Stabilization of ultra-small gold nanoparticles in a photochromic organic cage: modulating photocatalytic CO2reduction by tuning light irradiation

Article abstract of DOI:10.1039/d0ta12195a

Synthesis and stabilization of ultra-small metal nanoparticles (MNPs) composed of a few atoms are of paramount importance in modulating their material properties based on quantum confinement effects. The highly reactive surface of small MNPs tends to aggregate, resulting in bigger particles and subsequent deterioration of the catalytic activity. In this work, we exploited a dithienylethene (DTE) based photochromic organic cage (TAE-DTE) for thein situstabilization of ultra-small Au NPs (Au@TAE-DTE) (<2 nm) through a unique “reverse double-solvent approach (RDSA)”. The Au@TAE-DTE showed similar photochromic properties to the TAE-DTE and shuttled between two metastable photoisomers, Au@TAE-DTE-O and Au@TAE-DTE-C, based on DTE ring opening and closing upon treating with visible and UV-light, respectively. The Au@TAE-DTE hybrid showed visible light (λ= 400-750 nm) driven catalytic activity for photochemical CO₂reduction to CO. Importantly, irradiating with light of the full range (λ= 250-750 nm) allowed for co-existence of both photoisomers which thereby showed wide spectrum absorption as compared to individual photoisomers, consequently displaying substantially enhanced performance for the photocatalytic CO₂reduction. Further, the real-time progress of the CO₂reduction reaction and corresponding reaction intermediates was detected by anin situDRIFT experiment.

Full text of DOI:10.1039/d0ta12195a

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Stabilization of ultra-small gold nanoparticles in
a photochromic organic cage: modulating
photocatalytic CO2 reduction by tuning light
irradiation†
Cite this: J. Mater. Chem. A, 2021, 9,
5780
Ashish Singh, ‡ Parul Verma, ‡ Debabrata Samanta,
Anupam Dey,
*
Jyotirmoy Dey and Tapas Kumar Maji
Synthesis and stabilization of ultra-small metal nanoparticles (MNPs) composed of a few atoms are of
paramount importance in modulating their material properties based on quantum confinement effects.
The highly reactive surface of small MNPs tends to aggregate, resulting in bigger particles and
subsequent deterioration of the catalytic activity. In this work, we exploited a dithienylethene (DTE)
based photochromic organic cage (TAE-DTE) for the in situ stabilization of ultra-small Au NPs (Au@TAE-
DTE) (<2 nm) through a unique “reverse double-solvent approach (RDSA)”. The Au@TAE-DTE showed
similar photochromic properties to the TAE-DTE and shuttled between two metastable photoisomers,
Au@TAE-DTE-O and Au@TAE-DTE-C, based on DTE ring opening and closing upon treating with visible
and UV-light, respectively. The Au@TAE-DTE hybrid showed visible light (l ¼ 400–750 nm) driven
catalytic activity for photochemical CO2 reduction to CO. Importantly, irradiating with light of the full
range (l ¼ 250–750 nm) allowed for co-existence of both photoisomers which thereby showed wide
spectrum absorption as compared to individual photoisomers, consequently displaying substantially
enhanced performance for the photocatalytic CO2 reduction. Further, the real-time progress of the CO2
reduction reaction and corresponding reaction intermediates was detected by an in situ DRIFT experiment.
Received 16th December 2020
Accepted 24th January 2021
DOI: 10.1039/d0ta12195a
rsc.li/materials-a
stabilization of MNPs.5,9–13 However, the large pore sizes of
MOFs/COFs are oen associated with the drawbacks of irreg-
Introduction
ular distribution, leaching, and regrowth of MNPs eventually
leading to suppressed performance of MNPs.14 Thus, stabiliza-
tion of ultrasmall MNPs (<2 nm) is a challenging task which
impedes their application in the domain of catalysis.
The synthesis and stabilization of ultrasmall metal nano-
particles (MNPs) or nanoclusters (MNCs) have gained consid-
erable attention in recent years owing to their wide range of
applications in the eld of catalysis, photophysics, chemical
biology and therapeutics.1–3 The performances of MNPs are
mainly size dependent due to the “quantum size effect”3–8 and
small MNPs (<2 nm) are considered to be in the boiling state
because of their high surface free energy and therefore show
enhanced catalytic activity as compared to the large MNPs (>2
nm).4,8 In contrast, small MNPs tend to aggregate and conse-
quently hinder the catalytic activity as well as stability in pro-
longed reactions. In general, porous materials having
functionalized pore surfaces such as metal/covalent organic
frameworks (MOFs/COFs), silica-based materials and various
capping agents are being employed for the growth and
Recently, discrete organic cages (OC) bearing guest acces-
sible cavities have emerged as promising candidates for stabi-
lization of ultrasmall sized MNPs.15–19 Xu and co-workers
demonstrated an excellent method called the “reverse double-
solvent approach (RDSA)” for in situ stabilization of ultra-ne
Pd NPs in organic cages and the resulting hybrid showed high
dispersibility and ability to catalyze organic reactions effi-
ciently.17 Similarly, we envisioned that the stabilization of
ultrane gold NPs in an organic cage15 using the RDSA could be
a novel pathway to develop visible-light-driven photocatalysts
for the CO2 reduction reaction.16
It is noteworthy that photochemical CO2 reduction to solar
fuels with high selectivity and efficiency has been an intense
research topic in recent times as it does not only address global
warming but also provides a long-term solution for the shortage
of energy and carbon resources.20–24 However, this is a complex
process where multiple electron and proton transfers are
involved and therefore poses signicant challenges to the
Molecular Material Laboratory, Chemistry and Physics of Material Unit, School of
Advanced Materials (SAMat), Jawaharlal Nehru Centre for Advanced Scientic
Research, Jakkur, Bangalore-560064, India. E-mail: tmaji@jncasr.ac.in; Web: http://
www.jncasr.ac.in/tmaji
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ta12195a
‡ These authors contributed equally.
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researcher towards developing materials for photocatalytic CO2
reduction with high efficiency and selectivity.25–28 To this end,
discrete organic cages that can absorb visible light, exhibiting
high dispersibility, could be an elegant design to display
impressive photocatalytic activity towards CO2 reduction.29,30 In
this context, photochromic organic cages (POCs) could be an
ideal system for realizing photocatalytic properties as they
exhibit a broad absorption from UV to the visible region as
a result of photo-responsive isomerization.29,31–37 Thus, photo-
catalytic activities of such photochromic cages can be inu-
enced by modulating the irradiation light.38 Further, owing to
small cavity sizes, suitably functionalized POCs can act as
a template for stabilization of ultra-small metal nanoparticles
(NP@POCs) (example; Ag, Au and Cu) using the RDS approach.5
These NP@POC hybrids will not only provide catalytic sites but
also enhance the light-absorbing properties owing to their
localized surface plasmon band in the visible region, thereby
enhancing the photocatalytic activity.17 Importantly, the pho-
tocatalytic activity of organic cages towards CO2 reduction is
under-explored and needs to be developed rationally.39
Fig.
1
(a) Photo-isomers (TAE-DTE-O and TAE-DTE-C) of the
photochromic organic cage. (b) UV-Vis spectra in acetonitrile (10–5 M).
(c) FESEM and (d) TEM image for TAE-DTE-O.
real-time progress of the CO2 reduction reaction has been
tracked by in situ Diffuse Reectance Infrared Fourier Trans-
form (DRIFT) spectroscopic study.
In this work, we have exploited a dithienylethene (DTE)
based photochromic organic cage27 (Scheme 1, Fig. 1a), namely
TAE-DTE, for the in situ stabilization of ultra-small Au nano-
particles (<2 nm) through a reverse double-solvent approach
(RDSA).17,40,41 Interestingly, the size of Au NPs in the TAE-DTE
cage was found to be approximately 1.3 nm which was signi-
Results and discussion
cantly smaller than that in an earlier report.29 The gold nano- Synthesis and characterization of the photochromic organic
composite (Au@TAE-DTE) showed photochromism similar to cage (TAE-DTE) was reported recently,29 and we have followed
the bare cage and displayed distinguished UV-Vis absorption a similar procedure with slight modication (see the ESI,
spectra for their metastable photo-isomers. Next, the photo- Scheme S1 & S2†). In brief, the TAE-DTE cage was synthesized
isomer, Au@TAE-DTE-O (DTE open form), showed appreciable via Schiff base condensation between 4,40-(peruorocyclopent-
visible-light-driven photocatalytic activity for CO2 reduction to 1-ene-1,2-diyl)bis(5-methylthiophene-2-carbaldehyde)
(DTE-
CO (rate ¼ 56 mmol gꢀ1 hꢀ1, selectivity >78%). More impor- dialdehyde) and 2,20,200-triaminotriethylamine (TAE) followed
tantly, co-existence of both photoisomers, Au@TAE-DTE-O and by reductive amination using concentrated aqueous NaBH4 (see
Au@TAE-DTE-C (DTE closed form), in full-range light (250–750 the ESI† for details). The POC was characterized using various
nm) displayed wide absorbance and therefore activity and spectroscopic techniques, and the characterization data were
selectivity towards photocatalytic CO2 reduction was increased found to be in good agreement with the literature report (see the
signicantly (rate ¼ 105 mmol gꢀ1 hꢀ1, selectivity >85%). The ESI, Fig. S1–S4†).29 The TAE-DTE-O formed an ultra-ne stable
dispersion in common organic solvents (chloroform, methanol
and ethyl acetate) as conrmed by the Tyndall effect (Fig. S5†).
Notably, this dispersion cannot be distinguished by the naked
eye and looks similar to the solution state (Fig. S5†). However,
a poor dispersion was observed in water due to the presence of
hydrophobic peruoro substitutions on the exterior surface.42
Field Emission Scanning Electron Microscopy (FE-SEM) studies
of the TAE-DTE-O showed the nano spherical morphology (100–
150 nm) which was further supported by Transmission Electron
Microscopy (TEM) analysis (Fig. 1c and d). PXRD measurements
showed a broad peak at 2q ¼ 18.5ꢁ (4.79 A), suggesting the
˚
absence of long-range ordering in the TAE-DTE-O cage
(Fig. S6†).
The yellow coloured TAE-DTE-O changed to pink coloured
photoisomers (TAE-DTE-C) upon UV irradiation for 10 seconds
due to DTE ring-closing and the opposite was achieved upon
visible light irradiation for 25 seconds as a result of DTE ring
Scheme 1 Schematic representation of a photochromic organic cage
opening (Fig. 1a). UV-Vis spectra for both the photoisomers
(10ꢀ5 M) were recorded in acetonitrile and found to be similar
and stabilization of ultra-small gold nanoparticles toward visible light
driven photocatalytic CO2 reduction with the hybrid cage material.
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to the literature report.29 TAE-DTE-O showed broad absorbance cage using the RDSA, aiming to exploring the photocatalytic
in the UV-region (250–330 nm), whereas TAE-DTE-C showed activity of the hybrid material towards CO2 reduction (Fig. 2a).43
additional absorption in the visible region (450–580 nm) with In the RDSA, TAE-DTE-O (55 mg) possessing hydrophobic
an absorption maximum at 520 nm (Fig. 1b). The optical cavities was dispersed in water (10 ml), and the metal precursor
HOMO–LUMO band gap was calculated from the Tauc plot (AuCl3$6H2O) (15 mg) was dissolved in a hydrophobic solvent
derived from the absorption spectra and found to be 3.15 eV for (CH2Cl2) (15 ml) and added to the TAE-DTE-O dispersion. The
TAE-DTE-O and 2.25 eV for TAE-DTE-C (Fig. S7†). Next, the reaction mixture was stirred for 3 h. The metal precursor
quantum efficiency for the conversion of TAE-DTE-O to TAE- reached inside the POC through hydrophobic–hydrophobic
DTE-C and vice versa upon UV and visible light irradiation interactions between the metal solution and POC cavity. Finally,
was calculated to be 62% and 53%, respectively (see the ESI, the reaction mixture was treated with highly concentrated
Fig. S8–S9†). The maximum conversion of TAE-DTE-O to TAE- aqueous NaBH4 (30 mg in 100 ml) and further stirred for 3 h.
DTE-C in the photo-stationary state (PSS) was observed to be This resulted in the reduction of metal ions (Au3+) to ultra-ne
77.4%. The PXRD pattern and FE-SEM and TEM images of TAE- Au nanoparticles. Interestingly, the Au@TAE-DTE-O hybrid also
DTE-C were found to be similar to those of the TAE-DTE-O formed a stable and ne dispersion similar to the POC as
(Fig. S6
& S10†). Further, an optimized structure was conrmed through the Tyndall effect (Fig. 2b and S14†). The
computed for both TAE-DTE-O and TAE-DTE-C (Fig. S11 & PXRD pattern of Au@TAE-DTE-O showed a distinguishable
S12†). The time-dependent density functional theoretical (TD- peak at 38.9ꢁ, which conrmed the presence of gold NPs in the
DFT) computation, at the D3-B3LYP/6-31G(d) level, suggested cubic phase (Fig. S15†).44 The FT-IR spectrum of AuTAE-DTE-O
that the TAE-DTE-O was found to be stabilized by 54 kcal molꢀ1 was observed to be similar to that of the TAE-DTE-O cage,
as compared to the TAE-DTE-C. The HOMO–LUMO band gap suggesting non-covalent stabilization of Au NPs in the POC
for the TAE-DTE-O was calculated to be 3.66 eV which was (Fig. S16†). The loading amount of Au in the Au@TAE-DTE-O
reduced signicantly to 2.72 eV for the TAE-DTE-C (Fig. S13†). hybrid was determined by inductively coupled plasma-atomic
The UV-Vis transition in TAE-DTE-O (lmax ¼ 290 nm) appeared emission spectroscopy (ICP-AES) and found to be 2.2 wt%.
due to p / p* electronic transition from the HOMOꢀ1 to the The high-resolution TEM (HR-TEM) image indicated the
LUMO+2 (296 nm, f ¼ 1.07) (Fig. S11†). In addition to this type uniform distribution of ultrasmall sizes of Au NPs (<2 nm) on
of transition, TAE-DTE-C also exhibited a low-energetic p / p* spheres (Fig. 2d and e). Notably, lattice fringes of 0.213 nm were
transition at 537 nm from the HOMO to the LUMO with rela- observed in HRTEM images, indicating the orientation of cubic
tively lower intensity (f ¼ 0.24) (Fig. S12†).
Au NPs in the (111) plane (Fig. 2e).44 The high-angle annular
Next, we attempted in situ growth and stabilization of ultra- dark-eld scanning transmission electron microscopy (HAADF-
small gold nanoparticles (Au NPs) (<2 nm) in the TAE-DTE-O STEM) image of Au@TAE-DTE-O hybrid also showed spherical
Fig. 2 (a) Schematic representation for in situ growth and stabilization of ultrasmall gold nanoparticles (Au NPs) inside the organic cage. (b)
Au@TAE-DTE-O in acetonitrile (left) and the Tyndall effect for the same sample (right). (c) Deconvoluted XPS spectrum for Au NPs. Morphology
of Au@TAE-DTE-O: (d) low resolution TEM image (inset shows Au NPs inside spheres); (e) high resolution TEM image (inset picture shows lattice
fringes); HAADF-STEM images: (f) low resolution and (g) high resolution. (h) Histogram for the particle size of Au NPs obtained by HAADF-STEM.
5782 | J. Mater. Chem. A, 2021, 9, 5780–5786
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morphology (ꢂ150 nm), wherein the gold metal NPs were
embedded inside the spheres (Fig. 2f). The high resolution
HAADF-STEM images of Au@TAE-DTE-O exhibited uniform
dispersion of the ultrane Au NP matrix (Fig. 2g) and corre-
spondingly, the average diameter of Au NPs was found to be
ꢂ1.3 nm (Fig. 2h). The elemental mapping indicated the
uniform distribution of Au, S, C, N, and F in the Au@TAE-DTE
hybrid (Fig. S17†). Notably, the absence of the chlorine element
(Cl) corresponds to the gold salt ensuring the presence of
metallic gold rather than the ionic one. The energy dispersive X-
ray (EDX) study demonstrated 2.2% Au NPs in Au@TAE-DTE-O,
which is in agreement with that found by the ICP-AES analysis
(Fig. S18†). Next, X-ray photoelectron spectroscopy (XPS) study
was performed on the Au@TAE-DTE-O. Importantly, two peaks
at 84.95 eV and 88.56 eV represented two distinct spin–orbit
pairs, Au 4f5/2 and Au 4f7/2, respectively, conrming the pres-
ence of metallic gold (Au(0)) in the POC (Fig. 2c).45 Notably, the
negative core level shi in the XPS spectrum of Au(0) as
compared to the literature report29 further justied the down-
sizing of the Au NPs.46 Deconvolution of the XPS spectra of
Au@TAE-DTE-O also showed signature peaks for other
elements such as sulphur (154.37 eV and 164.43 eV), carbon
(285.73 eV and 286.81 eV), nitrogen (400.5 eV) and uorine
(689.25 eV) (Fig. S19†).
The photochromic behaviour and UV-Vis absorption spectra
of Au@TAE-DTE-O and Au@TAE-DTE-C were found to be
similar to those of the TAE-DTE-O and TAE-DTE-C, respectively,
indicating that the intrinsic photochromic properties of the
POC are unaffected by the Au NPs (Fig. S20†). However, we did
not observe any additional plasmon absorption band for Au
NPs, which can be attributed to the smaller sizes of the NPs, and
a similar observation has been reported earlier as well.6,47 Owing
to the wide absorption and presence of ultra-small Au NPs, the
Au@TAE-DTE hybrid could be a promising photocatalyst for the
CO2 reduction reaction.
We performed Mott Schottky (MS) analysis to assess the
viability of the TAE-DTE-O for the photocatalytic CO2 reduction
(see the ESI, Fig. 3a). The MS measurements were carried out at
different frequencies and the positive slope of the obtained Cꢀ2
values (vs. the applied potential) suggested the typical n-type
semiconductor nature of the TAE-DTE-O. The at band poten-
tial for TAE-DTE-O was found to be ꢀ0.92 V w.r.t. the RHE at pH
¼ 7 which was more negative than the reduction potential of
CO2 to CO (ꢀ0.52 V vs. NHE) as well as CH4 (ꢀ0.24 V vs. NHE)
(Fig. 3b). Electronic band structures vs. RHE at pH ¼ 7 were
elucidated for TAE-DTE-O on the basis of the bandgap and at
band positions. This clearly indicates the feasibility of the TAE-
DTE-O to carry out the CO2 reduction reaction (Fig. 3b).
Next, we evaluated the potential of the Au@TAE-DTE hybrid
for photocatalytic CO2 reduction in a CO2 saturated solution of
CH3CN and H2O (3 : 1 ratio) using triethylamine (TEA) as
a sacricial hole scavenging agent (see the ESI†). The amount of
the photo reduced products was quantied by GC-MS analysis.
Visible light (l > 400 nm) driven CO2 reduction performed with
the Au@TAE-DTE-O showed the formation of 620 mmol gꢀ1 CO
in 11 hours with an average rate of 56.4 mmol gꢀ1 hꢀ1 (Fig. 3d).
Notably, a small amount of CH4 (135 mmol gꢀ1 in 11 hours) and
Fig. 3 Photochemical CO2 reduction: (a) Mott–Schottky plot for TAE-
DTE-O at different frequencies. (b) Bandgap alignment to illustrate the
feasibility of the materials for catalyzing the CO2 reduction reaction. (c)
Optimized CO2 binding site in Au@TAE-DTE-C. CO2 reduction reac-
tion using Au@TAE-DTE-O under (d) visible light (400–750 nm) and (e)
full-range light (250–750 nm). (f) Nyquist plot for charge transfer
resistance under light and dark conditions. (g) In situ FT-IR experiment
for CO2 reduction using Au@TAE-DTE-O under visible light irradiation.
(h) Photocurrent graph under light and dark conditions.
a negligible amount of H2 gas (39 mmol gꢀ1 in 11 hours) were
also detected. However, the selectivity for CO, CH4 and H2
formation was calculated to be 78.3%, 17.4%, and 4.3%,
respectively. Importantly, GC-MS analysis of the liquid aliquot
of the reaction mixture conrmed the absence of liquid prod-
ucts such as HCOOH, HCHO or CH3OH as a result of CO2
reduction. Next, the CO2 reduction experiment was performed
upon irradiating with light of the full range (250–750 nm).
Interestingly, almost double the amount of CO formation (1157
mmol gꢀ1 in 11 hours) with a rate of 105.18 mmol gꢀ1 hꢀ1 was
detected with the full range irradiation (Fig. 3e). At the same
time, a small amount of CH4 (185 mmol gꢀ1 in 11 hours) and
insignicant amount of H2 (21 mmol gꢀ1 in 11 hours) were also
observed along with CO formation. Notably, the selectivity of
the CO formation is noted to be impressive, and the relative
percentage of CO, CH4 and H2 formation was found to be
85.1%, 13.4%, and 1.5%, respectively. In order to understand
the substantially enhanced catalytic activity of Au@TAE-DTE-O
in full-range light, we have recorded the UV-Vis absorption
spectrum upon irradiating Au@TAE-DTE-O with full-range light
(250–750 nm) for 20 minutes (Fig. S21a†). This showed the
characteristic absorption peaks for both Au@TAE-DTE-O (UV
region) and Au@TAE-DTE-C (visible region). The UV-Vis spec-
trum was also recorded aer completion of photocatalysis in
full-range light and found to be similar as observed aer 20
minutes of irradiation, illustrating that the maximum concen-
tration of photoisomers present in the catalytic medium was
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reached within 20 minutes (Fig. S21b†). The percentage of compared to the photochromic organic cage (TAE-DTE) was
photo-isomers in full-range was calculated based on absorbance validated by the excited-state lifetime measurements and charge
in the visible region and found to be 65% (Au@TAE-DTE-O) and separation studies (impedance study and photocurrent
35% (Au@TAE-DTE-C) (Fig. S21c & d†). However, the existence measurements) (Fig. 3f and h, S28). The excited-state lifetime for
of a mixture of photoisomers in the full-range coherently both Au@TAE-DTE-C and TAE-DTE-C was measured upon
resulted in a wide-range of absorbance from the UV to the exciting at 510 nm, and the corresponding decay was analyzed at
visible region that subsequently exhibited better CO2 reduction 650 nm (Fig. S28†). The lifetime for TAE-DTE-C and Au@TAE-
as compared to individual visible or UV light irradiation. DTE-C was found to be 1.29 ns and 0.45 ns, respectively (Table
Interestingly, cumulative absorbance of photoisomers was S1†). Fast decay aer Au NP stabilization is indicative of the fast
nicely covered under the global solar spectrum (Fig. S21c†). electron transfer from the cage to the Au centre, supporting the
Next, an isotopic labelling experiment performed under a 13CO2 enhanced catalytic activity of Au@TAE-DTE-C. Next, the lower
atmosphere for the Au@TAE-DTE-O hybrid conrmed that the resistance for Au@TAE-DTE-C/TAE-DTE-C (DTE closed form)
produced CO and CH4 originated from CO2 (Fig. S22 & S23†). compared to Au@TAE-DTE-O/TAE-DTE-O noted in the imped-
Importantly, the real-time progress of the CO2 reduction reac- ance study implies the smaller electron transfer resistance due to
tion was monitored by in situ experiments using Diffuse the enhanced p conjugation in the former case. Notably, both the
Reectance Infrared Fourier Transform spectroscopy (DRIFTs) photo-isomers of the Au@TAE-DTE hybrid exhibited a smaller
(Fig. 3g).48 The DRIFT spectra were recorded at a time interval of semi-circle radius compared to their corresponding photo-
5 minutes upon continuous visible light irradiation. Two peaks isomers of bare cages, indicative of lower charge-transfer resis-
were detected at 1552 cmꢀ1 and 1367 cmꢀ1 for the COOH* and tance aer Au NP encapsulation (Fig. 3f). The photocurrent for
COOꢀ* intermediates, respectively, which were characteristic Au@TAE-DTE-O under visible light irradiation was around 2.5
intermediates for the CO/CH4 formation.49 Peaks at 1463 cmꢀ1 times higher compared to the pristine TAE-DTE-O cage, corrob-
and 1685 cmꢀ1 could be assigned to the symmetric and asym- orating the formation of the Au/TAE-DTE Schottky junction that
metric stretching of the HCO*3, respectively. Most importantly, could have helped to separate the photogenerated electron–hole
stretching and bending vibrations for the surface adsorbed CO* pairs and subsequently result in higher charge transfer efficiency
intermediate were observed at 2062 cmꢀ1 and 1748 cmꢀ1
.
from the cage to Au NPs in the Au@TAE-DTE-O hybrid (Fig. 3h).
The experimental results clearly demonstrated that the Au
48
Notably, increasing intensity of the CO* peak was observed with
reaction time, suggesting accumulation of the CO on the cata- NPs play a crucial role in the photocatalytic CO2 reduction.
lytic surface. Additionally, two peaks appearing at 1037 cmꢀ1 Therefore, we optimized the model structure for the Au NP
and 1080 cmꢀ1 were assigned to CH3O* and CHO*, respectively, stabilized POC to evaluate the thermodynamic feasibility and
which are crucial intermediates formed during CH4 forma- catalytic centre in the Au@TAE-DTE for the CO2 reduction
tion.49 Thus, reaction intermediates in the course of CO2 reaction (see the ESI, Fig. S29†). The DFT computational
reduction identied by the DRIFT study clearly indicated CO investigation was performed at the D3-B3LYP/6-31G(d) level,
formation along with a measurable amount of CH4 as well.
and two gold atoms were considered for the optimization in the
Further, various control experiments were performed to cavity of TAE-DTE-O and TAE-DTE-C, in view of reducing the
evaluate the role of individual components in the photo- computational cost.50 The study revealed that the cavity of TAE-
catalysis. This has established that the catalyst (Au@TAE-DTE), DTE-O and TAE-DTE-C was able to stabilize Au atoms by
TEA, CO2, and light are necessary components in order to releasing 1.69 and 1.71 kcal molꢀ1 of free energy, respectively,
realize the photocatalytic CO2 reduction reaction (Fig. S24†). via the non-bonding weak interactions from N and S atoms
The photocatalytic experiment was also performed with the (Fig. S29†). More importantly, the CO2 adsorption site in
recovered Au@TAE-DTE catalyst for four consecutive cycles, and Au@TAE-DTE-C was found to be thermodynamically favourable
the catalytic activity was found to be retained as observed for the by 6.20 kcal molꢀ1 at the Au centre and therefore likely to act as
rst cycle, indicating the excellent catalytic recyclability a catalytic centre during the CO2 reduction (Fig. 3c). This was
(Fig. S25†). TEM analysis of the recovered catalyst displayed further supported by the fact that the LUMO of the gold nano-
a similar morphology as that observed for the as-synthesized particles was found to be of lower energy as compared to the
Au@TAE-DTE-O, indicating excellent stability of the photo- LUMO of the POC (TAE-DTE-O/TAE-DTE-C) which illustrates
catalyst (Fig. S26†). Further, we have also examined the poten- facile electron transfer from the POC to the Au centre
tial of the as-synthesized cage (TAE-DTE-O) for photocatalytic (Fig. S30†). The possible catalytic site at the bare TAE-DTE-C was
CO2 reduction upon irradiating with both visible (l ¼ 400–750 also determined by performing DFT calculations considering
nm) and full range (l ¼ 250–750 nm) light and the corre- individual hetero-atoms of the POC as the CO2 binding sites.
sponding CO formation in 12 hours was observed to be 17 mmol The secondary nitrogen of the TAE-DTE-C was observed to be
gꢀ1 and 360 mmol gꢀ1, respectively (Fig. S27†). However, it is the energetically most favourable binding site for the CO2
noteworthy that the photochemical CO2 reduction to CO using molecule (Fig. S31†).
the photochromic organic cage was observed to be 100%
selective and no other product was detected.
Conclusions
The above results unambiguously indicated the important
role of the Au NPs in the photocatalytic CO2 reduction. Moreover, In conclusion, we have utilized a photochromic organic cage
the enhanced photocatalytic activity of the Au@TAE-DTE hybrid (TAE-DTE) as a platform for the in situ growth and stabilization
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
TKM acknowledges the SERB, Dept. of Science and Technology
(DST), Govt. of India for nancial support (project no. CRG/
2019/005951). AS thanks the SERB for funding. PV acknowl-
edges the Council of Scientic and Industrial Research (CSIR),
Govt. of India, for the fellowship.
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