UiO-66-NH2Metal-Organic Frameworks with Embedded MoS2Nanoflakes for Visible-Light-Mediated H2and O2Evolution

Article abstract of DOI:10.1021/acs.inorgchem.0c01030

Hydrogen evolution from water splitting by means of a photocatalytic approach is an ideal future energy source and free of fossil reserves, in contrary photocatalytic O2 evolution remains a bottleneck due to high over potential and low efficiency. For reasonable use of solar light, photocatalysts must be sufficiently stable and efficient toward harvesting of sunlight from both theoretical and practical viewpoints. In this regard, here we have prepared MoS2-modified UiO-66-NH2 MOF through a facile hydrothermal technique and evaluated its efficiency toward photocatalytic H2 and O2 evolution by water splitting in the presence of sacrificial agents. A couple of similar type of analyses have been studied previously; however, this analysis represents a diverse scientific approach on the basis of interfacial contact toward reveal the actual potential of nanoflakes MoS2 as well as UiO-66-NH2. In this regard the as-synthesized photocatalyst was well-characterized by XRD, FTIR, UV-vis diffuse reflectance spectra, FESEM, HRTEM, XPS, and BET analysis techniques, which provide sufficient evidence toward successful synthesis of the pristine materials and efficacious anchorage of MoS2 on the active surface of UiO-66-NH2 by the ionic interaction between Zr-O and S/Mo. Among the synthesized photocatalysts (3 wt percent) MoS2/UiO-66-NH2 shows the optimum outcome toward H2 and O2 evolution, i.e., 512.9 μmol/h (4.37 times greater than bare UiO-66-NH2) and 263.6 μmol/h (4.25 and 11.32 times greater than bare UiO-66-NH2 and MoS2, respectively). The superior performance obtained by the composite is due to the synergistic effect of pristine UiO-66-NH2 and MoS2 which proceeds through a type-II interband alignment for the facile channelization of excitons. This investigation will bestow a beneficial blue-print to construct challenging photocatalysts and to find out the paramount performance toward photocatalytic water redox reaction.

Full text of DOI:10.1021/acs.inorgchem.0c01030

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UiO-66-NH2 Metal−Organic Frameworks with Embedded MoS2
Nanoflakes for Visible-Light-Mediated H2 and O2 Evolution
Satyabrata Subudhi, Gayatri Swain, Suraj Prakash Tripathy, and Kulamani Parida*
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ABSTRACT: Hydrogen evolution from water splitting by means
of a photocatalytic approach is an ideal future energy source and
free of fossil reserves, in contrary photocatalytic O2 evolution
remains a bottleneck due to high over potential and low efficiency.
For reasonable use of solar light, photocatalysts must be sufficiently
stable and efficient toward harvesting of sunlight from both
theoretical and practical viewpoints. In this regard, here we have
prepared MoS2-modified UiO-66-NH2 MOF through a facile
hydrothermal technique and evaluated its efficiency toward
photocatalytic H2 and O2 evolution by water splitting in the
presence of sacrificial agents. A couple of similar type of analyses
have been studied previously; however, this analysis represents a diverse scientific approach on the basis of interfacial contact toward
reveal the actual potential of nanoflakes MoS2 as well as UiO-66-NH2. In this regard the as-synthesized photocatalyst was well-
characterized by XRD, FTIR, UV−vis diffuse reflectance spectra, FESEM, HRTEM, XPS, and BET analysis techniques, which
provide sufficient evidence toward successful synthesis of the pristine materials and efficacious anchorage of MoS2 on the active
surface of UiO-66-NH2 by the ionic interaction between Zr−O and S/Mo. Among the synthesized photocatalysts (3 wt %) MoS2/
UiO-66-NH2 shows the optimum outcome toward H2 and O2 evolution, i.e., 512.9 μmol/h (4.37 times greater than bare UiO-66-
NH2) and 263.6 μmol/h (4.25 and 11.32 times greater than bare UiO-66-NH2 and MoS2, respectively). The superior performance
obtained by the composite is due to the synergistic effect of pristine UiO-66-NH2 and MoS2 which proceeds through a type-II
interband alignment for the facile channelization of excitons. This investigation will bestow a beneficial blue-print to construct
challenging photocatalysts and to find out the paramount performance toward photocatalytic water redox reaction.
than water reduction potential (H+/H2 = −0.41 V, pH 7).4
Therefore, the exploitation of visible-light active photocatalyst
1. INTRODUCTION
Water splitting facilitates cleavage of H−O−H bond by
is very important for effective absorption and conversion of
solar energy for environmental remediation. Despite the great
efforts devoted to design highly efficient photocatalysts, the
photocatalytic results are significantly limited due to narrow
light absorption and swift electron−hole recombination. In this
regard, the development of heterojunction (basically p−n
heterojunction)-based photocatalyst has garnered more
attention to resolve these issues due to its effective electron−
hole anti-recombination capabilities.5
In this aspect, metal−organic framework (also called porous
coordination polymers (PCPs)) are regarded as an interesting
type of porous hybrid crystalline material due to its flexible
porosity and viability for post-synthetic modification. These
different energy source, i.e., electrical (electrolysis), thermal
(thermolysis), or light (photolysis). Among these methods, the
photolysis of water in the presence of a photocatalyst can be
treated as a promising technique for both H2 and O2 evolution
in which the most important criteria is the alignment of redox
potential of water with electronic bandgap of photocatalyst.1
The conversion of photon energy to chemical energy in the
presence of heterogeneous photocatalyst by the photo-induced
decomposition of H2O has attracted global attention as an
ecofriendly method to mitigate future energy catastrophe and
environmental issues. It is quite difficult to achieve one-pot
water splitting under visible-light illumination due to uphill
nature of the reaction, so sacrificial reagents such as methanol
or silver nitrates are used as hole or electron scavenger,
respectively, to evaluate the photocatalytic activity of the
photocatalyst.2,3 In this regard, photocatalyst should fulfill the
thermodynamic requisites during each half cell reaction
(hydrogen and oxygen evolution) of photocatalytic water
splitting, i.e., the valence band (VB) potential must be more
positive than water oxidation potential (H2O/O2 = 0.82 V, pH
7) and the conduction band potential must be more negative
Received: April 8, 2020
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features formulate MOF as an efficient candidate toward
various applications such as gas storage and separation,
membrane separation technology of various wastewater,
chemical sensing, drug delivery, and catalytic transformation
of organic molecules.6−8 In order to get water-stable and
visible-light-active MOF, two common strategies are applied,
i.e., (i) introduction of functional group (−NH2, −SH) based
linker as the bridging unit with metal clusters on the basis of
hard/soft acid/base (HSAB) principle and (ii) heterojunction
with visible-light-active photocatalyst.6,8−10 In the current
study, we have employed both the approach to make a
suitable and efficient photocatalyst. From the viewpoint of
band structure, the conduction band (CB) of MOF is
constructed by the empty outer orbitals of the metal center,
and the valence band is mainly contributed to by the outer
orbitals of organic linkers. The linker effectively absorbs solar
light and transfers the energy to the metal center, which is
termed ligand to cluster/metal charge transfer (LCCT/
LMCT).11 Moreover MOFs exhibited a suitable band gap as
well as a reasonable band edge potential which are the two
fundamental requirements for water redox reaction and
possesses long-term stability against photocorrosion under
light illumination in aqueous solution.12−18 Mori’s group has
first employed MOF toward the hydrogen evolution reaction.
Garcia et al.19 reported water splitting by Zr-based MOFs such
as UiO-66 and UiO-66-NH2 for first time under UV-light
illumination in 2010. Lin et al.20 has carried out the visible-
light-driven H2 evolution reaction by incorporating Pt in the
active pores of MOFs. Rosseinsky et al.21 has carried out H2
evolution reaction by Al-containing porphyrin MOF. However,
the outcomes of these experiments were not up to the mark.
Afterward, most efforts have been focused to enhance the
photocatalytic activity of MOF by coupling with plasmonic
metals and doping with metals/nonmetals by hydrothermal
method.22
The researchers has also provide their immense attention
toward metal sulfide systems, i.e., basically toward MoS2
(MS)23,24 in the field of electrocatalysis, pollutant degradation,
and so on. MoS2 is an intrinsically p-type semiconductor and is
generally arranged in a way such that the hexagonally packed 6-
fold coordinated Mo atoms are covalently bonded with two
atomic layers of trigonal chalcogens in a S−Mo−S fashion. The
S−Mo−S layers were bonded with each other by weak van der
Waal’s forces of attraction. The suitable molecular arrangement
and good crystalline property is unable to resolve unsat-
isfactory the photocatalytic property which is mainly due to
low charge carrier density as well as low conductivity between
the adjacent S−Mo−S layers of MS.25 However, when it is
coordinated with any other photocatalyst it then enhances the
light-absorption capacity and charge separation efficiency of
the primary photocatalyst to a greater extent. Various works
have been proposed in this regard, such as MoS2/CdS, MoS2/
CeO2, MoS2/CaIn2S4, and MoS2/g-C3N4.26,27
Table 1. Performed Photocatalytic Applications of MoS2-
Based UiO-66-NH2 Photocatalyst
photocatalytic activity in the
presence of 300 W xenon lamp
(light source)
photocatalyst
H2 evolution O2 evolution time ref
EY-sensitized 5 wt %
MoS2 QDs/
1
186.37 μmol
3 h
28
29
UiO-66-NH2/G
1.5 wt % MoS2/UiO-
66/CdS
3 wt % MoS2/UiO-66-
NH2 (present work)
2
3
650 μmol
1 h
1 h
512.9 μmol
263.6 μmol
composite very complicated. In this work, we have reported
nanoflakes MoS2 embedded UiO-66-NH2 (MS/UNH)-based
heterojunction with intermediate weight percent variation of
MoS2 in comparison to previous works and study its
application toward photocatalytic H2 evolution and O2
evolution reactions. The analysis outcome shows superior
activity in comparison to EY-sensitized MoS2 QDs/UiO-66-
NH2/G photocatalyst and almost equivalent activity in
comparison to that of the MoS2/UiO-66/CdS photocatalyst.
The science behind the augmented photocatalytic activity is
mainly attributed to the surface contact of coupled photo-
catalyst as well as the availability of the active surface of the
formed composite toward photocatalytic reaction. The
successful interaction between nanoflake MS and UNH
facilitates the photoabsorption potential as well as the
photostability of the material, thus suppressing the photo-
exciton recombination. The successful interaction provides a
type-II interband alignment in which the internal electric field
created at the junction of two material causes separation of
photogenerated charge carriers more efficiently thus enhancing
the photocatalytic activity. Thus, the as-synthesized MS/UNH
would open up a new direction in MOF-based heterojunctions
toward various environmental photocatalytic applications
under visible-light irradiation.
2. EXPERIMENTAL SECTION
2.1. Chemicals Utilized. Zirconium chloride (ZrCl4), 2-amino-
1,4-benzene dicarboxylic acid (H2BDC-NH2), sodium molybdate
(Na2MoO4·2H2O), and thiourea (CH4N2S) were purchased from
sigma Aldrich. Methanol, ethanol, and N,N-dimethylformamide
(DMF) were obtained from Merck and were used without further
purification.
2.2. Synthesis of UNH. UiO-66-NH2 photocatalyst was
synthesized via hydrothermal method as reported previously.30 In a
typical preparation procedure, equimolar amounts (7 mmol) of
zirconium chloride and 2-amino terephthalic acid (ATA) were stirred
separately in DMF (40 mL) solution for 30 min. The formed
zirconium chloride and ATA solutions were mixed and was stirred for
more 30 min. Then, so formed suspension solution was undergo for
hydrothermal treatment for 24 h at 120 °C. After the time, is over it
was cooled to room temperature, and the sample was alienated via
centrifugation. Next, the sample was rinsed several times with
methanol to eliminate the remaining unreacted metal salts/organic
moieties. For pore activation, it was kept in methanol for 72 h and was
separated by centrifugation followed by drying at 100 °C for 12 h, and
the obtained product was named UiO-66-NH2 (UNH).
It is well-known that both MoS2 (MS) and UiO-66-NH2
(UNH) have suitable band edge potential, yet their unaided
utilization shows less efficiency toward the water redox
reaction due to high charge recombination. However, their
heterostructure composite can work as a promising photo-
catalyst with enhanced activity. Hao et al. and Shen et al.
reported EY-sensitized MoS2 QDs/UiO-66-NH2/G and
MoS2/UiO-66/CdS hybrid with enhanced photocatalytic
activity toward H2 evolution,28,29 as presented in Table 1;
however, the quaternary and tertiary junctions make the
2.3. Synthesis of MS. Pristine MoS2 was synthesized by
hydrothermal method as reported previously.26 In a typical
preparation procedure, 1:5 mmol sodium molybdate and thiourea
were stirred separately in 80 mL of EtOH/H2O (1:3) solution and
were sonicated for 30 min followed by constant stirring for 30 min.
Then, the so-formed suspension solution was undergo for hydro-
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Scheme 1. Schematic Representation of the Synthesized (i) MS, (ii) UNH, and (iii) MS/UNH (1, 3, and 5 wt %)
Figure 1. (a) XRD and (b) FTIR of UNH, MS, and MS/UNH (1, 3, and 5 wt %)
C
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Figure 2. continued
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Figure 2. XPS peaks of (a) C 1s, (b) Zr 3d, (c) N 1s, (d) O 1s, (e) Mo 3d, and (f) S 2p.
thermal treatment by a Teflon-lined stainless-steel autoclave for 24 h
at 210 °C. After the time is over, it was cooled at room temperature,
and the sample was alienated via centrifugation. Then, the sample was
rinsed three times with the EtOH/H2O (1:3) solution. It was oven-
dried at 80 °C for 12 h, and the obtained product was named MoS2
(MS).
2.4. Synthesis of MS/UNH. MS/UNH heterojunction photo-
catalyst was prepared by adding as prepared MS in required weight
percentage during UNH synthesis as described above. This procedure
was followed to prepare three composites, i.e., MS/UNH (1, 3, and 5
wt %), by weight percent variation of MS. The detailed synthesis
method is illustrated in Scheme 1.
catalyst. After 1 h of reaction, the H2 and O2 evolution by
synthesized materials was depicted in the respective sections.
4. CHARACTERIZATION
4.1. Structural Characterization. XRD analysis of UNH,
MS, and the series of MS/UNH were carried out to obtain the
crystal structure and phase purity, as provided in Figure 1. The
obtained result shows various sharp crystalline peaks for UNH
and was well-matched with the reported literature values.29 For
MS, three sharp peaks were observed at 14.1, 33.3, and 58.9°.
The peaks obtained at 14.1° represent the (002) diffraction
peak,30 which suggests the existence of a hexagonal phase of
MS having lattice constants a = b = 0.311 and c = 1.211 and
the space group p63/mmc.26 Moreover, the XRD analysis
concluded that there is no impurity in crystal lattice. In
composites, the peak intensity decreases, and blue-shifting was
observed in comparison to both UNH and MS as depicted in
Figure 1a. The decrease and shifting in peak positions well
represents the coordination of MS with UNH in MS/UNH
photocatalyst. Moreover, the presence of MS peaks also
indicates the fabrication of MS on exposed surface of UNH
instead of open pores and was further verified from FESEM
analysis as depicted in respective sections. Additionally, FTIR
analysis was carried out to study various vibrational modes of
the functional group present in the photocatalyst as shown in
Figure 1b. The broad and sharp bands at 1626 and 3432 cm−1
represent the bending and stretching vibration of the H−O−H
linkage.31 The peak at 467 cm−1 represents the characteristic
Mo−S stretching vibration.32 In addition, the peak at 3477 and
3340 cm−1 represents the stretching frequency for asymmetric
and symmetric vibration of −NH2 group of UNH. The peaks
at 1642 cm−1 represents the N−H bending vibration, and the
peak at 1244 cm−1 denotes the C−N stretching of aromatic
3. PHOTOCATALYTIC EXPERIMENTAL ANALYSIS FOR
WATER SPLITTING
The photocatalytic efficiency of the as synthesized photo-
catalysts was tested toward water redox reaction by using a
sealed quartz batch reactor containing 0.02 g of the
photocatalyst in 20 mL of various scavengers. For the H2
evolution reaction, a 10% methanol solution was used, whereas
0.05 M AgNO3 solutions were used for the O2 evolution
reaction. The solution was stirred to prohibit the coagulation
in the presence of 300 W Xe arc lamp (cutoff filter > 420 nm)
as a visible-light source. The Xe arc lamp was kept 8.7 cm away
from the reaction solution. Prior to the initiation of the
photocatalytic reaction, the suspension solution was treated
with N2 gas for 1 h to remove the dissolved gases. The water
displacement method was employed to collect the evolved H2
and O2 gas during the reaction and was analyzed with 5 Å
molecular sieves and a thermal conductivity detector by GC
analysis (Agilent Technology). Control photocatalytic experi-
ments were carried out in absence of catalyst and light which
show no result and confirm the importance of light and
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Table 2. XPS Analysis of UNH, MS, and MS/UNH
binding energy
C 1s
ref
UNH
288.96 eV
288.91 eV
0.05 eV
(OC−O) of ATA
286.30 eV
286.11 eV
0.19 eV
284.99 eV
MS/UNH
difference
reason
284.82 eV
0.17 eV
CC
34
35
34
34
26
26
C−NH2
N 1s
O 1s
Zr 3d
UNH
401.76 eV
401.29 eV
0.47 eV
NH2
400.63 eV
400.26 eV
0.37 eV
399.52 eV
399.51 eV
0.01 eV
MS/UNH
difference
reason
+
+
−NH3
−NH2
UNH
532.14 eV
532.04 eV
0.10 eV
MS/UNH
difference
reason
Zr−O bond
UNH
185.61 eV
185.50 eV (3d
0.11 eV
183.13 eV
MS/UNH
difference
reason
)
182.10 eV (3d
0.03 eV
)
5/2
3/2
Zr4+
Mo 3d
MS
227.17 eV
227.83
0.66 eV
(S 2S)
229.98 eV
233.11 eV
233.33 eV (3d
0.22 eV
234.09 eV
234.84 eV
0.75 eV
MS/UNH
difference
reason
230.38 eV (3d5/2
)
)
3/2
0.40 eV
Mo4+
Mo6+
S 2p
MS
162.09 eV
162.89 eV (2p
0.80 eV
163.29 eV
163.65 eV (2p
0.36 eV
MS/UNH
difference
reason
)
)
1/2
3/2
S−2
Figure 3. (a) FESEM image of MS/UNH, (b, c) HRTEM image of MS/UNH, (d) SAED pattern of MS/UNH
amines. A small band at 1510 cm−1 signifies the CC moiety
vibration of the aromatic ring. However, no eye-catching
shifting was observed in any MS/UNH composite. The above
observed FTIR data shows coupling between MS and UNH.
The architecture of heterojunction interface may inhibit the
recombination of photogenerated excitons by their proper
channelization and hence accelerate the surface reaction
kinetics. The balanced alignment of energy bands estimates
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the chemical characteristics of the heterojunction and
consequently the deviation in energy states. Therefore, we
have carried out high-resolution XPS analysis of UNH, MS,
and MS/UNH to inspect the constituent elements with respect
to their binding energy (BE) and the interfacial interaction
between semiconductors which is depicted in Figure 2. In the
first case, the variation in BE between UNH and MS/UNH
was compared, in which the XPS spectra of C 1s disclosed
three sharp peaks in both parent UNH (284.99, 286.30, and
288.96 eV) and MS/UNH (284.82, 286.11, and 288.91 eV)
which are in good agreement with CC, C−NH2, and sp2-
bonded carbonyl group of ATA, respectively. Similarly, the N
1s spectra of UNH are deconvoluted in to three peaks at
399.52, 400.63, and 401.76 eV that represent −NH2, −NH3,
toward the electron beam. Moreover, the presence of above
elements was further supported by XPS analysis in the
respective section.
4.2.2. BET Analysis. The surface areas and pore volumes of
UNH and MS/UNH were calculated from nitrogen
adsorption−desorption isotherms analyzed by BET and BJH
characterization techniques as shown in Figure 4. The obtained
+
and NH2 respectively, whereas for MS/UNH the −ve
shifting in BE was observed as represented in Table 2. The BE
of Zr is splitted in to two sharp peaks that represent the
existence of Zr4+ states. Second, the variation in BE between
MS and MS/UNH was compared, in which the XPS spectra of
Mo was split in to four deconvoluted peaks at 227.83, 230.38,
233.33, and 234.84 eV for MS/UNH, in which the S 2s peak
was located at 227.83 eV, and Mo4+ was represented by 3d5/2
and 3d3/2 spin state at BE 230.38 and 233.33 eV, respectively,
and the Mo6+ state at 234.84 eV. S shows two deconvoluted
peaks for 2p3/2 and 2p1/2 at 162.89 and 163.65 eV, respectively,
that typically represent the existence of the S2− state. This
comparison analysis indicates that in composite MS/UNH the
BE of Zr, N, and O has shifted toward a lower BE as compared
to its pristine material, whereas the BEs of Mo and S of MS
have shifted toward a positive BE as compared to its
components present in neat MS. This opposite tendency
indicates that after heterojunction formation the electron
density increases on the surface of UNH, whereas it decreases
at MS surface and confirms the channelization of photoexcited
electrons from MS to UNH.33 Additionally ICP-OES analysis
result confirms existence of 0.97, 2.83, and 4.67 wt % of MS in
MS/UNH (1, 3, and 5 wt %), respectively, and hence provides
sufficient evidence about the stability of surface interaction
between MS and UNH.
4.2. Morphological Characterization. 4.2.1. FESEM and
HRTEM Analysis. FESEM and HRTEM analysis of the
synthesized photocatalyst were carried out to study the surface
topology and elemental composition as depicted in Figure 3.
The SEM image clearly illustrates that nanoflake MS were
wrapped on the exposed surface of pristine UNH due to the
strong interfacial contact. This intimate interaction may
facilitate the fast transfer of photogenerated charge carriers
and simultaneously inhibits the charge recombination rate
which ultimately causes the enhancement of photocatalytic
efficiency. The lattice fringe for UNH was not observed in the
composite which is mainly due to the charging of nanoparticles
under the influence of electron beam36 and for MS the poor
crystallinity might be the reason. However, the formation and
morphology of pristine MS was confirmed from the FESEM
and HRTEM analysis result as provided in Figure S1. Although
the HRTEM images of MS/UNH composite does not contain
any lattice fringes of MS, the existence of MS is clearly
observed from the EDX analysis and color mapping data as
provided in Figures S2 and S3. This demonstrates the presence
and uniform distribution of Zr, Mo, S, C, N, and O elements in
the composite.28,29 Moreover the SAED pattern (Figure 3d) of
MS/UNH was provided in which the crystalline property was
not observed due to the sensitivity of the UNH material
Figure 4. BET surface area and BJH pore volume distribution (inset)
of UNH and MS/UNH (3 wt %).
graphical isotherm, i.e., a typical H3 hysteresis loop,
demonstrates type-IV isotherms for both UNH and MS/
UNH. The hysteresis loop appears for UNH at relative high
pressure (p/p0), which indicates the accumulation of pores.36
The surface areas for UNH and MS/UNH were found to be
831.488 and 1017.36 m2/g, respectively. The increase in
surface area in (3 wt %) MS/UNH in comparison to that of
pristine UNH is might be due to the nanoflakes morphology of
MS as provided in Figure S1. The observed enhanced surface
area for 3 wt % MS/UNH reveals the important reason behind
the higher photocatalytic activity. Moreover, BJH analysis
result proves the presence of micropores in the as-synthesized
pristine UNH and composite MS/UNH as shown in Figure 4
(inset). However, after subsequent modification with MS, the
pattern of the isotherm was preserved, and the pore size
remains constant. However a slight decrease in pore volume
was observed (UNH = 0.40 and 0.97 nm, MS/UNH = 0.38
and 0.87 nm). As it is well-observed from the SEM and
HRTEM analysis that MS does not occupy any pores of UNH
and only integrated on the surface of pristine UNH; hence, the
decrease in pore volume may be due to the presence of trapped
recrystallized ATA residue in the pores of UNH. In brief, the
increase in surface area is due to the close electrostatic
interaction of Zr with Mo−S group that increases the active
surface of the pristine UNH. Although MS loading causes the
increase in surface area and consequently increases the
photocatalytic activity, the 5 wt % composite shows less
catalytic activity in comparison to the 3 wt % composite which
might be due to wrapping of the active surface of UNH by
agglomeration and hence leads to its less availability toward
participation in photocatalytic reactions.
4.3. Optical Characterization. A comparative study of
UV−vis diffuse reflectance spectra (DRS) spectra of the
synthesized photocatalysts has been depicted in Figure 5. The
observed data shows that there is a continuous red shift in the
absorption edges with gradual increase in the weight
percentage of MS as observed for MS/UNH composite,
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Figure 5. (a) Diffuse reflectance spectra of UNH, MS, and MS/UNH (1, 3, and 5 wt %). (b) Tauc plot of UNH. (c) Tauc plot of MS.
which implies that the band gap was precisely controlled
according to the variation of molar ratio of MS. There is also a
change in color from yellow to dark brown was observed with
the increasing weight percentage of MS for the prepared
composite which might reduces the reflection of light and
increases the absorption capacity of visible light as observed in
UV−vis spectra. The band gap energy for UNH and MS were
observed as 2.67 and 1.86 eV by extrapolating the straight
portion of (αhυ)1/2 versus photon energy (hυ) according to
the Kubelka−Munk equation from the UV−vis diffuse spectra
as depicted in eq 1.
and at 365 nm for π−π* transition of ATA and Zr−O cluster.
The broad band observed at around 578 and 676 nm for MS
are corresponds to k- and k′-point of Brillouin’s zone, which
can be observed in MS/UNH and provides sufficient evidence
about the chemical interaction between the parent enti-
ties.32,37,38
4.4. Electrochemical Characterization. Electrochemical
analysis is an important analytical technique that uses electrical
stimulation to study the chemical reactivity of the photo-
catalyst. More specifically it analyses the redox reactions that
provides vital information related to kinetics and mechanism of
the reaction.
αhν = A(hν − Eg)n/2
(1)
EIS analyses of the photocatalyst were carried out to analyze
charge transfer activity as well as interfacial charge migration
resistance. As is well-known, the smaller the arc diameter of
Nyquist plot, the higher the charge separation efficiency,
leading to superior electrical conductivity. In the present
investigation, a smaller arc was formed by (3 wt %) MS/UNH
in comparison to that of parent UNH, which indicates
introduction of MS to UNH is very helpful to improve charge
separation efficiency and hence its photocatalytic activity. In
addition, current versus potential measurement (LSV) was
further performed to investigate the photogenerated charge
transfer and mechanism of the photocatalyst. The analysis was
carried out for parent UNH, MS, and MS/UNH at 5 mV s−1 in
an appropriate potential range as depicted in Figure 6b. The
LSV spectrogram reveals the p- and n-type semiconductor
character of MS and UNH, respectively. However, MS/UNH
shows both cathodic and anodic characteristics with enhanced
current density which represents the successful formation of
p−n heterojunction architecture by the composite.
where α = proportionality constant, h = planks constant, ν =
frequency of light, A = absorption coefficient, Eg = band gap,
and n = type of electronic transition.
Moreover, the band gap observed for composite 3 wt % MS/
UNH was 2.55 eV. This narrower band gap not only increases
the photon absorption capacity but also promotes the lifetime
of active species (e− and h+) which is well-supported by
photoluminescence (PL) analysis as shown in Figure S4 and
has been explained in a separate section. Except for band gap,
the finding of band edge potential is very important to estimate
the catalytic potential toward proposed applications. After
getting the corresponding VB and CB for UNH and MS, their
complementary band edge potentials were calculated from the
UV−vis analysis by applying the eq 2 as follows:
Eg = VB − CB
(2)
Moreover, the UV−vis spectrum shows two sharp bands for
UNH at 265 nm for n−π* transition of amino group of 2-ATA
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Figure 6. (a) EIS representation. (b) LSV representation of UNH, MS, and (3 wt %) MS/UNH. (c, d) Mott−Schottky representation of UNH and
(3 wt %) MS/UNH. (e) Transient photocurrent representation of UNH, M, and (3 wt %) MS/UNH. (f) TRPL representation of UNH and (3 wt
%) MS/UNH.
kT
e
Mott−Schottky analysis was carried out to determine the
2 Vapp − V −
(
)
fb
−2
electronic band structure, band edge potential, and hence the
charge transfer pathway to propose the photocatalytic
mechanism by the synthesized photocatalyst. In this regard,
Mott−Schottky analysis was carried out in 0.2 M Na2SO4 for
UNH and MS/UNH and 0.5 M H2SO4 for MS to learn the
type of semiconductor catalyst (p-type/n-type). Figures 6d and
S5) represents the Mott−Schottky spectrogram for parent
UNH and MS with flat band potentials −0.73 and +1.56 V by
drawing a tangent to the x-axis of the respective Mott−
Schottky plot. Normally, the graph is plotted between the
⇒CSC
=
ND ∈ ∈0 eA2
(3)
where k = Boltzmann’s Constant, Vfb = flat band potential, T =
absolute temperature, absolute temperature, e = electric charge
in C, ND = donor density in cm3, ∈ and ∈0 = dielectric
constant and dielectric constant in vacuum, and A = area in
cm2.
After getting the flat band potential, the corresponding band
edge potentials for MS were found to be VB = 1.78 eV and CB
= −0.08 eV, and for UNH, these were found to be VB= 2.03
eV and CB = −0.64 eV by following eq 4:
space charge capacitance (CSC) versus applied potential (Vapp
)
by using eq 3:
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lifetime of photoexcited electrons as represented in Figure 6f
which was fitted to a biexponential model equation:
⇒E(NHE,pH 7)
= EAg/AgCl − 0.059 × (7 − pH of the electrolyte) + 0.198
R(t) = A1 exp{−t/τ1} + A2 exp{−t/τ2}
(7)
(4)
where R = normalized emission intensity, t = time after pulsed
laser excitation, A = amplitude, and τ = exciton lifetime of
individual component. The average lifetime (τavg) of two
exponential decay is indispensable to illustrate the overall
TRPL character and can be calculated by eq 8:
Moreover, the types of semiconducting character calculated
for MS and UNH were p- and n-type, respectively, by
following eqs 5 and 6:35
i
j
y
z
k T
q
1
2
b
j
z
⇒
=
j−E + Efb −
z
(p‐type)
A1τ12 + A2τ22
A1τ1 + A2τ2
j
j
z
z
C2
q ∈ ∈0 N
(5)
a
k
{
τ
=
avg
(8)
i
j
j
y
k T
q
1
2
z
z
b
where τ1 and τ2 are electron-trapping time and formation of
electron−hole pairs, respectively.42,43 The fluorescence life-
times were acquired by fitting the decay profile (Table 3) with
⇒
=
jE − Efb −
z
(n‐type)
j
j
z
z
C2
q ∈ ∈0 N
(6)
d
k
{
where kb = Boltzmann’s constant, T = absolute temperature, E
= applied potential, Nd and Na = donor density and acceptor
density, respectively, C = space charge capacitance, and q =
electronic charge, ∈ and ∈0 = dielectric constant and dielectric
constant in vacuum.
Table 3. Exponential-Curve-Fitted TRPL Parameters of
UNH and (3 wt %) MS/UNH
τ1
τ2
sample
UNH
(3 wt %)
MS/UNH
(ns)
(ns)
A1
A2
τavg
χ2
In the Mott−Schottky spectrogram, UNH shows a −ve
spectrogram having a flat band potential of −0.73 V and that of
MS shows a +ve spectrogram having a flat band potential of
1.56 V (Figure S5). The composite MS/UNH shows an
inverted-V shape slope structure with corresponding +ve and
−ve shifting in comparison to their pristine counterpart; hence,
it indicates a higher donor density and better transfer of charge
carrier efficiency. Furthermore, the positive shifting in the flat
band potential of both UNH and MS implies the bending of
corresponding band edges to facilitate the transfer of charge
carriers at the heterojunction interface and further proves the
formation of p−n heterojunction architecture by the
composite.
0.36
0.49
4.81
3.21
6600.48
7947.89
16.40
79.25
0.504
0.657
1.203
1.217
two exponential terms (eq 8). Apparently, the average excited-
state lifetime of UNH and (3 wt %) MS/UNH is 0.504 ns
(504 ps) and 0.657 ns (657 ps), respectively.43,44 The average
longer lifetime of (3 wt %) MS/UNH indicates decrease in
radiative recombination and increase the lifetime of photo-
generated excitons. This investigation further supports the
experimental outcomes of the above-mentioned analysis.
5. PHOTOCATALYTIC WATER REDOX REACTION
Transient photo current response for UNH, MS, and (3 wt
%) MS/UNH was characterized with several on−off cycles
under visible-light irradiation, to investigate the separation
efficiency of photoexcited charge carriers as depicted in Figure
6e. As photocurrent is mainly due to diffusion of photo-
generated electrons to the back contact, in the meantime holes
are taken up by the hole acceptor in the electrolyte.39,40 The
smallest photocurrent density for pristine MS (inset) and
UNH represents rapid recombination of photogenerated
electron−hole pairs; however, comparatively higher photo-
current density of (3 wt %) MS/UNH represents the superior
lifetime of photogenerated electron−hole pairs in p−n
junction material.41 Moreover, this experiment provides
adequate evidence of effective exciton separation and sup-
pressed recombination of electron−hole pairs after the
introduction of MS in UNH due to interfacial interaction by
p−n heterojunction formation.
Moreover, the stability of photogenerated active species was
further supported by PL spectroscopy experiment, which was
carried out at room temperature with an excitation wavelength
of 360 nm. The less intense PL intensity for the composite
MS/UNH represents good exciton separation, superior charge
transfer, and recombination inhibition in comparison to
pristine UNH which is well matched with the observed
photocatalytic activity results. Moreover, the analysis result
shows better charge separation for 5 wt % in comparison to
that of 3 wt % still shows less activity which might be due to
agglomeration of MS on the surface of UNH. In addition, time
resolved photoluminescence (TRPL) analysis of UNH and (3
wt %) MS/UNH were carried out to study the emission
The synthesized materials were employed toward photo-
catalytic H2 and O2 evolution to evaluate their catalytic
efficiency. Pristine UNH has suitable band edge redox
potential for the water redox reaction, but it shows only
117.46 μmol/h of H2 and 61.59 μmol/h of O2 evolution which
might be due to the fast recombination of charge carriers.
However, the visible-light-active pristine MS have the suitable
VB potential for O2 evolution and unfavorable CB potential for
H2 evolution. The p−n junction composite, i.e., MS/UNH,
shows considerably enhanced consequences due to improved
exciton separation. The 3 wt % MS/UNH shows optimum
outcome toward H2 and O2 evolution, i.e., 512.9 μmol/h (4.37
times greater than bare UNH) and 263.6 μmol/h (4.25 and
11.32 times greater than bare UNH and MS respectively),
which can be attributed to high active surface area and
inhibition of exciton recombination. Additionally, other
composites, i.e., 1 and 5 wt % MS/UNH, generate a relatively
smaller amount of product under the same reaction conditions.
The decrease in photocatalytic activity for 5 wt % MS/UNH
might be due to limited charge separation and aggregation of
MS particles on the large surface of UNH that causes a
decrease in the area of active sites. In addition, a parallel
photocatalytic experiment toward H2 and O2 evolution
reaction was carried out by physically mixing UNH and MS,
i.e., (3 wt %) MS + UNH (physical mixture). The comparative
less activity obtained for the physical mixture in comparison to
its corresponding composite is mainly due to the physical
adsorption by weak van der Waal interactions instead of ionic
interfacial interaction between the pristine MS and UNH;
J
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Figure 7. (a) Histograms of H2 and O2 production under visible-light irradiation for 1 h. (b) Recyclability study of (3 wt %) MS/UNH toward H2
and O2 production.
hence, it shows no synergistic effect. However, hydrothermally
prepared composite materials, i.e., (3 wt %) MS/UNH, show
comparatively higher activity due to the existence of electro-
static interfacial interaction which provides optimum support
toward inhibition of charge recombination by heterojunction
interface formation. To validate the photocatalytic capability a
comparison analysis table has been provided in Table S1.
Furthermore, to verify the photostability, the best photo-
catalyst, i.e., 3 wt % MS/UNH, was subjected to four
consecutive H2 and O2 evolution cycles for 1 h each, and
the obtained results were depicted in Figure 7b. The
conversion efficiency of (3 wt %) MS/UNH toward H2 and
O2 evolution was estimated by the formula eqs 9−13 as stated
below.
where ΔHc = combustion heat of H2 (kJ/mol) [H2O ⇌ 2H+ +
1/2O2; ΔHc = 285.8 kJ/mol, r = radius of the circle, i.e., 1.5
cm].
6. MECHANISTIC APPROACH
On the basis of characterization analysis outcome, possible
photocatalytic mechanistic pathway by MS/UNH toward
water redox reaction is depicted in Scheme 2. The excellent
photocatalytic efficiency of MS/UNH is due to the
comparatively high active surface area (suitable for effective
adsorption of adsorbent) and superior exciton separation
efficiency (suitable for effective photocatalytic reaction) due to
maximum interfacial contact. The high active surface area of
UNH in composite is basically responsible for H2 generation as
it contains potential Lewis acid sites (Zr4+) and base sites
(−NH2), and hence acts as a bifunctional acid−base catalyst.
In contrast, water is amphoteric in nature and plays an active
role as a Bronsted acid and base. The bifunctional ionic
property of UNH provides active sites, i.e., Zr4+, and sites
(−NH2) for the physisorption of amphoteric water molecule
followed by photocatalytic H2 evolution.45 Besides the above-
mentioned two factors, the band edge potentials of the
respective photocatalyst are one of the crucial prospect from
which the accurate mechanistic pathway can be explained.
From the UV-DRS and Mott−Schottky analyses, the band
edge potentials for UNH were determined to be EVB = 2.03 eV
and ECB = −0.64 eV, and for MS, they were determined to be
EVB = 1.78 eV and ECB = −0.08 eV. There are two possible
ways of charge transfer by MS/UNH, i.e., (1) double charge
transfer and (2) p−n heterojunction to describe the charge
flow mechanism toward water redox reaction. In the case of
(1) double charge transfer, if the e−−h+ flow follows this
mechanism, then the electron will migrate from the CB of
UNH to the CB of MS. At the same time, the migration of the
hole takes place from the VB of UNH to the VB of MS
resulting in a high charge recombination process. Moreover,
the formed potential at the CB of MS is not favorable to
reduce water (H2O/H2 = −0.41 eV) due to insufficient
standard reduction potential (−0.08 eV). Hence this
mechanistic pathway is unfavorable with double charge
transfer. Therefore, the second approach, i.e., (ii) p−n
heterojunction charge transfer pathway, is more acceptable to
describe the redox process and can be explained on the basis of
the (a) electronic environment of photosensitized MS/UNH
5.1. Apparent Conversion Efficiency (ACE) for H2
Evolution.
stored chemical energy (SCE)
incident light intensity (ILI)
⇒ACE =
× 100
(9)
⇒ACE
moles of H2 produced·ΔHc
=
·100
150 mW cm−2(spherical surface area on which light falls)
(10)
512.9 μmol/h × 285.8 kJ/mol
× 100
⇒
150mW cm−2 × πr2
0.04071 W
=
× 100 = 3.84%
150 × 3.141 × (1.5)2 mW cm−2
(11)
5.2. ACE for O2 Evolution.
⇒ACE
moles of O2 produced × ΔHc
=
150 mW cm−2 × (spherical surface area on which light falls)
(12)
263.6 μmol/h × 285.8 kJ/mol
150 mW cm−2 × πr2
⇒
0.02092 W
=
= 1.97%
150 × 3.141 × (1.5)2 mW cm−2
(13)
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advantage of this analysis, as it will provide maximum
interfacial contact toward UNH. Additionally, from XPS
analysis the change of electron density provides sufficient
evidence toward active species migration, which is only
possible due to type-II heterojunction type surface contact
across the interface that suppress the active species
recombination. Moreover, work functions (Φ), Fermi level
(Ef) coalition, and band edge potential are crucial parameters
to get in-depth insight of a p−n heterojunction system. In our
study, the Ef of MS and UNH was found to be −6.79 and
−3.96, respectively, as presented in Table S2. The carrier
density of a semiconductor is inversely proportional to the
slope of their Mott−Schottky plot. As is well-observed in the
Mott−Schottky study, the composite shows both +ve and −ve
shifting for respective counterparts, indicates the downward
and upward shifting of the corresponding band edges to make
the thermodynamically balanced Fermi level with the
formation of a space charge region for the effective separation
of exciton pairs. When the Fermi level is thermodynamically
balanced, the space charge region exerts force by the captured
excitons through a built-in electric field toward the interfacial
region of the p−n junction, which effectively enhances the
photocatalytic activity of the composite.
Scheme 2. Schematic Representation of Proposed p−n
Heterojunction Mechanism for Photocatalytic Water Redox
Reaction
The mechanism has been shown in detail in Scheme 2,
showing before- and after-contact exciton channelization.
Before contact, the electrons and holes will migrated from n-
UNH to p-MS and from p-MS to n-UNH, respectively,
followed by the formation of a depletion zone between the
contact face of MS and UNH. The exciton migration will
continue until the occurrence of e−−h+ equilibrium and that
causes band bending by shifting of Fermi level. The band
bending creates a built-in potential (2.18 eV) between the
contact face to enhance the e−−h+ separation efficiency. After
contact, the exciton channelization occurs in suitably prepared
path as projected in Scheme 2. The projected electron−hole
channelization inhibits the prospect of recombination and
provides suitably active species to get accumulate at respective
band edges that served as a commanding tool toward the
photocatalytic water redox reaction.
6.2. Role of Favorable Band Edge Potential of
Photocatalyst (MS/UNH) toward Photocatalytic Water
Redox Reaction. It is well-known that the availability of
active species alone is not enough to carry out any
photochemical reaction, so the important factor is the
reduction potential of band edges at which the excitons are
available. In our case, MS exhibited a trace amount of H2 due
to unsuitable band edge potential, whereas the band edge
potential of UNH is suitable toward photocatalytic H2 (H2O/
H2 = −0.41 eV at pH 7) and O2 (H2O/O2 = 0.82 eV at pH 7)
evolution. After the chemical combination of MS and UNH,
the synergistic effect shows the best catalytic performance
toward both H2 (512.9 μmol/h) and O2 (263.6 μmol/h)
evolution due to suitable band edge potential as well as the
least probability of exciton recombination. The water redox
reaction (eqs 14−26) has been provided below.
and (b) role of favorable band edge potential of photocatalyst
(MS/UNH) toward photocatalytic water redox reaction.
6.1. Electronic Environment of Photosensitized MS/
UNH. To determine the mystifying science of heterojunction
mechanism, determination of electronic environment of the
photocatalyst is very crucial which can be determined from
UV−vis, Mott−Schottky, FESEM and XPS analyses. Here the
UV−vis analysis of the composite shows favorable band gaps
of pristine MS and UNH for heterojunction and proves the
visible-light-active property. Next to it, Mott−Schottky analysis
result shows p-type behavior of MS and n-type nature of UNH
which made them compatible to form a type-II heterojunction
type surface contact across the interface, and the coupled
structure of MS and UNH shows an inverted V-shape
structure, which suggests the p−n type characteristics of the
composite is also observed from the potential (I) versus
current density (V) graph as shown in Figure 6d. Next, FESEM
analysis reveals the nanoflake structure of MS which is a basic
6.2.1. Equation for H2 Evolution.
⇒MS/UNH + hυ → MS(h+)/UNH(e−)
(14)
⇒H2O + h+ → H+ + OH−
(15)
⇒2UNH(e−) + 2H+ → H2
(16)
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•
⇒CH3OH + h+ → CH2OH + H+
Authors
(17)
(18)
Satyabrata Subudhi − Centre for Nanoscience and
⇒•CH2OH + (h+) → CH2O + H+
⇒CH2O + OH + h+ → HCOOH + H+
⇒HCOOH → HCOO− + H+
⇒HCOO− + h+ → CO2 + H+
Nanotechnology, Siksha ‘O’ Anusandhan (Deemed to be
University), Bhubaneswar, Odisha 751030, India
Gayatri Swain − Centre for Nanoscience and Nanotechnology,
Siksha ‘O’ Anusandhan (Deemed to be University),
Bhubaneswar, Odisha 751030, India
•
(19)
(20)
Suraj Prakash Tripathy − Centre for Nanoscience and
Nanotechnology, Siksha ‘O’ Anusandhan (Deemed to be
University), Bhubaneswar, Odisha 751030, India
(21)
(22)
⇒2H+ + e− → H2
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01030
6.2.2. Equation for O2 Evolution.
⇒MS/UNH + hυ → MS(h+)/UNH(e−)
⇒MS(h+) + Ag+ + hυ → Ag2+
(23)
(24)
Notes
The authors declare no competing financial interest.
⇒2Ag+2 + 2H2O + hυ → Ag O2 + 4H+
(25)
(26)
2
ACKNOWLEDGMENTS
■
⇒2h+ + Ag O2 + hυ → 2Ag+ + O2
The authors are very thankful to the Siksha ‘O’ Anusandhan
(Deemed to be University) management for their encourage-
ment and constant support.
2
7. CONCLUSION
Herein we have successfully synthesized a MS/UNH
heterojunction hybrid photocatalyst by anchoring nanoflakes
MS onto octahedral surface of UNH by an effective
hydrothermal route. The observed analysis manifests superior
photocatalytic activity toward water redox reaction for the
composite in contrast to pristine MS and UNH which is
mainly due to an increase in active surface area and maximum
interfacial contact that makes available photo-excited charge
carriers, owing to the efficient charge separation of electron−
hole pairs at suitable band edge potential of the heterojunction
compound. The superiority of the active species for the
composite is well-supported by EIS spectra, LSV, and transient
photo current analyses. Moreover, the hydrophilicity of the
−NH2 group as present in ATA moiety is a most important
factor of the enhanced photocatalytic activity. The proposed
heterojunction photocatalyst with superior activity, stability,
recyclability and excellent photoreceptive nature needs more
investigation to explore the full potential of UNH and MS
toward energy and the environment.
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Kulamani Parida − Centre for Nanoscience and
Nanotechnology, Siksha ‘O’ Anusandhan (Deemed to be
University), Bhubaneswar, Odisha 751030, India;
orcid.org/0000-0001-7807-5561; Phone: +91-674-
2351777; Email: paridakulamani@yahoo.com,
kulamaniparida@soa.ac.in; Fax: +91-674-2350642
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