One pot solvent assisted syntheses of Ag3SbS3nanocrystals and exploring their phase dependent electrochemical behavior toward oxygen reduction reaction and visible light induced methanol oxidation reaction

Article abstract of DOI:10.1039/d0dt01012j

A huge variety of silver based ternary sulfide semiconductors (SCs) have been considered for the sustainable advancement of renewable energy sources. Herein, we have synthesized two important classes of newly emerging semiconductor nanocrystals (NCs) Ag3SbS3 (SAS), i.e. hexagonal and monoclinic by simply tuning the solvent polarity, of which the second one has been synthesized in a phase pure NC for the first time by the thermal decomposition of silver and antimony based dithiocarbamate (～N-CS2-M) complexes. Interestingly, these two systems exhibit two different semiconducting (SC) properties and band gaps; hexagonal SAS has a p type (Eg ～ 1.65 eV) whereas monoclinic SAS has an n type (Eg ～ 2.1 eV) character. For the first time ever we have designed a reducing working electrode (i.e. cathode) by modifying the rotating disc electrode (RDE) with hexagonal SAS that exhibits excellent electrochemical oxygen reduction reaction (ORR) activity (Eonset = 1.09 V vs. RHE and average number of electron transfer: 3.89) comparable to that of the highly expensive Pt/C (Eonset = 0.88 V vs. RHE and average number of electron transfer: 3.92). Density functional theory (DFT) investigation confirms the corroborations of experimental data with theoretical implications. In addition, the electrode fabricated from monoclinic SAS acts as an efficient photoanode which exhibits higher photoelectrochemical (PEC) methanol oxidation reaction (MOR) activity under illumination in alkaline medium compared to that of standard TiO2 grown on an indium tin oxide (ITO) coated glass slide. On illumination, the relative photocurrent density at the onset potential has been obtained to be 845 which is a very significant experimental output with respect to any other TiO2 or Pt?TiO2 based photocatalysts for this application. The physicochemical stability and reusability of both materials were supported by 50 hours of extended electrochemical chronoamperometric measurements and powder XRD and the TEM analyses after electrocatalysis. This study explores a possible pathway for designing simple and less expensive but catalytically efficient silver based ternary sulfide NC systems for developing an SC material to reduce the energy crisis in the near future.

Full text of DOI:10.1039/d0dt01012j

View Article Online
View Journal
Dalton
Transactions
An international journal of inorganic chemistry
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: J. Satra, U. K.
Ghorui, P. Mondal, G. R. Bhadu and B. Adhikary, Dalton Trans., 2020, DOI: 10.1039/D0DT01012J.
Volume 46
Number 10
14 March 2017
Pages 3073-3412
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Dalton
Transactions
An international journal of inorganic chemistry
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
rsc.li/dalton
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
ISSN 0306-0012
COMMUNICATION
Douglas W. Stephen et al.
shall the Royal Society of Chemistry be held responsible for any errors
N-Heterocyclic carbene stabilized parent sulfenyl, selenenyl,
and tellurenyl cations (XH+, X = S, Se, Te)
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
rsc.li/dalton

Page 1 of 43
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT01012J
One pot solvent assisted syntheses of Ag3SbS3 nanocrystals and exploring their phase
dependent electrochemical behavior toward oxygen reduction reaction and visible light
induced methanol oxidation reaction
Jit Satra,a Uday Kumar Ghorui,a Papri Mondal,a Gopala Ram Bhadu,b Bibhutosh
Adhikary*a
*
Corresponding authors
a
Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur,
Howrah 711 103, West Bengal, India
Email: bibhutosh@chem.iiests.ac.in
b
Department of Analytical and Environmental Science Division and Centralized Instrument
Facility, Gijubhai Badheka Marg, Bhavnagar 364021, Gujarat, India
1

Dalton Transactions
Page 2 of 43
View Article Online
DOI: 10.1039/D0DT01012J
Abstract
A huge variety of silver based ternary sulfide semiconductors (SCs) have been considered for the
sustainable advancement of renewable energy source. Herein, we have synthesized two
important classes of newly emerging semiconductor nanocrystal (NC) Ag3SbS3 (SAS) i.e.
hexagonal and monoclinic by simply tuning the solvent polarity, of which the second one have
been synthesized in a phase pure NC for the first time by the thermal decomposition of silver and
antimony based dithiocarbamate (~N-CS2-M) complexes. Interestingly, these two systems
exhibit two different semiconducting (SC) properties and band gaps; hexagonal SAS has p type
(Eg~1.65 eV) whereas monoclinic SAS has n type (Eg~2.1 eV) character. For the first time ever
we have designed a reducing working electrode (i.e. cathode) by modifying rotating disc
electrode (RDE) with hexagonal SAS which exhibits excellent electrochemical oxygen reduction
reaction (ORR) activity (Eonset=1.09 V Vs. RHE and average number of electron transfer: 3.89)
comparable to the highly expensive Pt/C (Eonset=0.88 V Vs. RHE and average number of electron
transfer: 3.92). Density Functional Theory (DFT) investigation confirms the corroborations of
experimental data with theoretical implications. In addition, the electrode fabricated by
monoclinic SAS acts as an efficient photoanode which performs higher photoelectrochemical
(PEC) methanol oxidation reaction (MOR) activity under illumination in alkaline medium
compared to standard TiO2 grown on indium tin oxide (ITO) coated glass slide. On illumination
the relative photocurrent density at onset potential has been obtained to be 845 which is a very
significant experimental output with respect to any other TiO2 or Pt@TiO2 based photocatalyst
for this application. The physiochemical stability and reusability of both materials were
supported by fifty hours of extended electrochemical chronoamperometric measurements and
powder XRD, TEM analyses after electrocatalysis. This study explores a possible pathway for
2

Page 3 of 43
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT01012J
designing a simple and less expensive but catalytically efficient silver based ternary sulfide NC
systems for developing an SC material to reduce the energy crisis in near future.
Introduction
Designing efficient, low cost and stable hybrid catalysts is vital for the development of futuristic
fuel cells. In this context cathodic electrochemical oxygen reduction reaction (ORR) and anodic
photo oxidation of alcohol as fuel are two critical reactions that explore the conclusive efficiency
of a fuel cell in terms of low over potential and high output of energy.1-12 In case of ORR,
Platinum (Pt) has been widely used as a preferred electrocatalyst in a liquid feed fuel cell
(LFFC).13-14 Although ultrafine Pt nanoaparticles having higher surface energies leads to fast
aggregation that suffers from poor reduction, retard kinetics and instability of the catalysts.15-17
Most importantly the very high cost of Pt based electrocatalyst has a negative impact on
commercializing fuel cells. But, there was always an urge to replace Pt with an efficient
alternative candidate cheaper than it. Therefore vast researches are going on with an aim to
reduce the use of platinum in catalytic processes without losing their activity and stability. Very
recently two dimensional, three dimensional binary and ternary metal chalcogenides like WSe2,
CoS2, MoS2, MoSe2, NiCo2S4, FeNiS2, CuCoS4, CoInS4 etc. have attracted more attention due to
their electrochemical properties having facile synthetic procedure, active surface edges, basal
planes, high surface area and especially low cost.18-28 Not only that, mixed valence metal sulfides
have shown notable stability in aqueous alkaline medium and better redox chemistry than metal
oxides. We must consider that ternary sulfide has higher redox reaction sites than binary due to
its closely packed array of S2- throughout crystal structure.29-31On the other hand
photoelectrochemical (PEC) organic substrate oxidation reaction leads to hydrogen generation
3

Dalton Transactions
Page 4 of 43
View Article Online
DOI: 10.1039/D0DT01012J
through water splitting. For example PEC methanol oxidation reaction (MOR) can replace water
oxidation for source of electrons to reduce proton (2H+ + 2e = H2) as it inhibits kinetic
limitations for water oxidation. However it has been challenging to carry out MOR by
photogenerated charge carriers in terms of activity and stability. MOR studies on widely used
semiconductor TiO2 have indicated methanol adsorption process followed by the formation of a
CH3O• radical and its subsequent oxidation with a valence band hole.32-34 As TiO2 has a large
band gap, it has been considered to be a poor solar absorber as it shows photocurrent density less
than 1 mA. Though many efforts have been done by coupling TiO2 with noble metal such as Pt
to optimize the enhancement of photocurrent density but it remains a challenge to develop such
semiconductor having convenient band diagram aligned with the redox potential of the desired
PEC reaction and stability. Ag3SbS3 (SAS) is a newly emerging class of semiconductor due to its
narrow band gap and high surface area. SAS has already been used in the field of nonlinear
optics, piezoelectricity, silver ion conductor, solar absorber and photocatalyst for dye
degradation.35-44 Recently, Huang et al. have studied morphology tunable growth of SAS
nanocrystal (NC) by changing solvent polarity and its facet dependent application in PEC water
splitting reaction as well as in photovoltaic field.44 On the other hand Wang et al in one of their
pioneering work had explored a single source precursor approach for synthesizing copper
antimony sulfide with different band gap on changing ligand molecule.45 Further, Sb belongs to
heavy metal elements that does not react with any species suspended in water and form
compound so it has been convenient for us to make use of SAS in aqueous electrolytic solution.41
Moreover, for many applications surface functionalization of NC has been done to improve
electrochemical conductivity as shorter ligands may alter the electronic properties by forming
active centers on the surface and ease the redox process.46,47
4

Page 5 of 43
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT01012J
Here we report for the first time a unique synthetic procedure to develop SAS based
electrocatalyst for ORR and photo induced MOR separately. By simply tuning solvent polarity,
we have synthesized the same stoichiometric composition of SAS nanomaterial but different
crystal systems (hexagonal and monoclinic) and semiconducting (SC) properties (p type and n
type). We have explained the whole ORR catalytic process by hexagonal SAS with the
thermodynamical approach by density functional theory (DFT) calculations. By comparing with
commercial Pt/C we have found that our material is slightly superior in terms of onset, half wave
potential, mass activity and durability than Pt/C. Beside this, we have also focused on MOR as a
model alcohol oxidation reaction on the surface of superior monoclinic SAS (n type) NC over
TiO2 under visible light illumination. Due to low band gap, high charge carrier density and
surface functionalized active sites of SAS, diffusion of methanol into catalytic interface and
subsequently oxidation could occur at as low as 20 mM concentration of methanol. This study
might open a strategy in developing semiconductor NC that can replace expensive metal
catalysts for ORR and MOR in fuel cell.
Experimental section
Chemicals and Experimental Methods
Details of materials, physical measurements, device fabrication for PEC-MOR, ORR and
electrochemical characterization for both ORR
and PEC MOR have been illustrated in
supporting information. Triethyl ammonium salt of piperidinedithio carbamate ligand
(C5H10NC{S}S-Et3NH+) (Pip-dtc), Ag(Pip-dtc) and Sb(Pip-dtc)3 complex have been prepared by
previously reported method.48-50
5

Dalton Transactions
Page 6 of 43
View Article Online
DOI: 10.1039/D0DT01012J
Syntheses of SAS NCs from precursor complexes
Hexagonal SAS NCs
Here 1 mmol each Ag(Pip-dtc) (white) and Sb(Pip-dtc)3 (pale yellow) was added to a mixture of
20 mL (60 mmol) of OAm and 30 ml (125 mmol) of Dodecanethiol (DDT) (OAm/DDT=1/1.5)
in a two necked 100 ml RB flask. To homogenize, the mixture was ultrasonicated for 30 minutes.
0
Then it was heated to 100 C for 15 minutes to degas by vacuum pump followed by heated to
0
0
220 C under argon insertion. During the temperature change from 100 C to 200 0C a distinct
colour transformation of reaction mixture was noticed as very pale yellow complex mixture
turned into dark brown followed by reddish brown (Fig. 1). The mixture was then annealed at
220 0C at inert atmosphere for 1h. Then the reaction was cooled to room temperature and stirring
was still continued for another 30 minutes. Nanoparticles were precipitated by adding excess
anhydrous ethanol. The resulting solution was centrifuged (10000 RPM) to separate the product.
The nanoparticles were further dispersed in toluene to remove attached impurities and then it was
re-precipitated by excess ethanol. This purification process was repeated 5 times to get the pure
desired product.
Monoclinic SAS NCs
1 mmol each Ag(Pip-dtc) and Sb(Pip-dtc)3 were loaded in a two necked 100 ml round bottom
(RB) flask in which only 50 ml (150 mmol) of oleylamine (OAm) is added. Here the colour
change follows; very pale yellowish mixture to deep orange and finally reddish orange between
0
0
100 C to 200 C (Fig.1). In this preparation, the remaining reaction as well as purification
procedure was same as mentioned above.
6

Page 7 of 43
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT01012J
100 0C
150 0C
200 0C
150 0C
200 0C
100 0C
Fig. 1. Colour change during the course of reaction for hexagonal(above) and
monoclinic(below).
To study the intrinsic electronic property of the as synthesized NCs in aqueous solution we have
carried out a ligand exchange reaction. At first, 1.0 g of Na2S·9H2O was dissolved in 50 ml of
dimethyl formamide (DMF) solvent and then 100 mg of each NC was dispersed in 100 ml
toluene by ultrasonication. Each time 10 ml of DMF solution was added into organic ligand
toluene solution in every 15 minutes interval and this mixture was kept under ultrasonicated for
5h under room temperature. At last the NCs were purified by centrifugation in toluene followed
by ethanol and the process was repeated 5 times. During the course of ligand exchange the
colour of the dispersed solution was changed from dark brown to black.
3. Results and discussion
The phase pure structure of two different crystalline systems of SAS has been characterized by
powder X-Ray diffraction (XRD) measurement. Fig. 2. (a) and (b) demonstrate the entire sharp
and distinct diffraction peaks of the two different SAS (NCs) which has been confirmed by the
following; (a) JCPDS NO. 21-1173 (for hexagonal) and (b) JCPDS NO. 25-1187 (for
monoclinic). The space group for the hexagonal phase is assigned to R3c(161) and for
monoclinic phase is P21/c(14). The 012 plane for the hexagonal SAS is further designated to
SAED pattern (Fig. 3d) where as for the monoclinic SAS NCs, the 130 plane is appeared at the
7

Dalton Transactions
Page 8 of 43
View Article Online
DOI: 10.1039/D0DT01012J
SAED pattern (Fig. 3j). The phase purity for both the samples has been confirmed by the
observation that no other characteristic peaks of Ag2S, AgSbS2 and Sb2S3 have appeared on
diffraction patterns.
(b)
(a)
Fig. 2. Powder X-ray diffraction pattern of (a) Hexagonal and (b) Monoclinic.
Stoichiometric composition and elemental mapping of both phases are determined by Electron
Dispersive X-ray Analysis (EDX) (Fig. S2). EDX spectra and stoichiometric composition are
shown in Fig. S2. (a) hexagonal (b) monoclinic SAS. Elemental mapping for Ag, Sb and S of
hexagonal SAS is shown in Fig. S2. (c), (d) and (e) and of monoclinic SAS in Fig. S2. (f), (g)
and (h). This study confirms the composition and morphology.
Transmission Electron Microscopic (TEM) and Scanning Electron Microscopic (SEM) images of
hexagonal and monoclinic SAS are shown in Fig. 3. TEM images reveal the high crystallinity
and phase purity having continuous lattice fringes for both the hexagonal prism and monoclinic
ortho-prism like particle. Fig. 3(a) shows low magnification TEM images for hexagonal
prismatic particle and the inset shows particle size distribution (on Gaussian fitting) that reveals
8

Page 9 of 43
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT01012J
the average diameter for the hexagonal prismatic particle is about 100 ± 4 nm. Fig. 3(b) shows
the high magnification image of the particle having clearly visible lattice fringes on it. HRTEM
image of Fig. 3(c) shows inter planar spacing obtained from fringe pattern which is 0.396 nm
(for 012), completely consistent with the standard value of ‘d’ spacing for the corresponding
JCPDS 21-1173. Fig. 3(d) illustrates the SAED pattern for the corresponding 012 plane. SEM
image (Fig. 3e) confirms the shape and morphology obtained from TEM i.e. the hexagonal
prismatic like particle assigning the characteristic planes (inset). The possible atomic
arrangement and the position of crystalline plane (012) in unit cell have been manifested in Fig.
3(f). Similarly Fig. 3(g) shows the particle distribution of monoclinic SAS in low magnification.
The size distribution of the monoclinic SAS particle has been represented by the Gaussian plot at
inset of Fig. 3(g). As the rod shaped particles have also a 2D facet, so we had to calculate the
length along with breadth to explore the size distribution and on doing so we got two distribution
histograms. For the length and breadth the average sizes are ~550 ± 5 nm (inset of Fig. 3g) and
142 ± 2 nm (inset of Fig. 3h) respectively. HRTEM image (Fig. 3i) clearly shows the inter-planar
distance for monoclinic SAS is 0.398 nm (for 130) that are again consistent with the
corresponding JCPDS 25-1187. Fig. 3(j) represents the SAED pattern of the NC indicating the
characteristic crystalline plane (130). The SEM image for the monoclinic NC (Fig. 3k) confirms
the orthoprismatic morphology of particle. Further the structure of unit cell (Fig. 3l) having
crystalline plane (130) has been obtained by combining idea from XRD, TEM and SEM
experiments as earlier. The well-defined crystalline shape and morphology of SAS has been
obtained by changing polarity and composition of solvent (Scheme 1) as obtained from XRD,
TEM and SEM results.
9

Dalton Transactions
Page 10 of 43
View Article Online
DOI: 10.1039/D0DT01012J
(a)
(a)
(b)
(b)
100< ≤105 nm
5 nm
100 nm
(c)
(c)
(d)
(d)
nm
d012 = 4 X 0.396
(012)
0.5 nm
2 nm
(e)
(e)
(f)
(f)
100 nm
10

Page 11 of 43
Dalton Transactions
View Article Online
(a)
(b)
DO1I: 4100.1<039/≤D10D4T401n01m2J
(g)
(h)
540< ≤554 nm
20 nm
100 nm
(c)
(i)
(d)
(j)
d130= 4 X 0.398 nm
(130)
0.5 nm
2 nm
(e)
(k)
(f)
(l)
100 nm
11

Dalton Transactions
Page 12 of 43
View Article Online
DOI: 10.1039/D0DT01012J
Fig. 3. TEM and SEM image analyses for hexagonal and monoclinic SAS. For hexagonal: (a)
TEM image confirms size distribution of quasi spherical particle (inset), (b) High magnification
image, (c) Fringe pattern analysis by HRTEM image confirms the inter planar spacing
(0.396nm), (d) SAED pattern analysis confirms the characteristic plane (012), (e) SEM image of
the corresponding hexagonal prism like particle and its equivalent model assigning possible
crystal facets (inset), (f) (012) crystal facet (orange) and possible atomic arrangement in unit cell
for the hexagonal system [the atoms are assigned as; Ag(blue), Sb(bottle green) and S(red)]. For
monoclinic: (g) TEM image confirms length size distribution of rod shaped particle (inset), (h)
High magnification image confirms the breadth size distribution of particle, (i) HRTEM image
confirms the inter planar spacing (0.398nm), (j) SAED pattern analysis confirms the
characteristic plane (130), (k) SEM image of the corresponding monoclinic ortho-prism particle
and its equivalent model assigning possible crystal facets, (l) Possible atomic arrangement and
(130) crystal facet (purple) in unit cell for the monoclinic system [the atoms are assigned as;
Ag(blue), Sb(bottle green) and S(red)].
In the solvent composition like the mixture of OAm and DDT in a certain ratio (here OAm/DDT
=1/1.5)51 hexagonal SAS was obtained. It has been well studied that DDT was usually used as
sulfur (S) source and stabilizer for the growth of metal sulfide nanocrystals.52 On the other hand
OAm shows it’s versatility as reducing agent, stabilizer and capping agent.53 In the hexagonal
unit cell, all the three S atoms bonded with Sb gets outward of the wall of the lattice which might
be stabilized by the dipole interaction of –SH of DDT and Sb-S bond while OAm worked as a
capping agent only. In case of monoclinic system it gets orthoprism (h>k, 0) [here (130)] like
structure (Fig. 3k). The (130) facet having all the three Sb-S bonds are aligned with the plane but
Ag-S bond is off the plane outward to wall of the lattice. As the dissociation of the –NH2 of
12

Page 13 of 43
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT01012J
OAm on {010} [crystallographic equivalent of 130 plane] containing Ag-S is faster than DDT so
here OAm acted as stabilizer as well as a capping agent for the synthesis of monoclinic NCs.54
The syntheses of NCs and ligand exchange process are illustrated in Scheme 1.
Scheme 1. Schematic manifestation of the preparation of (a) hexagonal SAS and (b) monoclinic
SAS NCs and corresponding ligand exchange process.
High-resolution XPS spectra of Ag 3d, Sb 3d, and S 2p for the as-prepared SAS samples are
obtained using C 1s as the reference at 284.6 eV (Fig. 4). Binding energy peaks of Ag 3d5/2 and
Ag 3d3/2 in as-prepared hexagonal SAS sample are at 367.72 eV (Ag 3d5/2) and 373.74 eV (Ag
3d3/2) (Fig. 4a) and for monoclinic SAS the peaks are at 367.91 eV (Ag 3d5/2) and 373.94 eV
13

Dalton Transactions
Page 14 of 43
View Article Online
DOI: 10.1039/D0DT01012J
(Ag 3d3/2) (Fig. 4d). Binding energy peaks of Sb 3d were located at 529.06-530.40 (Sb 3d5/2) and
538.39-539.65 eV (Sb 3d3/2) for hexagonal SAS (Fig. 4b) and at 529.17-530.53 eV (Sb 3d5/2) and
538.53-539.80 eV (Sb 3d3/2) (Fig. 4e) for monoclinic SAS respectively. For both the cases the
binding energies of Sb are slightly higher than that of Sb in Sb2S3,55 indicating a strong bonding
of Sb−S in both SAS samples. There are no peaks at 531−532 eV confirms there are no such
oxidized SbOx55 and adsorbed oxygen at the surface of samples which further proves the phase
purity of the samples. The high-resolution spectra of S 2p show two peaks for S at 161.24-161.87
eV (for S 2p3/2) and 163.09 eV (for S 2p1/2) for hexagonal SAS (Fig. 4c) and at 161.44-162.09
eV (for S 2p3/2) and 163.20 eV (for S 2p1/2) (Fig. 4f) that are attributed to a single doublet from
S−Sb bonds.56 The XPS peaks reside between 162.2-163.8 eV are ascribed to metal-sulfur bond
having less coordinating divalent sulfide (S-2) ions at the surface. This result is reflected on the
binding energy of Sb which is slightly lower in hexagonal than monoclinic SAS and this is due
to the stabilization process of off-pane Sb–S in 012 facet by –S of DDT.57 On the other hand the
binding energies of all the elements are higher in monoclinic than hexagonal due to its crystalline
saturation as obtained from the crystal structure. Here Ag-S bond might get stabilized through
polar interaction by –NH2 of OAm only. Most importantly, all XPS results confirm the presence
of following oxidation states: Ag(I), Sb(III) and S(II) which subsequently proves the
stoichiometric composition of SAS.
14

Page 15 of 43
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT01012J
(a)
(b)
(d)
(c)
(e)
(f)
Raw data
15

Dalton Transactions
Page 16 of 43
View Article Online
DOI: 10.1039/D0DT01012J
Fig. 4. XPS spectra of hexagonal and monoclinic SAS. Figure (a), (b) and (c) are for hexagonal
SAS. (d), (e) and (f) are for monoclinic SAS. Binding energy curves for all data are fitted with
Gaussian-Lorentzian function.
To confirm the arrangement of atoms in the crystal, the Raman spectroscopic measurement for
both the phases has been done and is shown in Fig. S4 (a) hexagonal and (b) monoclinic. In
addition the detailed explanation has also been depicted in supporting information. Also the
surface area characterized by nitrogen adsorption/desorption analysis has been demonstrated and
explained in supporting information. (Fig. S5 (a) hexagonal and (b) monoclinic). The BET
surface area obtained as follows: 230.10 m2g-1 for hexagonal SAS and 252.85 m2g-1 for
monoclinic SAS.
On the basis of the excellent physical properties of both materials, at first we have performed a
solution based electrocatalytic ORR experiment to explore their response individually and we
have obtained that hexagonal SAS does have a notable catalytic activity toward ORR but
monoclinic SAS does not (Fig. 5a). Both ORR plots are corresponding to the disk current at 1600
RPM respectively. Importantly, further evaluating the retarding MeOH crossover effect on ORR
by adding 3(M) MeOH into O2 saturated 0.1(M) KOH solution, we have distinctly observed that
hexagonal SAS has significant ORR selectivity and MeOH tolerance much higher than
commercial Pt/C but here the observation of responses of monoclinic SAS (Fig. 5b) toward
MeOH surprises us most and leads us to make use of this material in MOR. These interesting
results have driven us to explore the phase dependent catalytic activities for two different energy
applications.
16

Dalton Transactions
Page 17 of(4a3)
View Article Online
DOI: 10.1039/D0DT01012J
(b)
Fig. 5. (a) Comparative ORR activity study for both hexagonal and monoclinic SAS. (b)
Chronoamperometric measurements in O2 saturated 0.1(M) KOH for different catalyst before
and after addition of 3(M) MeOH.
Then we have evaluated the electronic properties of both SAS semiconductor NCs which depend
upon the band edge position and band gap. Here conduction band edge is associated with the Ag
5s orbital whereas the valence band edge is produced by Sb 5p and S 3p orbital.55 The valence
band energy maxima is associated with electrons having vector k=0. The transitions between
valence band maxima to conduction band minima are indirect in nature whereas the band edges
reside in other places of Brillouin zone.58 To obtain the semiconducting properties, probable
band position and band gap we have performed electrochemical Mott-schottky analyses59-61
combining with electronic spectra of both the materials. Electronic spectra are shown in Fig. S6.
(a) hexagonal and (b) monoclinic SAS. Their band gap has been obtained to be 1.65 eV for
hexagonal and 2.1 eV for monoclinic SAS. The absorption edge for monoclinic resides around
600 nm so deep red colour of monoclinic SAS was observed but in case of hexagonal some blue
shift occurs due to decrease in Sb content (obtained from EDX analysis) and so the colour has
been obtained as deep brown. This kind of phase dependent band intensity variation occurs due
17

Dalton Transactions
Page 18 of 43
View Article Online
DOI: 10.1039/D0DT01012J
to electronic delocalization and covalent character.60 The large orbital of Sb leads to less orbital
overlapping with S and also the electronegativity difference between Sb and S define the less
covalent and more ionic property of SAS NCs than any other mixed-valence ternary transition
metal sulfide NCs. The typical Mott-schottky analyses are shown in Fig. S7 ((a) hexagonal, (b)
monoclinic and (c) comparative demonstration of band energy diagram). The negative slope of
the plot (a) indicates that the hexagonal SAS has p type and the positive slope of the plot (b)
confirms monoclinic SAS has n type semiconducting property. The flat band potentials (EFB) are
as follows: 2.65 V for p type hexagonal SAS which corresponds to valence band whereas -1.02
V Vs. RHE for n type monoclinic SAS which refers to conduction band by convention. It is to be
noted that in case of light induced MOR low band gap of monoclinic SAS favors PEC reaction
than TiO2 having large band gap. On the other hand, though hexagonal SAS has lower band gap
than monoclinic SAS but it does not help in MOR because the charge carrier helps in oxidation
is hole which belongs to valence band and the valence band maxima for hexagonal SAS resides
at 2.65V, that is far away from the onset potential of MOR which starts from 0.23V vs RHE. The
same argument might be implanted for the non-participation of monoclinic SAS for ORR. Here
the responsible charge carrier is electron in the conduction band and the conduction band minima
for monoclinic SAS is at -1.02V which is quite distant from the potential of ORR (1.23V) and
hence monoclinic SAS does not favor ORR. Moreover photoluminescence (PL) lifetime study
(Fig. 6) reveals that the probability of electron hole pair recombination is very much less in case
of monoclinic than hexagonal SAS after visible light excitation. Lifetime parameter investigation
(in picosecond) on incorporating bi-functional decay relation: F(t)= A1 exp(-t/ + A2 exp(-
t/+shows that the band edge decay ( has been influenced higher than the decay in deep
18

Page 19 of 43
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT01012J
trap states (he value of for hexagonal and monoclinic SAS are 7.95 and 2356.04 ps
whereas the value of for the same are 1.25 and 53.21 ps.
(a)
(b)
(d)
(c)
Fig. 6. Electronic and corresponding PL spectra for (a) Hexagonal and (c) Monoclinic. Life time
measurements for the same (b) and (d).
19

Dalton Transactions
Page 20 of 43
View Article Online
DOI: 10.1039/D0DT01012J
Fourier Transform Infra-Red (FTIR) spectra of individual organic ligands along with the NCs
before and after exchange of ligands are shown in Fig. S8.
Nyquist plots and the corresponding Bode plots have been obtained from Electrochemical
Impedence Spectroscopy (EIS) measurements which are shown in Fig. S9 and S10. Fig. S9 (a)
represents comparative impedance study for ORR between commercial Pt/C and hexagonal SAS,
(b) shows corresponding Bode plot. The same sequence is maintained at for MOR also (Fig.
S10). Here the absence of higher frequency semicircle and the presence of only lower frequency
semicircle confirm, both ORR and MOR are kinetically controlled process but not the mass
transfer process.64 The data are tabulated in Table S1. The values of solution resistance (RS) are
near about equal as all the EIS were obtained in 0.1 (M) KOH and the small RS value indicate
less ohmic loss in electrolyte. The hexagonal SAS exhibits slightly lower charge transfer
resistance (Rct) than Pt/C which means faster charge transfer happens on semiconducting surface
than on the metallic surface for ORR.65 Similar result is reflected for MOR between monoclinic
SAS and TiO2, but here the difference in Rct is much higher than ORR. Also the higher
frequency for both the SAS NCs in Bode plots indicate the lifetime of electrons is shorter i.e.
faster charge transfer kinetic has been taken place than in case of Pt/C and TiO2.66
Before get into the whole electrochemical procedure for ORR and MOR in KOH medium, we
have tested and purified the commercial KOH. The procedure of testing of trace elements in
KOH and the purification process has been depicted in supporting information. The amount of
trace element obtained by Inductively Coupled Plasma Mass Spectroscopy (ICPMS) has been
tabulated in Table S2. The ORR polarization curves in commercial and pure KOH have been
shown in Fig. S11.
20

Page 21 of 43
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT01012J
At first we have performed a cyclic voltammetry (CV) for ORR within the potential range 0-1.25
V Vs. RHE under saturated N2 and then O2 atmosphere in 0.1(M) KOH (Fig. S12). It has been
shown that hexagonal SAS is very much prone to response to ORR. The comparative study
between hexagonal SAS and commercial Pt/C is shown in Fig. 7(a) and the corresponding bar
diagram for onset potential (Eonset) and half wave potential (E1/2) are shown in Fig. 7(b). The
values of Eonset and E1/2 for SAS are 1.09 and 0.86 V whereas for Pt/C, 0.88 and 0.73 V Vs. RHE
respectively and this superiority of our material encouraged us to perform ORR. LSV curves in
Fig. 7(c) illustrate the limiting rotating ring disc electrode (RRDE) speed (1625 RPM) for
maximum current density and also the first order kinetics has been obtained from the
corresponding linear Koutecky-Levich (K-L) plots67,68 (inset in Fig. 7c) for 0.4, 0.5, 0.6 and 0.7
V Vs. RHE. Solving the K-L equation, number of electron (n) transfer has been calculated as
follows: 3.85, 3.88, 3.90 and 3.94 for SAS (Fig. 7d) and 3.90, 3.90, 3.93 and 3.95 for Pt/C at the
corresponding potentials (Fig. S13). The kinetics of the ORR has been defined by Tafel slopes69-
71 (Fig. 7e) which indicates that the ease of adsorption and activation of O2 on SAS over Pt/C.67
The result was obtained to be 50.30 and 58.52 mV dec-1 for hexagonal SAS and commercial Pt/C
respectively. Moreover ECSA normalized specific activity and mass activity at 0.75 V Vs. RHE
for both hexagonal SAS and commercial Pt/C are compared (Fig. 7f) and again the hexagonal
SAS is far superior than Pt/C. Data are tabulated in Table S3.
21

Page 22 of 43
Dalton Transactions(b)
(a)
View Article Online
DOI: 10.1039/D0DT01012J
(d)
(c)
(e)
(f)
22

Page 23 of 43
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT01012J
Fig. 7. (a) ORR polarization curve for commercial Pt/C and hexagonal SAS. (b) Onset potential
and half wave potential bar diagram for materials. (c) ORR polarization curve for different
rotational speed and K-L plot (inset). (d) The number of electron transferred bar diagram within
potential range 0.3-0.8 V Vs. RHE. (e) Tafel slopes for materials. (f) Specific and mass activity
bar diagram for materials.
Comparing between ring current and disk current along with percentage of H2O2 yield Vs. n are
shown in Fig. 8(a) and (b). It is shown that the H2O2 (%) is lower for hexagonal SAS (12.1 %)
than Pt/C (14.6 %) as the number of electron transferred approaches to near four. It has been
confirmed that ORR leads to higher extent of water formation in case of our material than Pt/C.
In order to investigate the long term stability and reproducibility; chronoamperometric
measurement and CV with 5000 cycles have been allowed to run. Fig. 8(c) shows that the
relative current density reaches to 69% for SAS whereas it is 55% for commercial Pt/C after the
whole course of chronoamperometric study for 50 hours. Fig. 8(d) shows reproducible nature of
ORR activity by our material and that has been persistent from 1st cycle to 5000th cycle. Further
to evaluate stability and reusability, we have characterized our materials by XRD and TEM
analyses after electrochemical experiments (Fig. S1a and S3a) indicating there are no substantial
changes of phase as well as morphology which eventually support the stability and reusability for
the concerned application. Durability test was also performed for Pt/C (Fig. S14) which also
supports superior durability of our material. Further we have evaluated electrochemical active
surface area (ECSA) from double layer charge capacitance (Cdl) by performing CV in O2
saturated 1(M) KOH at different scan rates ranging from (20-100 mV/sec) for both hexagonal
SAS and commercial Pt/C72 (Fig. S15). The capacitive currents were measured at 1.2 V Vs. RHE
and the corresponding linear plots are shown in Fig. 8(e). We can see that Cdl gets increased by
23

Dalton Transactions
Page 24 of 43
View Article Online
DOI: 10.1039/D0DT01012J
hexagonal SAS compared to Pt/C which reflects in the value of ECSA. Interestingly we have
found a linear correlation between BET, Cdl, and ECSA (Table S4) as expected and has been
demonstrated by the bar diagram shown in Fig. 8(f). Loss of normalized ECSA(%) for both
hexagonal SAS and Pt/C have been shown in Fig. S16. Comparative literature study for complete
electrochemical ORR by various ternary metal sulfides NC is illustrated in Table S5.
(b)
(a)
(c)
(d)
24

Page 25 of 43
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT01012J
(f)
(e)
Fig. 8. (a) Ring current and disk current density obtained from RRDE test. (b) H2O2 % and
number of electron transferred for hexagonal SAS and Pt/C. (c) Comparative
chronoamperometric stability measurement for fifty hours. (d) Durability test by LSV curves for
1st and after 5000th cycle for ORR on hexagonal SAS. (e) The capacitive currents are measured
under different scan rates (20, 40, 60, 80, 100 mV.sec-1). (f) Comparative bar diagram for BET
surface area, Cdl and ECSA for corresponding material.
To have a clear theoretical insight about the ORR activity of hexagonal SAS we have performed
density functional theory (DFT) calculation for the overall reaction pathways on the catalytically
active site (Fig. 9). The whole computational methods are described in supporting information.
From the HRTEM and SAED pattern analysis; (012) plane has been constructed for the SAS
slab. The computational hydrogen electrode (CHE) method has been employed to predict the
overall electrode process i.e. potential limiting step and theoretical limiting potential. Here from
the energy profile diagram the limiting step might be OOH* formation (1st step) and the
associated free energy (G) have been obtained to be -1.0 eV. The limiting electrode potential
25

Dalton Transactions
Page 26 of 43
View Article Online
DOI: 10.1039/D0DT01012J
that keeps all the elementary reaction steps exothermic is called working potential (U) of
fabricated electrode by the corresponding catalyst.24 Here above 1.0 V the limiting step gets
endothermic and this is close consistent with the experimental onset potential (1.09 V) which is
higher than the theoretical working potential (0.73 V) for Pt/Cu(100). Moreover the calculated
binding energy for O2 on the catalytic surface has been obtained as -0.85 eV that is quite close to
that of -1.10 eV on Pt catalyst73. Further the reduction of OOH* to O* and not to OOH- has been
proven by the thermodynamical investigation as OOH* to OOH- is endothermic by 0.22 eV
whereas OOH* to O* is exothermic by -0.25 eV. The possible adsorption sites for intermediate
species (OOH*, O* and OH*) are shown in Fig. 9(a), (b) and (c) after optimization. The
corresponding relative free energy diagram for different intermediate species at U= 0, U= 1.0
and U=1.23 V have been shown in Fig. 9 (d), (e), (f) and (g).
(b)
(c)
(a)
26

Page 27 of 43
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT01012J
(e)
(d)
(g)
(f)
Fig. 9. Side view for the adsorption of the intermediate species (a) OOH*, (b) O*, (c) OH* on the
012 plane of hexagonal SAS. Comparison of the free energy (G) diagram of ORR on hexagonal
SAS (012) plane at U=0 (d, e), U=1.0 (f) U=1.23 V (g).
TiO2 has widely been used photocatalysts due to its non-toxicity, low cost, moderate efficiency
and stability. TiO2-P25 consists of anatase (75%) and rutile (25%) phases which are both
photoactive and due to lower conduction band edge of rutile than anatase, electron might get
transferred from anatase to rutile to reduce the electron hole pair recombination possibility. But
the main problem for TiO2 is its large bad gap which restricts TiO2 for being used as a
27

Dalton Transactions
Page 28 of 43
View Article Online
DOI: 10.1039/D0DT01012J
photocatlyst under visible light.74 On the other hand to suppress the surface poisoning, neutral
CO removal is required which is formed during MOR along with other unwanted oxygenated
species. The energy required for desorption of neutral CO by dissociative fashion is about 2.2-
3.0 eV which corresponds to visible to near UV light75. In our case, forward current (If) to
backward current (Ib) ratio (If/Ib) is obtained to be 8.30 which is satisfactorily high for removing
CO and other oxygenated species. Moreover the monoclinic SAS semiconductor NC exhibits
exceptional photosensitivity and stability in MeOH under visible light illumination and this is
mainly due to its convenient band gap as well as its high charge carrier. From the slope of the
Mott-schottky plot, using the equation; Slope = 2/eN, [where, dielectric constant of NC)
= 17.17, permittivity of vacuum) = 8.854 X 10-12 m-3.kg-1.S4.A2, e (electronic charge) = 1.6
X 10-19 C, N = charge carrier] the charge carrier has been obtained as 1.23 X 1017 and this is
quite high value of charge carrier that could accelerate a PEC reaction efficiently. We have
addressed the entire possible important determining factors by doing comparative study of
electro-oxidation of MeOH under visible light irradiation on the surface of monoclinic SAS and
TiO2 separately. At first we have measured CV in the Ar saturated mixture of electrolyte
containing 0.1 (M) KOH and 0.1 (M) MeOH to make sure about the oxidative response of our
material. Fig. 10(a) shows two distinct peaks at around 0.6 V which might correspond to MeOH
adsorption on the surface and the higher peak starts at 0.9 V continues to 1.1 V due to oxidation
of MeOH. After evaluation of oxidative response we have compared our material with most
extensively used semiconductor TiO2 by performing CV under dark (Fig. 10b). We can see our
material produces ~10 times higher current density than TiO2 (1.8/0.18) under dark. Thereafter
we have evaluated the whole MOR further under illumination. Compared to dark, current density
increases about 25 times under illumination of visible light which is shown in Fig. 10(c). Then
28

Page 29 of 43
Dalton Transactions
View Article Online
DOI: 10.1039/D0DT01012J
from I-V curve (Fig. 10d) under chopped light illumination for MOR, nearly 20 times higher
photocurrent density was obtained in case of our material than mesoporous TiO2. Upon
illumination of visible light onto the electrode surface, charge separation occurs between valence
band (VB) and conduction band (CB). The hole generated in VB is responsible for PEC
oxidation of methanol whereas electron resides on CB moves through the circuit and produces
photocurrent. From the onset potential, increase in PEC potential eases the charge separation
which continuously facilitates the overall redox reaction and finally whole procedure resulted in
substantial increase of photocurrent density. Enhancement of relative photocurrent density under
light irradiation [(ILight-IDark/IDark) × 100] has been the key determining factor which has been
obtained to be 845 at onset potential (0.73 V Vs. RHE) for SAS which are much higher than any
other TiO2 based catalyst used in MOR so far. Moreover the position of valence band maxima of
a semiconductor which is responsible for electrochemical oxidation of any organic substance,
resides at 1.08 V (Vs. RHE) for SAS whereas it resides at 3.0 V (Vs. RHE) for TiO2 (at pH 13)
and the conventional onset potential for MOR starts at 0.23 V Vs. RHE (Fig. S7c). The most
probable product of whole oxidation process might be formaldehyde because the FTIR
spectroscopic titration (liquid) has been done throughout one hour at a definite time interval
which is shown in Fig. S17. The corresponding peak identification has been illustrated in
supporting information. This confirms the oxidation reaction might get ceased with the formation
of aldehyde within this potential window. The characteristic peak for hydroxyl group of
methanol (-OH) gets lowered with time as expected. The relative study in terms of stability and
durability (Fig. 10e) between our material and TiO2 by chronoamperometric measurements76,77
(at 0.5 V Vs. RHE) reveals that relative photocurrent density becomes 52% for SAS and 7% for
TiO2 after 50 hour experiment which confirms that our material can carry the stable energy
29