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Oxygen insensitive thiol–ene photo-click chemistry for direct imprint lithography of oxides


Abstract and Figures

UV-nanoimprint lithography (UV-NIL) is a promising technique for direct fabrication of functional oxide nanostructures. Since it is mostly carried out in aerobic conditions, the free radical polymerization during imprinting is retarded due to the radical scavenging ability of oxygen. Therefore, it is highly desirable to have an oxygen-insensitive photo-curable resin that not only alleviates the requirement of inert conditions but also enables patterning without making substantial changes in the process. Here we demonstrate the formulation of metal-containing resins that employ oxygen-insensitive thiol–ene photo-click chemistry. Allyl acetoacetate (AAAc) has been used as a bifunctional monomer that, on one hand, chelates with the metal ion, and on the other hand, offers a reactive alkene group for polymerization. Pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), a four-arm thiol derivative, is used as a crosslinker as well as an active component in the thiol–ene photo-click chemistry. The FT-IR analyses on the metal-free and metal-containing resin formulations revealed that the optimum ratio of alkene to thiol is 1 : 0.5 for an efficient photo-click chemistry. The thiol–ene photo-click chemistry has been successfully demonstrated for direct imprinting of oxides by employing TiO2 and Ta2O5 as candidate systems. The imprinted films of metal-containing resins were subjected to calcination to obtain the corresponding patterned metal oxides. This technique can potentially be expanded to other oxide systems as well.
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Oxygen insensitive thiolene photo-click
chemistry for direct imprint lithography of oxides
Ravikiran Nagarjuna,
Mohammad S. M. Saifullah
and Ramakrishnan Ganesan *
UV-nanoimprint lithography (UV-NIL) is a promising technique for direct fabrication of functional oxide
nanostructures. Since it is mostly carried out in aerobic conditions, the free radical polymerization during
imprinting is retarded due to the radical scavenging ability of oxygen. Therefore, it is highly desirable to
have an oxygen-insensitive photo-curable resin that not only alleviates the requirement of inert
conditions but also enables patterning without making substantial changes in the process. Here we
demonstrate the formulation of metal-containing resins that employ oxygen-insensitive thiolene
photo-click chemistry. Allyl acetoacetate (AAAc) has been used as a bifunctional monomer that, on one
hand, chelates with the metal ion, and on the other hand, oers a reactive alkene group for
polymerization. Pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), a four-arm thiol derivative, is
used as a crosslinker as well as an active component in the thiolene photo-click chemistry. The FT-IR
analyses on the metal-free and metal-containing resin formulations revealed that the optimum ratio of
alkene to thiol is 1 : 0.5 for an ecient photo-click chemistry. The thiolene photo-click chemistry has
been successfully demonstrated for direct imprinting of oxides by employing TiO
and Ta
candidate systems. The imprinted lms of metal-containing resins were subjected to calcination to
obtain the corresponding patterned metal oxides. This technique can potentially be expanded to other
oxide systems as well.
1. Introduction
Due to its simplicity, versatility, and specic reactivity, click
chemistry has gained signicant interest in various elds
including organic transformations, surface functionalization,
polymeric materials, and biological applications.
reactions including alkyneazide, DielsAlder reactions, thiol
ene, thiolyne, etc., have been classied under click chemistry,
as these reactions directly yield an adduct without giving any
other byproduct.
Among them, thiolene click is an interesting
reaction occurring between commonly available reactive
alkenes and thiol groups, which is known to be tolerant to the
presence of oxygen and moisture when the oxygen concentra-
tion is less than that of thiol.
Thiolene reaction is also called
as hydrothiolation, in which the thiol group adds across the
carboncarbon double bond in anti-Markovnikov orientation.
Generally, the hydrothiolation reaction can proceed under
a variety of conditions such as thermal catalytic processes, and
Hydrothiolation via photo-clicking is
attractive as it proceeds well at room temperature reaction
conditions. In a typical thiolene click reaction, the photo-
generated electron from a photo-initiator is transferred to the
thiol group to produce a thiyl radical, which adds across
a carboncarbon double bond and thus transferring the radical
to the alkene. Abstraction of a hydrogen radical from the
neighborhood thiol to this carbon radical accomplishes the
hydrothiolation, while simultaneously propagating the chain
Fabrication of micro/nanostructures of oxides is of para-
mount importance in various applications including photo-
electrochemical cells,
photovoltaic cells,
Several techniques including photolithography,
electron-beam lithography,
electro-hydrodynamic lithog-
direct write assembly,
selective surface wetting,
dip-pen nanolithography,
nanoimprint lithography
among others
have been utilized towards fabrica-
tion of metal oxide micro/nanostructures. Each of these tech-
niques has its own merits as well as limitations. Among the
various available techniques, NIL has been regarded as the next
generation fabrication approach due to its advantages such as
simplicity, low-cost, high-delity, high-resolution, non-
dependence on the optical diraction limit, compatibility with
various substrates and ability to imprint arbitrary structures
over at and curved surfaces.
The descendants of NIL like
Department of Chemistry, Birla Institute of Technology and Science (BITS) Pilani,
Hyderabad Campus, Jawahar Nagar, Shameerpet Mandal, Hyderabad, 500078,
India. E-mail:
Institute of Materials Research and Engineering, A*STAR (Agency for Science,
Technology, and Research), 2 Fusionopolis Way, #08-03 Innovis, Singapore 138634,
Republic of Singapore
Electronic supplementary information (ESI) available. See DOI:
Cite this: RSC Adv.,2018,8,11403
Received 26th February 2018
Accepted 16th March 2018
DOI: 10.1039/c8ra01688g
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and roll-to-at
imprint lithog-
raphy techniques have augmented its high-throughput capa-
bility while maintaining high-precision. The NIL process can be
broadly classied into two categories: thermal and UV. Thermal
NIL requires temperature cycling, which reduces the
throughput, aects the mold life, and leads to line-width vari-
ation owing to the dierences in thermal expansion coecients
of the mold, substrate, and resist.
On the other hand, UV-NIL
is an attractive room temperature process that does not require
any temperature cycling, and thereby allows high-throughput
As opposed to the fabrication of polymeric micro/
nanostructures, fabrication of oxides has inherent challenges
in the materials design as the metal-containing polymers and
ceramics do not possess workable characteristics for direct
patterning. Therefore, the material design has to be customized
as per the requirement of each technique. In the case of
nanoimprinting, solgel approach was initially employed that
utilized hydrolysis followed by condensation of metal alkoxides
or halides.
The conventional solgel approach suered
low yield due to the lack of mechanical strength of the imprints
during the demolding step. Nanoparticle suspensions have also
been used for direct nanoimprinting with a soPDMS mold.
This approach has been used to directly fabricate three-
dimensional structures of multicomponent oxides like indium
tin oxide and nickel ferrite.
Photosensitive ethylhexanoate-
based oxide patterning has been reported that utilizes photo-
breakable precursors rather than photo-curable monomers.
Recently, polymerizable solgel approach (PSG) has emerged as
a promising method for rapid direct nanoimprinting of a host
of metal oxides such as TiO
, ZrO
amongst others.
The PSG approach utilizes metal-methacrylate liquid precursors
that undergo in situ thermal- or photo-induced free radical
polymerization during the nanoimprinting step. One of the
major advantages of this approach is the spin-coatable liquid
phase metal-containing resin that can easily ll up the pattern
features of the mold at near-ambient pressure due to capillary
action. Polymerization solidies the metal-containing precursor
and decreases the surface energy to facilitate easy and clean
demolding. This approach has been found to harness the
advantages and obviates the limitations of the conventional sol
gel approach. The PSG approach has been shown to be suitable
for high-throughput step-and-ash imprint lithography for
various oxides
and to synthesize supported oxide photo-cata-
Despite the high potential, the PSG approach based on
free radical polymerization of methacrylates is vulnerable to
radical scavenging by oxygen that could aect the degree of
polymerization, which in turn could eect on the quality of
imprinting. This warrants purging/bubbling of inert gas into
the metal-containing resin before patterning in order to elimi-
nate any dissolved oxygen.
As thiolene click chemistry can full the requirements such
as oxygen-insensitivity, spin-coatability, and room temperature
photo-crosslinking, it is deemed to be attractive for UV-NIL.
This chemistry has recently been shown for its potential for
imprinting polymeric structures over at and curved surfaces.
However, the merits of the thiolene photo-click chemistry have
so far not been realized for oxide nanoimprinting. Here, we
show the augmentation of the PSG approach by employing the
oxygen-insensitive thiolene click chemistry towards fabrica-
tion of metal oxide micro/nanostructures (Scheme 1). The
design of the photo-curable metal-containing resin, its photo-
curability and patternability are discussed by choosing tita-
nium alkoxide as the precursor to fabricate TiO
structures. TiO
was chosen as the principal oxide candidate, as
it is one of the most studied semiconducting oxides that
possesses huge potential for various applications including
energy, health, and environment.
Finally, the thiolene
approach was also studied for patterning Ta
in order to
ascertain its potential towards structuring other metal oxides.
2. Experimental section
2.1 Materials and methods
Titanium(IV) isopropoxide, tantalum(V) ethoxide, allyl acetoa-
cetate (AAAc), pentaerythritol tetrakis(3-mercaptopropionate)
(PETMP), 2-(methacryloyloxy)ethyl acetoacetate (MAEAA), 2-
hydroxy-2-methylpropiophenone (HMP), 1H,1H,2H,2H-per-
uorodecyltrichlorosilane, and toluene were purchased from
Sigma Aldrich and used as-received. Polydimethylsiloxane
(PDMS) stamps were fabricated from Sylgard 184 purchased
from Sigma Aldrich.
Scheme 1 Schematic representation of the overall process involving thiolene click chemistry for imprinting of metal oxides.
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2.2 Resin formulation
The TiO
resists were prepared using requisite amounts of
precursors as mentioned in Table 1. Briey, one equivalent of
was added and thoroughly stirred with 4 equivalents
of AAAc in a glass vial inside a nitrogen glove box. The color of
the solution rapidly turned into red that indicated the forma-
tion of Ti(AAAc)
complex. To this complex, either half equiva-
lent or one equivalent of the PETMP cross-linker was added and
the resultant mixture was shaken well on a vortex stirrer for
2 min to obtain a clear solution. To this mixture, 3 wt% (with
respect to the AAAc and PETMP) of HMP photo-initiator was
added to formulate the imprintable resin, which was diluted
with toluene in 1 : 1 (wt/wt) ratio to decrease the viscosity. The
resist was also prepared in a similar manner by taking
tantalum(V) ethoxide as the alkoxide source.
2.3 Imprinting of metal oxide
First, PDMS stamps were fabricated following the standard
procedure from Sylgard 184. Briey, the silicon rubber base pre-
polymer and cross-linker of Sylgard 184 were thoroughly mixed
in 10 : 1 weight ratio and the resulting mixture was poured over
polycarbonate master mold kept in a Petri dish. This assembly
was degassed in a vacuum desiccator to remove any trapped air
bubbles. The assembly was the subjected to 70 C for 1 h to
induce cross-linking of the precursor. Finally, the PDMS elas-
tomeric stamp was peeled ofrom the master mold. The
fabricated PDMS stamps were treated with 1H,1H,2H,2H-per-
uorodecyltrichlorosilane for 5 h inside an evacuated desiccator
in order to decrease the surface energy that facilitates easy
demolding. The water contact angle of the uorinated PDMS
stamp was found to be 130as opposed to 105for freshly
prepared PDMS, indicating the reduction in surface energy.
In a typical nanoimprinting process, the metal-containing
resin solution was spin-coated over a silicon wafer (20 mm
20 mm) at 1000 rpm for 45 s. The PDMS mold was then placed
on top of the wet thin lm and a slight pressure was applied.
This assembly was subjected to UV irradiation with a Hg vapor
lamp (125 W) for 20 min to induce crosslinking of the mono-
mers. Aer UV exposure, the PDMS stamp was carefully
demolded and the imprints were subjected to calcination at
designated temperatures for 1 h to obtain metal oxide micro/
2.4 Characterization
The thiolene photoreaction was following using FT-IR spec-
trophotometer (JASCO FTIR 4200). TGA analyses were carried
out using Shimadzu DTG-60 dierential thermal analyzer in air
atmosphere to follow the degradation prole and the residual
oxide mass present in the metal-containing resins. XRD
patterns of the thin lm samples calcined at dierent temper-
atures were measured at a scan rate of 1min
using Rigaku
Ultima IV X-ray diractometer equipped with Cu K
A). Diuse reectance spectroscopy analysis of the thin lm
samples calcined at dierent temperatures was performed
using JASCO V-670 UV-visible spectrophotometer. The surface
morphology of the imprints before and aer calcination was
characterized using a Carl-Zeiss ULTRA-55 eld emission
scanning electron microscope (FE-SEM).
3. Results and discussion
A judicious choice of precursors is the key to successful
implementation of the thiolene click chemistry to realize oxide
nanofabrication. In this regard, we chose pentaerythritol
tetrakis(3-mercaptopropionate) (PETMP) as the cross-linker
that has four thiol arms. Allyl acetoacetate (AAAc) was chosen
as the bifunctional reagent that on one hand chelates with the
metal center through the acetoacetate group and on the other
hand possesses the reactive alkene group for the click reaction.
When the metal center is chelated with more than one AAAc, it
could also act as a cross-linker, which would lead to the
formation of highly branched polymeric network. The chemical
structures of the various components used in this study are
shown in Fig. 1.
To probe the thiolene click reaction under UV irradiation,
we formulated a resin containing PETMP and AAAc in 1 : 4 ratio
mixed with 3 wt% of 2-hydroxy-2-methylpropiophenone (HMP)
photo-initiator. This metal-free resin was spin-coated over a pre-
cleaned silicon wafer and studied with the Fourier transform
infrared (FT-IR) spectroscopy to follow the thiolene photo-click
chemistry (Fig. 2a). As seen from the gure, the characteristic
SH stretching peak was observed at 2570 cm
, conrming the
presence of free thiol groups in the resin formulation. In
addition, an intense peak at 1738 cm
due to the carbonyl
group stretching and a weak peak at 1632 cm
due to the
alkene group were also observed. Aer subjecting the lm to
20 min of UV irradiation, it remained wet, indicating poor
Table 1 Proportions of various components used in the dierent
TA 1.0 4.0 0.0 0.156
TAP-0.5 1.0 4.0 0.50 0.208
TAP-1 1.0 4.0 1.00 0.277 Fig. 1 Chemical structures of the various components used for
imprinting of metal oxides.
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eciency of the thiolene click-reaction in this system. The FT-
IR spectrum obtained aer irradiation was almost identical to
that before UV irradiation. We postulated that this could be due
to the unfavorably positioned functional groups or reduced
probability of stoichiometric encountering of the allyl and thiol
groups in the thin lm. Therefore, we decreased the amount of
PETMP to half (PETMP and AAAc in 1 : 8 ratio) to provide more
alkene double bonds per SH group so as to enhance the
probability of the reaction at the thiol group sites. Interestingly,
the irradiated lm became dry to yield a smooth photo-cured
lm. The FT-IR corresponding to this composition revealed
the complete disappearance of the SH stretching peak,
strongly corroborating the occurrence of thiolene click reac-
tion (Fig. 2b).
To enable the thiolene chemistry for oxide nanoimprinting,
a photo-curable metal-containing resin formulation was
prepared by mixing titanium-allyl acetoacetate complex
] with PETMP and HMP. Table 1 shows the three resin
formulations that were used for photo-curability studies. The
rst composition (TA), which does not contain PETMP could
undergo [2 + 2] photo-addition of alkene double bonds to yield
cyclobutane derivative. This composition yielded a dry lm aer
UV irradiation. However, the lm was found to be brittle and
easily aked oof the silicon wafer. This could be due to the
excessive photo-curing of the four-arm alkene-functionalized
that caused residual stress in the thin lm.
In the
second composition (TAP-0.5), 1/8 equivalent of PETMP with
respect to Ti(AAAc)
was introduced to keep the eective avail-
able thiol groups per alkene as 0.5. This formulation aer UV
irradiation was found to yield a dry and smooth lm that did
not suer any peeling obehavior. This could be attributed to
the cushioning eect provided by the optimum cross-linking of
PETMP that circumvented cracking of the lm.
In the third composition (TAP-1), 1/4 equivalent of PETMP
with respect to Ti(AAAc)
was used in order to keep the thiol to
alkene ratio 1 : 1. Interestingly, the lm obtained with this
composition was found not fully dry aer 20 min of UV irradi-
ation. This is similar to the poor photo-curing observed with the
metal-free resin containing stoichiometric ratio of allyl to thiol.
Thus, for the remaining characterization and imprinting
studies, we utilized the optimized composition of TAP-0.5.
Fig. 3 shows the FT-IR analyses using TAP-0.5 to probe the
thiolene photo-click reaction. As seen from the spectra, the
characteristic ester and keto carbonyl groups are observed at
1745 and 1718 cm
, respectively. Compared to the metal-free
resin, pristine Ti(AAA)
showed two additional characteristic
peaks at 1610 and 1532 cm
that can be attributed to the
bidendate chelation of the enol form of AAAc.
In addition, the
carboncarbon double bond of allyl group appeared at
1633 cm
. In the case of TAP-0.5, in addition to the above
mentioned stretching peaks, a new peak at 2572 cm
sponding to the thiol appeared. Aer UV light irradiation, the
thiol peak became nearly invisible, while the intensity of allyl
double bond peak diminished. These changes clearly indicate
the occurrence of click reaction between the thiol and allyl
Fig. 2 FT-IR analyses of thiolene photo-curing of resins containing
PETMP and AAAc in the ratio of (a) 1 : 4 (TAP-1) and (b) 1 : 8 (TAP-0.5)
showing that the latter composition is amenable to the click chemistry.
Fig. 3 FT-IR study of thiolene photo-addition in the titanium-con-
taining resin TAP-0.5.
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groups in this system. It is noteworthy that the chelation with
the metal remains intact even aer the UV irradiation, which
conrms that the metal is entrapped inside the cross-linked
polymeric network.
Fig. 4a shows the thermogravimetric analysis (TGA) of TAP-
0.5 performed in air. A steady mass loss was observed
between 100 and 500 C due to the removal of residual solvent,
low molecular weight monomers, and polymers. No further
mass loss was observed above 500 C, suggesting the complete
removal of organics present in the resin. The residual inorganic
mass corresponding to TiO
at 500 C was found to be 10.4%
which is in close agreement with the theoretical estimated value
of 11.9%.
To conrm the phases obtained aer calcination of the UV-
cured TAP-0.5 lms, X-ray diraction (XRD) and Raman spec-
troscopy were performed. The calcination temperature was
varied from 450 to 650 C at a constant duration of 1 h to
monitor the evolution of the phases and the corresponding XRD
patterns are presented in Fig. 4b. As seen from the gure, the
TAP-0.5 lms heat-treated at 450 and 500 C revealed the
formation of phase pure anatase. The broad peaks indicate the
nanocrystalline nature of TiO
obtained at these temperatures.
Above 500 C, the (101) anatase peak was found to intensify with
narrowing of the peak, indicating the growth of nanocrystallites
to larger crystals (JCPDS 89-4921). At and above 600 C,
formation of a small amount of rutile phase is indicated by the
appearance of (110) plane at 2q¼27.4. Applying the Scherrer's
formula to the (101) plane of anatase TiO
, the crystallite size of
the samples calcined at 450, 500, 550, 600 and 650 C are
calculated to be 6.7, 7.1, 12.0, 15.7, and 15.6 nm, respectively.
The Raman spectra of the TiO
samples calcined at dierent
Fig. 4 (a) Thermogravimetric analysis of TAP-0.5 resin. (b) XRD patterns of TiO
obtained by calcination of photo-cured TAP-0.5 thin lms at
various temperatures. (c) Diuse reectance spectra and (d) the corresponding KubelkaMunk plots of TiO
thin lms prepared at dierent
Fig. 5 FE-SEM images of imprinted lines of TAP-0.5 (a and b) before
and (c and d) after calcination.
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temperatures looked identical (Fig. S4). A major peak at
121 cm
along with weak peaks at 365, 482, and 603 cm
observed in all cases that can be attributed to the respective
vibrational symmetries of E
, and E
modes of anatase TiO
Due to the presence in minute
quantity, the Raman peaks of rutile TiO
were not observed in
the samples calcined at 600 and 650 C.
The semiconducting nature of the oxide thin lms obtained
through the thiolene approach has been studied using diuse
reectance spectroscopy. The percentage reectance and their
corresponding KubelkaMunk plots of TiO
calcined at various
temperatures are shown in Fig. 4c and d, respectively. The
KubelkaMunk factor (K) is given by K¼(1 R)
/2R, where Ris
the % reectance. The intersection of the extrapolated linear
portion of the curve in (Kenergy)
versus energy plot with the
energy axis yields the band gap of the semiconductor material.
For direct band gap materials, the value of nis taken as 2, while
for indirect band gap material the value is taken as 1/2. Since
is an indirect band gap material, the value of nused was
1/2. The KubelkaMunk plots show the band gaps of TiO
calcined at various temperatures to be in the range of 3.3 to
3.45 eV. These values agree well with the band gap of anatase
reported in the literature (3.2 to 3.4 eV) and thereby con-
rming the semiconducting nature of the obtained thin
The metal-containing resin formulations were studied for
their suitability for imprinting under laboratory conditions. A
polydimethylsiloxane (PDMS) stamp was pressed over the spin-
coated TAP-0.5 lm and irradiated with UV light. Aer
demolding, the morphology of the photo-cured imprints before
and aer calcination was characterized using eld-emission
scanning electron microscopy (FE-SEM) and the images are
shown in Fig. 5. The PDMS somold used in this study
possessed a feature size of 2.1 mm line and 1.8 mm space
(Fig. S5). Aer patterning of TAP-0.5, the width of the
imprinted line and space was found to be 1.7 and 2.3 mm,
respectively. The shrinkage of the imprinted lines may be
attributed to the absorption of the residual solvent by PDMS
mold and polymerization. Post-calcination, the lines were
further shrunk to 420 nm and the spaces widened to 3.5 mm.
On the whole, the TiO
lines were shrunk to 25% compared to
the as-imprinted lines, which is similar to the values previously
reported PSG-based literature on TiO
The shelf-life of the AAAc-based titanium resin was poor and
thereby warrants immediate imprinting of the formulation. We
speculated that this could be due to the vulnerability of tita-
nium for hydrolysis, which was not averted by the chelated
AAAc. Therefore, we briey studied the shelf-life and imprint-
ability of a titanium resin in which AAAc was replaced with 2-
(methacryloyloxy)ethyl acetoacetate (MAEAA). This formulation
Fig. 6 (a) Thermogravimetric analysis of tantalum-containing resin. (b) XRD patterns, (c) diuse reectance spectra and (d) KubelkaMunk plots
of Ta
obtained at 450 and 700 C.
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was found to have a good shelf life of more than a month.
Furthermore, the successful direct imprinting using this
MAEAA-based thiolene resin conrmed that the approach can
be extended to other chelating monomers as well (Fig. S6).
To substantiate the applicability of this thiolene approach
for patterning other oxides, Ta
was chosen as the candidate
due to its potential applications in semiconducting devices.
Although tantalum is a pentavalent metal, we used 4 equiva-
lents of AAAc in order to match with the corresponding titanium
precursor. The resultant resin was found to have a good shelf
life of more than a month. The PETMP ratio to the AAAc was
xed in the similar fashion to that used in TAP-0.5 and the FT-IR
analyses before and aer UV light exposure were performed
(Fig. S7). The results revealed the disappearance of the SH
stretching peak, indicating the occurrence of thiolene photo-
click chemistry. Fig. 6 shows the TGA, XRD and diuse reec-
tance spectra of the tantalum-containing resin. The TGA prole
was observed to be in similar nature to that of TAP-0.5. The
complete mass loss was found to occur at 530 C and the
residual inorganic mass corresponding to Ta
was observed
to be 20.1%. The XRD pattern of Ta
was found to be broad,
indicating its amorphous nature at this temperature. Therefore,
the UV-cured lm was calcined at 700 C to obtain crystalline
. The XRD analysis of this sample revealed well-dened
orthorhombic crystalline phase of Ta
(JCPDS no. 79-
The diuse reectance and the corresponding Kubelka
Munk plots revealed a shallow spectrum for the 450 C calcined
sample, conrming the amorphous nature of the Ta
. The
thin lm calcined at 700 C was found to exhibit a band gap of
4.26 eV, which is similar to that reported in the literature.
UV imprint lithography of the tantalum-containing resin was
performed and the FE-SEM images corresponding to the as-
imprinted and calcined samples are shown in Fig. 7. The line
and space feature sizes of the as-imprinted patterns were found
to be 1.7 and 2.1 mm, respectively. Aer calcination, the lines
were shrunk to 470 nm and the spaces extended to 3.4 mm.
The cross-sectional view of FE-SEM images of the as-imprinted
and calcined samples showed the heights to be 1.7 mm and
410 nm, respectively. Thus, the lateral and vertical shrinkages
were found to be 75% that is similar to the case of TiO
. The
very similar imprinting behavior of tantalum-containing resin
shows the capability of the thiolene approach to be a potential
route for fabricating nanostructures of a host of oxides that
could nd applications in various devices.
4. Conclusion
In summary, thiolene photo-click chemistry has been
successfully applied to the fabrication of metal oxide micro/
nanostructures via imprinting. When PETMP and AAAc were
mixed to keep the allyl to thiol ratio as 1 : 1, the FT-IR studies
revealed poor eciency of the thiolene photo-click chemistry.
When allyl to thiol ratio was kept as 1 : 0.5, the thiolene photo-
click chemistry was found to be ecient under ambient
conditions, possibly due to the enhanced probability of the
reaction between SH group and the abundant alkene. The
metal-containing resin consisted of chelated titanium alkoxide
with AAAc (or MAEAA) with suitable proportion of PETMP cross-
linker, and HMP photo-initiator. With the optimal amount of
allyl to thiol ratio as 1 : 0.5 (TAP-0.5), the resin exhibited
excellent photo-curability to yield stable, crack-free, and dry
thin lm. The XRD and Raman analyses revealed the formation
of anatase TiO
phase at temperatures below 550 C, above
which a small amount of rutile formation was conrmed with
XRD. The diuse reectance spectroscopy revealed the semi-
conducting nature of the thin lm TiO
. The imprinting studies
demonstrated the patternability of the titanium-containing
resin to fabricate micro/nanostructures of TiO
. Successful
extension of this thiolene photo-click chemistry for patterning
conrmed the leeway of this approach towards fabrica-
tion of various functional oxide nanostructures.
Conicts of interest
There are no conicts to declare.
R. G. thanks the Department of Science & Technology (SERB/F/
825/2014-15) for the nancial aid. He also thanks Department of
Science and Technology fund for improvement of science and
technology infrastructure (DST FIST; SR/FST/CSI-240/2012) to
procure FT-IR, Raman microscopy, and UV-visible spectroscopy.
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... As such, we observe that the decrease in intensity of the FTIR band at 2,572 cm −1 ( Figure 3A) after UV-curing shows characteristic bond between thiolenes and allyl ethers (Nagarjuna et al., 2018). Based on the obtained Raman spectra of cured NOA73 ( Figure 3B) and taking into account the findings reported in the literature (Mongkhontreerat et al., 2013), we can conclude that the main components in NOA73 are triallyltriazinetrione and tris [2-(3-mercaptopropionyloxy)ethyl]isocyanurate in a molar proportion of at least 1:5, using 2,2-dimethoxy-2phenyl-acetophenone as photoinitiator and triallyl isocyanurate as crosslinking catalyst (Sanoria et al., 2015). ...
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Chromate conversion coatings have been in service for decades providing robust corrosion protection to a wide variety of aluminum alloys. However, it is also known that anti-corrosive coatings containing Cr6+ contributes to DNA damage, cause cancer and are not environmentally friendly. Consequently, regulatory restrictions over the use Cr6+ were established to mitigate the environmental damage and health problems. To answer to this hurdle and to meet the emergent need for environmentally friendly anti-corrosive coatings, we have successfully developed an innovative coating that combines anti-corrosive, low electrical resistance, and self-healing properties. First, we present two different coatings, that aim to display low electrical resistance properties: one containing only graphene and the other containing Zn nanoparticles and graphene. Confocal laser imaging and SEM microscopy was used to observe the morphology of the coatings. The electrical resistance was measured using the 4-wire connection Kelvin method. We compare the anticorrosive response for both coatings under neutral salt spray test (NSSt). Raman spectroscopy was performed before and after to understand the effect of NSSt corrosive species on the coatings. Then, we select the coating with lower electrical resistance, and we program on it a self-healing mechanism to boost its life service. Finally cyclic voltammetry is performed to confirm the excellent blocking properties of the tested coatings. All the coatings presented in this work are applied on aluminum AA 2024T351 and the optimal spray parameters for nanofillers dispersion are obtained. Our findings show great potential for preventing corrosion and compatibility with fully automated large-scale applications in different fields such as aerospace, automotive, construction, submarines and many more.
... Also, the peak at 1633 cm -1 of the carbon-carbon double bond in CAB modified with TMPTA and acrylate groups was observed to decrease the density of the peak of the carbon-carbon double bond since most of the acrylate groups were spent cross-linking with free -SH or other acrylate groups. Nagarjuna et al. have reported a similar spectrum in their films prepared using thiol-ene click reaction under UV rays (Nagarjuna et al. 2018). ...
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In the present study, a novel, eco-friendly, and simple polymeric adsorbent was obtained from cellulose acetate butyrate (CAB) and cysteine (Cys) to remove silver (I) ions in the presence of a thiol-ene click reaction under UV rays. Accordingly, firstly, CAB was modified with acrylate groups to turn it into a photocurable resin. Then, the acrylate modified CAB obtained with the monomer having cysteine and triacrylate group was crosslinked by thiol-ene click reaction under UV rays to obtain an eco-friendly adsorbent. The adsorbent acquired –S–, –COO and –NH2 groups as a result of this reaction and these functional groups are known to have the ability to selectively couple with Ag(I) ions. The adsorbent was evaluated for the selective removal of Ag(I) ions from aqueous samples characterized by SEM, SEM-EDAX, and FTIR. To determine the most suitable conditions, the effect of important parameters such as pH and contact time was investigated. It was determined that the most suitable isotherm for adsorption was the Langmuir isotherm (R: 0.999). The efficacy of our eco-friendly adsorbent was investigated in a real stream and lake waters and very good results were achieved. Graphic abstract
... Fig. 5 shows the FT-IR spectra of 1,3-propane dithiol and crosslinked membranes of 1.30Q0.50Pr. The absorption band at 2557 cm −1 was attributed to the stretching of thiol group which is almost the same as the reported value [40]. After crosslinking by thiol-ene reaction, this absorption band diminished. ...
In this study, we report the synthesis and properties of new anion exchange membranes using m-polybenzimidazole (m-PBI) as polymer backbone attached with quaternary ammonium group and alkene group as side chains. These ionic m-PBIs were crosslinked by thermal thiol-ene reaction. The ion exchange capacity (IEC) values and crosslinking degrees were controlled independently by the amount of attached quaternary ammonium groups and alkene groups, respectively. m-PBI based AEMs with IEC from 1.51 to 2.50 mmol/g exhibit hydroxide conductivity of 14 to 58 mS/cm at 80 °C. With similar IEC values, crosslinked membranes absorb less water but show similar hydroxide conductivity and better dimension stability than uncrosslinked membranes. A crosslinked membrane with an IEC value of 2.46 mmol/g can retain 84% of its pristine hydroxide conductivity after being immersed in 1.0 M sodium hydroxide solution at 60 °C after 720 h. These membranes in hydrated condition also show mechanical robustness with tensile strength of 10.8 to 14.6 MPa. From the high hydroxide conductivity, enhanced alkaline stability and good mechanical properties, crosslinked ionic m-PBIs are promising candidate as anion exchange membranes for fuel cell application.
Thermally conductive (TC) adhesive tapes consisting of adhesives and TC fillers are widely used as thermal interface materials because of their tack properties for temporary fixation and high peel strength for permanent bond. However, the trade-off between peel strength and thermal conductivity with increasing filler loading impedes the manufacture of high-performance TC tapes. Therefore, a technique to realize high thermal conductivity of TC adhesives with lower filler loading is crucial. In this study, epoxy-based adhesives (EAs) was used to fabricate TC adhesive tapes. To secure high peel strength of EAs, a cyclic carbonate-terminated oligomer (CCO) was synthesized and employed as a component. The simultaneous reactions between epoxy resin, CCO and curing agent were analyzed using FTIR spectroscopy. Thanks to tack, the partially cured EAs could be used to fabricate tapes which were temporarily fixed and then permanently bonded via complete curing. Next, EAs and h-BN fillers were admixed to produce TC adhesive tapes. Because h-BN fillers were partially aligned to the in-plane direction by bar-coating, the in-plane thermal conductivity of the TC-1 adhesive containing only 20 wt% h-BN fillers was as high as 2.73 W/m K, which was slightly lower than that (2.97 W/m⋅K) of the TC-4 adhesive containing 35 wt% h-BN fillers. In addition, the peel strength of the TC-1 adhesive tape reached 2307 (± 64) gf/in.
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In this study, new anion exchange membranes (AEM) based on crosslinked polybenzimidazole (m-PBI) with quaternary ammonium groups, crosslinkable allyl groups, and hydrophobic ethyl groups as side chains are synthesized and characterized. The AEMs are crosslinked by thermal thiol-ene reaction using a dithiol crosslinker. The ion exchange capacity (IEC) values and crosslinking density were controlled by the number of quaternary ammonium groups and allyl groups, respectively. The introduction of ethyl groups improved the solubility of ionic PBIs even at very low IEC values by eliminating the hydrogen bonding interaction of imidazole rings. This method allows ionic PBIs with broad IEC values, from 0.75 to 2.55 mmol/g, to be prepared. The broad IEC values were achieved by independently controlling the numbers of quaternary ammonium groups, allyl groups, and hydrophobic ethyl groups during preparation. The crosslinked ionic PBIs revealed hydroxide conductivity from 16 to 86 mS/cm at 80°C. The wet membranes also showed excellent mechanical strength with tensile strength of 12.2 to 20.1 MPa and Young's Modulus of 0.67 to 1.45 GPa. The hydroxide conductivity of a crosslinked membrane (0.40Q0.60Et1.00Pr, IEC = 0.95 mmol/g) decreased only 7.9% after the membranes was immersed in a 1.0 M sodium hydroxide solution at 80°C for 720 h. A single fuel cell based on this membrane showed a maximum peak power density of 136 mW/cm² with a current density of 377 mA /cm² at 60°C. Abstract
The study aims to obtain simple, transparent, and flexible coatings that are produced using an environmentally-friendly method, have either positive or negative charge density on their surfaces, and have antibacterial properties. For this purpose, poly(ethylene glycol) diacrylate (PEGDA), trimethylolpropane triacrylate (TMPTA), and trimethylolpropane tris(3-mercaptopropionate) (TMPTMP) were crosslinked using the thiol-ene click reaction in the presence of UV light. Ammonium salt is known to have antibacterial properties due to its positively charged surface and was added to the coating formulation to positively charge the surface of the glycidyltrimethylammonium chloride (GTMAC) coating and negatively charge the sodium 3-mercapto-1-propanesulfonate (3-MPS) surface. The thiol-ene click reaction was used in the coatings that were prepared by using the base formulation and adding 3-MPS while the thiol-ene and thiol-epoxy click reactions were used together for the addition of GTMAC to the formulation. The structural characterization of the coatings was made using FTIR, the thermal behaviors of the coatings were determined using thermogravimetric analysis (TGA), and the morphologies and chemical compositions of the coatings were examined using SEM and SEM-EDAX images. The antibacterial activities of the coatings were tested using the inhibition zone method against Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria that have different cell wall properties and can serve as models for similar bacteria.
The control of gate dielectric properties of vinyl-addition polynorbornene copolymers bearing pendant vinyl groups for OFET applications is demonstrated through photoinitiated thiol–ene click chemistry.
CsPbBr3 quantum dots (QDs) - NOA63 nanocomposites (NCs) are fabricated by using a UV-initiated thiol−ene “click” reaction, and the hydrophobic CsPbBr3 QDs are successfully embedded in the NOA63 matrix without chemical decomposition. The CsPbBr3 QDs - NOA63 NCs are utilized as a piezoelectric layer for piezoelectric nanogenerators (PENGs), which are compared with PENGs based on NOA63 and on CsPbBr3 QDs. The average output voltages of the PENGs based on the CsPbBr3 QDs - NOA63 NCs is significantly larger than that of the PENGs with CsPbBr3 QDs alone by a factor of 5.55. The significantly higher output voltages of the PENGs based on the CsPbBr3 QDs - NOA63 NCs is attributed to the very large piezoelectric coupling efficiency due to the utilization of NOA63. Moreover, such PENGS are found to have advantages such as an effective stress transfer, a morphology passivation effect and an enhanced piezoelectric response, and chemical compatibility with CsPbBr3 QDs.
Present study reports on the sorption study of chromium(VI), cadmium(II) ions and methylene blue dye by pristine, defatted and carbonized Nigella sativa L. seeds from aqueous solution. The removal of oil from pristine Nigella sativa L. (PNS) seeds was carried out by defatting the Nigella sativa with acetone and N,N-dimethylformamide and then labelled ANS and DNS, respectively. Thereafter the defatted ANS and DNS adsorbents were carbonized at 600 ºC for 2 h under nitrogen and labelled as CANS and CDNS. The results of pristine, defatted and carbonized seeds were compared. The removal of Cr(VI), Cd(II) and methylene blue dye from aqueous solutions was investigated by varying adsorbate concentration, solution pH, reaction contact time and temperature of the solution. The SEM images indicated that the surface morphology of PNS was irregular, whilst ANS and DNS had pores and cavities. CANS and CDNS was heterogeneous and had pores and cavities. FTIR spectroscopy showed that the adsorbents surfaces had bands that indicated a lot of oxygen containing groups. The pH of the solution had an influence on the removal uptake of Cr(VI), Cd(II) and methylene blue. The sorption of Cr(VI) decreased when pH of the solution was increased due to different speciation of Cr(VI) ions whilst the removal of Cd(II) and methylene blue increased when solution pH was increased. Pseudo first order kinetic model well described the adsorption of Cr(VI), Cd(II) and methylene blue onto PNS. On the other hand, the kinetic data for ANS, CANS, DNS and CDNS was well described by pseudo second order. Furthermore, the removal mechanism onto PNS and ANS was better described by Freundlich multilayer model. The CANS, DNS and CDNS fitted Langmuir monolayer model. Thermodynamic parameters indicated that the sorption processes of Cr(VI), Cd(II) and methylene blue was endothermic and effective at high temperatures for all adsorbents. The ΔSº and ΔHº had positive values this confirmed that the sorption of Cr(VI), Cd(II) and methylene blue onto all adsorbents was random and endothermic, respectively. The values of ΔGº confirmed that the sorption of Cr(VI), Cd(II) and methylene blue on all adsorbents was spontaneous and predominated by physical adsorption process. The CANS had highest adsorption capacity of 99.82 mg/g for methylene blue, 96.89 mg/g for Cd(II) and 87.44 mg/g for Cr(VI) followed by CDNS with 93.90, 73.91 and 65.38 mg/g for methylene blue, Cd(II) and Cr(VI), respectively. The ANS capacities were 58.44, 45.28 and 48.96 mg/g whilst DNS capacities were 48.19, 32.69 and 34.65 mg/g for methylene blue, Cd(II) and Cr(VI), respectively. PNS had the lowest sorption capacities at 43.88, 36.01 and 19.84 mg/g for methylene blue, Cd(II) and Cr(VI), respectively.
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It is a subject of exploration whether the phase pure anatase or rutile TiO2 or the band alignment due to the heterojunctions in the two polymorphs of TiO2 plays the determining role in efficacy of a photocatalytic reaction. In this work, the phase pure anatase and rutile TiO2 have been explored for photocatalytic nitroarenes reduction to understand the role of surface structures and band alignment towards the reduction mechanism. The conduction band of synthesized anatase TiO2 has been found to be more populated with electrons of higher energy than that of synthesized rutile. This has given the anatase an edge towards photocatalytic reduction of nitroarenes over rutile TiO2. The other factors like adsorption of the reactants and the proton generation did not play any decisive role in catalytic efficacy.
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Micropatterning of quantum dot (QD)-polymer nanocomposites exhibiting optically and electrically programmable properties in a large area with an organized array has been of great importance in various applications using photonic and optoelectronic devices. Here, we report the fabrication of a micropatterned QD-polymer composite exhibiting superior photoluminescence (PL) properties employing thiol-ene reaction and imprinting lithography techniques. The presence of positive charge on the QDs enhances the dispersion interaction between the QDs, and the presence of positively charged QDs containing allylic groups enables the QDs to form strong chemical binding to the polymer matrix. This phenomenon was confirmed using three different QDs; a neutral QD (QD-TOH), positive QD (QD-TTMA), and positive QD containing allylic groups (QD-TDMA-Ene). The synergetic effect of charge repulsion and chemical binding observed in our system induces a high PL with robustness of micropatterns. Moreover, the PL intensity linearly depends on the QD content in the polymer matrix, indicating easy controllability of the system, which can be used as a light-emitting source.
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New UV-cured allyl-eugenol networks have recently been developed as efficient antibacterial materials by using "click" chemistry. In recent years, multifunctional zinc oxide nanoparticles (ZnO NPs) have increasingly been used as effective antimicrobial inorganic compounds. The originality of this work lies in combining ZnO NPs with eugenol-based networks so as to enhance their antibacterial properties. This investigation highlights a straightforward one-step route to sustainable hybrid materials whose antibacterial properties against Staphylococcus aureus and Echerichia coli are improved without altering the network properties. These new hybrid materials with bioactive surface could be used for many biomedical applications.
Conjugated microporous polymers (CMPs) are a class of crosslinked polymers that combine permanent micropores with π- conjugated skeletons and possess three-dimensional (3D) networks. Compared with conventional materials such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), the CMPs usually have superior chemical and thermal stability. CMPs have made significant progress in heterogeneous catalysis in the past seven years. With a bottom-up strategy, catalytic moieties can be directly introduced into in the framework to produce heterogeneous CMP catalysts. Higher activity, stability, and selectivity can be obtained with heterogeneous CMP catalysts in comparision with their homogeneous analogs. In addition, CMP catalysts can be easily isolated and recycled. In this review, we focus on CMPs as an intriguing platform for developing various highly efficient and recyclable heterogeneous catalysts in organic reactions. The design, synthesis and structure of these CMP catalysts are also discussed in this focus review.
Dimensionally stable 1-D, 2-D and 3-D high aspect ratio crystalline metal oxide nanostructures are fabricated using soft nanoimprint lithography with inks comprised of nanoparticle (NP) dispersions in solvent or in sol-gel precursors for the metal oxide. Crystalline TiO2 and indium tin oxide (ITO) NP dispersions in solvent are imprinted using a solvent permeable patterned poly(dimethylsiloxane) (PDMS) stamp to yield robust crystalline nanostructures that are dimensionally stable to calcination (less than 8% linear shrinkage in imprinted feature heights upon heat treatment at 500 °C). Inks comprised of 80% crystalline NPs dispersed in 20% sol-gel binder are patterned using thermal- or UV-assisted imprinting with a PDMS stamp. The composition and physical properties of the dimensionally stable imprinted metal oxides (TiO2 and ITO) can be altered by varying the composition of the ink. Rapid printing of high aspect ratio nanostructures and sub-100-nm features are easily realized. Residual layer free, direct imprinting of isolated features is achieved by using an ink with the appropriate surface energy to ensure dewetting at stamp-substrate interface. The technique is extended to create 3-D mesh nanostructures by deploying a simple layer-by-layer imprint strategy. TiO2 3-D mesh nanostructures are robust and mechanically stable to calcination at temperatures of 1000 °C, which results in an anatase to rutile transition. The direct fabrication of high quality dimensionally stable metal oxide nanostructures opens the door to solution based and roll-to-roll processing of robust and efficient inorganic electronic, optical, and energy generation and storage devices.
The requirement of high-temperature calcination for titanium dioxide in (solid-state) dye-sensitized solar cells (DSSCs) implies challenges with respect to reduced energy consumption and the potential for flexible photovoltaic devices. Moreover, the use of dye molecules increases production costs and leads to problems related with dye bleaching. Therefore, fabrication of dye-free hybrid solar cells at low temperature is a promising alternative for current DSSC technology. In this work the authors fabricate hierarchically structured titania thin films by combining a polystyrene-block-polyethylene oxide template assisted sol–gel synthesis with nano-imprint lithography at low temperatures. The achieved films are filled with poly(3-hexylthiophene) to form the active layer of hybrid solar cells. The surface morphology is probed via scanning electron microscopy and atomic force microscopy, and the bulk film morphology is examined with grazing incidence X-ray scattering. Good light absorption by the active layer is proven by UV–vis spectroscopy. An enhancement in light absorption is observed and ascribed to light scattering in mesoporous titania films with imprinted superstructures. Accordingly a better photovoltaic performance is found for nano-imprinted solar cells at various angles of light incidence.
Although known for more than 40 years in the polymer chemistry field, the photochemical radical thiol–ene addition (PRTEA) has been recently recognized as a chemical reaction with click characteristics. Photoinitiation enables spatial and temporal control of this highly efficient reaction, bridging simple organic chemistry with high-end materials synthesis and surfaces functionalization. In this minireview, we focus on the latest contributions based on the PRTEA for the synthesis of chemical precursors for silica and transition metal oxides (TMO) based materials. We summarize the mechanism of the PRTEA, the development of new families of photoinitiators and how this extremely simple approach has spilled over into the materials science arena with clear success. In particular, PRTEA adds to the collective efforts for building a reliable and straightforward chemical toolbox for surface modification and the production of sol–gel precursors, nanoparticles and thin films. The excellent perspectives for simple molecular and supramolecular building block synthesis opens up a rational synthetic route for the design and integration of these components in multipurpose platforms.
Encoded microparticles are high demand in multiplexed assays and labeling. However, the current methods for the synthesis and coding of microparticles either lack robustness and reliability, or possess limited coding capacity. Here, a massive coding of dissociated elements (MiCODE) technology based on innovation of a chemically reactive off-stoichimetry thiol-allyl photocurable polymer and standard lithography to produce a large number of quick response (QR) code microparticles is introduced. The coding process is performed by photobleaching the QR code patterns on microparticles when fluorophores are incorporated into the prepolymer formulation. The fabricated encoded microparticles can be released from a substrate without changing their features. Excess thiol functionality on the microparticle surface allows for grafting of amine groups and further DNA probes. A multiplexed assay is demonstrated using the DNA-grafted QR code microparticles. The MiCODE technology is further characterized by showing the incorporation of BODIPY-maleimide (BDP-M) and Nile Red fluorophores for coding and the use of microcontact printing for immobilizing DNA probes on microparticle surfaces. This versatile technology leverages mature lithography facilities for fabrication and thus is amenable to scale-up in the future, with potential applications in bioassays and in labeling consumer products.
Micropatterned TiO2 nanorods (TiO2NRs) via three-dimensional (3D) geometry engineering in both microscale and nanoscale decorated with graphene quantum dots (GQDs) have been demonstrated successfully. First, micropillar (MP) and microcave (MC) arrays of anatase TiO2 films are obtained through the sol-gel based thermal nanoimprinting method. Then they are employed as seed layers in hydrothermal growth to fabricate the 3D micropillar/microcave arrays of rutile TiO2NRs (NR), which show much-improved photoelectrochemical water-splitting performance than the TiO2NRs grown on flat seed layer. The zero-dimensional GQDs are sequentially deposited onto the surfaces of the microscale patterned nanorods. Owing to the fast charge separation that resulted from the favorable band alignment of the GQDs and rutile TiO2, the MP-NR-GQDs electrode achieves a photocurrent density up to 2.92 mA cm(-2) under simulated one-sun illumination. The incident-photon-to-current-conversion efficiency (IPCE) value up to 72% at 370 nm was achieved on the MP-NR-GQDs electrode, which outperforms the flat-NR counterpart by 69%. The IPCE results also imply that the improved photocurrent mainly benefits from the distinctly enhanced ultraviolet response. The work provides a cost-effective and flexible pathway to develop periodic 3D micropatterned photoelectrodes and is promising for the future deployment of high performance optoelectronic devices.
TiO2 synthesized by polymerizable sol–gel approach is used to demonstrate the photoreduction of Cr(VI) in aqueous medium. Fe3O4 is chosen as the magnetically recoverable support, onto which, 10, 20, 30 and 50% of TiO2 have been dispersed following the polymerizable sol–gel approach. All the synthesized materials are characterized using XRD, SEM, diffuse reflectance spectroscopy and BET surface area measurement. Different hole-scavengers (oxalic acid, ammonium oxalate and ethanol) and an electron scavenger (potassium periodate) have been studied to compare the efficiency of Cr(VI) photoreduction in nitrogen as well as air atmosphere. With 5 mM of oxalic acid as the hole-scavenger, the TiO2/Fe3O4 materials have demonstrated superior activity to the non-supported bulk TiO2 towards Cr(VI) photoreduction in aerobic atmosphere. Particularly, 30% TiO2/Fe3O4 has shown the highest Cr(VI) photoreduction rate among the lot. The 30% TiO2/Fe3O4 catalyst has also demonstrated good recoverability as well as recyclability.