Selective Synthesis of Bismuth or Bismuth Selenide Nanosheets from a Metal Organic Precursor: Investigation of their Catalytic Performance for Water Splitting

Article abstract of DOI:10.1021/acs.inorgchem.0c02668

The development of cost-effective, functional materials that can be efficiently used for sustainable energy generation is highly desirable. Herein, a new molecular precursor of bismuth (tris(selenobenzoato)bismuth(III), [Bi(SeOCPh)3]), has been used to pr

Full text of DOI:10.1021/acs.inorgchem.0c02668

pubs.acs.org/IC
Article
Selective Synthesis of Bismuth or Bismuth Selenide Nanosheets
from a Metal Organic Precursor: Investigation of their Catalytic
Performance for Water Splitting
Shumaila Razzaque, Malik Dilshad Khan,* Muhammad Aamir, Manzar Sohail, Sanket Bhoyate,
Ram K. Gupta, Muhammad Sher, Javeed Akhtar, and Neerish Revaprasadu*
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ABSTRACT: The development of cost-effective, functional materials that
can be efficiently used for sustainable energy generation is highly desirable.
Herein, a new molecular precursor of bismuth (tris(selenobenzoato)-
bismuth(III), [Bi(SeOCPh)3]), has been used to prepare selectively Bi or
Bi2Se3 nanosheets via a colloidal route by the judicious control of the
reaction parameters. The Bi formation mechanism was investigated, and it
was observed that the trioctylphosphine (TOP) plays a crucial role in the formation of Bi. Employing the vapor deposition method
resulted in the formation of exclusively Bi2Se3 films at different temperatures. The synthesized nanomaterials and films were
characterized by p-XRD, TEM, Raman, SEM, EDX, AFM, XPS, and UV−vis spectroscopy. A minimum sheet thickness of 3.6 nm
(i.e., a thickness of 8−9 layers) was observed for bismuth, whereas a thickness of 4 nm (i.e., a thickness of 4 layers) was observed for
Bi2Se3 nanosheets. XPS showed surface oxidation of both materials and indicated an uncapped surface of Bi, whereas Bi2Se3 had a
capping layer of oleylamine, resulting in reduced surface oxidation. The potential of Bi and Bi2Se3 nanosheets was tested for overall
water-splitting application. The OER and HER catalytic performances of Bi2Se3 indicate overpotentials of 385 mV at 10 mA cm−2
and 220 mV, with Tafel slopes of 122 and 178 mV dec−1, respectively. In comparison, Bi showed a much lower OER activity (506
mV at 10 mA cm−2) but a slightly better HER (214 mV at 10 mA cm−2) performance. Similarly, Bi2Se3 nanosheets were observed to
exhibit cathodic photocurrent in photoelectrocatalytic activity, which indicated their p-type behavior.
als have shown promising applications for renewable energy
conversion devices and are potential candidates to address the
energy crisis.5
INTRODUCTION
■
Layered materials, when thinned down to their atomic limits,
are referred to as two-dimensional (2D) materials that exhibit
unique properties as compared to their bulk counterparts. The
thinning of the bulk layered material may result in enhanced
mechanical,1 conductive,2 and optoelectronic properties,3
which differ from the parent material. Among the layered
materials, the remarkable improvement in properties of
graphene has encouraged researchers to explore other 2D
materials, which also show interesting and exceptional
properties.4 Furthermore, the availability of layered materials
in different oxidation states and the type of chalcogenide (S,
Se, or Te) provide more flexibility in the optimization of the
desired properties.
Recently, the ever-increasing energy demand has shifted the
research focus toward the development of suitable nanoma-
terials for energy harvesting. The use of hydrogen as an
alternative green fuel is a viable option; however, obtaining
hydrogen in large quantities is a challenge. Water is the
cheapest source of hydrogen; however, the process of water
splitting to obtain hydrogen requires suitable catalysts as the
process is thermodynamically not feasible. The use of cost-
effective nanomaterials for efficient water splitting by evading
expensive metals, i.e., ruthenium, gold, or platinum, is
necessary for industrial applications. Two-dimensional materi-
Bismuth, from the pnictogen family, is of particular interest
as it shows a layered structure in both unary and binary forms
(i.e., bismuthene or Bi2Se3). Both Bi and Bi2Se3 exist in the
rhombohedral crystal structure, with band gaps varying
between 0.35 and 0.99 eV, depending on the thickness of
the sheets.6 Bismuth has been considered as a potential
material for energy storage and catalysis applications because
of its high environmental stability and unique electronic
properties. It shows a high theoretical volumetric capacity of
3765 mAh cm−3 due to its high density, which makes it a
highly suitable anodic material replacing other well-inves-
tigated 2D materials, such as black phosphorus (2266 mAh
cm−3) and graphite (837 mAh cm−3).7 Similarly, the narrow
energy gap and the layered structure of Bi2Se3 make it a
Received: September 11, 2020
Published: January 19, 2021
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suitable material for high-performance infrared detectors and
thermoelectric applications,8 and it has recently been
demonstrated to be a reference three-dimensional topological
insulator,9 with an insulating bulk gap of 0.3 eV and metallic
surface states consisting of a single Dirac cone.10 The unusual
surface of Bi2Se3 exhibits an unconventional spin texture,
electron dynamics, and stability characteristic.9b It has great
potential to serve as a conductive substrate to enhance HER
and supercapacitor performances.11 Furthermore, the layered
structure of Bi2Se3 allows easy intercalation between layers,
offering a high conducting and charging lane in electrode
materials.
Various synthetic approaches have been developed for the
synthesis of these materials.12−16 Bi2Se3 nanosheets/nanodiscs
have been synthesized by the multisource colloidal route,12
microwave synthesis,13 solvothermal/ionothermal route,14 and
exfoliation.15 On the other hand, bismuth nanomaterials are
usually prepared by the reduction of different bismuth salts by
employing different reducing agents/conditions or by exfolia-
tion.16 However, there is no report of the preparation of
bismuth from metal−organic precursors, and only a few such
complexes have been used so far for the synthesis of Bi2Se3
nanosheets. The molecular precursor route is desirable because
the presence of preformed bonds between the metal atom and
the chalcogenide atom in single-source precursors often
provides better control over size and morphology.17 The
only complexes available to date to prepare Bi2Se3 nanosheets
and/or films are 2-pyridyl selenolates,18 diselenoimidophos-
phinate,19 dialkyldiseleno phosphate,20 and dialkyldiselenocar-
bamate complexes.21 However, molecular precursors contain-
ing phosphorus may cause phosphorus contamination in the
final product or formation of a phosphate product
exclusively.20 Similarly, the synthesis of diselenocarbamate
precursors involves the use of highly unstable, obnoxious, and
expensive CSe2, which is commercially unavailable and very
difficult to synthesize.22
The colloidal method using metal−organic precursors
provides easy control over reaction parameters and better
reproducibility. A range of options are available for selecting
different capping agents such as primary amines, carboxylic
acids, and thiols, and highly crystalline products can be
achieved in a short duration of time. Since a single starting
material is being used here for the preparation of Bi or Bi2Se3
nanosheets and thin films, the flexibility of the starting material
makes the process facile and comparatively economical. The
metal−organic complexes are generally air- and moisture-
stable, which makes their handling and storage easy.
Furthermore, the mild reaction conditions offer a range of
temperatures at which reactions can be performed without
requiring state-of-the-art equipment or stringent requirements.
Herein, the synthesis of a new selenium-based complex of
bismuth, i.e., tris(selenobenzoato)bismuth(III) [Bi-
(SeOCPh)3], is reported by a facile route. The synthesized
complex is versatile as it was successfully employed for the
preparation of bismuth or bismuth selenide nanosheets by a
solvothermal route, whereas the decomposition of the
precursor by the vapor deposition method yielded Bi2Se3
films. The colloidally synthesized Bi and Bi2Se3 nanosheets
were also tested for electrocatalytic water splitting.
EXPERIMENTAL SECTION
■
Materials. All reagents, i.e., BiCl3, NaBH4, C6H5COCl, Se, 1-
octadecene (ODE), oleylamine (OLA), trioctylphosphine (TOP),
ethanol, acetone, and THF, were obtained from Sigma Aldrich.
Synthesis of [Bi(SeOCPh)3] Complex. NaHSe was synthesized
by treating 6.0 mmol of elemental Se powder with 6.0 mmol of
NaBH4 in ethanol (50.0 mL) under a nitrogen atmosphere at room
temperature. The decolorization of the solution from reddish to
colorless within 8−10 min of stirring indicates the generation of
NaHSe. After a further 15 min of stirring, benzoyl chloride in
equimolar quantity was injected slowly into freshly prepared NaHSe
solution. An immediate appearance of yellow color indicates the
generation of the selenobenzoate ligand. After the complete addition
of acid chloride, the reaction mixture was stirred continuously for
another half an hour to ensure completion of the reaction. After half
an hour of stirring, BiCl3 (0.48 g, 2.0 mmol) dissolved in ethanol
(20.0 mL) was slowly added while stirring. The reaction was
continued for approximately 30 min, after which the formed
precipitate was filtered and washed with an excess of ethanol. The
schematic reaction for the synthesis of the selenobenzoate complex of
bismuth is shown in Scheme 1. The recrystallization of the precipitate,
Scheme 1. Preparation of the Selenobenzoate Ligand and
the Tris(selenobenzoato)Bi(III) Complex
using THF, yielded a yellowish powder. Melting point: 177−178 °C;
elemental analysis calcd (%) for elemental analysis for C21H15O3BiSe3,
found: C 33.39, H 1.97, Bi 27.18; calcd (%): C 33.17, H 1.99, Bi
27.45.
Synthesis of Bi2Se3 Nanosheets. Oleylamine (8.0 mL) was
heated to 200 °C under a nitrogen environment. Once the desired
temperature was reached, the tris(selenobenzoato)bismuth(III)
complex (0.40 mmol), dispersed in ODE (3.0 mL), was injected
into preheated OLA under vigorous stirring. A decrease in
temperature (≈15−20 °C) was noted upon injection, and the
solution instantly turned brownish-black. The temperature was
quickly readjusted to 200 °C, where it was maintained for 1 h, after
which the heating source was removed and the solution cooled to
room temperature. A 40.0 mL mixture of acetone and methanol (1:1)
was added to the cooled solution, and the precipitated Bi2Se3
nanosheets were washed and separated by centrifugation.
Synthesis of Bismuth. Bismuth was prepared by following a
similar procedure, as used for the synthesis of Bi2Se3 nanosheets,
except that the precursor was dispersed in trioctylphosphine (TOP)
instead of ODE. It was noted that the color of the complex starts to
change when the complex was dispersed in TOP. The dispersed
complex was immediately injected into the preheated OLA (at 200
°C), and the reaction was continued for 1 h. There was no dispersion
of particles; instead, the particles flocculate immediately, forming a
solid chunk that precipitated out of the solution. The precipitate was
washed and separated by centrifugation using an acetone and
methanol (1:1) mixture. The solid chunk was crushed with a spatula
to a powdered form, which was then used for further analysis.
Deposition of Bi2Se3 Thin Films. Bismuth selenide films were
deposited on glass substrates. The surface of glass substrates was
cleaned by sonicating in HNO3, washed with deionized water, and
then rinsed in acetone. The AACVD setup consists of an ultrasonic
humidifier (to generate aerosol) and a carbolite tube furnace. For the
deposition of thin films, a two-necked round-bottom flask (RBF)
containing a freshly prepared solution of tris(selenobenzoato)-
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Figure 1. (a) p-XRD of Bi2Se3 nanosheets synthesized in oleylamine, (b) TEM image of the stacked nanosheets, (c) HRTEM showing lattice
fringes, (d) SAED pattern, and (e, f) XPS spectra of Bi 4f and Se 3d, respectively, along with the fitted peaks.
bismuth(III) (0.3 mmol) in chloroform (20.0 mL) was placed over
the humidifier. The reactor tube, containing eight glass substrates, was
inserted into the tube furnace. Reinforced tubings were used to
RESULTS AND DISCUSSIONS
■
Among the monochalcogeno-carboxylic acids, various com-
plexes of thiocarboxylic acid have been reported, but there are
only a few reports of the congeners containing heavier
chalcogenide atoms (i.e., Se or Te), probably due to their
instability and the handling difficulties associated with them.
Selenobenzoic acid is highly sensitive and cannot be isolated as
it is easily oxidized to its dimer, i.e., dibenzoyl diselenide.
However, it can be stabilized by the formation of a salt or in
solution under inert conditions. In the present study, the
bismuth−selenobenzoate complex was prepared by a new
method, which is comparatively more efficient than a
previously reported method.23 The previous method required
a reaction between an alkali metal (M = Na, K) and Se to
prepare M2Se, in ammonia at −70 °C for 12 h.24 Our synthetic
protocol can be performed at room temperature in ethanol.
NaBH4 replaced the highly pyrophoric alkali metals. Moreover,
the reaction can be completed within a short duration of time,
i.e., almost in half an hour. Elemental analysis and TGA were
connect the gas inlet to one neck of the RBF and the reactor tube by
the other neck. The ultrasonic humidifier generated the aerosol, which
was then transferred to the heating zone of the furnace by argon. The
flow rate was adjusted to 180 sccm, and the deposition of Bi2Se3 films
took place inside the heating chamber on the surface of the heated
glass substrates.
Characterization. Elemental analysis was performed using a
Thermo Scientific Flash 2000 Organic Elemental Analyzer. The
decomposition behavior of the precursor was observed by a Mettler-
Toledo thermogravimetric system. P-XRD was obtained from a
Bruker D8 Discover Diffractometer. A Talos F200X microscope,
operating at 200 kV and equipped with an FEI ceta camera, was used
to capture TEM and HRTEM images. Carbon coating of thin films
was performed on an Edwards coating system E306A. Scanning
electron microscopic (SEM) images and EDX analyses were obtained
from Philips XL30 FEG SEM. Raman analysis was performed using a
Renishaw 1000 Micro-Raman System, and PerkinElmer Lambda 1050
instrument was used for the UV−vis−NIR spectrum.
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used to characterize the final product. Repeated attempts to
recrystallize this product to determine the X-ray crystal
structure were unsuccessful as the product starts to
decompose.
which are arranged in the order of Se(I)−Bi−Se(II)−Bi−Se(I)
sheets.29 The superscripts (I) and (II) are used to differentiate
between the two Se atoms, which are arranged in different
surroundings (Figure S2, SI). The unit cell is described as a
quintuple layer (QL) and the covalent character predominates
within the QL, whereas two adjacent QLs are linked to each
other with weak van der Waals forces, giving rise to a highly
anisotropic structure. The weak interactions between the layers
allow the generation of mono to a few QLs under judicious
reaction conditions. The crystalline nature of these sheets was
investigated by HRTEM and SAED analyses. Clear lattice
fringes in the HRTEM image (Figure 1c) were observed, and
the lattice spacing corresponds to the (015) plane. The
crystallinity and high quality of nanosheets were also evident
from the well-defined spots as observed in the SAED pattern
(Figure 1d). Figure S3 (SI) shows the stacking of thin
nanosheets, and due to the thin nature of the sheets, they are
flexible enough to buckle and fold to acquire different types of
shapes. The nanosheets were of variable sizes and range
between tens of nanometers to hundreds of nanometers. The
broad peaks in the p-XRD pattern also indicate the presence of
very thin nanosheets of Bi2Se3.
Similarly, the surface composition and chemical states of the
synthesized Bi2Se3 nanosheets were further analyzed by XPS
analysis. The survey spectrum of Bi2Se3 shows the presence of
Bi and Se along with carbon, nitrogen, and oxygen (Figure S4,
SI). The sample was synthesized in oleylamine; therefore, in
this case, the comparatively intense C 1s peak at the binding
energy of 284.5 eV indicates that the carbon content may
include both adventitious carbon and carbon from the capping
agent, i.e., oleylamine, present at the surface of the material.
The surface capping by oleylamine is further evidenced by the
presence of the N 1s peak at the binding energy of 398.6 eV.
Bi2Se3 nanosheets can be oxidized easily at room temperature,
which was evident from the presence of the O 1s peak at a
binding energy of 530.2, although due to surface passivation,
the extent of oxidation was lower. The Bi 4f spectrum was
fitted with Bi 4f7/2 and Bi 4f5/2 peaks present at binding
energies of 157.7 and 163.1 eV, respectively. The minor fitting
peaks centered at 158.7 and 164.1 eV may show slight
oxidation of the Bi2Se3 nanosheets (Figure 1e). It is worth
noting that the binding energies of Bi 4f peaks in Bi2Se3 are
blue-shifted as compared to elemental bismuth due to charge
transfer from Bi to Se.30 The Se 3d spectrum can be fitted
perfectly by Se 3d5/2 and Se 3d3/2 peaks present at binding
energies of 53.3 and 54.1 eV (Figure 1f), respectively.
The decomposition behavior of the complex was analyzed
by TGA (Figure S1, SI). The decomposition of the precursor
took place in three steps, and the precursor indicates high
thermal stability in the solid state. A substantial weight loss
(almost ∼37%) was detected in the first step, in the
temperature range of 160−240 °C, which, possibly, shows
the loss of C6H5 moieties. A further 12% decrease in mass was
detected in the second step, which infers exclusion of the
carbonyl moiety, leaving behind the BiSe3 unit. Finally, two
such fragments undergo a transformation from BiSe3 to Bi2Se3
with a little loss of Se (9% weight loss) at high temperature,
and schematically it is shown in Scheme S1.25 The complex
decomposes completely around ≈385 °C, and a final residue of
∼42% was obtained, which corresponds to the formation of
Bi2Se3 (theoretically ca. 43%).
Bi2Se3 Nanosheets. The bismuth selenide nanosheets
were synthesized at 200 °C by decomposition of tris-
(selenobenzoato)bismuth(III) (dispersed in ODE) in OLA.
The reaction mixture turned brownish-black immediately upon
injecting the complex in oleylamine, which may indicate the
formation of nanomaterials. Similarly, it was observed that the
addition of the complex in OLA, even at room temperature,
starts changing the color of the solution to brownish-black,
indicating the decomposition of the complex; however, the
product formed was amorphous. Hence, to obtain Bi2Se3
nanosheets with reasonably high crystallinity, a reaction
temperature of 200 °C was used. It shows that OLA, besides
acting as a surface-passivating agent, also initiates the
degradation of the precursors. The degradation of the
selenobenzoate complexes in the presence of primary amines
may undergo a similar decomposition pathway, as indicated by
Chin et al. for primary amine-assisted decomposition of
thiocarboxylate complexes.26 Previously, decomposition of the
silver selenobenzoate was observed even at room temperature
in oleylamine by Vittal and co-workers.27 The decomposition
is accelerated by the primary amine, which acts as a nucleation
initiator as well. In this way, the nucleation and the growth
steps are separated, which is preferable to produce
monodispersed nanomaterials.28
The diffraction pattern of Bi2Se3 nanosheets is shown in
Figure 1a. The pattern observed in the p-XRD of the
nanosheets shows a good resemblance with the intensity
profile of the standard pattern, with the highest intensity peak
along the (015) plane. The intense and slightly broad peaks
indicate that the synthesized nanosheets comprise thin sheets.
The diffraction pattern matches well with the standard Bi2Se3
rhombohedral phase (ICDD # 01-089-2008). No peaks
belonging to elemental bismuth or selenium were observed.
The nanosheets were relatively thin, as observed by the
TEM images (Figure 1b), where overlapped thin layers of
Bi2Se3 can be seen clearly; however, due to stacked sheets, a
single separate nanosheet was not observed. The stacked layers
of the nanosheets can be seen clearly (indicated by the arrows
in Figure 1b), which can be distinguished based on the color
complexion of the layers, as in comparison to thickly stacked
sheets, mono to few nanosheets seem fairly transparent to the
electron beam. The anisotropic nature of the unit cells
determines the preferential growth into nanosheets. The basic
unit cell of layered Bi2Se3 is composed of five atomic layers,
The thickness of the Bi2Se3 nanosheets was determined by
AFM analysis (Figure S5, SI). The AFM image indicates the
formation of nanosheets in various sizes, and the dimensions of
some sheets were in microns. The height profiles at different
points indicate a thickness of almost 4 nm. A single layer of
Bi2Se3 is composed of covalently bonded five-atom Se−Bi−
Se−Bi−Se chains, and the thickness of a single-layer Bi2Se3
slab along the [001] direction is 0.96 nm. Furthermore, it has
been reported that the determination of the height profile by
AFM is exaggerated by almost 0.2−1 nm due to the overlayer
of the capping agent used in the synthesis.31 Therefore, the
observed height profile of 4 nm is nearly equal to 3−4 layers of
Bi2Se3 nanosheets.
UV−vis−NIR spectroscopy was used to determine the
optical properties of the Bi2Se3 nanosheets. The absorption
spectrum of well-dispersed Bi2Se3 nanosheets (Figure 2a) in
acetone resulted in a continuous broadband with λmax at 660
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nm. A broad absorption behavior was previously observed for
Bi2Se3 nanosheets, where it was reported that the broadness
might be attributed to the local surface plasmon resonance
(LSPR).32 The energy gap of the Bi2Se3 nanosheets is
estimated to be 1.0 eV (Figure 2b). A shift in the energy
gap is probably due to the decreased thickness of the sheets as
compared to the parent bulk counterpart. The Raman analysis
was performed using a 514 nm laser excitation at 25% power.
1
The characteristic vibration modes for Bi2Se3 are A1g out-of-
plane (72 cm−1), Eg2 in-plane (130 cm−1), and out-of-plane
A2g1 (174 cm−1).33 However, due to the broadness of the peak
centered at 126 cm−1, and the relatively high intensity, the
other peaks were amalgamated and only appeared as one
prominent peak (Figure 2c). The shift toward a lower value
and the increased width of the peak can be attributed to
phonon softening and enhanced electron−phonon coupling in
a thin-layered structure.34 Similarly, for layered materials, the
thickness of the nanosheets significantly affects the width,
position, and shape of the vibrational mode.35
Figure 2. (a) UV−vis−NIR absorption spectrum of Bi2Se3 nano-
sheets; the inset shows (b) the band gap by the Tauc plot and (c)
Raman shift for Bi2Se3 nanosheets.
Bismuth Nanosheets. Bismuth was synthesized by the
decomposition of the metal−organic precursor in OLA in the
Figure 3. (a) p-XRD analysis of Bi nanosheets, (b) EDX analysis, (c, d) TEM images, (e) survey scan and (f) high-resolution Bi 4f spectrum.
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Figure 4. AFM image showing height profiles of the Bi nanosheets at different points.
presence of TOP. It was noted that the dispersion of the
complex in TOP resulted in a change of color of the precursor,
which shows the interaction of TOP with the precursor. The
dispersed precursor was immediately injected into preheated
OLA, and the nature of the residue was observed by p-XRD.
The p-XRD analysis indicates the formation of bismuth rather
than Bi2Se3. The peaks were sharp and matched accurately to
the complex starts to change after ≈5 min to brownish-black.
After 15 min of sonication, acetone was added and the
decomposition product was washed and separated by
centrifugation. The p-XRD analysis of the black powder
obtained showed the formation of an amorphous product
(Figure S7, SI). It shows that the complex undergoes
decomposition even at room temperature, though the resulting
product is amorphous. TOP has been used previously as a
reducing agent, as trivalent phosphorus in TOP can be easily
oxidized from trivalent to the pentavalent state. Lee et al.
converted graphene oxide to reduced graphene using TOP,
which itself was converted to TOPO during the reduction
process.36 Mews et al. reported the synthesis of Bi nano-
particles by reducing BiCl3 and Bi[N(SiMe3)2]3 precursors
using only TOP. 31P-NMR spectroscopy confirmed the
complete oxidation of TOP to TOPO.37 However, there is
no report on the reduction of bismuth-based molecular
precursors to elemental bismuth.
Molecular precursors have been used previously for selective
phase-tuning of semiconducting materials by varying exper-
imental conditions, in both colloidal and solventless routes.17c,d
Here, the metal-to-selenium (i.e., Bi−Se) bond or the
selenium-to-carbon (Se−C) bond was selectively dissociated
using TOP or ODE to prepare bismuth or Bi2Se3 nanosheets,
respectively. The sample was analyzed by EDX analysis to
observe the presence of selenium, which might indicate the
formation of Bi2Se3 as an impurity phase; however, the EDX
analysis shows the presence of only bismuth (Figure 3b). The
SEM micrographs did not show any definite morphology, and
̅
rhombohedral Bi (ICDD # 01-085-1330) with the R3m space
group (Figure 3a). It was interesting to note that, although
selenium is directly attached with bismuth in the complex,
there was no indication of the formation of Bi2Se3, even as a
minor impurity phase.
The fact that the decomposition of the precursor gave Bi2Se3
nanosheets when dispersed in ODE shows that the formation
of bismuth exclusively is primarily due to the reducing effect of
TOP. The interaction of TOP with the complex was
investigated by UV−vis analysis. The complex was dissolved
in chloroform to monitor the absorption behavior, and a
greenish-yellow-colored solution was obtained. A strong
absorption was observed below 500 nm for the complex
(Figure S6, SI). When a small quantity of TOP was added to
the solution, the color of the solution started to change from
greenish-yellow to brownish. It shows that the complex begins
to decompose in the presence of TOP. The absorption
spectrum of the solution after the addition of TOP further
confirms the decomposition of the complex as the absorption
curve flattens completely.
Later, the complex was dissolved in TOP at room
temperature and sonicated for about 15 min. The color of
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particles with different sizes were randomly oriented (Figure
S8, SI). To have a better idea of the morphology, the sample
was also analyzed by the TEM analysis, which shows
aggregated particles with a mixed morphology (Figure 3c,d).
Interestingly, the formation of sheet-like structures was also
observed, along with some smaller particles. Sheets were of
cubic to rectangular morphology and mostly agglomerated
with other particles with an irregular morphology.
sizes.41 The probable reason might be that though the layered
structure of bismuth is similar to graphene sheets in graphite,
the interaction between layers is stronger, which causes a close
competition between exfoliation and downsizing. It is
anticipated that employing softer conditions/ligands may
yield bismuth nanosheets with a narrow size distribution and
thinner dimensions.
Bi2Se3 Thin Films. Since colloidal synthesis provided
different products by changing the reaction parameters, the
rationale behind investigating the precursor’s decomposition
by the vapor deposition method is to examine the effect of the
synthetic route on the final product. The precursor can behave
differently under different synthetic conditions, and since
chalcogenides have high partial pressure and bismuth can be
reduced under mild reaction conditions, it is worth
investigating whether bismuthene films can be deposited
from the same precursor at elevated temperatures.
Bismuth is easily oxidized in the air; therefore, the surface
composition and chemical state of the synthesized Bi were
further analyzed by XPS analysis (Figure 3e,f). The survey
spectrum of Bi clearly demonstrates the presence of bismuth
along with carbon and oxygen (Figure 3e). The C 1s peak at a
binding energy of 284.4 was attributed to the presence of
adventitious carbon typically found on the surface of materials
exposed to air.38 Although oleylamine and trioctylphosphine
were used as surfactants, their presence on the surface is ruled
out by the fact that there is an absence of peaks for binding
energies of nitrogen or phosphorus. Since no other peak was
detected in the C 1s spectrum, it indicates that there is an
absence of any bond between bismuth and carbon. As the
surface of bismuth was not well-capped by the surfactants, it
became prone to surface oxidation, and the extent of oxidation
was relatively greater than Bi2Se3 nanosheets. Consequently,
the O 1s peak at a binding energy of 530.2 is attributed to
lattice oxygen, combined with bismuth to form oxide. The
relatively low intensity of the O 1s peak, in comparison to pure
Bi2O3, indicates that there is partial surface oxidation and
bismuth was not completely converted to its oxide. Besides
atmospheric oxygen, the use of solvents, such as methanol and
acetone for washing, may also have contributed to the surface
oxidation of bismuth. Likewise, to further confirm the chemical
state of Bi, a high-resolution spectrum shows the splitting of
the main peak for Bi 4f (Figure 3f). The deconvoluted peaks at
156.5 and 162 eV correspond to the bismuth, whereas peaks
centered at 158.7 and 164.1 eV, respectively, represent
oxidized bismuth due to the surface oxidation of nanosheets,
indicating the presence of both elemental bismuth and
oxidized bismuth, in agreement with the previously reported
literature.39 It has also been reported that polishing the surface
of bismuth can reduce the oxide layer/content significantly.40
Due to the agglomeration and irregular morphology, the
TEM images were inconclusive in determining the actual
morphology of bismuth nanoparticles, i.e., whether they are
nanosheets or have a particle-like morphology. Therefore,
AFM was used to further investigate the morphology of
bismuth. As shown in Figure 4, the AFM image also confirms
the presence of agglomerated particles with highly diverse
shapes and sizes. The sizes of the particles vary between less
than 100 nm to microns. Due to agglomeration, the height
profiles at different points also showed variation, with the
lowest thickness of 3.6 nm to the highest thickness of 8.3 nm.
The theoretical monolayer thickness of bismuth is 0.395
nm,39a which shows that the sheets with the lowest thickness
are composed of 8−9 layers of bismuth. The stacking of
smaller sheets over larger sheets results in increased height
profiles, as shown in Figure 4.
The deposition was carried out on the glass substrates by
AACVD. The complex was readily soluble in common organic
solvents, such as CHCl3, THF, and toluene. It was observed
that the complex starts to decompose in THF after a while
during the generation of an aerosol, and toluene aerosol
requires a comparatively longer duration to finish; hence,
chloroform was used as a solvent for deposition. As observed
by the TGA analysis, the complex decomposes completely
around 385 °C; therefore, a deposition temperature between
400 and 500 °C was used. A light deposition was observed on
the slides placed at the start of the hot zone, but a thicker
deposition on the slides placed at the center to the end of the
glass tube was detected. All films were black, compact, and
showed moderate adherence (can be scratched easily by a
spatula). A closer examination of the film also suggested the
presence of some particulate crystallites at the film surface. It
may indicate that the decomposition of the precursor did not
entirely occur on the heated substrates, rather slightly above
the surface of the substratesa phenomenon also referred to
as snowing of the filmsresulting in poorly adherent films
with a particulate nature. All deposition experiments were
carried out until complete evaporation of the solution, which
took almost 30 min.
The diffraction patterns of the films deposited at different
temperatures are shown in Figure S9, SI. At 400 °C, only an
intense peak at 2θ ≈ 31.1° was observed, which matches with
the Bi2Se3 phase (ICDD # 01-085-0519). The films are highly
crystalline and textured along the (221) plane, as indicated by
the single intense peak detected in this plane. At 450 °C, other
peaks were also observed, but they appeared only as minor
peaks due to their low relative intensity as compared to the
high-intensity peak along the (221) plane. At 500 °C, the
intensity of this peak further increases and no other peaks were
observed, probably masked by the intense peak along the
(221) plane. The broad hump observed in all cases is due to
the amorphous glass substrate. The observations in p-XRD
patterns show that the deposition temperature may have only
little effect on the growth of the crystallite size and
morphology, as the preferred orientation of the crystallites in
thin films deposited at different temperature remains the same.
The effect of temperature on size, morphology, and
stoichiometry was analyzed by SEM analysis. At a deposition
temperature of 400 °C, the formation of granular particles,
which were uniformly distributed on the glass substrate, was
observed (Figure S10a,b, SI). The particles had a broad size
distribution. The films were slightly Se-enriched, and EDX
The harsh conditions (i.e., use of oleylamine and high
temperature) may have prevented the formation of sheets with
uniform size distribution due to their breakdown. A similar
observation was reported by Pumera et al., where the
exfoliation of bulk bismuth in different organic solvents
resulted in the formation of particles with diverse shapes and
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analysis indicates an average Se/Bi ratio of 1.73, which is
somewhat higher than the required stoichiometry of 1.50 for
Bi2Se3 (Figure S11a, SI). The homogeneity of the deposited
films was investigated by elemental mapping, which shows a
uniform distribution of bismuth and selenium over the
substrate (Figure S12a−c, SI).
The growth at a higher temperature of 450 °C indicates only
a slight alteration in the size of crystallites. A small increase in
the size of crystallites can be observed, but generally, the films
showed a similar morphology (Figure S10c,d, SI). The
stoichiometry of the films also changed, and an average Se/
Bi ratio of 1.65 was obtained from the EDX analysis. The films
are still selenium-rich but to a lesser extent as compared to the
films deposited at a temperature of 400 °C (Figure S11b, SI).
The elements were homogeneously distributed, as indicated by
the elemental mapping (Figure S12d−f, SI).
Further increase in the deposition temperature to 500 °C led
to the deposition of layered or sheetlike structures (Figure
S10e,f, SI), which appear to form clusters. The structure of
Bi2Se3 consists of layers that are interconnected by weak van
der Waal’s forces. The high temperature ruptures the delicate
forces and results in splitting the three-dimensional structure
into clusters of a few layers. The stoichiometry was also
significantly affected, as the films deposited were deficient in
selenium with an average Se/Bi ratio of 1.37, as shown by EDX
(Figure S11c, SI). The loss of chalcogens at higher
temperatures is a well-known observation due to their high
partial pressure, and this trend is clearly shown in the
stoichiometry of all deposited films deposited at different
temperatures. The homogenous distribution of Bi and Se in
films was indicated by EDX analysis (Figure S12g−i, SI).
Electrochemical Performance of Bi2Se3. The detail
regarding the experimental setup is provided in the Supporting
Information. A potential application of the Bi2Se3 nanosheets,
synthesized by the solvothermal method, was tested for both
hydrogen and oxygen evolution in an alkaline medium. Figure
5a shows a polarization curve for the Bi2Se3 electrode. The
Bi2Se3 electrode displayed an overpotential of 385 mV at 10
mA cm−2. A blank test of the substrate without Bi2Se3 was
performed to supplement that the electrocatalytic performance
originates from the active Bi2Se3 rather than the substrate
(Figure S13a, SI). It was observed that, for Ni foam to obtain a
current density of 10 mA cm−2, an overpotential of 440 mV is
required, which clearly indicates that the catalytic activity is
coming from Bi2Se3 nanosheets. The observed overpotential
was compared with other commonly used oxides or
chalcogenide-based nonprecious OER catalysts, suggesting
that the synthesized catalyst is better than or comparable to
the electrocatalysts prepared by different methods (Table S1).
To determine the OER kinetics of the Bi2Se3 catalyst, the Tafel
slope was calculated by plotting the potential versus the log of
current density. As seen in Figure 5b, the Tafel slope for the
Bi2Se3 electrode was calculated to be 122 mV dec−1.
The electrochemical stability of the Bi2Se3 electrode was
studied using cyclic voltammetry and chronoamperometry.
Figure 5c indicates the polarization curves of the Bi2Se3
electrode at the 1st, 1000th, and 2000th cycles. As evident
from the polarization curves at various cycles, the Bi2Se3
electrode is electrochemically stable up to 2000 cycles. The
electrochemical stability was further tested using chronoam-
perometry. As shown in Figure 5d, the current density of the
Bi2Se3 electrode was constant for almost 18 h of study.
Bifunctionality of the Bi2Se3 electrode as an electrocatalyst
for the HER process was also tested in an alkaline medium.
Figure 6a shows the polarization curve of the Bi2Se3 electrode.
Figure 6. (a) Polarization curve, (b) corresponding Tafel slope, (c)
cyclic stability, and (d) chronoamperometry measurements of the
Bi2Se3 electrode for HER.
The Bi2Se3 electrode required an overpotential of 220 mV to
achieve a current density of 10 mA cm−2 with a Tafel slope of
178 mV dec−1 (Figure 6b). The Ni foam showed an
overpotential of 319 mV at 10 mA cm−2, suggesting that
catalytic activities are originating from the Bi2Se3 (Figure S13b,
SI). Luo et al. reported that pristine Bi2Se3 nanosheets required
an overpotential of 508 mV to attain a current density of 10
mA cm−2, whereas Au-decorated Bi2Se3 nanosheets needed an
overpotential of 380 mV to achieve the same current density.42
Yang et al. reported a current density of 0.8 mA cm−2 for pure
Figure 5. (a) Polarization curve, (b) corresponding Tafel slope, (c)
cyclic stability, and (d) chronoamperometry measurements of the
Bi2Se3 electrode for OER.
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Bi2Se3 at 300 mV, which can be significantly enhanced by
coupling it with MoSe2 nanosheets.43 Pumera et al. prepared
Bi2Se3 nanosheets by electrochemical exfoliation; however, the
exfoliated Bi2Se3 nanosheets were catalytically inactive toward
HER and OER.44 It has been proposed that the high activity of
Bi2Se3 is a result of surface oxidation of Bi2Se3 nanosheets.45
The oxidized layer forms a heterojunction and results in the
readjustment of the band structure and provides better charge
transfer. The oxidized surface could also provide more active
edge sites for high HER performance.43 Thus, the synergistic
effect of surface oxidation is responsible for the better
performance of the Bi2Se3 nanosheets. A comparison of the
HER activity of Bi2Se3 nanosheets with other nonprecious
electrocatalysts is shown in Table S2. The durability of the
Bi2Se3 electrode as an HER catalyst was also studied using
cyclic voltammetry (Figure 6c) and chronoamperometry
(Figure 6d). As seen from both studies, the Bi2Se3 electrode
indicated a highly stable performance up to 2000 cycles of
study and over 24 h of chronoamperometric study. Our results
suggest that the Bi2Se3 electrode is highly durable and efficient
for both OER and HER processes.
Figure 8. (a) Polarization curve, (b) Tafel slope, (c) stability test, and
(d) EIS plots at various potentials for the OER activity of bismuth
nanosheets.
An electrolyzer was fabricated using two Bi2Se3 electrodes as
the anode and cathode for overall water-splitting studies. The
fabricated electrolyzer required a cell voltage of 1.9 V to attain
a current density of 10 mA cm−2 (Figure 7a). The performance
Bi electrode is electrochemically stable up to 1000 cycles.
Electrochemical impedance spectroscopy (EIS) was performed
to determine the overall series resistance of the sample (Figure
8d). It is well-known that the smaller the radius of the
semicircle, the lower will be the resistance. The graph indicates
an inverse relation between the arc radius of each sample and
the applied voltage. The significant decrease in radius with the
increase in applied voltage suggests that, at higher voltage,
bismuth leads to more effective charge separation and faster
interfacial charge transfer.
The electrocatalytic performance of the Bi electrode for the
HER process was also tested in an alkaline medium. Figure 9a
shows the polarization curve of the Bi electrode. The bismuth
electrode required an overpotential of 214 mV to achieve a
current density of 10 mA cm−2 with a Tafel slope of 158 mV
dec−1 (Figure 9b). The Ni foam showed an overpotential of
319 mV at 10 mA cm−2, suggesting that catalytic activities are
originating from the Bi (Figure S7b, SI). A comparison of the
Figure 7. (a) Polarization and (b) chronoamperometry plots for
overall water splitting of a two-electrode electrolyzer using a Bi2Se3/
Bi2Se3 electrocatalyst couple. The inset in (a) displays an optical
image of the Bi2Se3/Bi2Se3 electrocatalyst showing generated O2 and
H2.
of the electrolyzer was further tested using chronoamperom-
etry (Figure 7b). As seen in the chronoamperometry plot, the
Bi2Se3-based electrolyzer displayed considerable electrochem-
ical stability for about 20 h.
Electrochemical Performance of Bi. Electrocatalytic
performance of bismuth nanosheets was also tested for both
hydrogen and oxygen evolution in an alkaline medium. Figure
8a shows a polarization curve for the OER activity of the Bi
electrode. The Bi electrode displayed an overpotential of 506
mV at 10 mA cm−2. It indicates that Bi2Se3 nanosheets have
better OER activity as compared to bismuth nanosheets. The
observed overpotential was also compared with other
commonly used oxide or chalcogenide-based nonprecious
OER catalysts to compare with other electrocatalysts prepared
by different methods (Table S1). Likewise, the Tafel slope was
calculated by plotting potential versus the log of current
density to determine the OER kinetics of the Bi nanosheets.
The Tafel slope for the Bi electrode was estimated to be 175
mV dec−1 (Figure 8b). The electrochemical stability of the Bi
electrode was studied using cyclic voltammetry. Figure 8c
indicates the polarization curves of the Bi electrode at 1st and
after 1000 cycles. As evident from the polarization curves, the
Figure 9. (a) Polarization curve, (b) Tafel slope, (c) stability test, and
(d) chronoampere test for bismuth nanosheets.
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HER activity of Bi with other nonprecious electrocatalysts is
shown in Table S2. The durability of the bismuth electrode as
an HER catalyst was also studied using cyclic voltammetry
(Figure 9c) and chronoamperometry (Figure 9d). As seen
from both studies, the Bi2Se3 electrode indicated a stable
performance up to 1000 cycles of study, and after an initial
decrease in the current density, which may be probably due to
the surface oxidation of bismuth, a steady state was indicated
for over 18 h of chronoamperometric study.
Photoelectrochemical Water Reduction. Colloidally
synthesized Bi2Se3 nanosheets deposited on FTO-glass
substrates were used for photoelectrochemical studies. The
complete detail regarding electrode preparation and electro-
chemical setup for photoelectrochemical investigation is shown
in the Supporting Information. Bi2Se3 nanosheets were
observed to exhibit cathodic photocurrent. The chronoam-
perometric performances of the Bi2Se3/FTO photocathodes,
obtained at open-circuit potential (OCP) as a function of time,
under simulated sunlight illumination are shown in Figure 10a.
Once the current response was stable, the light was cut off at
regular intervals. It was evident that a significant cathodic
current was generated for H2 generation only when the light
was illuminated at the surface of the Bi2Se3/FTO electrode.
When the light was cut off, the photocurrent density
instantaneously became negligible. This infers that the current
generated was only due to the illuminated light and not
attributed to any intrinsic properties of the Bi2Se3 nanostruc-
tures.46 The photocathodic current generated due to H2
evolution for the Bi2Se3/FTO electrode was in the range of
−48.5 to −56.3 μA cm−2.
The LSV curves obtained with the Bi2Se3/FTO electrode are
displayed in Figure 10b. The enhancement in the cathodic
current density was observed for the Bi2Se3/FTO electrode
over the entire potential range tested. The change in current
density in the dark was from −28.0 μA cm−2 at 0 V to −87.7
μA cm−2 at 0.6 V, while under simulated solar light, the current
density was −109.8 μA cm−2 at 0 V and reached up to −300.5
μA cm−2 at 0.6 V.
The Bi2Se3/FTO photocathode stability was also assessed
under dark and sunlight illumination, as displayed in Figure
10c. The electrode showed a very stable response over 600 s of
testing time. The dark current was very stable, while a noisy
photocurrent was observed due to H2 evolution and
accumulation of H2 bubbles at the surfaces in the cases of
both of the electrodes.47 A small decrease in photocurrent
response was observed over time, which could be recovered
once the solution was stirred to remove the H2 bubbles from
the electrode surface. Thus, the Bi2Se3/FTO photocathode
acted as an efficient water reduction catalyst in the neutral
sodium sulfate solution and could be a promising candidate for
cathodic water-splitting applications.
Figure 10. (a) Chronoamperometric measurements with the Bi2Se3/
FTO electrode at OCP using chopping 1 sun simulated illumination,
(b) LSV curves (vs SHE) with the Bi2Se3/FTO electrode in dark and
under continuous 1 sun simulated illumination, and (c) chronoam-
perometric stability measurements with Bi2Se3/FTO at OCP in dark
and under continuous 1 sun simulated illumination.
a reducing agent, and the decomposition of the complex in the
presence of TOP was studied by UV−vis spectroscopy and p-
XRD. When 1-ODE or oleylamine was used as a dispersion
medium, Bi2Se3 nanosheets were formed exclusively. Oleyl-
amine has a dual role, i.e., it acts as a capping agent and
initiates the degradation of the complex. XPS analysis indicated
that the handling of the samples in normal atmospheric
conditions and the use of solvents such as methanol and
acetone are probably responsible for the surface oxidation of
both materials. The height profiles by AFM suggested that
Bi2Se3 nanosheets were composed of almost 3−4 layers,
whereas Bi nanosheets were composed of 9−10 layers.
Likewise, the decomposition of the precursor in the vapor
phase resulted in the formation of Bi2Se3 films. The
stoichiometry of the thin films, as determined by EDX, varies
significantly (from selenium-rich to selenium-deficient) with a
change in temperature. The crystallites’ size also increases with
an increase in temperature; however, the films showed poor
adherence to the substrate and nonuniformity.
CONCLUSIONS
■
A facile synthetic route was used to prepare a mono-
selenobenzoate complex of bismuth (tris(selenobenzoato)-
bismuth(III)) at room temperature. The synthesized molecular
precursor was used for the preparation of Bi or Bi2Se3
nanosheets by the colloidal method. The reactions were
performed at relatively mild reaction conditions. The precursor
decomposes at room temperature in the presence of
oleylamine or in TOP, yielding amorphous Bi2Se3 or Bi,
respectively. Therefore, a range of temperatures can be used to
prepare the desired product. It was observed that TOP acted as
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Bi2Se3 nanosheets showed high activity for electro- and
photoelectrochemical water splitting. The OER and HER
catalytic performances indicate overpotentials of 385 mV at 10
mA cm−2 and 220 mV, with Tafel slopes of 122 and 178 mV
dec−1, respectively. The electrodes indicated a highly stable
performance up to 2000 cycles of study and over 24 h of
chronoamperometric study. In comparison, Bi showed a much
lower OER activity, requiring an overpotential of 506 mV to
achieve a current density of 10 mA cm−2 with a Tafel slope of
175 mV dec−1. However, the HER performance was slightly
better than Bi2Se3 nanosheets, indicating an overpotential of
214 mV at 10 mA cm−2 with a Tafel slope of 158 mV dec−1.
Similarly, Bi2Se3 nanosheets also showed significant photo-
electrochemical catalytic properties for water splitting,
indicating the material’s high potential as a cheap source for
the production of hydrogen from water.
Sanket Bhoyate − Department of Chemistry, Pittsburg State
University, Pittsburg, Kansas 66762, United States
Ram K. Gupta − Department of Chemistry, Pittsburg State
University, Pittsburg, Kansas 66762, United States;
orcid.org/0000-0001-5355-3897
Muhammad Sher − Department of Chemistry, Allama Iqbal
Open University, Islamabad 44000, Pakistan
Javeed Akhtar − Department of Chemistry, Materials
Laboratory, Mirpur University of Science & Technology
(MUST), Mirpur 10250, AJK, Pakistan; orcid.org/0000-
0001-5938-3315
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02668
Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
■
ACKNOWLEDGMENTS
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02668.
The authors thank the National Research Foundation (NRF)
South African Research Chairs Initiative (SARChI) program
for financial support. M.D.K. also thanks for funding from the
European Union’s Horizon 2020 research and innovation
programme under the Marie Skłodowska-Curie grant agree-
ment No. 847413 for funding. Scientific work published as part
of an international co-financed project founded from the
programme of the Minister of Science and Higher Education
entitled "PMW" in the years 2020 - 2024; agreement no. 5005/
H2020-MSCA-COFUND/2019/2.
Electrochemical water splitting; photoelectrocatalytic
(PEC) water splitting; stepwise illustration of the
proposed decomposition pathway of the tris-
(selenobenzoato)bismuth(III) complex; Thermogravi-
metric analysis; TEM images showing stacking, buckling,
and folding of the Bi2Se3 nanosheets; Survey scan of
bismuth selenide; AFM image showing height profiles of
the Bi2Se3 nanosheets at different points; UV spectra of
the complex, before and after addition of TOP; p-XRD
pattern of Bi prepared by decomposition of precursor at
room temperature; SEM images of bismuth at different
magnifications; p-XRD, SEM, EDX and mapping of
Bi2Se3 thin fims; OER and HER polarization curves for
Ni foam; comparison of OER performance of Bi2Se3
nanosheets with previously reported nonprecious
electrocatalysts (PDF)
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AUTHOR INFORMATION
■
Corresponding Authors
Malik Dilshad Khan − Institute of Physical Chemistry, Polish
Academy of Sciences, 01-224 Warsaw, Poland; Department
of Chemistry, University of Zululand, Kwa-Dlangezwa 3880,
South Africa; Email: malikdilshad@hotmail.com
Neerish Revaprasadu − Department of Chemistry, University
of Zululand, Kwa-Dlangezwa 3880, South Africa;
orcid.org/0000-0001-5730-1232;
Email: RevaprasaduN@unizulu.ac.za
Authors
Shumaila Razzaque − Key Laboratory of Material Chemistry
for Energy Conversion and Storage, Ministry of Education,
School of Chemistry and Chemical Engineering, Huazhong
University of Science and Technology, Wuhan 430074, China
Muhammad Aamir − Department of Chemistry, Materials
Laboratory, Mirpur University of Science & Technology
(MUST), Mirpur 10250, AJK, Pakistan; orcid.org/0000-
0002-2386-3090
Manzar Sohail − Department of Chemistry, School of Natural
Sciences, National University of Science and Technology,
Islamabad 46000, Pakistan
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