A semiconducting gyroidal metal-sulfur framework for chemiresistive sensing

Article abstract of DOI:10.1039/c7ta02069d

The gyroid is an iconic structure that conjures up an intriguing 3D congener of the famous electronic systems of graphene and related 2D materials. Unlike the more accessible 2D graphitic systems, gyroidal metal-organic frameworks with demonstrated conductive properties remain unknown. We here report a semiconducting gyroidal net (denoted as HTT-Pb) that derives its rich electronic properties from the large organic π-electron system of a triphenylene core, highly polarizable Pb-dithiolene links, and robust Pb-oxo connections. In contrast to the generally encountered difficulty in crystallizing metal-thiolate networks, single crystals of HTT-Pb amenable to X-ray studies can be reliably obtained by regular solvothermal synthesis. The electronic conductivity of the framework solid is highly responsive to the water content in air, demonstrating potential use in chemiresistive sensing of humidity.

Full text of DOI:10.1039/c7ta02069d

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A semiconducting gyroidal metal-sulfur framework
for chemiresistive sensing†
Cite this: DOI: 10.1039/c7ta02069d
a
a
b
a
b
*
*
Jiahong Huang, Yonghe He, Ming-Shui Yao, Jun He, Gang Xu,
c
d
*
Matthias Zeller and Zhengtao Xu
Received 7th March 2017
Accepted 20th April 2017
DOI: 10.1039/c7ta02069d
rsc.li/materials-a
The gyroid is an iconic structure that conjures up an intriguing 3D and D surfaces) have of late attracted increasing interest. For
congener of the famous electronic systems of graphene and related example, structural modeling indicates that these surfaces can be
2D materials. Unlike the more accessible 2D graphitic systems, gyro- tiled with carbon polygons to generate carbon schwarzites (or
idal metal–organic frameworks with demonstrated conductive prop- Mackay–Terrones crystals) as 3D analogs of intensely studied
erties remain unknown. We here report a semiconducting gyroidal net graphene sheets. Since the initial proposition in 1991,2 numerous
(denoted as HTT–Pb) that derives its rich electronic properties from theoretical studies have been conducted to reveal the remarkable
the large organic p-electron system of a triphenylene core, highly stability and potentially intriguing electronic properties of these
polarizable Pb-dithiolene links, and robust Pb-oxo connections. In negatively curved graphitic carbon networks;3 the making of these
contrast to the generally encountered difficulty in crystallizing metal- 3D all-carbon crystals has, however, remained a challenge, and
thiolate networks, single crystals of HTT–Pb amenable to X-ray studies only illusive, sporadic occurrence has been reported so far. One
can be reliably obtained by regular solvothermal synthesis. The elec- major difficulty here lies in the very strong constituent C–C bond
tronic conductivity of the framework solid is highly responsive to the that frustrates the assembly of extended ordered structures, while
water content in air, demonstrating potential use in chemiresistive negatively curved molecules4 as discrete fragments of these carbon
sensing of humidity.
networks are well known.
The synthetic obstacle to all-carbon nets prompts us to target
more accessible systems of solid state materials for the explo-
ration of electronic properties in association with 3D minimal
surfaces. For this, the structural and functional versatility of
intensely studied metal–organic frameworks (MOFs) can be
advantageous.5 Unlike the intractable C–C bond for carbon
schwarzites, the very diverse metal–organic links offer a broad
range of bonding strengths to help accommodate the formation
of ordered solid state networks. Also, networks of a selected
topology (connectivity) can be assembled with a certain degree
of correlation with the shapes of the organic linkers and the
metal nodes. Notably, networks with structures associated with
minimal surfaces have since long been of interest in the MOF
eld.1g,6 For example, rigid tritopic molecules can sometimes
link up with metal nodes to generate a 3-connected net that is of
the gyroid topology [i.e., a (10,3)-a net].6a,b
Electrical conductivity, however, remains unexplored for
gyroidal MOF solids. In general, the largely insulating charac-
ters of MOF solids arise from the weak electronic interaction
across the molecular units,7 and polarizable ligands and metal
centers are oen used to enhance charge transport.8 Among
these, metal-thiolate links gure prominently,9 while strong
metal-amine and metal-aryloxide bonds have also been
explored.10 Our long-standing efforts to access semiconductive
The gyroid (G-surface) belongs to a family of triply periodic
minimal surfaces (TPMSs): it divides space into two equivalent,
interwoven channel domains and has everywhere a uniform
saddle shape with zero mean curvature (k1 + k2 ¼ 0) and negative
Gaussian curvature (k1k2 < 0; k1 and k2 being the two principal
curvatures). In addition to its frequent occurrence in various
materials systems (i.e., liquid crystals, block copolymers, and solid
state inorganics and frameworks),1 electronic properties associ-
ated with the gyroid and other minimal surfaces (e.g., Schwarz P
aSchool of Chemical Engineering and Light Industry, Guangdong University of
Technology, Guangzhou 510006, Guangdong, China. E-mail: junhe@gdut.edu.cn
bState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, Fujian, China.
E-mail: gxu@irsm.ac.cn
cDepartment of Chemistry, Purdue University, 560 Oval Drives, West Lafayette,
Indiana, 47907 USA
dDepartment of Biology and Chemistry, City University of Hong Kong, 83 Tat Chee
Avenue, Kowloon, Hong Kong, China. E-mail: zhengtao@cityu.edu.hk
† Electronic supplementary information (ESI) available: Experimental details and
general procedures; XRD, IR, Raman, EPR, XPS, TGA, optical absorption spectrum
and temperature dependent I–V curves. CCDC 1506170 contains the
supplementary crystallographic data for HTT–Pb. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c7ta02069d
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linker HTT and Pb(II) ions (denoted as HTT–Pb). Besides its
highly ordered structure and tunable semiconductive proper-
ties, its gyroidal topology represents a solid step toward the
synthesis and exploration of electronic systems related to peri-
odic minimal surfaces.
A solvothermal reaction (Scheme 1) of 2,3,6,7,10,11-hex-
akis(butyrylthio)triphenylene (HBuTT), Pb(OAc)2 (OAc: acetate)
and NaOH in a mixed solvent of ethylenediamine (EDA) and
methanol at 95 ꢀC yielded light-orange crystals of HTT–Pb.
HTT–Pb features a formula [Pb3OH0.5(HTT)]1.5ꢁ for the anionic
framework, with the negative charge being balanced by the
cations in the channel (e.g., EDX analysis indicated substantial
presence of Na+). The HBuTT molecules under the reaction
conditions hydrolyse in situ to form the HTT thiol species
(Scheme S1†): as a masked version of HTT, HBuTT thus serves
to slow down the reaction between the sulfur groups and the
Pb(II) ions, to promote crystallization as a result. By comparison,
if the free thiol of HTT is directly mixed with the Pb(OAc)2
solution, an amorphous precipitate was formed immediately,
and no signicant improvement in crystallinity was observed
even aer prolonged heating.
Scheme 1 Synthesis of the HTT–Pb framework solid.
networks7b,11 utilize polycyclic aromatic linkers (like in HTT,
2,3,6,7,10,11-triphenylene hexathiol)9i,10a,12 not only as a source
of highly polarizable p-electrons, but also as incipient frag-
ments of the above carbon schwarzites. Like the carbon
schwarzites, the strong metal–ligand links here hinder the
growth of ordered networks, and consequently, only poly-
crystalline powders or less-ordered solids were obtained. For
instance, a recent semiconductive MOF based on HTT and Pt(II)
features a 2D kagome lattice with distinct electronic properties,
but only polycrystalline samples with small grain sizes were
obtained.9i
Single-crystal X-ray diffraction revealed that HTT–Pb crys-
ꢀ
This is what makes the present single crystal structure so
worthy of note. This is a 3D network based on the dithiolene
tallizes in the cubic space group Pa3. The asymmetric portion of
the unit cell contains 1/3 of an O atom, one Pb(II) center and 1/3
Fig. 1 Single crystal structure of HTT–Pb. (a) The coordination environment around a pair of Pb3(m3-O) units; only one benzenoid ring of the
triphenylene unit is shown; the secondary interactions are shown as dashed lines. Interatomic distances (A˚): Pb1–S1, 2.714; Pb1–S2, 2.666; Pb1–
S10, 3.666; Pb1–S20, 3.258; the O/O contact is 2.659 A˚. (b) Two HTT linkers bonded to a Pb3(m3-O) pair, together with the topological
representation shown as fragments of two interpenetrating 3-connected, gyroidal nets (shown in yellow and purple, respectively). (c) An
overview of the single crystal structure of HTT–Pb with the two corresponding gyroidal nets. (d) Two parallel HTT linker molecules, each from
a separate gyroidal net, together with their topological equivalents (in yellow and purple, respectively). Pb atoms: green; S: orange; C: grey; O:
red.
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of an HTT ligand. The O atom is bonded to three crystallo- stable in water and common organic solvents (PXRD pattern
˚
graphically equivalent Pb(II) centers (Pb–O distance: 2.301 A; d in Fig. 2), and can be handled in air for several hours without
Pb–O–Pb angle: 110.0ꢀ) to form a Pb3(m3-O) core with the shape signicant degradation in crystallinity (see Fig. S2† for PXRD
of a squashed trigonal pyramid (Fig. 1a). The Pb–O distance is patterns; see also Fig. S3† for Raman spectra). With longer
quite short, similar to those found in the crystal structure of exposure to air (e.g., over 24 hours), the light orange, as-made
PbO,13 wherein the short Pb–O distances generate substantial crystals gradually darken, while the crystallinity degrades
semiconductive attributes in the solid state. The Pb(II) ion is signicantly, as shown by the weaker and broadened PXRD
also chelated to two S atoms of HTT with characteristic covalent peaks in Fig. S2.† The substantial timeframe (hours) of stability,
˚
Pb–S distances of 2.666 and 2.714 A. The trigonal Pb3(m3-O) core, however, allows electronic properties and device studies to be
together with the tritopic HTT linker, generates a three- conducted in air using the as-made sample of HTT–Pb.
connected network of the gyroid topology (srs net; Fig. 1b and
c).
Diffuse reectance measurement of an as-made HTT–Pb
solid (Fig. S4†) indicates a band gap of about 1.7 eV, which is
The inorganic node of the srs net, i.e., the Pb3(m3-O) core, smaller than that for most of the semiconducting lead-sulfur-
pairs up via distinct intermolecular interactions (Fig. 1a and b). organic network solids.14 The pressed pellet of HTT–Pb as
The two Pb3O pyramids are related by the 31 screw axis, and the measured in a two-probe setting exhibits an electrical conduc-
two O apices are only 2.7 A apart, indicating an O–H/O tivity of 1.1 ꢂ 10ꢁ6 S cmꢁ1 at room temperature. Values of the
˚
hydrogen bonding interaction. In addition, the two Pb3O pyra- variable-temperature conductivities of the same sample follow
mids are also linked by three symmetry-related Pb/S secondary the Arrhenius behavior, with an activation energy of 0.396 eV
˚
interactions (i.e., Pb/S1) at 3.507 A. The Pb center is also in (Fig. S5 and S6†). The low activation energy could be due to the
contact with two other S atoms (Pb/S distances: 3.258, 3.666 extrinsic resistance—e.g., from contact, grain boundary—being
˚
A); these two sulfur atoms being chelated to another Pb center dominant (as compared with the intrinsic resistance of the
within the same Pb3O unit, and they do not serve to link up framework solid), thus obscuring the temperature dependence
across the two Pb3O pyramids.
of the intrinsic, band-like conduction. The electrical conduc-
Two interpenetrating gyroid nets were found in the crystal tivity of HTT–Pb compares well with those of other semi-
structure of HTT–Pb, each residing in one of the two channel conductive metal–sulfur nets.9i,14a,15
domains molded by the gyroid surface (Fig. 1c). The three-fold
The semiconducting HTT–Pb, with its porous network and
symmetry of the HTT triphenylene core, whose centroid lies at water stability, suggests chemiresistive application. For this,
the fractional coordinates (0.40, 0.40, 0.40), coincides with the a moisture sensor device was conveniently fabricated by drop-
three-fold axis (e.g., the body diagonal of the unit cell) of the casting an ethanol suspension of ground HTT–Pb crystals
gyroid net. Across the two srs nets, distinct pairs of HTT onto an Al2O3 substrate with interdigital electrodes.10b The
˚
centroids at the distance of 6.94 A can be identied (Fig. 1d); the setup for sensing measurement was home-built as schematized
corresponding two triphenylene units are staggered, forming in Fig. 3a and S7.† A constant bias potential of 1.0 V was applied
a well-dened void that appears to be well-suited for the dock- via a Keithley source meter to the sensor device housed in a gas-
ing of planar aromatic guests (i.e., to afford an optimal stacking ow chamber. Air streams with different relative humidity were
˚
distance of 3.47 A with the triphenylene anks).
achieved by changing the relative rates of dry air ows through
Powder X-ray diffraction (PXRD) indicates the crystalline a water-lled bubbler and a dry tube, with the actual relative
phase purity of the HTT–Pb sample (Fig. 2). The HTT–Pb host
net remains upstanding aer being heated under vacuum at
150 ꢀC (PXRD pattern c in Fig. 2). The HTT–Pb crystals are also
Fig. 3 (a) The homemade setup for the humidity measurement (MFC:
Fig. 2 PXRD patterns (Cu Ka, l ¼ 1.5418 A˚): (a) calculated from the mass flow controller); (b) current response of a HTT–Pb based sensor
single-crystal structure; (b) as-made bulk sample; (c) sample from (b) to dry air and different RH (5–100%) at 25 ꢀC; (c) humidity-dependent
evacuated by using an oil pump at 150 ꢀC for 1 h; (d) sample from (b) responses; (d) curves of current vs. time of the HTT–Pb based sensor
after being soaked in deaerated water at rt for 24 h.
at various RH obtained by the DC reverse polarity method.
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humidity (RH) determined using a commercial humidity
sensor. A stable baseline current was observed under owing
dry air; when moist air was mixed in the ow, as shown in
Fig. 3b, the electrical current increased rapidly; reverting to dry
air, on the other hand, causes the current to drop sharply back
to the baseline current within about 6 seconds. Also, a broad,
monotonic response ranging from 5 RH% to 100 RH% was
observed. The signal strength is also remarkable, e.g. with
a baseline current at 10ꢁ12 A, the conductance increased by over
4 orders of magnitude at 90% RH, with a current as high as 2.7
ꢂ 10ꢁ7 A (Fig. 3b and c).
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This work was supported by the National Natural Science
Foundation of China (51402293, 21471037 and 61571140), the
NSF of Fujian (2015J01230), Guangdong Natural Science Funds
for Distinguished Young Scholars (15ZK0307), the Research
Fund for the Doctoral Program of Higher Education of China
(Grant 201244200120007), and the Research Grants Council of
HKSAR [GRF 11303414]. We also thank Prof. Wei-Xiong Zhang
at Sun Yat-Sen University and Prof. Qiao-Wei Li at Fudan
University for helping with the gures.
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