Thermally treated zeolitic imidazolate framework-8 (ZIF-8) for visible light photocatalytic degradation of gaseous formaldehyde

Article abstract of DOI:10.1039/d0sc01397h

The development of wide-spectrum responsive photocatalysts for efficient formaldehyde (HCHO) removal is highly desired yet remains a great challenge. Here we successfully converted zeolitic imidazolate framework-8 (ZIF-8), one of the most well-studied metal-organic frameworks (MOFs), from routine ultraviolet-driven to novel broad-spectrum-driven photocatalyst via a facile thermal treatment. The isocyanate groups (-NCO) formed in the thermally treated ZIF-8 (ZIF-8-T) is crucial in enabling the superior photocatalytic performance in formaldehyde degradation. Specifically, the best-performing ZIF-8-T sample showed around 2.1 and 9.4 times the HCHO adsorption amount and the solar photocatalytic degradation rate, respectively, of pristine ZIF-8. In addition, ZIF-8-T exhibited visible light (λ ≥ 400 nm) photocatalytic HCHO degradation performance, photo-converting 72percent and nearly 100percent of 20 ppm and 10 ppm HCHO within 1 hour, respectively. This work affords new insights and knowledge that inspire and inform the design and development of MOF-based photocatalysts with broad-spectrum responses for efficient air purification operations.

Full text of DOI:10.1039/d0sc01397h

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DOI: 10.1039/D0SC01397H
Thermally Treated Zeolitic Imidazolate Framework-8 (ZIF-8) for
Visible Light Photocatalytic Degradation of Gaseous Formaldehyde
ac
b
ac
ac
a
d
Tianqi Wang,‡ Yufei Wang,‡ Mingzhe Sun, Aamir Hanif, Hao Wu, Qinfen Gu,
e
*f
*b
*b
*ac
Yong Sik Ok, Daniel C.W. Tsang, Jiyang Li, Jihong Yu and Jin Shang
aSchool of Energy and Environment, City University of Hong Kong, Tat Chee Avenue,
Kowloon, Hong Kong, China.
bState Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of
Chemistry, Jilin University, Changchun 130012, China.
cCity University of Hong Kong Shenzhen Research Institute, 8 Yuexing 1st Road,
Shenzhen Hi−Tech Industrial Park, Nanshan District, Shenzhen, China.
dThe Australian Synchrotron (ANSTO), 800 Blackburn Road, Clayton, VIC 3168,
Australia.
eKorea Biochar Research Center, O-Jeong Eco-Resilience Institute (OJERI) & Division of
Environmental Science and Ecological Engineering, Korea University, Seoul 02841,
Republic of Korea.
fDepartment of Civil and Environmental Engineering, The Hong Kong Polytechnic
University, Hung Hom, Kowloon, Hong Kong, China.
‡
These authors contributed equally to this work.
Corresponding authors:
_________________________________
_
*Tel: +86 431 8516 8608, Fax: +86 431 8516 8608, Email: lijiyang@jlu.edu.cn (J.L.).
*Email: jihong@jlu.edu.cn (J.Y.).
*Tel: +852 2766 6045, Fax: +852 2334 6389, Email: dan.tsang@polyu.edu.hk (D.T.).
*Tel: +852 3442 7714, Fax: +852 3442 0688, Email: jinshang@cityu.edu.hk (J.S.).

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Abstract: The development of wide-spectrum responsive photocatalysts for
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t
formaldehyde (HCHO) removal is highly desired yet remains a great challenge. Here
we successfully converted zeolitic imidazolate framework-8 (ZIF-8), one of the most
well-studied metal-organic frameworks (MOFs), from routine ultraviolet-driven to
novel broad-spectrum-driven photocatalyst via a facile thermal treatment. The
isocyanate groups (−N=C=O) formed in the thermally treated ZIF-8 (ZIF-8-T) is crucial
in enabling the superior photocatalytic performance in formaldehyde degradation.
Specifically, the best-performing ZIF-8-T sample showed around 2.1 and 9.4 times the
HCHO adsorption amount and the solar photocatalytic degradation rate, respectively,
of pristine ZIF-8. In addition, ZIF-8-T exhibited visible light (≥400 nm) photocatalytic
HCHO degradation performance, photo-converting 72% and nearly 100% of 20-ppm
and 10-ppm HCHO within 1 hour, respectively. This work affords new insights and
knowledge that inspire and inform the design and development of MOF-based
photocatalysts with broad-spectrum responses for efficient air purification operations.

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DOI: 10.1039/D0SC01397H
Introduction
Formaldehyde (HCHO) is a criteria indoor air pollutant that poses significant
1
threats to human health. Driven by the public concerns, various technologies such as
adsorption and photocatalysis have been developed for the removal of HCHO from
indoor air.2-5 Adsorption is the most straightforward strategy for HCHO removal, but
it, by itself, is unable to ultimately eliminate HCHO. On the other hand, photocatalysis
can completely convert HCHO to harmless carbon dioxide and water under ambient
conditions upon appropriate light irradiation. However, the efficiency of
photocatalytic HCHO degradation is often not sufficiently high due to the limited
6
, 7
HCHO adsorption on conventional photocatalysts such as TiO and g-C N , which
2
3 4
have limited surface to bind pollutants. It is well acknowledged that adsorption is
crucial for efficient photocatalysis because many photocatalytic reactions necessitate
the short-range contact between catalysts and reactants. A porous structure in
photocatalysts can not only improve HCHO adsorption, but also elevate photocatalytic
efficiency due to the more abundant active chemical sites and the shorter electron-
transfer distance compared with non-porous photocatalysts.8-10 A hierarchical porous
structure is more attractive owing to the facilitation of mass diffusion and the
improved light transmittance.11-13 Therefore, a porous photocatalyst is desirable for
HCHO elimination.
One family of such promising candidate materials are metal-organic frameworks
(MOFs). MOFs are a class of porous materials composed of metal ions and organic
bridging ligands, which exhibit unique properties such as rich porosity, large specific
surface area, abundant active centers, remarkable reusability, and tunable
photocatalytic activity.14-17 One of the most outstanding advantages of MOFs over
traditional semiconductors is the high designability at molecular level, which is
enabled by rationally tuning metal ions or organic ligands. Therefore, MOFs have been
successfully employed as a new class of photocatalysts towards various photocatalytic
applications. For example, zeolitic imidazolate framework-8 (ZIF-8), one of the most
well-studied MOFs, has been utilized for photocatalytic bacterial disinfection under

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1
8
simulated solar irradiation and degradation of methylene blue dye under
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irritation.19 Hierarchical porous ZIF-8 has been readily prepared,20, 21 showing a
potential in facilitating pollutant elimination. However, the photocatalytic HCHO
1
8
degradation by ZIF-8 has been rarely studied. Due to the wide bandgap energy, ZIF-
can only be excited by UV light, which accounts for as low as 4% of solar spectrum.22
8
To better utilize natural sunlight and thus enable an efficient photocatalysis, a
photocatalyst is highly desirable that can be responsive to visible light, which
constitutes about 45% of sunlight.23
Doping (or co-doing) and dye sensitization are the common strategies to realize
the visible-light activation of wide-bandgap photocatalysts.24-26 However, these
strategies face limitations such as high material cost (e.g. upon transition metals used
o
as dopants), high energy consumption (e.g., doping temperature 700~1200 C), and
risk of secondary pollution (e.g., metal/dye leakage). Recently, the important role of
ligand types in extending the optical absorption of MOFs was discovered. For instance,
with aminoterephthalate instead of terephthalate as linker, the UiO-66 MOF showed
an increase in absorption band from 300 to 440 nm.27 More recently, Fu et al.
demonstrated a NH -MIL-125(Ti) MOF exhibited visible-light response by changing the
2
ligand from terephthalic acid to 2-aminoterephthalic acid.28 These studies also
emphasized the significant role of amine groups (NH ) within ligands in realizing
2
visible-light photocatalysis on MOFs. On the other hand, isocyanate (−N=C=O) groups
showed great potential to convert the UV-responsive semiconductors (e.g., TiO ) into
2
visible-light photocatalysts. For example, −N=C=O groups (2,4-diisocyanate, TDI)
anchored on TiO via –NHCOOTi– bonds could be excited by visible light and
2
transferred electrons to the conduction band of TiO , therefore, the optical absorption
2
of TiO /TDI complex was extended to visible light region, realizing photocatalytic dye
2
degradation under visible light irradiation.24, 29 Chen et al. also reported a −N=C=O
groups-modified TiO , which showed visible-light absorption and enhanced
2
photocatalytic performance towards 2,4-dichlorophenol degradation.30 They
explained it by a direct surface electron transfers from the lone pair electrons of N
atom and O atoms to the conduction band of TiO . Fan et al. reported a silver
2

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isocyanate (AgNCO) photocatalyst synthesized by the precipitation of m
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isocyanate N=C=O and Ag . The AgNCO photocatalyst exhibited visible-light
photocatalytic degradation ability and superior stability. Upon visible light irradiation,
the electrons transferred from Ag and pi bond to isocyanate group and converted it
2
−
−
from N –C≡O state to N =C=O state. Subsequently, electron-induced reactive species
were generated for photocatalytic degradation. These exciting works motivate us to
functionalize ZIF-8, a typical wide-bandgap and one of the most well-studied MOFs, to
be a visible-light-driven photocatalyst by tuning its 2−methylimidazole (C H N ) ligand.
4
6 2
We hypothesized that the −N=C− bonds containing 2−methylimidazole can be
modified to generate the −N=C=O functional groups through oxidation. Therefore, we
initiated to develop a facile strategy to enable ZIF-8 to exhibit visible light activity.
Herein, for the first time, we functionalized hierarchical porous ZIF-8 nanocrystals
as a visible-light-driven photocatalyst via a facile thermal treatment in air at a
o
temperature as low as 200 C (Scheme 1). The formation of new −N=C=O functional
groups (derived from −N=C− bonds in the organic ligands via oxidation) was discovered
in the thermally treated ZIF-8 (named ZIF-8-T), leading to an increase in the specific
surface area and a substantially expanded light absorption range from UV to infrared.
By coupling adsorption and photocatalytic oxidation, efficient HCHO elimination was
achieved by ZIF-8-T under visible light and simulated sunlight. Considering the
availability of various organic linkers and the possibility of modulating the composition
of MOFs, we believe this study can open up new opportunities to develop efficient
and wide-spectrum responsive MOF photocatalysts.

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DOI: 10.1039/D0SC01397H
Scheme 1. Schematic representation of ZIF-8-T for photocatalytic HCHO degradation.
Results and discussion
Characterization of the as-made materials
Fig. 1 (a) PXRD patterns and (b) UV-Vis DRS spectra of the as-prepared samples.
As shown in Fig. 1a, the powder X-ray diffraction (PXRD) pattern of the as-prepared
ZIF-8 is in good agreement with that of previously reported ZIF-8,32, 33 suggesting the
successful synthesis of pure phase ZIF-8. The PXRD patterns of ZIF-8-T samples are
consistent with that of pristine ZIF-8, indicating that the ZIF-8 framework was well
preserved after thermal treatment. Notably, the PXRD peaks of ZIF-8-T products
(especially ZIF-8-T3) are significantly stronger than that of ZIF-8, suggesting an
improved crystallinity caused by the thermal treatment. No significant changes in the
PXRD pattern of ZIF-8-T4 were observed when further prolonging the thermal
treatment to 7 h. The lattice parameters of ZIF-8 before and after thermal treatment

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were estimated by analyzing the PXRD patterns, with a=17.0130 Å for ZI
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a=17.0206 Å for ZIF-8-T-3. Therefore, the lattice structures of ZIF-8 remained almost
unchanged upon thermal treatment. As shown in the ultraviolet-visible diffuse
reflectance spectroscopy (UV-Vis DRS) spectra (Fig. 1b), all ZIF-8(-T) samples exhibit
strong absorption peaks at around 207 nm, which is attributed to the intra-ligand
charge transfer, as suggested by the very similar absorption profile of
2
−methylimidazole (H-MeIM) ligand (Supplementary Fig. S1). Pure ZIF-8 also shows a
broad absorption located at ca. 300 nm, which corresponds to the ligand-to-metal
charge transfer (LMCT).18 After the thermal treatment, ZIF-8-T samples exhibit a
remarkably improved light absorption especially in visible light region – a significant
absorption at 420 nm together with the minor LMCT signal at 300 nm. In particular,
the absorption range of ZIF-8-T3 photocatalyst even extends to above 700 nm,
indicating a broad-spectrum-responsive characteristic. Compared with the recently
reported MOFs-based visible-light photocatalysts (Supplementary Table S5), which
showed limited absorption in visible light region, the ZIF-8-T photocatalysts prepared
in this work exhibited sufficient visible light absorption, with the absorption band
being 715 nm. Thus, ZIF-8-T photocatalysts show great potential in using sunlight.

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DOI: 10.1039/D0SC01397H
Fig. 2 (a) In-situ FTIR spectra of ZIF-8 recorded at different temperatures in air, (b) C
s XPS spectra, (c) XANES spectra, and (d) Zn K-edge EXAFS spectra of ZIF-8 and ZIF-8-
T3.
1
To evaluate the chemical structure changes of ZIF-8 during the thermal treatment,
in-situ Fourier transform infrared (FTIR) analysis was carried out. As demonstrated in
-1
o
Fig. 2a, the peak of methyl groups (−CH ) at 1380 cm becomes weaker at 125 C, and
3
o
almost disappears when the temperature reaches 150 C, indicating the dissociation
of −CH groups. James and Lin also reported the disappearance of −CH groups when
3
3
o
34
thermally treating ZIF-8 at a 300 C in air or nitrogen. Meanwhile, an additional peak
-1
corresponding to isocyanate groups (−N=C=O) is observed at 2223 cm in the FTIR
o
spectra of the thermally treated ZIF-8 samples at 175 C, and it becomes more
o
remarkable at 200 C with increasing thermal treatment time. The FTIR peak location
of −N=C=O observed in this work matches well with the previous studies. For example,

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-1
the peak of −N=C=O groups was detected at 2225 cm in the FTIR spectrum o
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HMS catalyst at 200 C during gas adsorption. The FTIR peaks of −N=C=O groups in
-1
the absorbed isocyanate compound on Cu-ZMS-5 were observed at 2240 cm and
-1
o
o
36
2
204 cm at 100 C and 200 C, respectively. The −N=C=O groups on the surface of
-1 37
TiO showed a FTIR peak at 2260 cm . The above results suggest ZIF-8 was gradually
2
oxidized and the new −N=C=O groups were generated during the thermal treatment
process in air. On the other hand, when ZIF-8 was treated under N , its FTIR spectrum
2
(Supplementary Fig. S2) shows the absence of −N=C=O peak as well as the decrease in
the intensity of −CH peak, revealing that only the dissociation of −CH groups occurs
3
3
during the thermal treatment of ZIF-8 in inert atmosphere.
X-ray absorption spectroscopy (XPS) spectra of the as-prepared samples were
recorded to elucidate the elemental compositions and valence states. As shown in Fig.
2
2
b, the C 1s XPS spectrum of ZIF-8 can be divided into three peaks located at 284.4,
3
84.9, and 285.8 eV, which are attributed to C=C, C–sp , and C–N bindings,
1
6
respectively. As for the C 1s XPS spectrum of ZIF-8-T3, two additional peaks denoting
C=O bonds and C=N bonds from −N=C=O groups are observed at 287.1 eV and 287.6
3
8
3
eV, respectively. The peak area of C–sp (284.9 eV) in ZIF-8-T3 becomes significantly
smaller than that in the XPS spectrum of ZIF-8, suggesting the loss of –CH groups in
3
2
+
ZIF-8-T3. In addition, the same valence state of Zn in both ZIF-8 and ZIF-8-T3 was
confirmed by the Zn 2p XPS peaks at 1021.6 eV (Zn 2p ) and 1044.8 eV (Zn 2p ), as
3
/2
1/2
shown in Supplementary Fig. S3.
To understand the local geometric structure of ZIF-8 before and after the thermal
treatment, X-ray absorption spectroscopy (XAS), a powerful technique to determine
the coordination environment and valence state of the target atoms, was employed.
The Zn K edge X-ray absorption near edge structure (XANES) of ZIF-8-T3 is almost
overlapped with that of pristine ZIF-8 (Fig. 2c), suggesting the unchanged Zn chemical
state after thermal treatment. As shown in the extended X-ray absorption fine
structure (EXAFS) spectra (Fig. 2d), the presence of Zn–N bonds and the absence of
Zn–Zn bonds in both ZIF-8 and ZIF-8-T3 samples are observed, indicating that Zn atoms
are atomically dispersed and bonded with N atoms. As listed in the fitting results

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(
Supplementary Table S1), ZIF-8 and ZIF-8-T3 have very similar Zn-N coor
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number (3.97 for ZIF-8 and 3.95 for ZIF-8-T3) and bond length (1.997 Å for ZIF-8 and
.998 Å for ZIF-8-T3), suggesting that no changes occur to Zn–N bond. The energy
1
dispersive spectroscopy (EDX) measurements (Supplementary Fig. S4 and Table S2)
show that the atomic content of C in ZIF-8 decreases from 68.2% to 56.2% with the
heating time increasing from 0 to 7 h, while O content increases from nearly zero to
6
.27%. The EDX results further reveal the loss of methyl group and the incorporation
of oxygen into ZIF-8 framework after the thermal treatment. Considering the cleavage
of C–N connection is more easily to take place due to its lower bond dissociation
-1
-1
-1 39, 40
enthalpy (449 kJ·mol ) than that of C=C (602 kJ·mol ) and C=N (615 kJ·mol ),
it is
deduced that upon heating, the dissociation of −CH group from the imidazolate ligand
3
and the breakage of C–N bond occur, together with the generation of −N=C=O bonds.
On the basis of the above discussion, the mechanism of partial structure changes of
the pyrolyzed ZIF-8 is proposed and shown in Scheme 1.
Fig. 3 (a) Dark field TEM, (b) SEM, and (d) light field TEM of ZIF-8; (c) SEM, (e) light field
TEM, and (f) high resolution TEM of ZIF-8-T3.
The morphologies of ZIF-8 and ZIF-8-T samples were investigated by scanning
electron microscope (SEM) and transmission electron microscopy (TEM). As displayed

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in Fig. 3a-b, ZIF-8 nanostructures show a uniform rhombic dodecahedron morp
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with the particle size of approximately 90 nm, while the shape of ZIF-8-T3 particles is
slightly irregular (Fig. 3d-f). Abundant mesopores are observed in both ZIF-8 and ZIF-
8
-T3 samples (Fig. 3a, d, and e), with the size being ~3 nm in diameter, as shown in
Fig. 3f. The presence of micro-mesopores in ZIF-8(-T) photocatalysts was favorably
generated via the incomplete crystallization thanks to the relatively short reaction
time (1 h) during the material synthesis, making a hierarchical porous structure
supposed to facilitate guest adsorption and light transmittance.
Fig. 4 (a) N adsorption and desorption isotherms and (b) pore size distributions of ZIF-
2
8
and ZIF-8-T3.
As mentioned above, the adsorption capacity of photocatalyst is important for the
photocatalytic performance. Therefore, the porosity of ZIF-8 samples was studied by
N adsorption-desorption isotherms conducted at 77 K (Fig. 4). As anticipated, the
2
presence of hierarchical micro−mesopores is confirmed in all samples, as suggested
by the sharp slopes of the isotherms appearing at low relative pressure range and the
hysteresis loops at high relative pressure (i.e., 0.92~1.0), respectively. The pore size
distribution (Fig. 4b) also confirms the presence of micropores (1.0−1.7 nm) and
mesopores (~3.1 nm). It is well acknowledged that the hierarchical porous structure is
favorable for gas diffusion and thus accelerates the subsequent gas adsorption and
reaction. As shown in Supplementary Table S3, the specific surface area of ZIF-8-T3
2
−1
2
−1
(
1327 m ·g ) is slightly higher than that of ZIF-8 (1212 m ·g ), which is probably due
to the formation of interrupted cavities in ZIF-8 framework caused by the broken
bonds upon thermal treatment. The higher specific surface of ZIF-8-T can favor HCHO

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adsorption and offer more active sites for HCHO degradation.
DOI: 10.1039/D0SC01397H
Fig. 5 (a) Tauc plots, (b) Mott−Schottky plots, (c) valence band XPS spectra, and (d)
band structures of ZIF-8 and ZIF-8-T3.
To characterize the band structures of ZIF-8 and ZIF-8-T3, Tauc plots,
Mott−Schottky plots, and valence band XPS spectra were examined. As shown in the
Tauc plots (Fig. 5a), the bandgap energy of ZIF-8 and ZIF-8-T3 is accordingly estimated
to be 3.87 and 2.19 eV, respectively. The positive slopes of Mott−Schottky plots
confirm the n−type semiconductor nature of ZIF-8 and ZIF-8-T3 (Fig. 5b), and thus their
Femi levels are close to the flat potentials, which are examined to be −0.22 V and −0.32
V (vs. NHE), respectively. As displayed in the valence band XPS spectra (Fig. 5c), the
valence band (VB) maximum positions of ZIF-8 and ZIF-8-T3 are 2.63 eV and 2.61 eV
below the Femi level, respectively. The conduction band (CB) minimum positions are
4
1
accordingly calculated to be −1.67 and −0.48 V (vs. NHE). Consequently, a diagram
of the band structures is established as shown in Fig. 5d. The significant shift of CB and
the moderate shift of VB lead to the narrowed bandgap of ZIF-8-T3, making the photo-
excitation wavelength of ZIF-8-T3 fall into visible light region.

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Photocatalytic degradation of gaseous HCHO
DOI: 10.1039/D0SC01397H
Fig. 6 (a) Photocatalytic degradation of gaseous HCHO (20 ppm)
,
(b) the corresponding
CO evolution, and (c) HCHO conversion (evaluated by CO evolution in Fig. 6b) and
2
2
kinetics over the as-prepared samples under simulated sunlight (AM 1.5); (d)
photocatalytic degradation of gaseous HCHO (20 ppm) by ZIF-8 and ZIF-8-T3 under
visible light (inset is the CO evolution curve).
2
In comparison with pure ZIF-8, ZIF-8-T catalysts exhibited substantially enhanced
adsorption capacity and remarkably elevated photocatalytic degradation
performance towards gaseous HCHO. Fig. 6a shows that about 11.6% and 24.0% of
HCHO (20 ppm) were adsorbed by ZIF-8 and ZIF-8-T3, respectively, at equilibrium in
dark condition. The HCHO adsorption capacity ZIF-8-T3 is calculated to be 1.51
-1
-1
mmol·g , which is more than two times of that on pristine ZIF-8 (0.73 mmol·g ). The
improved HCHO adsorption by ZIF-8-T3 is possibly attributed to the larger specific
surface area and structure changes (i.e., formation of −N=C=O bonds and dissociation
of −CH groups). In addition, the less adsorption competition with CO may contribute
3
2
to the enhanced HCHO adsorption by ZIF-8-T3, as suggested by its lower CO absorbed
2

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-1
-1
amount (0.45 mmol·g at 1 bar) than that of ZIF-8 (0.60 mmol·g atDO
Supplementary Fig. S5).
The photocatalytic performance of the as-prepared samples was first examined
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(
under simulated sunlight (AM 1.5) irradiation. As shown in Fig. 6 and Supplementary
Table S4, only ~29% of HCHO was photocatalytically degraded within 1 h by pristine
ZIF-8. Impressively, a rapid decrease of HCHO concentration was observed once ZIF-
8
-T photocatalysts were employed. Among them, ZIF-8-T3 exhibited the highest
photocatalytic HCHO degradation activity, with the pseudo-first-order kinetics (k) of
−1
0
.0754 min and nearly 100% conversion of HCHO achieved within 1 h. The sunlight-
driven HCHO degradation rate by ZIF-8-T3 is about 9.4 times of that by pristine ZIF-8.
The efficient degradation of HCHO into CO and water by ZIF-8-T samples was
2
corroborated by the evolution of CO (CO ) (Fig. 6b), which was close to the
2
2
theoretical CO value corresponding to the degradation of HCHO. In addition,
2
compared with ZIF-8-T3, although the microporous mZIF-8-T3 offers a larger specific
surface area (Supplementary Fig. S6 and Table S3), it shows a lower photocatalytic
−1
efficiency towards HCHO degradation (k = 0.0321 min ) (Fig. S7). This result suggests
that the hierarchical micro−mesoporous structure of ZIF-8-T3 is beneficial to the
photocatalytic performance, which can be ascribed to the better HCHO diffusion and
light transmittance in ZIF-8-T3 photocatalyst.12 Zhang and co−workers have
demonstrated that the presence of hierarchical pores facilitated the molecular
1
3
reactants to reach the active sites on the porous interior. García−Benjume et al. have
reported that the hierarchical porous structure of anatase photocatalyst allowed
better light penetration and improved the photocatalytic performance of dye
degradation.11
To investigate the responsiveness to light spectrum, the best-performing ZIF-8-T3
was then irradiated by visible light for HCHO degradation. As shown in Fig. 6d, ZIF-8-
T3 exhibited an efficient visible-light-driven photocatalytic degradation of HCHO, with
−1
~
72% decomposition of 20 ppm HCHO within 1 h (k = 0.0247 min ). In addition, the
effect of initial concentration of HCHO on the photocatalytic efficiency of ZIF-8-T3 was
investigated. As shown in Fig. S8, the photocatalytic HCHO degradation efficiency

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increased with decreasing HCHO concentration. Especially, nearly 100%DOI
H
: 1
C
0.
H
1039/D0SC01397H
O
−1
removal (k = 0.0453 min ) was achieved by ZIF-8-T3 within 1 h with the initial HCHO
concentration of 10 ppm. By contrast, pristine ZIF-8 showed no visible light
photoactivity. The photocatalytic performance of ZIF-8-T3 is significantly better than
those of recently reported photocatalysts for HCHO degradation under similar
experimental conditions.6, 42-45 The stability and reusability of ZIF-8-T3 were
investigated by repeating the photocatalytic HCHO degradation with recycled ZIF-8-
T3, as shown in Fig. S9. After each run, the photocatalyst was collected and degassed
at 393 K for 12 h under vacuum to completely remove the absorbed HCHO as well as
products from photocatalytic reactions. Importantly, ZIF-8-T3 photocatalyst exhibited
excellent stability and reusability, as suggested by the almost unchanged
photocatalytic HCHO degradation efficiency after 5 cycles. Therefore, the ZIF-8-T
photocatalysts prepared in this work show a great potential for the real-world
application in HCHO decomposition and abatement.
Mechanisms of the enhanced photocatalytic activity of ZIF-8-T and reaction
pathways of photocatalytic degradation of HCHO
Fig. 7 (a) Photocatalytic HCHO degradation efficiency by ZIF-8-T3 and ZIF-8-T (N2)
under various irradiations and (b) voltammograms of ZIF-8-T3 and ZIF-8-T (N ) upon
2
visible light irradiation.
As mentioned above, the major changes of the chemical structure of ZIF-8 after
thermal treatment were the generation of −N=C=O functional groups and the
dissociation of −CH groups. To understand which one is the dominant factor for the
3

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improved photocatalytic activity of the thermally treated ZIF-8, we then car
D
r
O
ieI:
d
10
o
.1039/D0SC01397H
u
t
the visible light photocatalytic HCHO degradation by ZIF-8-T (N ). ZIF-8-T (N ) was
2
2
thermally treated under N atmosphere so that only the dissociation of −CH groups
2
3
took place, whereas the formation of −N=C=O functional groups via oxidation was
forbidden. As shown in Fig. 7a, in comparison with ZIF-8-T3 photocatalyst, ZIF-8-T (N2)
sample exhibited much lower photocatalytic efficiency for HCHO degradation under
both visible and simulated sunlight illuminations. Only 9.5% conversion of HCHO was
achieved by ZIF-8-T (N ) under visible light. Although around 37% HCHO was degraded
2
by ZIF-8-T (N ) under simulated sunlight, this efficiency is close to that of pristine ZIF-
2
8
(~29%). Moreover, the photocurrent intensity of the ZIF-8-T3 is significantly higher
than that of the ZIF-8-T (N ) upon visible light irradiation at different voltages (Fig. 7b),
2
revealing the better charge transfer ability of ZIF-8-T3. Therefore, it is the newly
generated −N=C=O groups rather than the dissociation of −CH groups that dictates
3
the outstanding photocatalytic capacity of ZIF-8-T photocatalyst, especially the visible-
light-induced photocatalytic performance.
−
Fig. 8 (a) EPR signals and (b) •O DMPO spin−trapping EPR spectra of ZIF-8-T3 with
2
and without simulated sunlight irradiation.
To elucidate the transfer pathway of photogenerated electron, we first monitored
the valence state of Zn in ZIF-8-T3 upon light irradiation using electron paramagnetic
resonance (EPR). The EPR spectra of ZIF-8-T3 recorded in air with and without sunlight
irradiation are shown in Fig. 8a. In comparison with the unirradiated ZIF-8-T3, two new
peaks appear at g = 2.0038 and g = 1.9600 in the EPR spectra of the irradiated one,

Page 17 of 26
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which are ascribed to the absorbed •O2− and the paramagnetic Zn+
c
e
n
t
e
r
s,
D
O
I
: 1
0
.1
039/D0SC01397H
respectively.46, 47 Therefore, it can be induced that under sunlight irradiation, the
electrons generated by ligand (via LMCT) or semiconductor (from VB to CB) would
2
+
+
−
transfer to Zn to produce paramagnetic Zn sites, as shown in Eq. (1)-(3). Then, •O
2
+
radicals are generated by electron transfer from Zn to O (Eq. (4)). We also argue that
2
the photogenerated electrons at semiconductor CB can directly react with O to form
2
−
•
O radicals due to the more negative CB potential of ZIF-8-T3 (-0.48 V vs. NHE) than
2
−
48
that of O /•O (-0.33V vs. NHE) (Eq. (5)). As expected, four characteristic peaks of
2
2
−
DMPO−•O adducts were clearly detected for ZIF-8-T3 under simulated sunlight
2
irradiation, whereas no signal was observed for ZIF-8-T3 in dark (Fig. 8b). However,
the generation of •OH radicals is thermodynamically forbidden due to the deficient
VB potential (1.71 V vs. NHE) for the conversion of H O to •OH (2.68 V vs. NHE). It is
2
not surprising that negligible signals of •OH radicals were observed for ZIF-8-T3 with
and without irradiation (Supplementary Fig. S10).
Fig. 9 (a) Effect of relative humidity on photocatalytic degradation of gaseous HCHO
(
20 ppm) by ZIF-8-T3 under simulated sunlight and (b) concentration of H O produced
2
2
by ZIF-8-T3 in a liquid-phase system under simulated sunlight.
It is well acknowledged that the performance of catalysts is strongly influenced by
4
2
water species. To understand the effect of water species on the photocatalytic
performance of ZIF-8-T3, the HCHO degradation experiments were carried out at low,
ambient, and high relative humidity (RH). Fig. 9a demonstrates that the RH
substantially affects the photocatalytic HCHO degradation efficiency. Although
moisture competes with HCHO for adsorption by ZIF-8-T3, as revealed by the

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decreased HCHO adsorption amount at higher RH, the presence of moistur
D
e
O
f
I:
a
10
v
.
o
1039/D0SC01397H
r
s
HCHO degradation significantly. This is ascribed to the important role of moisture in
the generation of H O , which is active for HCHO degradation, as shown in Eq. (7) and
2
2
(
8). The produced H O by ZIF-8-T3 in a liquid-phase system was then detected by
2
2
using p-hydroxyphenylacetic acid as the probe, with the total H O concentration of
2
2
−1
1
1.1 μmol·L irradiated by simulated sunlight for 1 h (Fig. 9b). It is noted that the
-1
photocatalytic HCHO degradation rate (k = 0.0754 min ) at ambient RH (~65%) is close
-1
to that (k = 0.0759 min ) at higher RH (~86%), suggesting ZIF-8-T3 photocatalyst can
work efficiently at ambient condition. In addition, the VB potential of ZIF-8-T3 is
positive enough to directly oxidize HCHO, as demonstrated by the study reported
7
−
+
earlier. Therefore, •O , H O , and h are the major reactive species responsible for
2
2 2
the photocatalytic HCHO degradation by ZIF-8-T3. Given the important role of
−
electron-induced reactive species (i.e., •O and H O ), the abundant photogenerated
2
2 2
electron−hole charge pairs and the high electron transfer efficiency in ZIF-8-T3 (Fig. 10)
are significantly favorable for the photocatalytic HCHO degradation performance.
Based on the discussion above, the reaction pathways under sunlight irradiation are
elaborated as the following equations and shown in Scheme 1:
-
+
ZIF-8-T3 + hv → e + h
(1)
2)
2
+
+
Ligand + Zn → Zn (LMCT)
(
2
+
-
+
Zn + e → Zn
(3)
(4)
(5)
(6)
(7)
(8)
(9)
+
2+
−
Zn + O → Zn + •O
2
2
-
−
e + O → •O
2
2
−
•
•
2
O + HCHO → CO + H O
2
2
2
−
+
-
O + H +H O → H O + O + OH
2
2
2
2
2
H O + HCHO → 3H O + CO
2
2
2
2
+
h + HCHO → CO + H O
2
2

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DOI: 10.1039/D0SC01397H
Fig. 10 Photocurrent responses of ZIF-8 and ZIF-8-T3 under simulated sunlight.
Conclusions
The wide-spectrum photocatalytic degradation of gaseous HCHO is for the first
time achieved by the −N=C=O groups-functionalized ZIF-8 MOF, which is fabricated via
o
a facile thermal treatment strategy at a relatively low temperature of 200 C. The
organic linkers in ZIF-8 are partially oxidized after thermal treatment, resulting in the
generation of −N=C=O functional groups. The optical absorption band of ZIF-8-T is
broadened from UV (~325 nm) to visible (~715 nm) region. Thanks to the introduction
of −N=C=O groups into ZIF-8 backbones, efficient photocatalytic degradation of
gaseous HCHO by ZIF-8-T is achieved upon sunlight and visible light irradiations. In
addition, the hierarchical porous structure of ZIF-8-T facilitates the HCHO adsorption
and light transmittance. Compared with pristine ZIF-8, ZIF-8-T photocatalyst shows
more abundant charge carrier density and more efficient electron transfer, which are
favorable for the photocatalytic performance. An LMCT process in ZIF-8-T during the
photocatalytic reaction is established. The photogenerated reactive species including
−
+
•
O radicals, H O , and h play the important role in HCHO degradation. This work
2
2 2
provides valuable insights into the development of visible-light-induced MOFs
photocatalysts and boosts the application of MOFs in air purification.
Experimental
Synthesis of hierarchical porous ZIF-8 and conventional ZIF-8

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Hierarchical porous ZIF-8 was synthesized by following a reported proced
D
u
O
r
I:
e
10
(
.1
n
039/D0SC01397H
o
4
9
thermal treatment in this procedure) with minor modifications. Herein, we used 1/4
volume of reaction solutions (i.e. 50 mL) than the reported ones. In brief, 0.7332 g of
Zn(NO ) ·6H O was dissolved in 50 mL methanol as solution A, and 1.6225 g of
3
2
2
2
−methylimidazole was dissolved in 50 mL methanol as solution B. Then, solution A
was rapidly added into solution B within 30 seconds and stirred for 1 h. After
centrifugation and washing with methanol for 5 times, the milky white hierarchical
porous ZIF-8 product was obtained. Conventional microporous ZIF-8 was synthesized
using the similar procedure as that of hierarchical porous ZIF-8, except that the mixed
solution was stirred for 20 h.
Synthesis of hierarchical porous ZIF-8-T and microporous ZIF-8-T
The hierarchical porous ZIF-8-T was obtained by thermally treating the as-
o
prepared hierarchical porous ZIF-8 at 200 C in air. To evaluate the effect of reaction
time, a series of hierarchical porous ZIF-8-T samples were prepared by varying the
thermal treatment time duration of 1, 3, 5, and 7 h. The products are denoted as ZIF-
8
-T1, ZIF-8-T2, ZIF-8-T3, and ZIF-8-T4, accordingly.
Since the pyrolysis atmosphere has significant influences on the chemical structure
of products, we also thermally treated the hierarchical porous ZIF-8 under N2
o
atmosphere at 200 C for 5 h. As anticipated, the oxidation of ZIF-8 and the generation
of oxygen-related bonds were forbidden. The product is denoted as ZIF-8-T (N2).
To understand the role of hierarchical pores in ZIF-8-T on the adsorption and
photocatalytic performance, we also fabricated microporous ZIF-8-T by using the as-
prepared conventional ZIF-8 as precursor, with the same procedure as that of ZIF-8-
T3. The product is named as mZIF-8-T3.
Characterizations
Powder X−ray diffraction (PXRD) patterns were recorded using an X−ray
diffractometer (X′Pert3 Powder, PANalytical, Netherlands) with Cu Kα radiation (40 kV,
4
0 mA). X-ray absorption spectroscopy (XAS) measurements were conducted at the
Australian Synchrotron (ANSTO) to evaluate the chemical state and configuration of
Zn. The morphologies of ZIF-8 photocatalysts were investigated by transmission

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electron microscopy (TEM) and high angle annular dark field scanninDO
g
I: 1
T
0.
E
1039/D0SC01397H
M
(HAADF−STEM) conducted using a high−resolution transmission electron microscope
(Tecnai F20, FEI Company, USA). Energy dispersive spectroscopy (EDX) was measured
by an EDX detector (Oxford Aztec Energy X−MAX 50) equipped in a scanning electron
microscope (FEI Quanta 450 FEG) to examine the chemical compositions of
photocatalysts. The indium film was used for sample loading to eliminate the influence
on carbon content from the conductive tape. The porosity of ZIF-8 samples was
determined by a 3Flex Surface Characterization Analyzer (Micromeritics Instrument
Corp., U.S.A.) using N at 77 K. Prior to the measurement, all samples were degassed
2
at 423 K for 12 h under vacuum using a VacPrep Degasser (Micromeritics Instrument
Corp., U.S.A.). To examine the chemical state of Zn during the photocatalytic reaction,
electron paramagnetic resonance (EPR) signals of Zn were recorded with a Bruker
E500 spectrometer at room temperature in air. Besides, EPR signals of spin−trapped
paramagnetic species (•O2− and •OH) were measured with corresponding N,
N−dimethyl pyrroline N−oxide (DMPO). In-situ Fourier transform infrared (FTIR)
spectra were collected with a FTIR spectrometer (NICOLET iS50, Thermo Scientific,
USA). To examine the recombination rate of electrons and holes, photoluminescence
(PL) spectra were recorded on a fluorescence spectrometer (FP−6500, Jasco, Japan)
with various excitation wavelength (λ ). The light absorption profiles of
ex
photocatalysts were examined by ultraviolet-visible diffuse reflectance spectroscopy
(UV-Vis DRS) recorded with an Lambda 950 spectrometer (PerkinElmer, U.S.A.).
Photoelectrochemical measurements including transient photocurrent responses,
electrochemical impedance spectroscopy (EIS), and Mott–Schottky plots were carried
out with a CHI 760E electrochemical workstation (Shanghai Chen Hua, China).
Photocatalytic degradation of gaseous HCHO
Photocatalytic degradation of gaseous HCHO was performed in a 400 mL
PLS−SXE300 Labsolar−IIIAG photoreactor (Perfectlight Co., Ltd, China). Typically, 50
mg of photocatalyst powder was uniformly dispersed on the watch glass inside the
reactor. Then an appropriate volume of HCHO solution (37 wt. %) was injected into
the reactor to reach the initial HCHO concentration of ca. 20 ppm. Prior to illumination,

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039/D0SC01397H
d
o
the reactor was heated at 40 C for 4 h to completely vaporize the HCHO solu
D
ti
O
o
I:
n
10
a
.1
n
to reach the adsorption equilibrium. A 300-W Xenon lamp (Newport−67005, Oriel
Instruments, U.S.A.) with appropriate filter(s) was vertically placed outside the reactor
to provide visible light or simulated sunlight irradiation. The reaction temperature was
o
maintained at 40 C by a water circulation bath in all photocatalytic experiments. At
given time intervals, the HCHO concentration was determined using a gas
chromatograph (Shimadzu GC−2010, Japan) equipped with a flame ionization detector
(FID). The kinetics of photocatalytic degradation of HCHO was determined by using
the pseudo−first−order kinetic equation ln(C /C) =kt, where C is the initial
0
0
concentration of HCHO, C is the HCHO concentration at time t, and k is the apparent
reaction rate constant.
Conflicts of interest
There are no conflicts to declare.
Acknowledgments
This research was financially supported by the National Natural Science Foundation of
China (21706224, 21671075), the Science and Technology Innovation Commission of
Shenzhen Municipality (JCYJ20180307123906004), the Research Grants Council of
Hong Kong (CityU 21301817, 11215518). We thank Dr. Yun Hau Ng for his invaluable
advices and comments throughout the project. The technical assistance from Ms.
Yuanmeng Tian, Mr. Zeyu Tao, and Ms. Shanshan Shang of School of Energy and
Environment, City University of Hong Kong are appreciated. The authors also would
like to acknowledge the research undertaken on the XAS beamline at the Australian
Synchrotron, and we thank Dr. Bernt Johannessen in assistance for XAS experiment.
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DOI: 10.1039/D0SC01397H
1
Graphical Abstract
2
3
The isocyanate groups (−N=C=O) derived from −N=C− bonds in the organic ligands
enables the thermally treated ZIF-8 to act as wide-spectrum photocatalyst.
4
5
6