A gigantic polyoxozirconate with visible photoactivity

Article abstract of DOI:10.1039/c7dt02013a

We present a gigantic polyoxozirconate nanocluster (Zr₁₈O₂₁(μ₂-OH)₂(OOCPh)₂₈), involving 6, 7 and 8-coordinated Zr atoms. It exhibits visible light driven photocatalytic activity for H₂ evolution. The current results could represent a prelude of polyoxozirconate-based photocatalysis.

Full text of DOI:10.1039/c7dt02013a

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A gigantic polyoxozirconate with visible
photoactivity†
Cite this: DOI: 10.1039/c7dt02013a
Received 2nd June 2017,
Accepted 7th July 2017
a
a
a
b
a
Tingting Xu, Xudong Hou, Yang Wang, Jun Zhang, Junxiang Zhang and
a
Bo Liu
*
DOI: 10.1039/c7dt02013a
rsc.li/dalton
We present a gigantic polyoxozirconate nanocluster (Zr18
OH) (OOCPh)28), involving 6, 7 and 8-coordinated Zr atoms. It
exhibits visible light driven photocatalytic activity for H evolution.
2
O21(µ -
2
2
The current results could represent a prelude of polyoxozirconate-
based photocatalysis.
Polyoxometalates (POMs) have attracted intensive research
attention in both science and industry owing to their rich
structures and electronic properties and hence have appli-
cations in diverse fields such as catalysis, medicine, and mag-
1
netism. A great deal of efforts in POMs have been devoted to
d-block elements in high oxidation states, typically for V, Mo,
2
W etc. Recently, research studies on Ti-based POMs with novel
structures and properties have been renewed. Zhang and co-
Fig. 1 Illustration of the development of polyoxozirconate family. Aqua
workers have reported fullerene-like polyoxotitanate {Ti42} and spheres: Zr; red spheres: O.
{
Ti } clusters which hold the record of the largest titanium-
5
2
3
oxo cluster to date. Nevertheless, the polyoxozirconates
POZs), being analogous to polyoxotitanate, are less studied
(
Zr have been reported as secondary building units in metal–
organic frameworks.
8
and their properties as well as applications are even rarely
explored. Low-nuclearity polyoxozirconates containing 2–5 Zr
8
Zirconia has been widely used as an insulating material
due to its wide band gap (5 eV). In contrast, titania with a
band gap of around 3 eV has been explored as a semi-
conductor photocatalyst broadly. Zr(IV) having d10 electron con-
atoms have been prepared as the precursors for ZrO
2
thin film
fabrication. The reported high-nuclearity POZs, including
Zr }, {Zr10}, and {Zr12}, can be considered as derivatives from
the octahedral Zr
Hetero-polyoxozirconates, such as [Co Zr O (pr) (Hbda) ]
4
{
9
5
6 8
O parent core (Fig. 1 and Table S1†).
2
c,9
figuration could be explored for photocatalytic applications,
6
6
8
12
6
6
if we are able to narrow the band gap and enable efficient
solar spectrum absorption. In this work, we report a gigantic
polyoxozirconate of [Zr O (OH) (OCCPh) ], which, to the
(
2 2
Hbph) ·(H bph) and Zr24-cluster substituted poly(polyoxo-
7
tungstate) have exhibited distinct properties from pure POZs
owing to interactions between Zr and hetero-components. It is
noteworthy that the {Zr24} core is not isolated from its parent
1
8
21
2
28
best of our knowledge, represents the largest polyoxozirconate
so far and exhibits visible light driven photocatalytic activity
for H evolution. Another {Zr } cluster coordinating with
6
compound. Besides, various Zr-oxo clusters, typically Zr and
2
18
2
−
10
SO
4
has been reported without any properties, and the
structure is completely different from POZr18
.
a
An assembly of ZrCl and benzoic acid with the assistance
of thiourea in dried acetonitrile gave rise to pale yellow hexa-
hedra of [Zr18O21(OH) (OOCPh)28] (Fig. S1,† denoted as POZr18),
2
Department of Chemistry, University of Science and Technology of China, Hefei,
Anhui 230026, P. R. China. E-mail: liuchem@ustc.edu.cn
4
b
School of Materials and Chemical Engineering, Anhui Jianzhu University, Hefei,
2
30601, People’s Republic of China
which crystallized in the space group P2 /C (a = 28.3272(9) Å,
1
†
Electronic supplementary information (ESI) available: Crystallographic details
b = 20.6226(6) Å, c = 35.7721(11) Å; α = γ = 90°, β = 95.824°). As
a reference, we failed to obtain POZr18 without adding thiourea
in the reaction. It is presumed that thiourea functioned as
and information of supplementary figures. Structural determination details are
summarized in Table S2. CCDC 1522943. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c7dt02013a
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alkaline to promote the formation of POZr18. The crystal struc-
ture was resolved from single crystal X-ray diffraction data
(
see details in the ESI†). The global structure of POZr18 con-
sists of a {Zr18 21(OH) } core terminated by twenty-eight bridg-
ing carboxylates with a molecular size of 1.9 × 1.8 × 1.6 nm
Fig. 2a). Among eighteen Zr atoms, two adopt a six-coordi-
O
2
3
(
nation mode and two are coordinated with eight oxygen atoms
as individually marked with yellow and pink polyhedra in
Fig. 2b; all other fourteen Zr atoms adopt a seven-coordination
mode (green polyhedra). The co-existence of 6, 7 and 8-coordi-
nation modes in one single POZ structure has not been found
in previous studies yet. Bond valence sum (BVS) calculations
revealed that the BVS values of O52 and O53 (1.18 for O52, 1.14
for O , respectively) are close to 1, which suggested that O
Fig. 3 (a) {Zr18(OH)
nium oxide cores ({Zr
angle emphasizing the pentagonal {Zr
2
O
21} cores in the POZr18 cluster. (b) Partial zirco-
53
52
9
O10}) in the POZr18 cluster (shadow in (a)), view
and O53 are the two µ
with blue spheres in Fig. S2.†
2
-bridging hydroxyl groups as highlighted
6 5
O
} substructure. (c–e) The
1
1
Here, BVS values were zirconium atom skeleton in POZr18
.
1
2
calculated using s = exp[(r − r)/B] and the Zr–O distances
0
are listed in Table S3.† Other twenty-one oxygen atoms adopt a
µ
3
-bridging mode to connect adjacent Zr atoms.
The bulk phase purity of POZr18 was examined and con-
Benzoate groups can be classified into three types accord- firmed by powder X-ray diffraction (Fig. 4a) and elemental ana-
ing to their coordination modes: one is mono-dentate (in light lysis (ESI†). The PXRD pattern was identical to the one simu-
blue circle); three individually chelate with a single Zr atom (in lated from the single crystal XRD data. The thermogravimetric
orange circles) and twenty-four benzoates bridge two adjacent analysis revealed that POZr18 is thermally stable up to 200 °C
Zr atoms (in green circle) (Fig. S2†). As clearly presented in (Fig. S4†). The optical absorbance of ZrO2 and a small POZ
2
Fig. 3a, the zirconium atoms in the {Zr18O21(OH) } core can be cluster mainly originated from ligand (O) and metal (Zr)
described as a C -symmetrical structure which has a secondary charge transfer (LMCT). The band gap of ZrO was determined
2
2
rotation axis through Zr10 and Zr
6
(Fig. S3†). In addition, the to be in close proximity to 5 eV (Fig. 4b), thereby only exhibit-
remaining skeleton except Zr10 can formally be divided into ing potential photoactivity in the ultraviolet region. In con-
two corner-sharing (Zr ) pentagonal {Zr Zr(µ-O) } building trast, the UV-Vis absorption spectrum of POZr18 indicated a
6
5
5
blocks, and a {Zr
Zr Zr(µ-O) } pentagon (Fig. 3b). Alternatively, the {Zr18} skel- an obvious absorption tail extended into the visible region.
eton can be disintegrated into four {Zr (Zr)} pentagons The optical band gap of POZr18 was estimated to be 2.75 eV
3 5
(µ-O) } group is attached side-on to the strong absorption centered at ca. 300 nm in the UV region and
{
5
5
5
(
{
Fig. 3c–e). Two {Zr
Zr10} units (Fig. 3d) and two {Zr10} units are further connected Obviously, the ligand of benzoates in POZr18 made no contri-
into {Zr } units via two vertex-sharing edges (Fig. 3e). {M (M)} bution to visible absorption. EDS analysis (Fig. S5†) indicated
5
g
(Zr)} pentagons share an edge to form according to the equation E = 1240/λ (λ = 450 nm) (Fig. 4b).
1
8
5
1
3
units have been found in polyoxomolybdates, cerium-oxo that there was no sulfur-related species in the POZr18 cluster,
nanoclusters
1
4
3a
and fullerene-like {Ti42}.
5
The {Zr (Zr)} which could cause band-gap narrowing. It has been reported
pentagon identified in POZr , however, has not been found that oxygen vacancies in the crystal have important impact on
1
8
1
5
in the other reported POZ families so far. It could be a the electronic properties.
The introduction of oxygen
new type of building unit to construct large and novel POZ vacancies could narrow the band-gap and enhance visible light
clusters.
photocatalytic activity. Electron spin resonance (ESR) was
employed to investigate whether there are some oxygen
vacancies in the POZr18 cluster. As displayed in Fig. S6,† the
line at g = 2.004 was identified as the electrons trapped in
Fig. 2 (a) Crystal structure of POZr18 (H atoms are omitted for clarity).
Atom color code: large spheres, Zr (different colors indicate different
coordination types); red O; gray C. (b) Polyhedral mode presentation of
Fig. 4 (a) XRD patterns of POZr18; (b) UV-Vis absorption spectrum of
POZr18. Polyhedral color code: yellow: ZrO
6
; green: ZrO
7
; pink: ZrO
8
.
2
POZr18 cluster (blue) and ZrO (red).
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1
6
4+
oxygen vacancies. The bulk Zr ions adjacent to the bulk Chinese Academy of Science, and the Fundamental Research
oxygen vacancies could capture the electrons resulting in the Funds of Central Universities (WK2060190053), and the Anhui
3
+
formation of Zr ions. The signal at g = 1.974 can be contri- Province Natural Science Foundation (1608085MB28).
3
+
17
buted by the bulk Zr ions located at axially symmetric sites.
Accordingly, we assumed that the visible absorbance origi-
nated from oxygen vacancies in bulk POZr18. POZr18 represents
the largest and the first visible-active polyoxozirconate to date.
The previous studies on polyoxozirconates mainly focused
on their structure, characterization and their applications as
precursors for ZrO preparation. The photocatalysis appli-
2
cation of POZs is restricted by their limited optical absorption.
As POZr18 exhibited visible absorption, we are encouraged to
Notes and references
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investigate its photocatalytic activity for H
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1
8
particles as the co-catalyst under visible light irradiation. It
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2
formed in a gas-phase photoreactor where TEA and H O
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vapors participated in photocatalytic reaction (see details in
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1
9
20
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a polyoxozirconate of POZr18, which represented the largest
and the first visible-active polyoxozirconate so far. POZr18 dis-
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visible light irradiation. In spite of the moderate photocatalytic
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