A green approach to the fabrication of a TiO2/NiAl-LDH core-shell hybrid photocatalyst for efficient and selective solar-powered reduction of CO2 into value-added fuels

Article abstract of DOI:10.1039/d0ta00104j

Exploring promising photocatalysts with high efficiency and selectivity for CO₂ reduction holds paramount significance for resolving the energy crisis and various environmental problems associated with traditional fossil fuels. Here, we rationally design a core-shell hybrid photocatalyst in which anatase TiO₂ hollow spheres serve as the core component and NiAl layered double hydroxide (LDH) nanoflakes serve as the shell component. The synthesis of the TiO₂/LDH core-shell hybrid involves hydrothermal and calcination treatments without the use of environmentally toxic solvents or surfactants. Assorted experimental results demonstrate that the TiO₂/LDH core-shell hybrid exhibits a strong light-harvesting ability, large surface area, porous structure, and extraordinary CO₂ adsorption capability. In addition, the unique core-shell geometric structure of the TiO₂/LDH hybrid results in a large interfacial contact area and thus provides a broader platform for efficient charge transfer. Benefiting from these structural and compositional features, the TiO₂/LDH core-shell hybrid exhibits remarkable CO₂ reduction activity, high selectivity (against water reduction), and more importantly, good stability during consecutive test cycles. Therefore, this work offers a promising approach to the rational design and fabrication of core-shell hybrid photocatalysts with potential applications in solar energy conversion and environmental protection.

Full text of DOI:10.1039/d0ta00104j

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A green approach to the fabrication of TiO2/NiAl-LDH core–shell hybrid photocatalyst
DOI: 10.1039/D0TA00104J
for efficient and selective solar-powered reduction of CO2 into value-added fuels
Wan-Kuen Jo, a Satyanarayana Moru, b Surendar Tonda a,*
a Department of Environmental Engineering, Kyungpook National University, 80 Daehak-ro,
Buk-gu, Daegu 41566, Republic of Korea.
b
Department of Electrical and Computer Engineering, Iowa State University, Coover Hall,
Osborn Drive, Ames, Iowa 50011, USA.
Corresponding Author:
S. Tonda, E-mail: surendar.t86@gmail.com; surendart@knu.ac.kr
Contact No.: +82 53 950 6584.
1

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DOI: 10.1039/D0TA00104J
Abstract
Exploring promising photocatalysts with high efficiency and selectivity for CO2 reduction
holds paramount significance for resolving the energy crisis and various environmental
problems associated with traditional fossil fuels. Here, we rationally design a core–shell hybrid
photocatalyst in which anatase TiO2 hollow spheres serve as the core component and NiAl
layered double hydroxide (LDH) nanoflakes serve as the shell component. The synthesis of the
TiO2/LDH core–shell hybrid involves hydrothermal and calcination treatments without the use
of environmentally toxic solvents or surfactants. Assorted experimental results demonstrate
that the TiO2/LDH core–shell hybrid exhibits strong light-harvesting ability, a large surface
area, porous structure, and extraordinary CO2 adsorption capability. In addition, the unique
core–shell geometric structure of the TiO2/LDH hybrid results in a large interfacial contact area
and thus provides a broader platform for efficient charge transfer. Benefiting from these
structural and compositional features, the TiO2/LDH core–shell hybrid exhibits remarkable
CO2 reduction activity, high selectivity (against water reduction), and, more importantly, good
stability during consecutive test cycles. Therefore, this work offers a promising approach to
the rational design and fabrication of core–shell hybrid photocatalysts with potential
applications in solar energy conversion and environmental protection.
Keywords: TiO2 hollow sphere; layered double hydroxide; core–shell architecture; hybrid
heterojunction; CO2 photoreduction
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1 Introduction
DOI: 10.1039/D0TA00104J
The ever-increasing consumption of nonrenewable fossil fuel sources has led to growing
concerns about global energy shortages and the unusual increase in atmospheric carbon dioxide
(CO2) levels causing global warming via the greenhouse effect.1,2 In this context, the
development of advanced technologies for CO2 capture and conversion is receiving
considerable interest within the scientific community. The sunlight-driven photocatalytic
transformation of CO2 into value-added fuels (e.g., CO, CH4, CH3OH, and HCHO), mimicking
natural photosynthesis, is regarded as a sustainable approach that would not only manage the
global carbon balance but also partly fulfill future energy demand.3-5 Despite great progress in
recent years, this photocatalytic CO2 conversion approach is still challenging because of the
extremely stable chemical bonds in CO2 molecules, the competing water reduction reaction,
and the complex conversion pathway involving the transfer of multiple protons and electrons.6,7
Therefore, robust catalysts with highly efficient solar-light utilization, strong reduction
capabilities, and enhanced charge separation and transport processes are needed.
Following the seminal discovery of photoelectrochemical CO2 conversion in aqueous
suspensions of semiconductors by Inoue et al. in 1979,8 assorted semiconductor materials have
been exploited as photocatalysts for the CO2 reduction reaction.9,10 Among them, titanium
dioxide (TiO2)remains the most widely exploited photocatalyst because of its high thermal and
chemical stability, cost-effectiveness, and ecofriendly character.11 Optimizing the
microstructure and morphology of TiO2 is critical to enhancing its photocatalytic performance.
Thus far, TiO2 materials with various morphologies (e.g., sheets, wires, tubes, and hollow
spheres) have been developed in efforts to improve the photocatalytic activity of TiO2.12-15 In
particular, TiO2 hollow spheres represent a substantial cutting-edge innovation for solar energy
conversion because of their peculiar characteristics, which include a large specific surface area,
low density, and superior light scattering and reflection properties for better light
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harvesting.16,17 However, analogous to other unitary photocatalysts, the intrinsic drawbacks of
DOI: 10.1039/D0TA00104J
TiO2, such as its ultraviolet (UV)-matching or wide bandgap and its rapid recombination of
photogenerated charge carriers, result in poor photocatalytic efficiency, thereby greatly
limiting the practical applications of pristine TiO2 materials irrespective of their size and
geometric shape.
Different strategies have been adopted to overcome the limitations of TiO2,including noble-
metal deposition,18 the use of carbon-based conducting materials,19 and the formation of
heterostructures with other semiconductors.20 Among these strategies, integrating TiO2 with
other narrow-bandgap semiconductors to form a hybrid heterostructure has thus far been the
most promising approach to boosting the photocatalytic efficiency of TiO2.21,22 In particular,
layered double hydroxides (LDHs) have received considerable attention as photocatalysts
because of their strong visible-light response, controllable particle size, and tunable metal
cation composition.23-25 In addition, abundant hydroxyl groups on the surface of LDHs promote
the adsorption of CO2 molecules, which is indispensable for photocatalytic CO2 conversion.26
Because of these impressive properties, LDHs are an excellent choice for coupling with TiO2
to construct a hybrid heterostructure that impedes the recombination of photoexcited electron–
hole pairs and effectively utilizes the full solar spectrum because of the combination of UV-
light-responsive TiO2 and visible-light-responsive LDHs. In addition, the intimate interfacial
contact area in hybrid photocatalysts is a key factor for facilitating charge transfer at the
interface. In this regard, the fabrication of core–shell hybrid photocatalysts has attracted
particular interest because the core–shell geometry can enlarge the interfacial contact area,
thereby providing a broader platform for efficient charge transfer.27,28 Nevertheless, the
fabrication of such efficient core–shell hybrid materials in an ecologically friendly, and
economical manner remains a formidable challenge.
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In the present work, we rationally integrated anatase TiO2 hollow spheres and NiAl-LDH
DOI: 10.1039/D0TA00104J
nanoflakes to construct a highly efficient TiO2/LDH hybrid system with a core–shell geometric
structure. The TiO2/LDH core–shell hybrid was obtained via a synthesis route involving
hydrothermal and calcination treatments but not involving environmentally toxic solvents or
surfactants. We assessed the photocatalytic activity of the fabricated TiO2/LDH core–shell
hybrid by monitoring their ability to produce value-added chemicals via CO2 reduction under
simulated-solar-light illumination. Importantly, as revealed in this work, the TiO2/LDH core–
shell hybrid displayed substantially improved CO2 reduction activity compared with TiO2
hollow spheres, LDH, and commercial P25 catalysts. The effect of the core–shell architecture
on the CO2 photoreduction performance of the TiO2/LDH hybrid was carefully investigated. A
possible mechanism for the notable CO2 reduction activity of the present TiO2/LDH core–shell
hybrid was proposed on the basis of the results of electron spin resonance (ESR) experiments.
The durability and photostability of the core–shell hybrid were also probed using successive
test cycles. To the best of our knowledge, this work represents the first report of the fabrication
of a TiO2/LDH core–shell hybrid by integrating TiO2 hollow spheres and NiAl-LDH
nanoflakes for the high-performance photoconversion of CO2 into value-added fuels.
2 Experimental section
2.1 Synthesis of catalysts
TiO2 hollow spheres were fabricated via a hydrothermal method followed by calcination
treatment using carbon spheres as templates. Carbon spheres were prepared by a hydrothermal
route based on the reported procedure with minor changes.29 In a typical procedure, 300 mg of
agarose powder was carefully added to 60 mL of preheated water (90 °C) under vigorous
stirring. After complete dissolution of the agarose powder, the obtained transparent precursor
solution was transferred into a 100 mL Teflon-lined autoclave reactor and heated at 180 °C for
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12 h. The resultant dark-brown precipitate was washed and rinsed several times with water and
DOI: 10.1039/D0TA00104J
finally dried in an oven at 80 °C to obtain the carbon spheres.
To prepare the TiO2 hollow spheres, 100 mg of the as-prepared carbon spheres were
dispersed in 60 mL of water with the assistance of ultrasonication. Thereafter, 1.76 mL of
titanium(IV) oxysulfate was added dropwise to the carbon-sphere suspension under constant
stirring, and the resulting mixture was agitated for 30 min to ensure adequate diffusion of the
titanium ions into the carbon surface layer. The suspension was then transferred to a 100 mL
Teflon-lined autoclave reactor that was subsequently heated at 180 °C for 2 h. The product
obtained after the hydrothermal reaction was collected by centrifugation, rinsed repeatedly with
water, and dried at 80 °C to obtain TiO2/C powder. The resultant TiO2/C powder was placed
into an alumina crucible with a cover and calcined at 450 °C (heating rate: 2 °C min−1) for 3 h
under an air atmosphere to obtain TiO2 hollow spheres (TiO2 HSs). A TiO2 nanoparticles (TiO2
NPs) reference sample was obtained via the same method but without the addition of carbon
sphere templates.
The TiO2/LDH core–shell hybrid was prepared via a hydrothermal approach. A specific
quantity of as-obtained TiO2 HSs powder was dispersed in 100 mL of water with the aid of
ultrasonication. Ni(NO3)2·6H2O (0.006 M) and Al(NO3)3·9H2O (0.002 M) were added to the
dispersion and magnetically stirred for 30 min. Urea (0.05 M) and NH4F (0.016 M) were
subsequently mixed with the aforementioned reaction mixture and agitated for another 30 min.
Afterwards, the mixture was transferred to a 150 mL Teflon-lined autoclave reactor that was
heated at 120 °C for 6 h. The resultant precipitate was collected by centrifugation, rinsed
repeatedly with water until the pH of the wash solution was neutral, and dried at 80 °C
overnight to obtain the TiO2/LDH core–shell hybrid. Pristine LDH was synthesized via a
similar procedure but without the addition of TiO2 hollow spheres.30 A TiO2 NPs/LDH
reference sample was synthesized via the same method as the TiO2/LDH core–shell hybrid
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except that the TiO2 HSs powder was replaced with the same amount of TiO2 NPs in the
DOI: 10.1039/D0TA00104J
synthesis procedure.
The experimental details for the materials, characterization techniques, and photocatalytic
activity tests are provided in the electronic supplementary information.
3 Results and discussion
3.1 Characterization of the photocatalysts
The morphology and microstructure of the prepared samples were determined by FESEM and
TEM analyses. The FESEM images in Fig. 1a and b show that the synthesized carbonaceous
products are nearly monodispersed, with a spherical morphology. The diameters of the carbon
spheres mainly fall within the range 250–350 nm, centered at ~320 nm. The TEM micrographs
(Fig. 1c and d) further demonstrate that the surface of these carbon spheres is smooth. The
FESEM (Fig. 2a) and TEM (Fig. 2b and Fig. S2) micrographs of the TiO2/C sample show
plentiful tiny particles of TiO2 uniformly deposited onto the surface of the carbon spheres.
Figure 2c illustrates the hollow-sphere morphology of TiO2 prepared at 450 °C, where the
highlighted areas in the image clearly show the hollow nature of the spheres. A high contrast
between the pale center and the dark edge of each sphere is observed in the TEM image of the
TiO2 (Fig. 2d), further confirming that each sphere is indeed hollow, with a wall thickness of
approximately 30 nm. A close observation of the surface by TEM analysis (Fig. 2d inset)
reveals that each hollow sphere is composed of numerous nanosized TiO2 particles, amongst
which numerous voids (or pores) are created. We inferred from these results that, after thermal
treatment of the TiO2/C sample at 450 °C, the carbon sphere templates were completely
removed, resulting in the formation of TiO2 HSs.
Interestingly, the TEM image of the TiO2/LDH sample (Fig. 3a) shows that the hybrid
exhibits a hierarchical flower-like core–shell architecture composed of a TiO2 hollow-sphere
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core and an LDH shell. This TEM image also shows numerous ultrathin LDHDOnIa: 1n0o.1f0l3a9k/De0sTA00104J
perpendicularly grafted throughout the TiO2 hollow-sphere cores, enabling the light to easily
pass through the LDH to reach the TiO2 core. The magnified TEM image of the TiO2/LDH
hybrid (Fig. 3b) further confirms the close integration between the TiO2 core and the LDH shell.
These results suggest that the ultrathin LDH nanoflakes in the reaction mixture are firmly
grafted onto the surface of TiO2 HSs during the hydrothermal treatment, which results in the
formation of a TiO2/LDH core–shell hybrid photocatalyst. The HRTEM image (Fig. 3c)
illustrates lattice fringes with d-spacings of 0.35 nm and 0.26 nm, which are assigned to the
(101) plane of anatase TiO2 and the (012) plane of LDH, respectively.30,31 In addition, scanning
transmission electron microscopy–high-angle annular dark-field (STEM-HAADF) elemental
distribution maps of the TiO2/LDH hybrid (Fig. 3d–h) indicate the presence of Ti, O, Ni, and
Al elements; among these elements, Ti and O are observed in the inner core, whereas Ni, Al,
and O are distributed homogeneously in the shell component, confirming the core–shell
architecture of the TiO2/LDH hybrid. Furthermore, the EDS spectrum of the TiO2/LDH core–
shell hybrid (Fig. S3) also indicates the presence of Ti, O, Ni, and Al (with a Ni/Al molar ratio
of ∼3.0), consistent with the mapping results.
XRD analysis was conducted to analyze the phase structure of the synthesized samples. The
typical diffraction pattern of TiO2 HSs in Fig. 4a well matches the reference data for anatase
TiO2 (JCPDS: 21-1272), where the reflections at 25.30°, 37.82°, 48.04°, 53.94°, 55.03°, and
62.69° are attributed to the (101), (004), (200), (105), (211), and (204) crystal planes,
respectively. All of the diffraction peaks are sharp and intense, suggesting that the anatase TiO2
HSs are highly crystalline. The XRD pattern of the pristine LDH shows diffraction peaks at 2θ
values of 11.58°, 23.28°, 34.98°, 39.46°, 46.83°, 61.02°, and 62.29°, corresponding to the (003),
(006), (012), (015), (018), (110), and (113), respectively, which are well indexed to hexagonal
NiAl-LDH (JCPDS No. 22-0452). All of the diffraction peaks that appeared in the XRD pattern
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of the TiO2/LDH core–shell heterostructure are in accordance with the peaks of TiO2 and NiAl-
DOI: 10.1039/D0TA00104J
LDH, indicating successful formation of the TiO2/LDH hybrid. Moreover, the magnified XRD
spectra in Fig. S4 clearly illustrate that the characteristic peak of TiO2 at 25.3° in the TiO2/LDH
core–shell is slightly shifted from its original position, perhaps because of the minor shrinkage
of TiO2 lattice structure after LDH deposition. A similar kind of shift in the diffraction peak is
also observed in previously reported core–shell hybrids.32,33
UV–vis DRS analyses were performed to characterize the light-harvesting properties of the
TiO2 HSs, LDH, and TiO2/LDH core–shell hybrid samples (Fig. 4b). Analogous to TiO2 NPs,
the TiO2 HSs exhibit a steep absorption edge at ~400 nm, reflecting its wide bandgap and
intrinsic UV-light-responsive characteristics. Meanwhile, LDH displays intrinsic optical
absorption bands in both the UV- and visible-light regions; the absorption band in the UV
region corresponds to ligand-to-metal charge transfer (LMCT), whereas the bands in the
visible-light region are related to d–d transitions.30,34 More specifically, the bands at 420 and
655 nm are associated with spin-forbidden transitions 3A2g (F) → 1T2g (D) and 3A2g (F) → 1Eg
(D), respectively; the other bands at 380 and 740 nm are related to spin-allowed transitions
3A2g (F) → 3T1g (P) and 3A2g (F) → 3T1g (F), respectively, which result from the d8 configuration
of Ni2+ ions in an octahedral geometry.34-36 Interestingly, typical visible-light absorption bands
of LDH and the characteristic UV-light absorption of TiO2 are also observed in the DRS profile
of the TiO2/LDH core–shell hybrid, reflecting the strong synergy between the TiO2 and LDH
in the resulting TiO2/LDH core–shell hybrid. Because of the presence of the UV-light-
responsive TiO2 HSs and the visible-light-responsive LDH, the target TiO2/LDH core–shell
hybrid can effectively utilize the full solar spectrum, which can subsequently lead to the
generation of the vast number of photoinduced charge carriers required for the photocatalytic
reactions.
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The FT-IR spectra of the TiO2 HSs, LDH, and the TiO2/LDH core–shellDOhIy: 1b0r.1i0d39/aDr0eTA00104J
presented in Fig. S5. The TiO2 HSs exhibit a broad band at 3000–3500 cm−1, which corresponds
to physically adsorbed water on the surface of TiO2.37 The typical bands at wavenumbers less
than 900 cm−1 are attributed to Ti−O−Ti and Ti−O stretching vibration modes.16 The FT-IR
spectrum of LDH shows a broad band at 3430 cm−1, which corresponds to the stretching
vibrations of surface hydroxyl groups in the LDH host layers and interlayer water molecules;
and the weak band at ~1640 cm−1 is assigned to the bending vibrations of water molecules.38
The sharp and intense band at approximately 1360 cm−1 is ascribed to bending vibrations of
−
NO3 species intercalated into the lamellar network.39 The characteristic bands at wavenumbers
less than 800 cm−1 are assigned to lattice vibrations of M–OH and M–O–M (M = Ni, Al).30
Importantly, the FT-IR spectrum of the TiO2/LDH core–shell hybrid displays characteristic
bands related to both TiO2 and LDH; the LDH bands are dominant, possibly because the
ultrathin LDH nanoflakes were grafted throughout the TiO2 hollow-sphere cores with intimate
interactions.
The detailed surface chemical states of the elements in the TiO2 HSs, LDH, and TiO2/LDH
core–shell hybrid samples were examined by XPS analysis. Among the survey XPS spectra
(Fig. S6), the spectrum of the bare TiO2 HSs shows peaks corresponding to Ti and O, whereas
that of LDH shows peaks related to Ni, Al, and O elements. Moreover, the survey XPS profile
of the TiO2/LDH core–shell hybrid demonstrates that the hybrid comprises Ti, Ni, Al, and O
elements, in accordance with the results obtained by EDS analysis. The high-resolution Ti 2p
spectrum of TiO2 (Fig. 5a) displays two symmetrical peaks at binding energy (BE) values of
458.4 and 464.2 eV, corresponding to Ti 2p3/2 and Ti 2p1/2, respectively, indicating that Ti
exists in the +4 valence state in the TiO2 HSs.40 Moreover, a notable negative shift in the Ti
2p3/2 and Ti 2p1/2 BE values was observed for the TiO2/LDH core–shell hybrid compared with
those for TiO2 HSs. Meanwhile, the Ni 2p spectrum of LDH (Fig. 5b) exhibits two typical
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peaks centered at 856.3 and 873.9 eV, corresponding to the spin–orbit couple of Ni 2p3/2 and
DOI: 10.1039/D0TA00104J
Ni 2p1/2, respectively.35 The concomitant shake-up satellite peaks indicate that Ni in the LDH
is in the high-spin divalent valence state.30 In contrast to the BEs of Ti 2p, a substantial positive
shift in the BEs of Ni 2p3/2 and Ni 2p1/2 is observed in the case of the TiO2/LDH core–shell
hybrid relative to that of LDH. The Al 2p spectra of both LDH and the TiO2/LDH core–shell
hybrid (Fig. 5c) exhibit a typical peak at 73.8 eV, confirming the trivalent oxidation state of Al
in the catalysts.30 The unusual peak at ~68.5 eV in Al 2p spectra is assigned to Ni 3p. As shown
in the O 1s spectra (Fig. 5d), the TiO2 reveals two peaks at 529.1 and 530.6 eV corresponding
to lattice O (OL; Ti–OL) species and surface-adsorbed water, respectively,16 whereas LDH
exhibits peaks at 531.1 and 532.2 eV, corresponding to OL species (M–OL; M = Ni, Al) and
hydroxide groups (O–H), respectively.38 Typical O 1s components related to both TiO2 and
LDH were also observed in the O 1s spectrum of the TiO2/LDH core–shell hybrid. The positive
and negative shifts in the BEs of Ni 2p and Ti 2p components, respectively, demonstrate solid
interactions between TiO2 HSs and LDH, which favor the transfer of photoinduced charges to
boost the CO2 reduction performance of the TiO2/LDH core–shell hybrid.41-43
Figure S7 displays the N2 adsorption–desorption isotherms and the corresponding BJH
pore-size distribution curves of the TiO2 NPs, TiO2 HSs, LDH, and TiO2/LDH core–shell
hybrid samples. According to the IUPAC classification,44 the isotherm of the TiO2 HSs (Fig.
S7a) is a type IV isotherm with a H4 hysteresis loop, implying the existence of narrow slit-like
pores. This interpretation is further supported by the corresponding BJH pore-size distribution
curve (inset of Fig. S7a), which reveals that TiO2 HSs exhibit a pore-size distribution in the
range 2–14 nm, with a mean pore diameter of approximately 6.5 nm. This pore structure is
consistent with the TEM results (Fig. 2d). Analogous to the TiO2 HSs, the TiO2/LDH core–
shell hybrid displays a type IV isotherm with a H4 hysteresis loop (Fig. S7b). Meanwhile, the
isotherm of LDH (Fig. S7c), classified as type IV with a H3 hysteresis loop, suggests the
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existence of slit-shaped mesopores. The measured Brunauer–Emmett–Teller (BET) specific
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surface area (SBET), average pore diameter, and total pore volume of all of the prepared samples
are summarized in Table 1. The results show that the SBET of the TiO2 HSs is much higher than
that of the reference TiO2 NPs (Fig. S7d). Notably, the SBET of the TiO2/LDH core–shell hybrid
is greater than that of the TiO2 HSs, consistent with results reported for other hybrid
heterostructures associated with TiO2 hollow spheres.41,45 Moreover, compared with the TiO2
HSs, the TiO2/LDH core–shell hybrid exhibits a relatively higher average pore diameter and
greater total pore volume, possibly because of the introduction of LDH, which exhibits a high
average pore diameter and total pore volume. Such good surface and porous properties of the
TiO2/LDH core–shell hybrid would favor the exposure of active sites and ion transport
channels, thereby improving photocatalytic performance.
The adsorption of CO2 onto the catalyst surface is considered a prerequisite step in the
photocatalytic CO2 conversion reaction.15 Hence, the CO2 adsorption capabilities of the
synthesized catalysts were investigated using CO2 adsorption isotherm studies; the results are
presented in Fig. 6a. Clearly, more CO2 adsorbs onto the TiO2 HSs than onto the reference TiO2
NPs, perhaps because of the large exposed surface area of TiO2 HSs, which favors CO2
adsorption. Interestingly, after LDH nanoflakes were grafted onto the TiO2 hollow sphere cores,
the resulting TiO2/LDH core–shell hybrid displayed improved CO2 adsorption capacity (0.48
mmol/g) compared with the TiO2 HSs (0.30 mmol/g) and the reference TiO2 NPs (0.14 mmol/g)
catalysts. The improved CO2 adsorption capacity of the TiO2/LDH core–shell hybrid is
attributed not only to the improved specific surface area and porous structure but also to the
existence of abundant basic hydroxyl (–OH) functional groups on the LDH surface, which can
facilitate chemical adsorption of acidic CO2 molecules.26,46 Therefore, the improved CO2
adsorption capability certainly accounts for the enhanced CO2 photoreduction performance of
the TiO2/LDH core–shell hybrid.
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TGA was conducted to examine the thermal stabilities of the prepared TiO2 HSs, LDH, and
DOI: 10.1039/D0TA00104J
TiO2/LDH core–shell hybrid samples. The TGA curve in Fig. 6b indicates that TiO2 HSs have
good thermal stability, with a total mass loss of only ~3.2% in the temperature range 30–800 °C.
The observed weight loss was attributed to the elimination of surface-adsorbed water and
residual organic species.37 The TGA plot of LDH shows two major mass-loss regions, where
the first weight loss at 30–200 °C corresponds to the loss of surface-adsorbed and interlayer
(chemisorbed) water molecules.30,47 The second mass loss between 200 and 375 °C is related
−
to the loss of interlayer NO3 species and dehydroxylation of the brucite-like layers.30 No
obvious weight loss occurs at temperatures greater than 375 °C, likely because of the formation
of metal oxides from the LDH.48 Similar mass-loss regions related to LDH are also observed
in the TGA plot of TiO2/LDH core–shell hybrid. Nonetheless, we estimated the actual content
of LDH in the TiO2/LDH core–shell hybrid to be 18.5 wt% on the basis of its TGA curve.
On the basis of the aforementioned experimental results, we proposed a formation
mechanism for the TiO2/LDH core–shell hybrid, which is schematically depicted in Scheme 1.
Carbon spheres, which serve as the templates, with an average diameter of ~320 nm were first
prepared by a simple hydrothermal route using agarose as a saccharide precursor. The surface
of these carbon spheres was quite hydrophilic because of the presence of OH and C=O
functional groups, as confirmed in previous reports.49,50 Upon addition of a titanium precursor
to the carbon sphere colloid solution, the titanium cations were easily bound with surface
functional groups of carbon spheres via electrostatic or coordination interactions.49 In the
subsequent calcination treatment, the inner part of the solid carbon spheres gradually dissolved;
meanwhile, the surface layers comprising the titanium ions were cross-linked and recrystallized
to form TiO2 HSs as replicas of the templates but with a slightly smaller size. During the
calcination treatment, numerous narrow-sized pores were created between the surface TiO2
nanoparticles, as confirmed by TEM observations and BET measurements (Figs. 2d and S7a).
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In the next step, the obtained TiO2 HSs served as nucleation sites for the in situ growth of LDH.
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In detail, Ni2+ and Al3+ ions from their respective precursors tended to bind to the surface of
the TiO2 HSs. Upon the addition of urea and ammonium fluoride to the reaction mixture, LDH
nanoflakes perpendicularly grew throughout the TiO2 hollow sphere cores under hydrothermal
conditions. As a result, the TiO2/LDH core–shell hybrid with intimate interactions between the
TiO2 hollow-sphere cores and the LDH shell was realized. Notably, deionized water was the
only solvent used in the synthesis; the use of toxic solvents or surfactants was deliberately
avoided.
3.2 Photocatalytic activity
The photocatalytic activities of the synthesized TiO2 HSs, LDH, and TiO2/LDH core–shell
hybrid catalysts were investigated in detail via a CO2 reduction assay under simulated solar-
light illumination. Figure 7a and b demonstrates the evolution of carbon-based products as a
function of irradiation time over all of the fabricated catalysts. Because of their adequate
conduction band-edge potentials (determined by valence-band XPS analysis discussed later)
for the water reduction process, all of the synthesized catalysts also generated H2 as a
competitive water reduction product (Fig. 7c). Under our experimental conditions, CH4 was
found to be an exclusive carbon-based product of CO2 reduction when P25 or TiO2 NPs or
TiO2 HSs were used as the photocatalyst. The reference TiO2 NPs catalyst showed minimal
CH4 production, with a total yield of 0.20 μmol for 5 h. A total CH4 production yield of 1.02
μmol was attained over the TiO2 HSs, which is 5.1 times greater than that attained for the
reference TiO2 NPs catalyst. This result clearly reveals the importance of the hollow-sphere
structure of TiO2 in achieving enhanced photocatalytic CO2 reduction performance. However,
the selectivity toward CH4 production against the competitive H2 production over TiO2 HSs
was only 78%. Among the tested catalysts, P25 exhibited poor CH4 production because of its
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previously discussed intrinsic drawbacks. Meanwhile, bare LDH generated a distinct carVbieownAr-ticle Online
DOI: 10.1039/D0TA00104J
based product, CO, with a total yield of 0.32 μmol, in accordance with our previous reports.30,38
As envisioned, after the LDH nanoflakes were grafted throughout the TiO2 hollow-sphere
cores, the resultant hierarchical TiO2/LDH core–shell hybrid displayed substantially enhanced
photocatalytic activity for CO2 reduction to CH4 compared with its counterpart, TiO2 HSs. This
target TiO2/LDH core–shell hybrid exhibits the highest CH4 evolution among the tested
samples, with a total yield of 5.14 μmol over 5 h, equating to a rate of 20.56 μmol g−1 h−1 (Fig.
7d), representing remarkable 25.7- and 5.0-fold improvements in CH4 production compared
with those of reference TiO2 NPs and TiO2 HSs, respectively. The activity results also indicate
that the TiO2/LDH core–shell hybrid produced a small amount of CO (2.48 μmol g−1 h−1), as
an additional solar fuel.
To verify the feasibility of carbon-based CH4 or CO production from surface organic
residues, a control experiment in the absence of CO2 or in the presence of Ar was carried out.
No CH4 or CO production was detected for the same TiO2/LDH core–shell hybrid after 5 h of
light illumination. A 13C isotope-labeled experiment was also conducted over TiO2/LDH core–
shell hybrid under the identical test conditions; the resultant products were analyzed via gas
chromatography–mass spectrometry (GC–MS). The peak at 8.62 min in the GC spectrum (inset
13
of Fig. S8) with a m/z value of 17 in the MS spectrum (Fig. S8) was attributed to CH4.51,52
Moreover, the MS spectrum also displays signals with m/z values of 29 and 45, corresponding
13
13
to CO (as the minor product) and CO2, respectively.53 These results demonstrate that the
carbon source was merely derived from the input CO2, ruling out the possibility of
contamination from other carbon sources. In addition, control experiments in the dark or
without a catalyst did not led to the production of hydrocarbon fuels, confirming that CO2
conversion was triggered solely by light irradiation in the presence of a catalyst. Furthermore,
the yield of H2 over the TiO2/LDH core–shell hybrid was only 0.52 μmol, which implies that
15

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the selectivity toward CH4 generation against H2 reached almost 95%. Notably, the VCiewHAr4ticle Online
DOI: 10.1039/D0TA00104J
production yield and selectivity (against water reduction) of TiO2 HSs were substantially
improved via coupling with the LDH, even though LDH alone did not generate any detectable
CH4 in the CO2 reduction reaction under identical test conditions. In addition, the TiO2/LDH
core–shell hybrid is far superior many of the reported TiO2-based catalyst systems with respect
to the rate of CH4 production, as summarized in Table S1. Furthermore, the apparent quantum
yield of TiO2/LDH core–shell hybrid for the formation of CH4 was calculated to be 0.92%,
which is also higher than the other state-of-the-art TiO2-based photocatalysts for CH4
production.18,54-57
The importance of the core–shell structure on the CO2 reduction performance was
investigated by comparing the solar fuel production rates of the target TiO2/LDH core–shell
hybrid with those of a similar TiO2 NPs/LDH hybrid with a different structure. Because the
TiO2 HSs were substituted with TiO2 NPs in the synthesis process (see the Experimental
section), the obtained TiO2 NPs/LDH hybrid does not possesses the core–shell morphology
(Fig. S9). The FESEM image (Fig. S9) also illustrates poor contact between the components
of the TiO2 NPs/LDH hybrid. As a result, the productivity and selectivity toward CO2 reduction
of the TiO2 NPs/LDH hybrid are inferior to that of the TiO2/LDH hybrid with core–shell
morphology. These results confirm that the core–shell geometric structure could enlarge the
interfacial contact area and thus provide a broader platform for efficient charge transfer, which
subsequently resulted in extraordinary photocatalytic CO2 reduction activity of the TiO2/LDH
core–shell hybrid.
Because the durability and photostability are indispensable parameters for a photocatalyst
with respect to its practical applications, four consecutive CO2 reduction test runs were carried
out using the TiO2/LDH core–shell hybrid. Fig. 8a clearly shows that the TiO2/LDH core–shell
hybrid maintains its remarkable activity after four successive test runs, with no apparent
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reduction in CH4 or CO production. This result implies that the TiO2/LDH core–DshOIe: l1l0.1h0y39b/rDi0dTA00104J
maintains impressive durability during prolonged CO2 reduction reactions. In addition, XPS
and XRD analyses were conducted to assess the stability of the target catalyst after CO2
reduction activity tests. Negligible changes were observed in the XPS spectra (Fig. S10a and
b) of the TiO2/LDH core–shell hybrid before and after the photocatalytic CO2 reduction tests,
affirming that its chemical structure was not altered during the reaction. Moreover, the lack of
an apparent difference in the XRD patterns (Fig. S11) of the fresh and reused TiO2/LDH core–
shell hybrid further evidences the stability of its phase structure. In addition, the SEM and TEM
images in Fig. S12a and b clearly illustrated that the core–shell geometry of the TiO2/NiAl-
LDH hybrid did not change after the photocatalytic CO2 reduction reaction. Hence, the
aforementioned results strongly imply that the present TiO2/LDH core–shell hybrid, as a highly
efficient and stable photocatalyst, holds great promise for practical hydrocarbon fuel generation.
3.3 Photocatalytic mechanism
To determine the underlying reasons for the extraordinary photocatalytic CO2 reduction
performance of the TiO2/LDH core–shell hybrid, we conducted various experiments and
carefully analyzed the results. The photocatalytic CO2 reduction reaction is well known to
primarily encompass the following fundamental processes: (i) photoabsorption; (ii) separation
and migration of photoexcited charge carriers; and (iii) CO2 adsorption and reduction.58,59
These factors are elucidated as follows.
The previously discussed UV–vis DRS results demonstrate that the TiO2/LDH core–shell
hybrid exhibited improved photoabsorption ability because of the presence of UV-light-
responsive TiO2 HSs and visible-light-responsive LDH. This broad range of photoabsorption
could generate the vast number of photoexcited charges necessary for the subsequent CO2
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reduction reaction. Hence, this property is likely one of the pivotal reasons for the improved
DOI: 10.1039/D0TA00104J
CO2 reduction efficiency of the TiO2/LDH core–shell hybrid system.
The separation–recombination rate of the photoexcited electrons and holes is one of the
decisive factors affecting the photocatalytic CO2 reduction efficiency because the low
recombination rate or high separation rate can enable more electrons to participate in reducing
adsorbed CO2 molecules.60 Hence, a comprehensive understanding of charge separation and
transport processes within the TiO2/LDH core–shell hybrid is critical. In this regard,
photoluminescence (PL) spectroscopy, a powerful tool to characterize the recombination
probability of photoexcited electrons and holes,61 was used to analyze the synthesized catalysts.
As shown in Fig. 8b, the PL emission intensity of the TiO2/LDH core–shell hybrid decreased
dramatically compared with those of the TiO2 HSs and the TiO2 NPs, which implies that
charge-carrier recombination was efficiently hindered. In addition, the PL emission intensity
of the target TiO2/LDH core–shell hybrid is inferior to that of the analogous TiO2 NPs/LDH
hybrid with a different morphology, consistent with its superior CO2 photoreduction
performance, as previously discussed. This result provides strong evidence that the core–shell
architecture of the TiO2/LDH hybrid could create large interfacial contact between TiO2 HSs
and LDH for fast charge transfer, effectively impeding the direct recombination of
photogenerated charge carriers.
Transient photocurrent response studies were also conducted to further verify the separation
and transport behaviors of photogenerated charge carriers in the TiO2/LDH core–shell hybrid.
Because the photocurrent is primarily generated by the diffusion of photogenerated electrons,
a strong photocurrent response implies greater separation efficiency of electron–hole pairs.60,62
Figure 8c displays the transient photocurrent responses of the TiO2 HSs, TiO2/LDH core–shell
hybrid, TiO2 NPs/LDH, and reference TiO2 NPs samples, as recorded during several on–off
cycles of intermittent light irradiation. The photocurrent response of the TiO2/LDH core–shell
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hybrid is several times higher than that of the TiO2 NPs, TiO2 HSs, and the TiO2 NPs/LDH
DOI: 10.1039/D0TA00104J
catalysts, which unequivocally demonstrates the greater separation of photoexcited charges in
the TiO2/LDH core–shell hybrid system. The greater extent of charge separation could certainly
enhance the CO2 photoconversion efficiency. This result is in accordance with the previously
discussed PL and CO2 reduction activity results.
To provide additional evidence for the efficient charge transfer in the TiO2/LDH core–shell
hybrid, we conducted EIS analysis. The arc diameter in the EIS profile reflects the charge-
transfer resistance of the electrode material: the smaller the arc diameter, the greater the charge
transfer efficiency.16,63 Figure 8d depicts the Nyquist plots for the TiO2 NPs, TiO2 HSs, TiO2
NPs/LDH, and TiO2/LDH core–shell hybrid samples. These Nyquist plots were fitted using the
equivalent circuits displayed in Fig. S13. The Randles circuit (which containing Warburg
element) was used for the fitting of the EIS curves of TiO2/LDH core–shell and TiO2 NPs/LDH
hybrids where diffusion behaviour along with solution resistance and charge transfer resistance
were observed. The simple charge transfer circuit was used for the EIS curves of TiO2 NPs and
TiO2 HSs where only solution resistance and charge transfer resistance were observed. The
charge transfer resistance values of TiO2 NPs and TiO2 HSs are about 14.8 and 10.6 kΩ,
respectively, as estimated by fitting the EIS data with equivalent circuit. Notably, the
TiO2/LDH core–shell hybrid displayed a significant decrease in the charge transfer resistance
(152 Ω) compared to TiO2 NPs, TiO2 HSs, and TiO2 NPs/LDH (420 Ω). The diminished charge
transfer resistance imparts the TiO2/LDH core–shell hybrid with increased electronic
conductivity, facilitating the migration of photoinduced electrons and holes. Thus, the EIS
results, together with the PL and photocurrent outcomes, demonstrate that the strong synergy
between the TiO2 HSs and LDH, induced by their efficient core–shell geometric structure,
promotes interfacial charge-transfer processes. Furthermore, these processes impede the direct
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recombination of photoexcited charges, which eventually results in the exceptional VCiewOAr2ticle Online
DOI: 10.1039/D0TA00104J
reduction performance of the TiO2/LDH core–shell hybrid toward the production of solar fuels.
In addition, the previously discussed BET analysis results demonstrate that the TiO2/LDH
core–shell hybrid maintains a high surface area and good pore structure, whereas the CO2
adsorption isotherm analysis results confirm that the TiO2/LDH core–shell hybrid exhibits
excellent CO2 adsorption capability, which is a prerequisite for the subsequent CO2 reduction
reaction by photoexcited charges. These results indicate that the enhanced surface area, pore
structure, and CO2 adsorption capability of the TiO2/LDH core–shell hybrid are also important
factors responsible for the extraordinary CO2 photoreduction activity. To elucidate the transfer
pathways of the photoinduced electrons and holes across the interface in the TiO2/LDH core–
shell hybrid, an investigation of the band-edge potentials of its constituents is essential. In this
study, the VB edge potentials of TiO2 and LDH were determined by VB-XPS from the intercept
of the tangent to the density of states at the Fermi edge as 2.73 and 1.6 eV, respectively (Fig.
S14), in good agreement with values reported previously.30,35,41,42 Meanwhile, the bandgap
energies of TiO2 HSs and LDH estimated from Tauc plots are 3.18 and 2.32 eV, respectively
(Fig. S15). On the basis of these VB edge potentials and the bandgap energies obtained from
Tauc plots (Fig. S15), the conduction band (CB) edge potentials were calculated to be −0.45
and −0.72 eV for TiO2 and LDH, respectively.
Because both the CB and VB potentials of TiO2 are more positive than those of LDH, the
charge transfer at the interface may follow the Z-scheme transfer or double-transfer route. If
the Z-scheme transfer route is assumed for the present core–shell hybrid system, then the CO2
reduction reaction and the water oxidation reaction would proceed on the CB of LDH and on
the VB of TiO2, respectively. This behavior can be confirmed by evaluating the generation rate
•−
of spin-reactive hydroxyl radicals (•OH) and superoxide radicals (O2 ) via the ESR technique.
5,5-Dimethyl-1-pyrroline N-oxide (DMPO), a typical spin-trap molecule, was used for
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DOI: 10.1039/D0TA00104J
•
•−
trapping the OH and O2 to form the DMPO−•OH and DMPO−O2•− adducts, respectively.64
Figure 9a and 9b display typical DMPO−•OH and DMPO−O2•− peaks of the TiO2 HSs, LDH,
and the TiO2/LDH core–shell hybrid under simulated solar-light irradiation. The lack of a
signal corresponding to DMPO−•OH or DMPO−O2•− under the dark condition for all of the
catalysts demonstrates that the spin-reactive species were indeed produced under stimulation
of solar light. A strong ESR signal with a 1:2:2:1 quartet corresponding to the DMPO−•OH
adduct was observed for the TiO2 HSs. By contrast, no obvious signal was detected for LDH
•
because of its unmatched VB potential to produce OH radicals. Surprisingly, the TiO2/LDH
core–shell hybrid displayed a substantial decrease in DMPO−•OH signal intensity compared
with the TiO2 HSs, perhaps because of the photoinduced hole transfer from the VB of TiO2 to
the VB of LDH. Meanwhile, a set of four peaks corresponding to the DMPO−O2•− adduct was
observed for all of the three catalysts; however, the intensity of the peaks for the TiO2/LDH
core–shell hybrid was much higher than that observed for TiO2 HSs and LDH. The increased
DMPO−O2•− signal intensity and the decreased DMPO−•OH signal intensity for the TiO2/LDH
core–shell hybrid indicate that the Z-scheme transfer route is not applicable to the present core–
shell hybrid system. Therefore, the charge transfer in the TiO2/LDH core–shell hybrid follows
the conventional double-transfer route, where the CO2 photoreduction process occurs on the
CB of TiO2 and the water oxidation reaction occurs on the VB of LDH. Notably, TiO2 HSs and
LDH exclusively produced CH4 and CO, respectively, whereas the TiO2/LDH core–shell
hybrid produced CH4 as the major product along with very small amount of CO, which also
supports the aforementioned conclusion that the CO2 reduction reaction occurs on the CB of
TiO2.
The band alignment of the TiO2/LDH core–shell hybrid and detailed conventional double-
transfer mechanism based on the previously discussed ESR results and band-structure analysis
are schematically illustrated in Fig. 10. When the TiO2/LDH core–shell hybrid is exposed to
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light, both TiO2 and LDH are excited, producing electron (e−) and hole (h+) chargDeOcI:a10r.r1i0e3r9s/Da0TtA00104J
the CB and VB, respectively. Because of the well-matched band structure and inner electrical
field established between TiO2 and LDH, the excited electrons transfer from the CB of LDH
to the CB of TiO2; meanwhile, the photoinduced holes tend to transfer from the VB of TiO2 to
the VB of LDH. As a result, the direct recombination of photoexcited electrons and holes is
substantially inhibited in both components. The charge transfer process also results in the
accumulation of holes in the VB of LDH and electrons in the CB of TiO2. The gathered holes
in the VB of LDH are trapped by H2O molecules to release protons (H+) and O2. Meanwhile,
under the assistance of the produced protons, the electrons accumulated in the CB of TiO2 can
react with CO2 molecules adsorbed onto the surface of the catalyst to produce solar fuels such
as CH4 and CO. Thermodynamically, the generation of CH4 from CO2 reduction is indeed
difficult compared with the generation of CO because the former requires 8 e− and 8 H+,
whereas the latter needs only 2 e− and 2 H+. Nonetheless, the CB potential of TiO2 (−0.45 eV
vs. normal hydrogen electrode (NHE), pH = 7) is slightly lower than the standard reduction
potential of CO/CO2 (−0.53 eV vs. NHE, pH = 7) but is higher than the standard reduction
potential of CH4/CO2 (−0.24 eV vs. NHE, pH = 7),10 suggesting that the electrons in the CB of
TiO2 are only capable of producing CH4. In addition, the double transfer mechanism suggests
that a substantial number of available electrons (required for CH4 formation) exist in the CB of
TiO2, which could also explain why CH4 was generated as a major product in the CO2 reduction
reaction by the TiO2/LDH core–shell hybrid. The leftover electrons in the CB of LDH after
charge transfer may be responsible for the observed production of a small amount of CO. The
reactions that occur during the photocatalytic CO2 conversion process to produce solar fuel are
summarized by the following equations:
TiO2/LDH + hν⟶TiO2/LDH (hV+B + eC―B)
TiO2/LDH (hV+B + eC―B)⟶TiO2 (e ― ) + LDH (hV+B)
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DOI: 10.1039/D0TA00104J
+
+
LDH hVB + 2H2O⟶O2 + 4H + 4e ― ; 퐸0 = +0.82 V
(
)
―
+
0
(
)
(Major)
TiO2 8e +8H +CO2⟶TiO2 +CH4 +2H2O; 퐸 = ―0.24 V
―
+
0
(
)
LDH
(Minor)
2e +2H +CO2⟶LDH + CO + H2O; 퐸 = ―0.53 V
―
+
0
(
)
(Minor)
TiO2 or LDH 2e +2H ⟶TiO2 or LDH + H2; 퐸 = ―0.41 V
4 Conclusions
In summary, we successfully fabricated the TiO2/LDH core–shell hybrid via ecologically
friendly and economical synthesis route. Notably, compared with TiO2 NPs, TiO2 HSs,
commercial P25, and TiO2 NPs/LDH with different morphologies, the synthesized TiO2/LDH
core–shell hybrid displays substantially enhanced photocatalytic CO2 reduction activity
favoring CH4 and CO production. The CO2 reduction activity of the TiO2/LDH core–shell
hybrid in terms of CH4 production is comparable, and in many cases superior, to that of the
numerous reported TiO2-based photocatalysts. This remarkable CO2 reduction activity of the
TiO2/LDH core–shell hybrid toward solar fuel generation is predominantly attributed to its
unique core–shell geometric structure, which provides an enhanced interfacial contact area for
efficient charge transfer, together with other advantageous attributes such as good light-
harvesting ability, a large surface area, porous structure, and outstanding CO2 adsorption
capability. Additionally, the TiO2/LDH core–shell hybrid demonstrates high stability during
successive test cycles, with no apparent change in the CO2 reduction activity.
Conflicts of interest
There are no conflicts to declare.
Acknowledgments
This work was supported by National Research Foundation of Korea grants funded by the
Korea government (MSIP) (No. 2017R1A4A1015628).
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Figure captions
DOI: 10.1039/D0TA00104J
Fig. 1 (a, b) FESEM and (c, d) TEM micrographs of the prepared carbon spheres.
Fig. 2 (a) FESEM and (b) TEM images of the TiO2/Csample. (c) FESEM and (d) TEM images
(inset is the magnified-TEM image) of the TiO2 HSs.
Fig. 3 (a) TEM, (b) magnified TEM, and (c) high-resolution TEM micrographs of the
TiO2/LDH core–shell hybrid. (d to h) STEM-HAADF elemental distribution maps of
constituent elements in the TiO2/LDH core–shell hybrid.
Fig. 4 (a) XRD and (b) UV-vis DRS spectra of the prepared samples.
Fig. 5 High-resolution XPS spectra of TiO2 HSs, LDH, and TiO2/LDH core–shell hybrid
samples. (a) Ti 2p, (b) Ni 2p, (c) Al 2p, and (d) O 1s.
Fig. 6 (a) CO2 adsorption isotherms and (b) TGA curves of the prepared samples.
Fig. 7 Time-dependent (a) CH4, (b) CO, and (c) H2 yields produced in the presence of all the
prepared samples. (d) Comparison of the photocatalytic CH4, CO, and H2 production rates over
the prepared samples upon 5 h of simulated-solar-light irradiation.
Fig. 8 (a) Recycling tests of CH4 and CO evolution over the TiO2/LDH core–shell hybrid. (b)
PL, (c) photocurrent, and (d) EIS profiles of TiO2 NPs, TiO2 HSs, TiO2 NPs/LDH, and
TiO2/LDH core–shell hybrid samples.
Fig. 9 DMPO spin-trapping ESR spectra of TiO2 HSs, LDH, and TiO2/LDH core–shell hybrid
samples for (a) DMPO−•OH and (c) DMPO−O2•− irradiated for 6 min.
Fig. 10 Schematic illustration of the proposed mechanism for photocatalytic CO2 reduction in
the TiO2/LDH core–shell hybrid system.
Scheme 1 Schematic representation of the synthesis of TiO2/LDH core–shell hybrid
photocatalyst.
29