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Supporting Information
for “Highly Efficient Visible Light-Induced O2 Generation by Self-Assembled
Nanohybrids of Inorganic Nanosheets and Polyoxometalate Nanoclusters”
Jayavant L. Gunjakar, Tae Woo Kim, In Young Kim, Jang Mee Lee, and Seong-Ju Hwang*
Center for Intelligent Nano-Bio Materials (CINBM), Department of Chemistry and Nano
Sciences, Ewha Womans University, Seoul 120-750, Korea
Figure S1│Structural study. Powder XRD patterns of the (a) ZCV-1, (b) ZCV-2, (c) ZCW-1,
and (d) ZCW-2 nanohybrids calcined at 300 °C.
: The effect of calcination at 300 C on the crystal structure of the as-prepared Zn-CrLDHPOM nanohybrids is examined with powder X-ray diffraction (XRD) analysis. As plotted
in Fig. S1, all the present nanohybrids calcined at 300 C commonly show the suppression of a
series of intense (00l) peaks at low angle region with a broad hump feature at 2 = 3040. The
disappearance of the (00l) reflections corresponding to basal planes strongly suggests the
destruction of intercalation structure after calcination at 300 C. However, even with the absence
of (00l) reflections, the in-plane (110) peak is still discernible at 2 = ~62 for all the present
nanohybrids calcined at 300 C. This observation demonstrates that, despite the frustration of
the pillared structure, a fraction of the Zn-Cr-LDH nanosheets is still maintained upto 300 C.
Figure S2│ XANES spectroscopy. (a) Zn K-edge and (b) Cr K-edge XANES spectra of the asprepared nanohybrids of (i) ZCV-1, (ii) ZCV-2, (iii) ZCW-1, (iv) ZCW-2, (vviii) their
derivatives calcined at 200 C, (ix) the pristine Zn-Cr-LDH, (x) ZnO/Cr2O3, and (xi) CrO3. (c) W
LIII-edge and (d) V K-edge XANES spectra of the as-prepared nanohybrids of (i) ZCW-1/ZCV1, (ii) ZCW-2/ZCV-2, (iiiiv) their derivatives calcined at 200 C, (v) WO2/V2O4,
Na2WO4·2H2O/V2O5, and (vii) V2O3.
(vi)
Figure S3│ XANES spectroscopy. (a) Zn K-edge and (b) Cr K-edge XANES spectra of the asprepared nanohybrids of (i) ZCV-1, (ii) ZCV-2, (iii) ZCW-1, (iv) ZCW-2, (vviii) their
derivatives calcined at 300 C, (ix) the pristine Zn-Cr-LDH, (x) ZnO/Cr2O3, and (xi) CrO3. (c) W
LIII-edge and (d) V K-edge XANES spectra of the as-prepared nanohybrids of (i) ZCW-1/ZCV1, (ii) ZCW-2/ZCV-2, (iiiiv) their derivatives calcined at 300 C, (v) WO2/V2O4,
Na2WO4·2H2O/V2O5, and (vii) V2O3.
(vi)
: The effects of hybridization and heat-treatment on the local crystal and electronic structures of
the host Zn-Cr-LDH material are investigated with XANES spectroscopy at Zn K-edge and Cr
K-edge. The Zn K-edge XANES spectra of the as-prepared ZCW and ZCV nanohybrids and
their calcined derivatives are plotted in Fig. S2a, as compared with those of the pristine Zn-CrLDH and bulk ZnO. The as-prepared ZCW and ZCV nanohybrids as well as the pristine Zn-CrLDH exhibit nearly identical edge positions to that of ZnO, indicating the divalent oxidation
state of zinc ions in these compounds. Like the pristine Zn-Cr-LDH compound, all of the asprepared ZCW and ZCV nanohybrids display a broad and intense main-edge peak A, a typical
spectral feature of the Zn-containing LDH compound.[1,2] This strongly suggests that the layered
lattice of the Zn-Cr-LDH nanosheets remains intact after the hybridization with POM
nanoclusters. For all the present Zn-Cr-LDHPOM nanohybrids, the heat-treatment at 200 C
gives rise to only a negligible spectral variation, indicating the maintenance of the Zn-Cr-LDH
lattice upon calcination at this temperature. As shown in Fig. S3a, the calcination of the asprepared  nanohybrids at 300 C does not induce a marked change of the edge energy,
indicating the maintenance of divalent Zn oxidation state after the heat-treatment. In contrast to
the reference ZnO, the ZCW and ZCV nanohybrids calcined at 300 C commonly exhibit the
poorly resolved XANES features in the energy region of 9660–9670 eV, strongly suggesting the
maintenance of Zn-Cr-LDH nanosheets. However, in comparison with the spectra of the asprepared nanohybrids, the present spectra of the 300 C-calcined materials display a significant
broadening of the peak A, reflecting the increase of structural disorder upon the calcination
process. The variation of POM content in the present nanohybrids has little influence on the
overall spectral features of the present nanohybrids, confirming negligible influence of guest
POM species on the local atomic arrangement of host Zn-Cr-LDH lattice.
Fig. S2b represents the Cr K-edge XANES spectra of the Zn-Cr-LDHPOM nanohybrids and
their calcined derivatives, as compared with several reference spectra. All of the present
materials show a pre-edge peak P corresponding to the dipole-forbidden 1s  3d transition,
whose intensity reflects sensitively the local structure and oxidation state of chromium ions.[3]
While a very intense pre-edge peak P is observable for the reference CrO3 compound with
tetrahedral Cr6+ ion, all the Zn-Cr-LDHPOM nanohybrids and the pristine Zn-Cr-LDH exhibit
only a weak intensity for the pre-edge peak P. This finding clearly demonstrates the presence of
Cr3+ ions in octahedral geometry for all the present nanohybrids, like the pristine Zn-Cr-LDH.[1]
All the present materials display several main-edge peaks A, B, and C, which are assigned as
dipole-allowed transitions from 1s orbital to unoccupied 4p ones. These main-edge spectral
features of the pristine Zn-Cr-LDH compound remain nearly unchanged upon the self-assembly
with POM nanoclusters and the heat-treatment at 200 C, underscoring the maintenance of the
Zn-Cr-LDH nanosheets after the hybridization and calcination processes. As shown in Fig. S3b,
the edge position of the nanohybrids calcined at 300 C remains nearly the same as that of the
reference Cr2O3, confirming the maintenance of trivalent oxidation state of chromium ions. In
contrast to the edge energy, the heat-treatment at 300 C induces significant spectral variations
like the appearance of the additional pre-edge peak P' and the main-edge feature A, as observed
in the reference Cr2O3. This finding reveals the transformation of the local structural
environment of chromium ions to the Cr2O3-type structure. Like Zn K-edge XANES spectra,
there is no significant change in Cr K-edge spectra upon the change of POM content, confirming
that the amount of guest POM species has little influence on the chemical bonding nature of the
host Zn-Cr-LDH lattice.
The local symmetry and oxidation state of guest POM nanoclusters in the present Zn-CrLDHPOM nanohybrids and their calcined derivatives are also examined using XANES analysis
at W LIII-edge and V K-edge. The W LIII-edge XANES spectra of the as-prepared ZCW
nanohybrids and their derivatives calcined at 200 and 300 C are plotted in Figs. S2c and S3c, as
compared with those of references WO2 and Na2WO4·2H2O. All of the present materials show
broad and intense white-line peak A corresponding to dipole-allowed transitions from the 2s
level to unoccupied 5d states.[4,5] Considering the fact that W 5d orbitals are separated into t2g (or
e) and eg (or t2) orbitals under an octahedral (or tetrahedral) crystal field, the observed broad
peak A consists of overlapped features related to 2s  5dt2g (or 5de) and 2s  5deg (or 5dt2)
transitions. Since the crystal field of tetrahedral symmetry is weaker than that of the octahedral
one,[47] the reference Na2WO4·2H2O with tetrahedral WO4 unit shows a narrower full-width-athalf-maximum (FWHM) for the peak A than the reference WO2 with WO6 octahedral units. Both
the ZCW nanohybrids display broader FWHM than Na2WO4·2H2O, confirming the maintenance
of the octahedral symmetry of W7O246 nanoclusters in the ZCW nanohybrids. Upon the heattreatment at 200 and 300 C, there is no remarkable change in the spectral feature of the asprepared nanohybrids, indicating negligible change of chemical bonding nature of
polyoxotungstate nanoclusters even with the collapse of intercalation structure.
The V K-edge XANES spectra of the as-prepared ZCV nanohybrids and their derivatives
calcined at 200 and 300 C are plotted in Fig. S2d and S3d, as compared with the reference
spectra of V2O4, V2O5, and V2O3. All of the present materials display not only a pre-edge peak P
corresponding to the 1s  3d transition but also main-edge peaks A, B, and C corresponding to
the 1s  4p transitions. The intensity of the pre-edge peak P is much stronger for the ZCV
nanohybrids than for the corundum-structured V2O3 containing VO6 octahedra. Since the
intensity of this pre-edge peak P is proportional to the deviation of local symmetry from
centrosymmetric octahedral one,[8,9] the observed high intensity of the pre-edge peak for the
ZCV nanohybrid strongly suggests the non-centrosymmetric local environment of vanadium
ions in these materials. All of the present ZCV nanohybrids exhibit similar positions of the mainedge peaks A, B, and C, which are similar to those of V2O5 but higher than those of V2O4. This
result provides strong evidence for the pentavalent oxidation state of vanadium ions in these
materials. The observed V K-edge XANES spectra of the present ZCV nanohybrids are fairly
similar
to
the
previously
reported
data
of
n-hexylammonium
decavanadate
(n-
C6H13NH3)6(V10O28)∙2H2O, confirming the stabilization of decavanadate nanoclusters in the
present nanohybrid materials.[810] The heat-treatments at 200 and 300 C have little influence on
the overall spectral feature of the as-prepared nanohybrids, indicating the maintenance of the
decavanadate nanoclusters after the calcination process.
Figure S4│ Surface morphological study of nanohybrids calcined at 300 °C. FE-SEM
images of the (a) ZCW-1, (b) ZCW-2, (c) ZCV-1, and (d) ZCV-2 nanohybrids.
: The crystal morphology of the Zn-Cr-LDHPOM nanohybrids calcined at 300 C are probed
with field emission-scanning electron microscopy (FE-SEM) analyses. As illustrated in the Fig.
S4, the heat-treatment at 300 C has little influence on the porous stacking structure and highly
anisotropic 2D shape of the LDH–POM nanohybrids, underscoring the high morphological
stability of these materials.
Figure S5│ EDS–elemental mapping analysis of the as-prepared Zn-Cr-LDH–POM
nanohybrids and their calcined derivatives.
EDS–elemental maps and (center) FE-SEM
images of the (a) ZCW-1, (b) ZCW-2, (c) ZCV-1, and (d) ZCV-2 nanohybrids. The data in the
left, middle, and right columns represent the FE-SEM images of the as-prepared nanohybrids and
their derivatives calcined at 200 and 300 °C, respectively.
Figure S6│HRTEM EDS–elemental line profile. (a) ZCW-1 and (b) ZCV-1 nanohybrids; red
(zinc), cyan (chromium), violet (oxygen), and green (tungsten/vanadium).
: The spatial distributions of component elements in the as-prepared Zn-Cr-LDHPOM
nanohybrids and their calcined derivatives are examined with EDSelemental mapping analysis.
As illustrated in the Fig. S5, all of the component elements, i.e. zinc, chromium, tungsten,
vanadium, and oxygen, are uniformly distributed in entire parts of the as-prepared nanohybrid
materials, clearly demonstrating the homogeneous hybridization of the Zn-Cr-LDH nanosheets
and POM nanoclusters without any phase separation.
The special elemental distributions of ZCW-1 and ZCV-1 nanohybrids are also investigated
with elemental EDS line profile using EDS machine installed with HR-TEM. As illustrated in
Fig. S6, the elemental EDS line profiles of the both nanohybrids clearly demonstrate the
homogeneous distribution of zinc, chromium, tungsten, vanadium, and oxygen elements along
the lines centered in the crystallites of the both nanohybrids. The heat-treatments at 200 and 300
C give rise to only a negligible frustration in the elemental distribution of the as-prepared ZnCr-LDHPOM nanohybrids, confirming the maintenance of intimate mixing between the two
components after calcinations. This finding provides strong evidence for no occurrence of any
phase separation upto 300 C.
ICP spectrometry and CHNS elemental analysis for the Zn-Cr-LDH–POM nanohybrids.
: The chemical compositions of the Zn-Cr-LDH–POM nanohybrids are quantitatively determined
with ICP and CHNS elemental analyses. The CHNS analysis reveals the presence of nitrogen
element in all the present nanohybrids, suggesting the incorporation of nitrate ions as a charge
compensator. Based on the ICP and CHNS elemental analysis, the chemical compositions of the
as-prepared
Zn-Cr-LDH–POM
nanohybrids
Zn0.64Cr0.36(OH)20.057(W7O39)0.89H2O0.09NO3,
0.07NO3,
are
determined
to
be
Zn0.68Cr0.32(OH)20.076(W7O39)0.15H2O
Zn0.66Cr0.33(OH)20.056(V10O28)0.80H2O0.09NO3,
and
Zn0.64Cr0.36(OH)2
0.061(V10O28)1.00H2O0.10NO3 for ZCW-1, ZCW-2, ZCV-1, and ZCV-2, respectively. This
result clearly demonstrates the tunability of the chemical composition of the present Zn-CrLDH–POM nanohybrid materials synthesized by the self-assembly method.
Figure S7│ N2 adsorption–desorption isotherm measurement for the calcined derivatives of
the Zn-Cr-LDH–POM nanohybrids. N2 adsorptiondesorption isotherms of the (a) ZCW-1,
(b) ZCW-2, (c) ZCV-1, and (d) ZCV-2 nanohybrids calcined at 300 °C.
Table S1. BET surface area of the Zn-Cr-LDHPOM nanohybrids calcined at 200 and 300 C
Sample
BET area (m2g1)
Sample
BET area (m2g1)
ZCW-1 calcined at 200 C
48
ZCV-1 calcined at 200 C
128
ZCW-1 calcined at 300 C
37
ZCV-1 calcined at 300 C
89
ZCW-2 calcined at 200 C
70
ZCV-2 calcined at 200 C
133
ZCW-2 calcined at 300 C
57
ZCV-2 calcined at 300 C
91
: The surface area and pore structure of the Zn-Cr-LDHPOM nanohybrids calcined at 300 C is
investigated with N2 adsorptiondesorption isotherm measurements. As shown in Fig. S7, the
heat-treatment at 300 C causes a significant weakening of the hysteresis, indicating the partial
destruction of mesoporous stacking structure at these temperatures. The surface areas of the
present Zn-Cr-LDHPOM nanohybrids are calculated based on the fitting analysis with the
BrunauerEmmettTeller (BET) equation. As summarized in Table S1, the calcination at 200
and 300 C induces the significant depression of the surface area, a result of the decrease of
interlayer spacing and the partial destruction of pillared structure.
Figure S8│ Pore size analysis for the calcined derivatives of the Zn-Cr-LDH–POM
nanohybrids. Pore size distribution curves of the Zn-Cr-LDHPOM nanohybrids of ZCV-1 (dot
dashed lines), ZCV-2 (dashed lines), ZCW-1 (solid lines), and ZCW-2 (dotted lines). The plots
(a), (b), and (c) represent the calculated curves of the as-prepared nanohybrids and their
derivatives calcined as 200 and 300 C, respectively.
: The pore-size distribution curves of the as-prepared Zn-Cr-LDHPOM nanohybrids and their
derivatives calcined 200 and 300 C are calculated on the basis of the Barrett–Joyner–Halenda
(BJH) method. As plotted in Fig. S8, all of the as-prepared nanohybrids have mesopores with a
broad pore size distribution of average diameter of ~3–8 nm. In comparison with the ZCV
nanohybrids showing somewhat broad distribution of pore size, the ZCW nanohybrids possess
much narrower distribution of pore size, implying the higher order of porous stacking structure
in the latter system. The heat-treatment at 200–300 C gives rise to a significant narrowing of the
pore size distribution of the present nanohybrids with the average diameter of ~3–3.5 nm. This
result indicates the improved ordering of mesopores via the removal of larger mesopores in the
as-prepared materials. Judging from the fact that all the present nanohybrids have much smaller
basal spacing of ~1 nm than the pore diameter, the mesopores in these materials are formed by
the house-of-cards stacking structure of layered crystallites, as suggested from the FE-SEM
results.
Figure S9│ Diffuse reflectance UV–vis spectroscopic analysis for the calcined derivatives of
the Zn-Cr-LDH–POM nanohybrids. Diffuse reflectance UV–vis spectra of the ZCW-1
(dashed-dotted lines), ZCW-2 (dot-dot-dashed lines), ZCV-1 (dashed lines) and ZCV-2 (dotted
lines) nanohybrids calcined at 300 C.
: The band structure and optical property of the Zn-Cr-LDHPOM nanohybrids calcined at 300
C are investigated with diffuse reflectance UVvis spectroscopy. As plotted in Fig. S9, the
visible light absorption ability of the as-prepared nanohybrids is well-maintained after the heattreatment at 300 C, indicating the maintenance of the electronic coupling between Zn-Cr-LDH
and POM components.
Figure S10│ PL spectroscopic analysis for the calcined derivatives of the Zn-Cr-LDH–
POM nanohybrids. PL spectra of the ZCW-1 (dot-dashed lines), ZCW-2 (dot-dot-dashed
lines), ZCV-1 (dashed lines), and ZCV-2 (dotted lines) nanohybrids calcined at 300 °C.
: The charge transfer between Zn-Cr-LDH nanosheets and POM nanoclusters after the
calcination at 300 C is investigated with photoluminescence (PL) spectroscopy. As plotted in
Fig. S10, the depressed PL signal of the present Zn-Cr-LDHPOM nanohybrids is wellmaintained upto 300 C, confirming the maintenance of the electronic coupling between Zn-CrLDH and POM components.
Figure S11│ Comparative photocatalyst tests of the calcined Zn-Cr-LDH–POM
nanohybrid with several references. Time-dependent photoproduction of O2 gas under visible
light illumination (λ  420 nm) by the ZCW-1 nanohybrid calcined at 200 C (close circles), the
pristine Zn-Cr-LDH (open circles), the pristine ammonium polyoxotungstate (triangles), the
pristine sodium metavanadate (squares), the physical mixture of Zn-Cr-LDH and ammonium
polyoxotungstate (inverse triangles), and the physical mixture of Zn-Cr-LDH and sodium
metavanadate (diamonds).
: The photocatalytic activity of the 200 C-calcined ZCW-1 nanohybrid is compared with those
of several references including the pristine Zn-Cr-LDH material, the pristine POM compounds
and their physical mixtures. As illustrated in Fig. S11, the 200 C-calcined ZCW-1 nanohybrid
is much more active for the photocatalytic generation of O2 molecules than the pristine Zn-CrLDH material, the pristine POM compounds, and their physical mixtures. The present results
obviously demonstrate the advantage of the hybridization with POM in improving the
photocatalytic activity of the pristine Zn-Cr-LDH.
Figure S12│ Consecutive photocatalytic activity tests. Time-dependent photoproduction of O2
gas under visible light illumination ( > 420 nm) by the (left) ZCW-1 and (right) ZCV-1
nanohybrids for (a) the 1st cycle, (b) the 2nd cycle, and (c) the 3rd cycle.
: To verify the photocatalytic stability and recyclability of the present Zn-Cr-LDHPOM
nanohybrids, the three consecutive tests of visible light-induced O2 evolution are carried out for
the most active nanohybrid materials of ZCW-1 and ZCV-1. The sacrificial agent AgNO3 is
added just after each run to compensate the consumption of this sacrificial agent. As plotted in
Fig. S12, both the ZCW-1 and ZCV-1 nanohybrids retain most of photocatalytic activity for
consecutive three cycles, clearly demonstrating the high stability of their photocatalytic activity.
Like the pristine Zn-Cr-LDH material, there occurs a slight depression of the photocatalytic
activity with proceeding the cycle. This phenomenon is generally observed for most of O2evolution photocatalysts including the pristine Zn-Cr-LDH material, a result of the
photodeposition of Ag on the surface sites of photocatalyst, leading to the masking of reaction
sites.
Figure S13│Structural and morphological study after photocatalytic activity tests. (Left)
Powder XRD patterns of the ZCW-1 and ZCV-1 nanohybrids restored after the three
consecutive photocatalytic activity tests. (Right) (a,b) FE-SEM images and (c,d) EDS elemental
mapping data of the (top) ZCW-1 and (bottom) ZCV-1 nanohybrids restored after the threeconsecutive photocatalyst tests.
: The effects of photoreaction on the crystal structure, crystal morphology, and chemical
composition of the ZCW-1 and ZCV-1 nanohybrids are investigated along with the
photodeposition of Ag metal on the surface of catalyst materials. As plotted in the left panel of
Fig. S13, both the ZCW-1 and ZCV-1 nanohybrids restored from the three-consecutive
photocatalytic reactions show a series of (00l) reflections and in-plane (110) peak, confirming
the maintenance of their original intercalation structure and the in-plane structure of the LDH
component after the photoreactions. In addition to these peaks of the nanohybrids, both the
photoreacted materials display newly appeared XRD peaks of Ag metal, confirming the
photodeposition of Ag on the surface of nanohybrids.
As illustrated in the FE-SEM results of the right panel of Fig. S13, the porous stacking
structures of the ZCV-1 and ZCW-1 nanohybrids remain nearly unchanged after the
photoreactions, confirming their high morphological stability. Also, the EDS mapping data of
ZCV-1 and ZCW-1 nanohybrids restored from the photoreactor clearly demonstrate the
homogeneous distribution of Zn, Cr, V, W, and Ag elements without any phase separation,
verifying the maintenance of hybrid structure composed of Zn-Cr-LDH and POM, and the
uniform deposition of Ag metal on the surface of the hybrid photocatalyst.
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