wavelength of 464 nm, and the results are normalized by non-purified orange-
water.
light illumination at the Sb2Se3/electrolyte (orange–water)
interface by generating a relevant cathodic photocurrent and are
spontaneously followed by the reactions shown in the inset of
-
Figure 7a. To verify the influence of oxygen radical (·O2 )
formation, the photodegradation was carried out in the oxygen-
removed electrolyte solution under high-purity nitrogen (N2)
condition, resulting in reduced photocurrent generation (Figure
S3).[36] The photodegradation performance was subsequently
evaluated by chronoamperometry measurement using the three-
electrode system, and the result exhibited the cathodic
photocurrent density of 1 mA cm-2 at -0.4 V (0.1 V vs. RHE), as
shown in Fig. 7b. In this study, a higher potential than 0 V vs. RHE
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consisting
of
hydrophobicity
membrane
(HM)
and
photodegradation membrane (PM) were applied to implement
overall water purification by overlapping an upper membrane with
the photodegradation function (PM) and a lower membrane with
oil-separating function (HM). The upper membrane can be
immersed in polluted solutions and photodegrade the water-
soluble pollutants due to non-selective permeability of the water
and oil parts. To continuously keep contacting the upper
membrane with water, the lower HM mesh confines the polluted
water. However, this approach requires independent
developments of two separated materials for a sustainable water
purification system.[1] To ideally achieve integrated water
purification with a single material, it must strategically have the
following properties: i) a superhydrophobic surface, ii) efficient
enough to perform photodegradation in the visible range, iii) a
high-crystalline semiconductor for efficient charge transport, and
iv) easily controlled growth on mesh-type substrate, as shown in
Scheme 2b. In this view, the versatile Sb2Se3 is suitable for
realizing the integrated water purification with hydrophobic and
photoelectrochemical functions (H&PM). Consequently, the side-
wall facets of Sb2Se3 nanorods exhibit hydrophobicity due to their
low surface energy resulting from the dangling bond-free
properties based on the crystallographic understanding and the
cohesive energy of water. In addition, some of hydrophilic parts
(top/bottom ends of nanorods) could inevitably contact the
electrolyte, which contains water-soluble-organics, due to its
gravitational pressure. However, the total surface energy of
membranes did not decrease enough to become hydrophilic. In
essence, the exposed surface states with dangling bonds at the
top/bottem ends of nanorods are expected to easily react with
components of electrolyte and function as the reactive sites in the
Scheme 2. Schematics of integrated water purification involving both water-
insoluble fluid (oil) separation and photodegradation of water-soluble organics.
(a) Conventional approach requiring double-layered membrane with separated
functional materials. (b) Bi-functional Sb2Se3-membrane performing integrated
water purification process.
Demonstration of integrated water purification (physical
water-oil separation and chemical photodegradation)
Some photoactive materials could exhibit superhydrophobicity
induced by additional fabrications, whereas few materials show
good photocatalytic activities with superhydrophobicity enough to
separate the mixture of water and oil.[38-40] Previously, the different
membranes were separately utilized through 2-step process,
which involved the oil separation step and the photodegradation
purification step. For this reason, independent attempts were
necessarily performed by a temporary solution that used different
membranes with separated purification functions.[1,2,7,41] As shown
in Scheme 2a, the approaches using double-layered membranes
Figure 8. Demonstration of the integrated water purification with prototype apparatus using Sb2Se3-membrane. (a) Schematics of the apparatus components for
the integrated water purification capable to oil-removing and photoelectrochemical degradation processes. (b) Conceptual model and photoimages of sequential
water-oil separation using Sb2Se3-membrane. (i) Hexane injection into the apparatus using a syringe, and (ii) its penetration through the membrane. (iii) Water
injection into the apparatus, and (iv) its confinement on the membrane owing to superhydrophobicity. (c) Conceptual figures and photoimages of the water
purification apparatus connected electrically to commercial CIGS solar cell. The inset shows the solar-driven organic degradation process.
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10.1002/cssc.202000438
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redox reactions of photoelectrochemical degradation. Additionally, the photovoltage of solar cell assisted the charge separation for
in the respect of reaction rates, the morphological state exposing
{0k0} planes indicates that the charge transport efficiency can be
enhanced along [0k0] direction that consists of strong covalent
bonds (Sb-Se) in 1D ribbon-like crystal structure. In this respect,
the correlation between bi-functionality and crystal orientation of
Sb2Se3 was experimentally demonstrated by determining the
PEC performance depending on the exposed crystal planes. The
LSV was carried out using three types of Sb2Se3 nanorods with
different orientation (Figure S4). For randomly oriented nanorods,
the PEC properties are shown in Figure 7a. Horizontally aligned
Sb2Se3 nanorods had no significant photo-response due to the
vertical transport of photocarriers along the longitudinal axis of the
nanorod. To achieve the cathodic current generation, the photo-
generated electrons in Sb2Se3 should be transferred into the
electrolyte. For Sb2Se3, charge can be preferentially transported
along the [010] direction towards the top end of the nanorods
under the external bias during photodegradation process.
Therefore, vertical Sb2Se3 nanorods exhibit good PEC
performance, and randomly oriented or inclined species show
high photo-activity.
To demonstrate the effectiveness of the integrated water
purifications using Sb2Se3-membrane, the prototype apparatus
was designed, where the membrane is located between
transparent top and bottom cylinders, as shown in the conceptual
design (Figure 8a) and the real picture (Figure 8c). The integrated
water purification was performed via sequential processes where
the water-oil separation was carried out (step 1), followed by the
photodegradation of confined-water in the upper cylinder (step 2).
In the first process, the selective permeability of Sb2Se3-
membrane about water and hexane were sequentially observed
with dynamic photographs during injection of these liquids with
visual distinction (Figure 8b). Figure 8 shows the sequential
water–oil separation process: blue-hexane is initially injected in
the top cylinder, passes through the Sb2Se3-membrane and is
then collected in the below containers [(i) and (ii)]. However, the
orange-water injected in the top cylinder is entirely well-confined
in the top cylinder on the filter [(iii) and (iv)]. These results confirm
that the intensive superhydrophobic Sb2Se3-membrane acts
excellently for physical filtration after permeating the oil.
Finally, the confined water in the top cylinder was
photodegraded by the photoactive Sb2Se3-membrane under
visible-light illumination, as shown in Figure 8c. Since the
photoelectrochemical reactions of Sb2Se3 can be operated at low
applied voltage (Figure 7b), the photovoltage of commercial solar
cell can be used to demonstrate the solar-driven integrated water
purification under only light illumination without the external
applied bias using potentiostat. The Sb2Se3-membrane and
commercial photovoltaic were illuminated for photodegrading
process, and then electrically connected as shown in Figure 8c.
In the photodegradation process, the photovoltage generated in
the solar cell was directly applied to the membrane to enhance
the photocatalytic reaction efficiency of the Sb2Se3. The
photodegradation of Sb2Se3-membrane operated only with solar
energy, without artificial electronic controller. This indicated that
the photodegradation of MO in water was carried out by
photocharges generated in Sb2Se3 under light illumination, and
the promotion of charge injection. The photovoltage of the used
solar cell was approximately 0.4 V, which was enough to drive the
purification process through chemical degradation as shown in
Figure 7c. Thus, the generated photocurrent could contribute to
the degradation of MO, and a long-term degradation test was
performed with only light illumination. Consequently, to
accomplish the integrated physical/chemical purification of the
water contaminated with oil and water-soluble organics, the
versatile binary chalcogenide material with the anisotropic crystal
structure was proposed, and the verification was achieved with a
single Sb2Se3-membrane based on its superhydrophobic and
photoactive characteristics.
Conclusions
In summary, the concept of the integrated water purification
using a single Sb2Se3 material with both superhydrophobicity and
photoactive characteristics was proposed and its feasibility was
proved. It was confirmed that the bifunctional Sb2Se3-membrane
sequentially drives the water–oil separation process and water-
soluble organic degradation, owing to the intrinsic anisotropic
crystal structure and photoelectrochemical performance. The
Sb2Se3 exhibited specific wettability depending on the {h00} or
{h0l} surface planes, which could be intentionally exposed by
different morphologies. The only vertical Sb2Se3 exposing the
{0k0} surface exhibited a low contact angles of <90°, but the {h00}
or {h0l} surfaces in the horizontal or inclined Sb2Se3 alignment
could make the superhydrophobicity. This hydrophobic surface
property achieved excellent water–oil separation by blocking the
passage of the water through the fabricated Sb2Se3 filter coated
on mesh substrates. Moreover, Sb2Se3 photocathode showed
photocurrent density of 1 mA cm-2 at the potential sufficient for
-
oxygen radical (·O2 ) generation in MO photodegradation. As a
result, the photodegradation of water-soluble organic was
achieved with the same Sb2Se3 membrane under visible-light
illumination after the physical separation. Therefore, Sb2Se3 with
its 1D ribbon-like crystal structure is a potential candidate
functionally having both crystallographically superhydrophobic
and visible photoactive characteristics for next-generation
versatile material in water purification research.
Experimental Section
Thermal evaporation of Sb2Se3 and control of wettability via
substrate surface modification
The Sb2Se3 nanostructures on Mo substrate as the source in
the thermal-transfer process were prepared by modifying the
previously reported method which involved the electrodeposition
of Sb-Se precursors and thermal-annealing process for
crystallization.[42,43] The pristine Sb2Se3 materials had pure crystal
with only one single phase. The synthesized sources (Sb2Se3
nanostructures) was thermally evaporated in a vacuum at 400 oC
onto various target substrates (glass, polyimide, and mesh-type
metal substrate) using rapid thermal annealing (RTA) system.
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The membrane substrate had a mesh-size of 100 µm and non-
selective permeability of water and oil.
flexible CIGS (Cu(In, Ga)Se2) solar cells provided by SolarFLEX
Ltd. were electrically connected to the purification system to apply
a suitable electric potential during the photoelectrocatalytic water
purification, and thus, the entire water purification system could
be self-driven by photovoltaic cells.
Basic characterization of material property
The crystallographic properties and the planes exposed to the
atmosphere of crystalline Sb2Se3 phases were investigated using
TEM (JEM-2100F) and high-resolution XRD (Bruker AXS D8
Discover with a Cu Kɑ radiation source). The morphological
investigations of the vertical and horizontal Sb2Se3 crystals were
performed using field-emission scanning electron microscopy
(FESEM, JSM-6700F, 10 kV). The surfaces of the platinum mesh
and Sb2Se3-membrane were also observed under optical
microscopy (Leica DM-2700M).
Acknowledgements
This research was supported by Samsung Research Funding &
Incubation Center of Samsung Electronics [grant number SRFC-
MA170206].
Wettability tests of the prepared Sb2Se3 and water-oil
separation performance
The contact angles of the water and hexane solutions on the
prepared Sb2Se3 were measured on various samples (Sb2Se3 on
the glass, polyimide) using a contact angle meter [Phoenix-
MT(A)]. The total surface energies of the samples from contact
angle measurements were calculated using a surface energy
calculator program. The water–oil separation performances of the
Sb2Se3-coated substrates were visually recorded in the form of
time-lapse optical photographs using a camera (Samsung SM-
G965NZPFKOO). To illustrate water purification via water–oil
separation and pollutant degradation, system of apparatus
Conflicts of interest
The authors declare no competing financial interest.
Keywords: Photoelectrochemical
superhydrophobicity • antimony selenide • catalysis
•
water purification
•
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Entry for the Table of Contents
FULL PAPER
Horizontally grown Sb2Se3 nanorods
with superhydrophobic and
photocatalytic properties facilitate to
operate both physical water-oil
separation and photocatalytic
degradation
Y. B. Kim, S. W. Cho, N. G.
Deshpande, S. H. Jung, D. S. Kim, K.
J. Park, H.-K. Kim, H. K. Cho*
Page No.XX – Page No.XX
Smart Bifunctional Sb2Se3 Nanorods
for Integrated Water Purification:
Insoluble Fluid Separation and
Photoelectrochemical Water-
soluble-organic Degradation
((Insert TOC Graphic here: max.
width: 5.5 cm; max. height: 5.0 cm))
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